Structure and Analysis Bridge Design brief

Moh-993

CSB-MM-Corrosion-Protection-2010.pdf

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Steel bridges – Material matters

Corus

Corrosion protection

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Contents 3 Introduction 4 Corrosion of structural steel 8 Influence of design on corrosion 10 Preparing for corrosion protection 15 Paint coatings 18 Metallic coatings 21 Appropriate specifications 24 Inspection and quality control 26 References and further reading

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1. Introduction

The use of steel for modern bridges has grown significantly over the last 30 years. Engineers and specifiers have recognised the benefits that steel offers as a construction material, which combined with imaginative designs has resulted in some striking bridges that have not escaped the public’s attention.

The use of steel in bridges goes back over 100 years. A notable example is the imposing Forth Rail Bridge in Scotland, which was completed in 1890. The scale and size of this significant landmark was a major achievement in construction engineering, and the structure has stood the test of time. The surface preparation and painting systems used on this bridge, and on similar old steel bridges, are quite primitive by modern standards and frequent maintenance is required to ensure a continued serviceable life.

Modern bridges currently have a design life requirement of 120 years, and the performance of the protective system is a critical factor. Furthermore, reductions in the number of repainting cycles have become significant in the evaluation of whole life costs.

There has been a widely held view that most steel bridges require frequent attention to maintain the original protective coating system. In reality, coating lifetimes to first major maintenance have progressively increased from 12 to 15 years to 20 to 25 years.

From the continued developments in coating technology, modern high performance coating systems may be expected to achieve lives to first major maintenance in excess of 30 years on thoughtfully designed steel bridges. In addition, the use of Weathering steel offers a very low maintenance alternative, and further information is provided in the separate ‘Weathering steel’ 1 publication in this Steel bridges – Material matters series.

Right: Forth Rail Bridge, Scotland Opposite: Hulme Arch Bridge, Manchester

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2.1 The corrosion process The corrosion of steel can be considered as an electrochemical process that occurs in stages. Initial attack occurs at anodic areas on the surface, where ferrous ions go into solution. Electrons are released from the anode and move through the metallic structure to the adjacent cathodic sites on the surface, where they combine with oxygen and water to form hydroxyl ions. These react with the ferrous ions from the anode to produce ferrous hydroxide, which itself is further oxidised in air to produce hydrated ferric oxide (i.e. red rust.) The sum of these reactions can be represented by the following equation:

4Fe + 302 + 2H20 = 2Fe203.H20 (Steel)+(Oxygen)+(Water) = Hydrated ferric oxide (Rust)

The process requires the simultaneous presence of water and oxygen. In the absence of either, corrosion does not occur.

However, after a period of time, polarisation effects such as the growth of corrosion products on the surface cause the corrosion process to be stifled. New, reactive anodic sites may be formed thereby allowing further corrosion. In this case, over long periods, the loss of metal is reasonably uniform over the surface, and this is usually described as 'general corrosion'. A schematic representation of the corrosion mechanism is shown in Figure 1 right.

Figure 1. Schematic representation of the corrosion mechanism for steel

2. Corrosion of structural steel

Fe++ O2 H2O

++ ++ ++

++ --------

electrons

anode

at anode at cathode combined

cathode

Fe Þ Fe++ + 2e-

O2 + 2H2O + 4e- Þ 4OH-

4Fe + 3O2 + 2H2O = 2Fe2O3.H2O

OH-OH-

++ - -

Corrosion of structural steel

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2.2 Localised corrosion Various types of localised corrosion can also occur, but these tend to be less significant for bridge steelwork.

2.2.1 Bimetallic corrosion When two dissimilar metals are joined together and in contact with an electrolyte, an electrical current passes between them and corrosion occurs on the anodic metal. Some metals (e.g. stainless steel) cause low alloy structural steel to corrode preferentially whereas other metals (e.g. zinc) corrode preferentially themselves, thereby protecting the low alloy structural steel. The tendency of dissimilar metals to bimetallic corrosion is partly dependent upon their respective positions in the galvanic series, the further apart the two metals in the series the greater the tendency, see Figure 2 right.

Another aspect that influences bimetallic corrosion is the nature of the electrolyte. Bimetallic corrosion is most serious for immersed or buried structures, but in less aggressive environments e.g. stainless steel bearings attached to weathering steel girders on a bearing shelf, the effect on the weathering steel girders is minimal and in most practical bridge situations, no special precautions are required. Note that bimetallic corrosion is seldom an issue for ordinary structural steel on bridges, as it is invariably painted which insulates the metal from the electrolyte thus breaking the circuit.

