Corrosion: Factors, Chemistry and Thermodynamics
CHEM 2040 - Organic Chemistry I
University of Cincinnati
June 2, 2024
Introduction
Corrosion is defined as the breakup of materials to its component atoms
after series of chemical reactions caused by its immediate surrounding or
immediate environment. This article is going to examine the dynamics
involving corrosion of materials. The corrosion of materials is attributed to
a number of factors that are contributed by the environment. Corrosion
contributes to loss of materials especially metallic in nature. The word
corrosion has often substituted the word chemical oxidation in most
literature. Metals are the most affected materials and this affects the
operations of most machinery made of metallic materials. The smooth
operations of metallic parts in a machine can be greatly hindered by
corrosion of the moving parts.
Definition of corrosion
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Metals are susceptible to chemical reactions which results in them being
eaten away through oxidation processes. This also results in the change of
composition of its atoms making the metal much weaker. Corrosion of
metal manifests itself in the form of rust. In his studies Brewster (2007)
states that the degree of corrosion also varies from material to material as
the chemical composition of every material differs from each other. There
are some materials that corrode due to mere exposure to moisture in the
air; while there are some metal are corroded after heavy exposure to
moisture in the form of water. There are several methods of mitigating the
effects of corrosion on materials. While they may vary in terms of actual
results, some of them have been proven to work. An example of a
preventive measure is to oil the moving parts and chromate conversion.
According to Clark (1992), corrosion is a long term, process and also
matters to the environmental factors. Cars found in coastal towns exhibit
heavy signs of corrosion due to the heavy humidity.
Corrosion in Other material
Polymers are the best examples of material other than metals that undergo
corrosion. There are changes in the structure of the polymer which results
in the weakening of the material if exposed to environmental elements
such as heat, rain and other elements. Polymer cracks when corroding and
loses weight. Corrosion in polymers is be due to the large size of the
molecules which reduce the tensile strength of the polymer and results in
further cracking and general disintegration. Apart from the environmental
factors, polymers can also be degraded when exposed to chemical agents
such as alkali and acid.
Factors contributing to corrosion
Corrosion is affected by many factors but the main ones include,
temperature, size of the metal, type of the metal, climate. Temperature
increases the rate of disintegration of a metal. Any rise in temperature
alters the chemical structure of the material. A hot and moist environment
is sure to have a great effect on the rate of corrosion and this explains the
reason why corrosion happens faster in coastal towns. Another factor that
affects corrosion is the size of the metal. A metal that displays thick
structure characteristics is more susceptible to corrosion. This fact is clearly
manifested during the treatment of the metal, the thinner sections of the
metal are often found to have a lower level of corrosion attack than the
rest of the metal which is thicker. When treating the metal, it is advised to
first identify the integrity of the metal and strong points of the major parts,
this will ensure that corrosion is detected first and preventive measures
taken in order to stop it from spreading to other parts of the metal.
Another factor that has an inclination towards the rate of corrosion on a
material is the presence of foreign material. Foreign materials are
manifested through soil, foreign residue and even atmospheric dust. Soil
that can be trapped between the materials can increase the rate of
corrosion as it becomes more and more humid as it absorbs atmospheric
moisture.
Chemistry of Corrosion
The chemistry of corrosion refers to the forces that drive the metals to
disintegrate from their original chemical structure, to become weaker.
Energy is required in order to liberate the metals from their original
compounds. Reactions occur as a result of swap over of electrons.
Corrosion is mostly contributed from an electrochemical progression on
the material. Environments with acid and one that is salty hasten the rate
at which materials are degenerated. Such environments support the
formation of electrolytic cells on the material that leads it to be eaten
away. Corrosion can also be presented in the formation of green sediments
in the materials. The temporal presence of corrosion in metallic form is
what causes the degeneration. It is necessary for the material to soak up
and store up energy that is necessary for the liberation of the material from
their original compound.
The environment is controlled by factors such as the alkalinity of solution
where the material could be exposed to. An alkaline environment is one
that is composed of excess hydrogen ions that could be present. The
hydrogen ions contribute to acidity and the hydroxyl ions contribute to the
alkalinity of the solution. The solution can also change in its other
properties without necessarily increasing in acidity. This can be shown by
the example whereby hydrogen ions can be formed in water by the
bringing in hydrochloric acid or sulfuric acid, both of which are immaterial.