The tendency for bimetallic corrosion is also influenced by the relative surface areas of the cathodic and anodic metals (Ac/Aa). In simple terms, the greater the Ac/Aa ratio, the greater the tendency for bimetallic corrosion.

Figure 2. General galvanic series

Anodic end (More prone to corrosion)

Magnesium Zinc Aluminium Carbon & Low Alloy (Structural) Steels Cast Iron Lead Tin Copper, Brass, Bronze Nickel (Passive) Titanium Stainless Steels 430/304/316 (In the passive state)

Cathodic end (Less prone to corrosion)

When in bimetallic contact, metals higher in the series corrode preferentially to metals lower in the series.

Bimetallic corrosion

Above: Holmfield Viaducts, M62/A1(M) Junction, Ferrybridge

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2.2.2 Pitting corrosion In some circumstances the attack on the original anodic area is not stifled and continues deep into the metal, forming a corrosion pit. Pitting more often occurs with low alloy structural steels in continually wet conditions or buried in soil rather than those exposed in air. Hence, pitting corrosion is rarely encountered on typical modern steel bridges.

2.2.3 Crevice corrosion Crevices can be formed by design detailing, welding, surface debris, etc. Available oxygen in the crevice is quickly used by the corrosion process and, because of limited access, cannot be replaced. The entrance to the crevice becomes cathodic, since it can satisfy the oxygen-demanding cathode reaction. The tip of the crevice becomes a localised anode and high corrosion rates occur at this point.

2.3 Corrosion rates The principal factors that determine the rate of corrosion of steel in air are:

2.3.1 'Time of wetness' This is the proportion of total time during which the surface is wet, due to rainfall, condensation etc. It follows, therefore, that for unprotected steel in dry environments (e.g. enclosures), corrosion will be minimal due to the low availability of water.

2.3.2 ‘Atmospheric pollution’ The type and amount of atmospheric pollution and contaminants (e.g. sulphates, chlorides, dust etc.)

Sulphates These originate from sulphur dioxide gas produced during the combustion of fossil fuels, e.g. sulphur bearing oils and coal. The sulphur dioxide gas reacts with water or moisture in the atmosphere to form sulphurous and sulphuric acids. Industrial environments are a prime source of sulphur dioxide.

Chlorides These are mainly present in marine environments. The highest concentration of chlorides is to be found in coastal regions and there is a rapid reduction moving inland. In the UK there is evidence to suggest that a 2 kilometre strip around the coast can be considered as being in a marine environment.

Both sulphates and chlorides increase corrosion rates. They react with the surface of the steel to produce soluble salts of iron, which can concentrate in pits and are themselves corrosive.

In a given local environment, corrosion rates can vary markedly, due to effects of sheltering and prevailing winds etc. It is the 'micro-climate' immediately surrounding the structure that determines corrosion rates for practical purposes.

Because of variations in atmospheric environments, corrosion rate data cannot be generalised. However, environments can be broadly classified, and corresponding measured steel corrosion rates provide a useful indication of likely corrosion rates. More information can be found in BS EN ISO 12944-2 2

and ISO 9223 3, see Figure 3 below.

Figure 3. Atmospheric corrosivity categories and examples of typical environments (BS EN ISO 12944-2 2 )

Corrosivity category and risk

Low-carbon steel Thickness loss µm

(see Note 1) Exterior

C1 very low ≤ 1.3 –

C2 low > 1.3 to 25

Atmospheres with low level of pollution. Mostly rural areas.

C3 medium

> 25 to 50 Urban and industrial atmospheres, moderate sulphur dioxide pollution. Coastal area with low salinity.

C4 high

> 50 to 80 Industrial areas and coastal areas with moderate salinity.

C5-I very high (industrial)

> 80 to 200 Industrial areas with high humidity and aggressive atmosphere.

C5-M very high (marine)

1µm (1 micron) = 0.001mm

1. The thickness loss values are after the first year of exposure. Losses may reduce over subsequent years. 2. The loss values used for the corrosivity categories are identical to those given in ISO 9223 3. 3. In coastal areas in hot, humid zones, the mass or thickness losses can exceed the limits of category C5-M. Special precautions must therefore be

taken when selecting protective paint systems for structures in such areas.

> 80 to 200 Coastal and offshore areas with high salinity.

Examples of typical environments In a temperate climate (informative only)

Corrosion of structural steel

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This page: Øresund Bridge, Copenhagen - Malmo (Photo courtesy of Arup)

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3. Influence of design on corrosion

The design of a structure can affect the durability of any protective coating applied to it. Old steel bridges designed with many small structural components and fasteners, e.g. bracings and rivets, are more difficult to protect than modern designs with large flat surfaces.