Corrosion affects mostly iron and steel after oxidation. There are metals
such as aluminum which react to environmental oxidation by forming an
outer shell which is a tough oxide that links itself to the metal surface and
thus limiting further exposure of the metal from further effects of
corrosion and oxidation agents. It is also important to note that corrosion
happens in areas where moisture is evident. Moist air can also cause
corrosion as in the case of most metal corrosions, this leads to the
phenomenon that is mostly referred to rusting of metals. This is explained
using the chemical equations below.
Fe ——-> Fe+2 + 2 e–
The quantity of water that can b complexed with the iron (III) oxide that is
also referred to as ferric oxide, this amount also determines the color of
rust or corrosion. In his experiments Talbot (2008), reports that corrosion
can be seen through many colors from yellow to orange and sometimes
orange brown. The next step of events is characterized by the presence of
moisture in the form of water and oxygen. The process of oxidation results
in the formation of ferric ions as illustrated below:
Fe+2 ————> Fe+3 + 1 e–
Reduction of oxygen is done through electrons that have been supplied
from the oxidation steps that have been shown above.
O2 (g) + 2 H2O + 4e– ——> 4 OH–
Formation of ferric oxide is aided by the combination of oxygen and ferric
ions. The oxide is filled with various amounts of water in its process of
hydration. This is because of the actual electrons is generated through
actual oxidation of the metal which in our case is iron. The electrodes are
conducted through the process of diffusion in the water and transferred to
another point where there is sufficient oxygen. Varshneya (2006) states
that he resultant reaction is an electrochemical cell whereby the iron acts
as an anode; oxygen gas bubbles act as the cathode while the aqueous
solution acts a salt bridge. Moisture increases the rate at which rust
formation occurs. A salty environment also increases the rate of
electrochemical rusting.
Corrosion Thermodynamics
Damage is determined by reactivity of local environment.
Thermodynamics has been used to estimate any theoretical activity of any
metal or alloy in a situation that supports corrosion. This can be done when
the chemical composition of the metal is known. It is important to note
that the environment where the metal is exposed to is just a variable which
can easily vary with time and set conditions. The environment plays a
major role in influencing the formation of rust. The environment to some
extent matches the micro environmental conditions. In his discussions this
can be best explained as the confined environment at the external shell of
the metal. Thee extent to which the metal or material is corroded is
determined by reactivity of this confined environment, which is also
determined by factors such as ion flow, number of pH cells ands deposits.
Corrosion Kinetics
Corrosion situations can be best explained using a number of
thermodynamic principles. The situation encompasses the stability of the
chemical species and the reactions linked to the process. If two metals are
in contact with each other, they can generate a voltage as in an
electrochemical cell. In his discussions Clark (2002) explains that the
galvanic series determines the anode from the cathode. Materials that are
higher in the galvanic series have a tendency to sustain a cathodic reaction.
Metals such as iron will tend to corrode when attached to graphite or
platinum. Varshneya (2006), states that as much as the galvanic series tries
to present a clear view, it cannot forecast the rate of corrosion. Balancing
processes that can take place over surfaces that are metallic; the resultant
potential from such reactions is often conciliation between different
equilibrium potentials of all reactions drawn in. Polarization is referred to
as the variation between potential (E) and each reaction equilibrium
potential, Eeq. This can be illustrated through the chemical equation
below:
h=E-Eeq
Where h is defined as over potential. The polarization can be referred to
as anodic, when the electrode process is stepped up in terms of speed of
reaction.
Corrosion Prevention by Electrochemical Methods
Corrosion in materials can be avoided by applying electrochemical
principles that are categorized under two classes. There are sacrificial
anodes and cathodic shielding by the application of currents. In the
sacrificial anodes, the process of corrosion is permitted to take place on a
metal piece that could be superfluous to the metal structure. An example
of this can be shown when apiece of zinc metal is attached to a steel hull
of a ship or boat. This can be shown by an Evans Diagram below.