The articulation of a bridge also influences its durability as leaking deck joints have often been the source of corrosion problems. Ideally, expansion joints should be avoided by the use of continuous and integral construction. However, if expansion joints are unavoidable they should be located away from the ends of the girders, and a positive non-metallic drainage system should be provided to convey any leaks away from the steelwork.

Detailing is important to ensure that the protective treatment can be applied to all surfaces, to avoid the creation of water and dirt traps that would accelerate corrosion, and could lead to pitting corrosion. Detailing is also important to ensure that future inspections and maintenance can be carried out effectively.

Guidance for the prevention of corrosion by good design detailing can be found in BS EN ISO 12944-3 4.

3.1 Access for coating application and maintenance Access to all surfaces to provide both the initial surface treatment and subsequent maintenance painting is essential. Narrow gaps, difficult to reach corners, and hidden surfaces should be avoided wherever possible. Similarly, clearance between connecting members at junctions, and the degree of internal angles at skewed web stiffeners should allow access for coating and inspection. Refer to Figure 4 opposite.

3.2 Copes A typical detail that is difficult to protect is a cope hole in a web stiffener. Unless the hole is very large, it is virtually impossible to blast clean the surface properly and to apply a protective treatment to the surface. Ideally copes should be avoided by using close fitting snipes and a continuous weld around the corner. Although this may form a moisture/dirt trap, it is considered a better detail than having a drainage path through a cope where the protection system is at its most vulnerable.

If cope holes are used, they should be circular and of at least 40mm radius, preferably more. Cope holes formed by 45º snipes should not be used. The weld will not be returned through the hole, which creates the additional problem of a narrow crevice.

3.3 Avoidance of moisture and debris traps Details that could potentially trap moisture and debris should be avoided where possible. Measures that can be taken include: • Grind flush welds on horizontal surfaces. • Curtail transverse web stiffeners short of the bottom flange. • Avoid using channels with toes upward. • Arrange angles with the vertical leg below the horizontal. • Avoid the use of ‘T’ section bearing stiffeners.

3.4 Crevices Crevices attract and retain water through capillary action, and should be avoided. Preloaded bolted joints pose a particular problem, so welded connections are preferable in terms of corrosion protection. However, crevice effects on preloaded bolted connections can be minimised by limiting the bolt spacing and edge distance, using flexible cover plates, and sealing the edges of the joint. Crevices at the intersections of cross bracings should be avoided by using a packing plate the same thickness as the web stiffener, and a single preloaded bolt through all three pieces.

3.5 Drainage and ventilation Provision should be made for adequate drainage and ventilation to enable the steel to dry out, e.g. minimise the ‘time of wetness’. Closely spaced girders should be avoided and deck run-off should be directed away from steel surfaces. In addition, the use of wide cantilevers with suitable drip details should be considered.

Left: Festival Park Flyover, Stoke Below: A63 Accommodation Bridge, A1(M) Ledsham

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Preferred detail Grind stiffener to avoid web to flange weld

45º snipe Not recommended

Angles with vertical leg below the horizontal

Provide clearances for access to all surfaces

Access for coating & inspection

25mm clearance (min)

Detail to avoid water & dirt traps

Curtail transverse web stiffeners short of the bottom flange.

min. 30mm

‘Bad’ ‘Good’

Single Preloaded bolt

Spacer plate

Curtail stiffener at bottom flange

≤ 5tw

40-50mm radius cope

max. 30º

Figure 4. Detailing for durability

Plan on skew stiffener

Influence of design on corrosion

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4. Preparing for corrosion protection

The application of a protective coating system is the most common way of preventing corrosion. The effectiveness of the system depends upon the initial surface condition, the coating materials, the application procedures, the access for application and the environment under which the work is done.

4.1 Initial surface condition Structural steel elements in new bridges are usually either hot rolled sections or fabricated plate girders. The initial steel surfaces normally comply with rust grades A or B according to BS EN ISO 8501-1 5. Material that is pitted, i.e. rust grades C or D, should be avoided if possible, since it is difficult to clean all the corrosion products from the pits during surface preparation.

4.2 Surface preparation Surface preparation is the essential first stage treatment of a steel substrate before the application of any coating, and is generally accepted as being the most important factor affecting the total success of a corrosion protection system.

The performance of a coating is significantly influenced by its ability to adhere properly to the substrate material. Residual millscale on steel surfaces is an unsatisfactory base to apply modern, high performance protective coatings and is therefore removed by abrasive blast cleaning. Other surface contaminants on the rolled steel surface, such as oil and grease are also undesirable and must be removed before the blast cleaning process.

The surface preparation process not only cleans the steel, but also introduces a suitable profile to receive the protective coating.