According to the Evens diagram, anode # 3 is shown as being protected
from corrosion and disintegration by the reaction of anode #2. Application
of this method can be shown in the Galvanized bolts, automobile steel
among other metal applications. We can again see that the galvanic series,
playing a role as characteristic coating that work on the principal of the
sacrificial anode are zinc on steel, aluminum and cadmium on steel. It is
important to note that cadmium is just below steel in the series; hence it
is limited in terms of “throwing power”. In his book Moran (2005) states
that throwing power can be best defined as the ability to shield metal from
corrosion over large distance.
Cathodic Protection through application of current: Voltage or cathodic
current is applied to the metal with the sole purpose of ensuring that the
metal requiring protection is kept at its cathodic region. A diagram
illustrating the whole process is as shown below:
The anode in this case can be consumable and made from the less costly
materials such as scrap metal. In his book, Riquelme (2009) concludes that
there are also incidences where the anode is not consumed at all and such
a case of cathodic protection is of steel in concrete that is reinforced. It has
been found that it is difficult to replace anodes. Varshneya (2006) states
that this method is limited by a phenomenon often referred to as stay
currents. This phenomena occurs when a short circuit path becomes is
present between the anode and cathode, whereby the current is
transmitted away. A good illustration of this would be the close proximity
of a second metal conductor to both anode and cathode. The short circuit
component has the effect of corroding the anode, hence limiting the
protection.
Methods of Control
Anodic protection: A current can be applied to a material in order to
induce the drawing out of electrons out of the metal until it reaches a state
where it penetrates into the passive region. The main advantage to this is
that it is much cheaper to run such a system. The main disadvantage of
such is that it is a complex system that requires specialized persons and
can easily become hazardous in a case where power fails. It can also
become unsafe as such a scenario can become uncontrollable. It is because
of this that such a system is not very popular.
Inhibitors: Inhibitors are implemented in the reduction or blocking of
corrosion as they adapt several mechanisms such as Adsorption inhibitors
which shield the metal from corrosion by adsorption on the metal that
could be presented to the electrolyte. Another technique of using
inhibitors is through the introduction of poisons that act as inhibitors
limiting hydrogen ion reduction or generation of cathodic reduction
involving hydroxyl ions. The third type of inhibitor mechanism is through
filming inhibitors which is through the formation of protective films
through the addition of particular ions that have high redox reaction
potential.
Types of Corrosive atmospheres
There are several types of corrosive atmospheres where they are defined
by the number of corrosive elements that could be present. It has been
found out that the more aggressive the corrosive environment, the higher
the importance in choosing a proper protective coating that can withstand
it. Basically there are four types of corrosion environments such as
industrial, marine, rural and indoor environments. A manufacturing
environment is full of contamination that comprises of sulfur and other
lethal compounds which aids in the formation of acid rain. When the rain
falls on metallic objects, it corrodes the metals as it combines with smog
and other gaseous compounds to form acidic rains. It is also important to
note that an acidic environment is also full of other compounds such as
chlorides that are even more corrosive. Corrosion caused by chloride
compounds is more pronounced than in other compounds.
Marine environment: In his research Kaesche (2004) concludes that the
degree of corrosion id determined by the quantity of salt deposited which
is also dependent on the speed of the wind which carries fine particles of
salt. An exposed metallic surface can cause a corrosive external metal shell.
The rate of corrosion has been observed to decrease as one gores further
away from the sea as the salt deposits become fewer and fewer because
of the effect of wind currents. Equipment that are exposed to the sea water
are the most affected as the continued splashing and spraying of sea water
subjects them to continued cycle of corrosive agents in the form of salt
crystals.
Rural: Metals rural atmospheres are exposed to the byproducts of
materials found in the farm. Such materials have been found to be
extremely corrosive. Waste products from animals, farm pesticides are just
a few examples of rural materials that can corrode metals.
Indoor: Indoor environments are the least severe corrosive environments
as factors such as wind and extreme heat are absent. However indoor
environments provide constant condensation which can be highly
corrosive to metals within the confines of the rooms. Indoor environments
should be properly ventilated in order to allow the flow of air in to the
rooms.
Factors affecting Atmospheric Pollution
Atmospheric pollution is determined by the moisture content in the
atmosphere, formation of rain or relatively high humidity. All these factors
play a very big role in atmospheric corrosion. Rain also has an important
role of washing away salt deposits that may have accumulated on exposed
surfaces but also on the other hand it has the effect of corroding metals
that if subjected to the metals for a longer time. Dew can become soaked
with salt from the oceans and hence thus offer an electrolyte that aids in
deterioration of metals.
Measurement of atmospheric factors
There are several techniques which govern how environmental factors will
be measured. In his studies Ojovan (2007) explains that the factors
measured are the pollutants in the atmosphere, amount of sediments
collected on the metal surface and the difference in the time interval
between the times that the atmospheric factors have had their effects on
the material. Factors such as the temperature, relative humidity and wind
direction can be easily measured but some factors such as the time of
wetness which is the period between the times that the water settles on
the metallic surface are hard to measure. The time of wetness is also the
time that the relative humidity is higher than eighty percent. Another
method of measuring atmospheric corrosion factors is through the
quantity of pollutants that are measured in terms of the rate of salt
deposition. Other methods include measuring the salinity of airborne
chlorides and measurement of atmospheric corrosivity through coupon
exposures.
Pipeline Corrosion
Pipelines deteriorate after some time which is affected by quality of the
pipe and environmental effects. Some pipes have been found to last
around seventy years if the quality of the metals was found to be high. This
lifetime can be increased by implementing coatings and also utilizing
methods such as Cathode protection of the metal comprising the pipeline.
Pipe line corrosion in oil & gas pipes (petroleum) & how to treat it
Corrosion in petroleum pipes is estimated to cost petroleum companies
about 5.4 Million dollars annually in form of failures and repairs. These
companies have resulted in the development of risk management
strategies that involve prevention of corrosion in the pipes. Mitigating
factors include, Cathode protection of the pipes, regular inspections
carried out to evaluate any cases of impending corrosion. Detection of
flaws is implemented through smart pigging which involves
Treatment of the flaws after detection is done through, coating of the pipe
and performance of cathode protection on the pipes.
Forms of Corrosion
There are several forms of corrosion that exists and are manifested
through the degree of attack and the visual observation that can be
recorded from the metals. Uniform corrosion reduces the size of the metal
as it becomes thinner and thinner over duration of time. This form of
corrosion attack can be prevented through coating of the metal, providing
cathodic protection and introduction of inhibitors.
Crevice Corrosion: This type of corrosion is linked with small volumes of
dormant solutions. This solution can cause holes, and lap joints in stable
metals and thus is referred to as crevice corrosion.
Pitting: This form of corrosion often leaves holes on the metal thus causing
weaknesses in the structure of the metal. This form of degradation leads
to loss of manufacturing apparatus to fail as numerous machine damage
leads to systematic structural loss of the metal. This form of corrosion is
also difficult to detect as symptoms show up after the full effect has been
detected. The other forms of corrosion have less impact but also
contribute to the weakening of the metal structure; they are Intergranular
corrosion, selective corrosion, erosion corrosion, stress-corrosion cracking
Galvanic Corrosion: This type of corrosion occurs when two metals are in
contact or could also be connected through electrical means. The metal
that has a lesser resistance becomes anodic, and because the corrosion is
between two different metals, it is thus referred to as galvanic corrosion.
Intergranular Corrosion: This type of corrosion is further heightened by
surface tension.
Selective Leaching: This process involves the selective elimination of zinc
away from alloys such as brass and other alloys.
Erosion Corrosion: This form of corrosion can be detected by the
appearance of grooves and rounded holes in the metals. It is also
characterized by short duration in which it attacks the metals and resultant
rapid degradation of the material.
Stress-Corrosion Cracking: This type of corrosion is characterized by
coincident cracking which can be manifested through fine cracks which can
be detected when viewed keenly. The metal is virtually un-attacked on the
surface but is attacked on the grain of the metal. Cracking appears through
the metal hence compromising its strength.
Corrosion testing & monitoring
There needs to be control measures in the mitigation of corrosion effects.
The close monitor of metallic characters can decrease the overall effects of
corrosion on metals. The benefits of such measures greatly outweigh the
costs of undertaking such strategies. These strategies should be done
periodically. The timely shut down of systems such as pipelines and
factories in order for monitoring to be done is important so as to detect
and mitigate corrosion effects. There are several systems in the market
that vary both in cost and complexity. We have hand held loggers that are
used in the surveillance of atmospheric corrosivity. There are also systems
that relay information directly and simultaneously to control rooms and
persons. Such information is important as they are able to pin point the
area where corrosion is about to affect the whole industrial system. These
methods are performed using high precision sensors which are able to
dwell on every detail within the metal structure hence prolong the life of
the metal.