4.3 Surface cleanliness Various methods and grades of cleanliness are presented in BS EN ISO 8501-1 5. This standard essentially refers to the surface appearance of the steel after abrasive blast cleaning, and gives descriptions with pictorial references of the grades of cleanliness. The standard grades of cleanliness for abrasive blast cleaning are:

Sa 1 – Light blast cleaning Sa 2 – Thorough blast cleaning Sa 21⁄2 – Very thorough blast cleaning Sa 3 – Blast cleaning to visually clean steel

Specifications for bridge steelwork usually require either Sa 21⁄2 or Sa 3 grades.

The cleaned surfaces should be compared with the appropriate reference photograph in the standard according to the specification.

Below: Jackfield Bridge, Shropshire

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Preparing for corrosion protection

Profile peaks less likely to protrude from thin coatings. Good mechanical adhesion. Recommended for thermally sprayed metal and high build paint coatings.

4.4 Surface profile and amplitude The type and size of the abrasive used in blast cleaning have a significant effect on the profile and amplitude produced. In addition to the degree of cleanliness, surface preparation should also consider 'roughness' relative to the coating to be applied. High build paint coatings and thermally sprayed metal coatings need a coarse angular surface profile to provide a mechanical key. This is achieved by using grit abrasives. Shot abrasives are used for thin film paint coatings such as pre-fabrication primers, but such coatings are rarely used on bridges. See Figure 5 below.

The surface treatment specification should describe the surface roughness required, usually as an indication of the average amplitude achieved by the blast cleaning process. Several methods have been developed to measure or assess the distance between the peaks and troughs of blast cleaned surfaces. These have included comparator panels, special dial gauges, replica tapes and traversing stylus equipment. Usually, comparators or replica tapes are used, and the relevant standards are BS EN ISO 8503-1 6, and BS EN ISO 8503-5 7 respectively.

4.5 Surface dust The blast cleaning operation produces large quantities of dust and debris that must be removed from the abraded surface. Automatic plants are usually equipped with mechanical brushes and air blowers. Other methods can utilise sweeping and vacuum cleaning.

However, the effectiveness of these cleaning operations may not be readily visible, and the presence of fine residual dust particles that could interfere with coating adhesion can be checked for using a pressure sensitive tape pressed onto the blast cleaned surface. The tape, along with any dust adhering to it, is then placed on a white background and compared to a pictorial rating. This method is described in BS EN ISO 8503-5 7.

Figure 5. Blast cleaning abrasive particles and steel surface profiles

Shot – Gives rounded profile Grit – Gives angular profile

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4.6 Additional surface treatments Sawn and flame-cut ends and edges need treatment to ensure that the coating adheres and is of sufficient thickness.

At outside arrises (i.e. the meeting between two surfaces) there is a potential problem when there is a sharp (i.e. 90°) edge, because the fluid coating will not cover it properly. Refer to Figure 6 below. Consequently, they should be smoothed by grinding or filing. It is generally considered sufficient to smooth the corner to a radius of about 2mm; chamfering to 45° is also effective, but it is difficult to avoid leaving some sharp edges when attempting this with hand tools. It can be argued that with modern high-build coatings and the use of stripe coats (an extra coat applied only locally), smoothing to a 1mm radius is adequate. Network Rail specifies a minimum radius of 3mm and this is considered by some steelwork contractors to be an onerous requirement.

In addition to the requirement for smoothing arrises, the Highways Agency’s Specification for Highway Works 8, specifies the application of one or more stripe coats for all external corners (and for welds and fasteners, for a similar reason).

The corners of rolled sections generally do not require grinding, as they are usually smooth as a result of the rolling process.

For the treatment of flame-cut surfaces, which are harder than the rolled surface, refer to Steel Bridge Group Guidance Note GN 5.06 9.

4.7 Site connections and splices Girder splices and connection details are often not given full protection in the shop, leaving the connection zones to be made good on site. A frequent consequence is that these zones are the least well prepared and protected, and are the first to show signs of breakdown. Hence, it is important to pay special attention to the corrosion protection of these areas.

4.7.1 Welded connections At welded connections, the key factors in ensuring the effectiveness of the coating system are the effectiveness of the protection before final coating. The areas local to welds are usually masked, to prevent them being coated. The masking stays in place until the joint is welded; this is not an ideal form of protection if there is prolonged exposure before welding.

After welding, it is essential that the joint surfaces, including the weld itself, are prepared to the specified standard of cleanliness and profile. Because of the contamination that occurs from the welding flux, particular attention needs to be paid to cleaning off all residues.