Protective Coating
This is the most widely used technique of controlling corrosion as the
coating separates the surfaces are subject to corrosion from the factors in
the environment that heavily cause corrosion. Coatings are important
when combined with other techniques of corrosion prevention such as
cathodic protection.
Effects of coating on corrosion
Coating assists in the control of corrosion as various coats that are resistant
to the corroding factors is highly wanted. Such coatings include powder
coatings, which are applied to metallic shells. Many types of coatings have
different characteristics that need to be understood before being applied
in any environment. Such considerations include the physical properties of
the metal where the coating will be applied, the mechanism associated
with the coating to be applied and the general environmental elements
that the metal is subjected to. Coating also assists in protecting the metals
from the environmental element such as heat and excess moisture.
Another type of coating used in the prevention of corrosion is through
electroplating which is mostly utilized through the passage of current in
liquid containing metal ions that are in a liquefied form. The other form of
coating is zinc coating which uses other the advantages found in zinc metal.
Zinc displays outstanding characteristics such aas its resistance to
corrosion in most interior and exterior environments. Zinc coating can also
be applied to most metals without any problem occurring hence making it
to be one of the best coating materials. The third type of coating that can
be applied on metals is through electroless plating which utilizes
electroless nickel. Pack cementation is widely used in present resistance to
ferrous alloys. The whole process of cementation includes aluminizing,
chromizing and introducing silicon to the metal. The other type of metal
includes metal cladding and thermal spraying which involves putting in
energy to the metallic surfaces in order for adhesion to occur. Plasma are
applied in the reduction of the high temperatures that happen in the
process. Corrosion prevention can also be done through physical vapor
deposition which is very clean whereby the coating is dumped over the
entire material concurrently as opposed to one particular area.
CP Design Systems
Cathodic protection designs are first implemented by conducting the
required testing and conduction of site evaluations. Cathodic protections
assist in the protection of structures as corrosion effects are mitigated
fully. This method utilizes metals that could be in contact with each other.
A metal that could be in touch with the other is fully induced with a
polarized current that is strained against the interface. Polarization of the
metals results in the metal achieving a homogeneous interface which is
able to withstand corrosion and corrosion effects.
Factors affecting CP Design: The design of a Cathodic protection system
involves many factors such as the selection of a material that fulfils the
requirements set for a suitable material that will act as the anode, the
physical dimensions of the material and the technique in which installation
of the galvanic methods. The design values of the cathode protection are
also to be taken into consideration; this includes the current densities
which is the current per unit surface area. Another factor that needs to be
taken to consideration is the coating effect in respect with the demands
placed on current.
Cathodic Protection
CP. shields metals by interfering with natural actions of the
electrochemical cells accountable for metal deterioration. CP. can be
implemented to manage the metal surfaces that are open to
environmental elements. This is done in order to provide the necessary
electrical current to the galvanic anode protection.
Conclusion
Corrosion has the effect of deleting the metallic structure which greatly
inhibits the performance of equipments and machines. Greater emphasis
on research into ways of reducing the effects of corrosion should be
developed in order to mitigate the financial cost that is experienced by
most persons especially those in the industrial sector.
List of References
Brewster, M. (2007) Fundamental aspects of corrosion films in corrosion
science. California, Springer.
Clark, E. (2002) Corrosion of Glass, Ceramics and Ceramic Superconductors.
Sydney, CRC Publishers.
Kaesche, H. (2004) Corrosion of metals: physicochemical principles and
current problems. Sydney, Longman Publishers.
Moran, C. (2005) Corrosion monitoring in industrial plants using
nondestructive testing and electrochemical methods. Perth, ASTM
International.
Ojovan, M. (2007) New Developments in Glassy Nuclear Waste forms. New
York, Nova Science Publishers.
Riquelme, R. (2009) Corrosion: understanding the basics. London, Oxford
Publishers.
Talbot, E. (2008) Corrosion science and technology. New York, CRC Press.
Varshneya, A.K. (2006) Fundamentals of inorganic glasses. London, Oxford
publishers.