Figure 6. Cross-section showing reduction in coating thickness at a corner (image courtesy of Steel Protection Consultancy)

Above: Docklands Light Rail, London

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The surfaces of welds themselves should not need any grinding if they comply with the requirements of BS EN 1011-2 10 for smoothness and blending into the parent metal. However, rough profiles, badly formed start-stops, sharp undercut and other defects such as adherent weld spatter should be removed by careful grinding. Particular attention needs to be paid to the blast cleaned profile because weld metal is harder and site blast cleaning is more difficult than shop blasting.

4.7.2 Bolted connections Preloaded bolted connections merit particular consideration, both of the surfaces that will remain exposed and of those that will not (e.g. the faying surfaces). The friction surfaces are usually either unpainted or metal sprayed without sealer. Hence, they need to be protected (usually by masking tape) until the parts are finally bolted together.

Attention should be paid to the removal of any adhesive used on the protective films for the faying surfaces, and to the removal of any lubricants used on the threads of bolts. Care should also be taken to avoid contamination of surfaces during bolting up. For example, older air-power wrenches tend to produce a fine oily /misty exhaust which may settle on the surface.

4.8 Surfaces in contact with concrete Surfaces in contact with concrete are usually, with the exception of a marginal strip at the edges of the interface, blast cleaned bare steel. The marginal strip should be treated as an external surface, except that only the shop coats need be applied. The width of the marginal strip should ideally be at least equal to the required cover to the reinforcement, for the same exposure condition. A width of 50mm is common.

Any aluminium metal spray on surfaces in contact with concrete needs to receive at least one coat of paint to prevent the reaction that may occur between concrete and aluminium. It is recommended that any shear connectors are positioned such that they (and their welds) do not lie within the marginal strip; they should also be protected against overspray of the coating.

4.9 Damage during handling During handling, turning and assembly, damage to edges and to surfaces by the use of sharp-toothed clamps must be avoided by taking precautionary measures, such as the use of lifting devices with soft renewable contact surfaces or properly designed lifting cleats. If damage does occur, it must by carefully blended out by grinding (and the full protective treatment restored, with specified overlaps between coats).

4.10 Cleanliness at site Just as surface cleanliness before first coating is fundamental to performance of the system, so is the cleanliness of painted surfaces prior to the application of subsequent coats. On site, thorough cleaning shortly before painting is always necessary to remove contamination accumulated over time and from construction activities including dust, grout leaks from concreting, and the products of blast cleaning, bolting and welding.

Preparing for corrosion protection

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“The method of application and the conditions under which paints are applied have a significant effect on the quality and durability of the coating.”

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Opposite: Infinity bridge, Stockton-on-Tees

5. Paint coatings

Paint systems for steel bridges have developed over the years to comply with industrial environmental legislation and in response to demands from bridge owners for improved durability performance. Previous five and six coat systems have been replaced by typically three coat alternatives, and the latest formulations have focussed on application in even fewer numbers of coats, but with increasing individual film thickness. Examples of this are epoxy and polyester glass flake coatings that are designed for high build thickness in one or two coat applications. Also single coat high build elastomeric urethane coatings, up to 1000 µm (microns) thick, have been used on several bridges in Scotland since 1988.

Modern specifications usually comprise a sequential coating application of paints or alternatively paints applied over metal coatings to form a ‘duplex’ coating system. The protective paint systems usually consist of primer, undercoat(s) and finish coats. Each coating ‘layer’ in any protective system has a specific function, and the different types are applied in a particular sequence of primer followed by intermediate/build coats in the shop, and finally the finish or top coat on site.

5.1 Primers The primer is applied directly onto the cleaned steel surface or, in the case of duplex systems, the sealed metal coating. Its purpose is to wet the surface and to provide good adhesion for subsequently applied coats. For primers applied directly to steel surfaces, these are also usually required to provide corrosion inhibition. There are two basic types of primer.

Primers pigmented with metallic elements anodic to steel. These primers are formulated so that, when a break in the coating (due to damage or local corrosion) exposes the steel substrate, the anodic metal corrodes sacrificially in preference to the steel. This effectively stifles steel corrosion and under-rusting of the primer until the anodic metal is exhausted. Zinc-rich primers are the most commonly used of this type.

Primers relying on the high adhesion and chemical-resistance properties of the binder. With these primers, good adhesion is obtained (provided that the surface is very thoroughly cleaned) and it is sufficient to prevent under-rusting at any break in the coating (due to damage). Two-pack epoxy primers are typical of this type. These primers may contain inhibitive pigments to interfere with the corrosion process. Zinc phosphate, for example, is a mildly inhibitive pigment and is widely used in modern primer formulations.

Paint coatings

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5.2 Intermediate coats Intermediate coats are applied to ‘build’ the total film thickness of the system. Generally, the thicker the coating the longer the life. Intermediate coats are specially designed to enhance the overall protection and, when highly pigmented, decrease permeability to oxygen and water. The incorporation of laminar pigments, such as micaceous iron oxide (MIO), reduces or delays moisture penetration in humid atmospheres and improves tensile strength. Modern specifications now include inert pigments such as glass flakes to act as laminar pigments. Undercoats must remain compatible with finishing coats when there are unavoidable delays in applying them.

5.3 Finish coat The finish coat provides the required appearance and surface resistance of the system. Depending on the conditions of exposure, it must also provide the first line of defence against weather and sunlight, open exposure, and condensation (as on the undersides of bridges).

5.4 Stripe coats Stripe coats are additional coats of paint that are applied locally to welds, fasteners and external corners. Their function is to build a satisfactory coating thickness at edges and corners where paint has a tendency to contract and thin upon drying. Specifications should indicate the type and number of stripe coats required and state when they are to be applied.

5.5 The paint system The various superimposed coats within a painting system have, of course, to be compatible with one another. They may be all of the same generic type or may be different, e.g. chemical resistant types, such as a recoatable polyurethane finish coat, may be applied onto epoxy primer and intermediate coats. However, as a first precaution, all paints within a system should normally be obtained from the same manufacturer and used in accordance with the manufacturer’s recommendations.

An important factor in the coating system is the definition and measurement of the dry film thickness (d.f.t.). Dry film thicknesses are generally checked on the complete paint system, although individual films may be checked separately. Usually, nominal dry film thicknesses are specified but sometimes minimum values are quoted.

For nominal dry film thicknesses, individual values less than 80% of the nominal thickness are not acceptable. Values between 80% and 100% are acceptable provided that the overall average (mean value) is equal to or greater than the nominal.

Specifications for minimum dry film thicknesses require careful paint application to avoid excessive film thickness. The ‘over application’ of paints can result in the formation of high stresses and may cause premature failure of the system. Wet film thickness (w.f.t.) checks may also be required during the application of the coating to check that a subsequent satisfactory dry film thickness will be achieved.

5.6 The application of paint coatings The method of application and the conditions under which paints are applied have a significant effect on the quality and durability of the coating. Standard methods used to apply paints to structural steelwork include application by brush, roller, conventional air spray and airless spray.

Paint coatings for bridges are usually spray applied. The paint is atomised into fine droplets and projected onto the surface to be protected where the droplets join together to form a continuous film. The atomisation can be accomplished in a number of ways.

In air spraying, the paint is atomised by mixing it with a stream of compressed air in a conventional spray gun. The paint can be either sucked into the air stream (as in the simple suction-cup gun used for application to small areas) or fed to the spray gun under pressure from a pressure pot. For ideal application, careful adjustments of the spray nozzle and air pressures must be made by a skilled operator, according to the consistency and composition of the paint product and the film thickness required.

Paint coatings

Below: SmithKline Beecham, Marlow

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For airless spraying, the paint is hydraulically compressed and, on release through a small orifice in an airless spray gun, it is atomised and projected onto the surface. By changing the orifice size and shape and by varying the hydraulic pressure, atomisation can be accomplished for a wide range of paint consistencies from thin to thick, to give a wide range of rates of deposition. The equipment required is much more expensive than for conventional air spraying, because it must withstand the much higher pressures involved. For conventional air spraying, the maximum air pressure will normally not exceed 100 psi (6.9 bar); for airless spraying, hydraulic pressures up to 4,000 psi (280 bar) may be required.

A variant of the above involves heating to reduce the consistency of the paint rather than adding dilutants. In this way greater film thickness per application is achieved. This method can be used for the application of solvent-free materials such as two-pack products, which can be mixed at the spray gun nozzle at the moment of application. The use of expensive equipment and highly skilled labour is necessary for the achievement of optimum results but is justified for the protection of large and important structures.

With modern high performance coatings, correct application has become increasingly important to achieve the intended performance. Industry has recognised this and established a training and certification scheme for paint applicators (ICATS – Industrial Coating Applicator Training Scheme). ICATS registration has subsequently become a mandatory requirement for work on Highways Agency and Network Rail bridges.

5.7 Conditions of application The principal conditions that affect the application of paint coatings are temperature and humidity. These can be more easily controlled under shop conditions than on site.

Temperature Air temperature and steel temperature affect solvent evaporation, brushing and spraying properties, drying and curing times and the pot life of two-pack materials, etc. Where heating is required, this should only be by indirect methods.

Humidity Paints should not be applied when there is condensation present on the steel surface or the relative humidity of the atmosphere is such that it will affect the application or drying of the coating. Normal practice is to measure the steel temperature with a contact thermometer and to ensure that it is maintained at least 3°C above the dew point.

However, moisture cured paints are available. These paints are specifically formulated for application in damp and humid conditions; reference should be made to the manufacturer’s data sheets for details of limiting conditions of application.

Right: Hunslett Moor footbridge, Leeds

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6. Metallic coatings

The two most commonly used methods of applying metallic coatings to structural steel are thermal (metal) spraying and hot-dip galvanizing. In general, the corrosion protection afforded by metallic coatings is largely dependent upon the choice of coating metal and its thickness and is not greatly influenced by the method of application.

6.1 Thermally sprayed metal coatings Thermally sprayed coatings of zinc, aluminium, and zinc- aluminium alloys provide long-term corrosion protection to steel structures exposed to aggressive environments. They are an important component of coating systems that are currently specified by Network Rail and the Highways Agency, and they are commonly used on steel bridge decks prior to surfacing with mastic asphalt systems.

For bridge components, thermally sprayed aluminium is usually preferred and it acts as a barrier coating. However, for rail bridges likely to be subjected to collision damage, zinc is often preferred due to its sacrificial nature.

The metal, in powder or wire form, is fed through a special spray gun containing a heat source, which can be either an oxygas flame or an electric arc. Molten globules of the metal are blown by a compressed air jet onto the previously grit blast cleaned steel surface. No alloying occurs and the coating consists of overlapping platelets of metal and is porous. The adhesion of sprayed metal coatings to steel surfaces is considered to be essentially mechanical in nature. It is therefore necessary to apply the coating to a clean roughened surface and blast cleaning with coarse grit abrasive is normally specified.

The pores are subsequently sealed by applying a thin organic coating, which penetrates into the surface. Sealers may be either un-pigmented, with colouring agents or aluminium flake. Typically specified coating thicknesses vary between 100-200 µm for aluminium and 100-150 µm for zinc. For most bridge applications, thermal spray coatings are over-coated with paint coatings (after the application of a sealer coating) to form a ‘duplex’ coating system. The combination of metal and paint in a duplex protective treatment has greater durability in comparison with that of the individual components.

Thermally sprayed metal coatings can be applied in the shop or at site. No drying time is required, they do not sag or run, and can be applied to the required thickness in a single operation. There is no limitation on the size of the workpiece that can be coated, as there is with hot-dip galvanizing, and since the steel surface remains cool, there are no distortion problems.

For further information on thermally sprayed metal coatings, refer to Steel Bridge Group Guidance Note GN 8.04 11.

6.2 Hot-dip galvanizing Hot-dip galvanizing is a process that involves immersing the steel component to be coated in a bath of molten zinc (at about 450oC) after pickling and fluxing, and then withdrawing it. The immersed surfaces are uniformly coated with zinc alloy and zinc layers that form a metallurgical bond with the substrate. The resulting coating is durable, tough, abrasion resistant, and provides cathodic (sacrificial) protection to any small damaged areas where the steel substrate is exposed.

As the zinc solidifies, it usually assumes a crystalline metallic lustre, often referred to as spangling. The thickness of the galvanized coating is influenced by various factors including the size and thickness of the workpiece, the steel surface preparation, and the chemical composition of the steel. The typical minimum average coating thickness for bridge girders is 85 µm. Thick steel parts and steels which have been abrasive blast cleaned tend to produce relatively thick coatings.

Since hot-dip galvanizing is a dipping process, there is obviously some limitation on the size of components that can be galvanized. However, ‘double-dipping’ can often be used when the length or width of the workpiece exceeds the size of the bath. The longest tank in the UK is currently 21m in length, the maximum double-dip dimension is 30m, and the maximum lift weight is 12 tonnes..

Some aspects of the design of structural steel components need to take the galvanizing process into account, particularly with regards the ease of filling, venting and draining and the likelihood of distortion. To enable a satisfactory coating, suitable holes must be provided in hollow sections to allow access for the molten zinc, the venting of hot gases, and the subsequent draining of zinc. Further guidance on the design of articles to be hot-dip galvanized can be found in BS EN ISO 14713 12.

The suitability of steels for hot-dip galvanizing should also be considered. Structural steel that is to be hot-dip galvanized should be clearly specified, by invoking the appropriate options in the material standards, e.g. Option 5 in BS EN 10025 13.

Figure 7. Cross-section through a thermally sprayed aluminium coating

Aluminium

Steel

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For many applications, hot-dip galvanizing is used without further protection. However, to provide extra durability, or where there is a decorative requirement, paint coatings are applied. The combination of metal and paint coatings is usually referred to as a 'duplex' coating. When applying paints to galvanized coatings, special surface preparation treatments must be used to ensure good adhesion. These include light blast cleaning to roughen the surface and provide a mechanical key, and the application of special etch primers or 'T' wash, which is an acidified solution designed to react with the surface and provide a visual indication of effectiveness.

For further information on hot-dip galvanizing, refer to Steel Bridge Group Guidance Note GN 8.03 14.

6.3 Bolts, nuts and washers The exposed surfaces of bolts, nuts and washers need to be protected to at least the same level as the primary members of steelwork. Hence, the full coating system should be applied after assembly of the bolted joint.

However, bolts, nuts and washers need some short-term corrosion protection during the construction period until the full coating system is applied. This normally takes the form of a metallic coating applied during manufacture of the components. The recommended approach is to specify hot- dip galvanized bolts, nuts and washers, as this provides the highest level of corrosion protection with a considerably thicker coating than either sherardizing or electroplating.

The number of edges and potential crevices at bolted joints make them particularly vulnerable to corrosion. Hence, the Highways Agency Specification for Highway Works 8 requires stripe coats to be applied to all exposed surfaces of bolts, nuts and washers.

For further information on the protective treatment of bolts, refer to Steel Bridge Group Guidance Note GN 8.02 15.

Figure 8. Cross-section through a typical hot-dip galvanized coating

Zinc layer

Zinc/Iron alloy layers

Steel

Metallic coatings

Below: Hot-dip galvanized steel bridge, Scotland (Photo courtesy of Forestry Civil Engineering)

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“The overall success of a protective coating scheme starts with a well-prepared specification.”

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7. Appropriate specifications

7.1 General The overall success of a protective coating scheme starts with a well-prepared specification. It is an essential document that is intended to provide clear and precise instructions to the contractor on what is to be done and how it is to be done. The specification should be drafted by someone with appropriate technical expertise, and it should be clear as to what is required, and what is practical and achievable. In detail the specification needs to consider the following aspects, and guidance on the issues to be addressed is provided throughout this brochure:

• The environment and access for future maintenance • The design of the bridge • The surface preparation • The coating system and application • Handling and transportation • Inspection and quality control • Health and safety

The majority of steel road bridges are protected according to the requirements of the Highways Agency, Series 1900 specifications (refer to section 7.2), and Network Rail have standard specifications for their rail bridges (refer to section 7.3). Alternative coating systems and methods may be specified for other bridges but the same standards and principles of good coating practices should similarly be applied.

7.2 Highways Agency specifications The Highways Agency’s requirements for new structures are described in the Manual of Contract Documents for Highway Works (MCDHW) 8:

• Volume 1: Specification for Highway Works (SHW), Series 1900: Protection of Steelwork Against Corrosion.

• Volume 2: Notes for Guidance on the Specification for Highway Works, Series NG 1900: Protection of Steelwork Against Corrosion.

These documents consider the environment, accessibility, required durability of the systems and finish colour. The factors to be taken into account when selecting an appropriate system are described below, and a summary table of suitable protective systems for bridges (Table 19/2B) is presented in Figure 9 on page 22.

7.2.1 Accessibility For the purposes of maintenance painting, new structures are described as either ‘Ready Access’ where there are limited restrictions for working, or ‘Difficult Access’ where a structure crosses a busy motorway or railway.

Left: Renaissance Bridge, Bedford

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7.2.2 Required durability The minimum requirements for coating systems are currently as follows:

• No maintenance for 12 years. • Minor maintenance from 12 years. • Major maintenance after 20 years.

7.2.3 Colour Reference is made to the BS 4800 16 range, description and any special finish e.g. gloss/low sheen.

7.2.4 Future amendments As a result of a research project the Highways Agency is considering removing the Aluminium Metal Spray (AMS) Type II system from the SHW 1900 Series at the next revision. The Highways Agency has also been actively testing & trialling coating systems that may be suitable alternatives to or, considered as replacements for the Type I and II systems. For indicative purposes, the draft revisions to the Highways Agency systems are summarised in Figure 10.

Prior to the next revision of the SHW 1900 Series, the use of these new systems on Highways Agency schemes will be given favourable consideration where suitable ‘Departure from Standard’ requests are made.

Appropriate specifications

System Access Metal 1st coat 2nd coat 3rd coat 4th coat Minimum total dry film Estimated cost

type type thickness of paint system (µm) £/m2 (2008)

Zinc phosphate Polyurethane

HB QD MIO, HB QD (2 pack) finish

| R – epoxy epoxy (2 pack) or MC

300 20 undercoat polyurethane

(2 pack) primer finish

Item 111 112 168 or 164

Zinc phosphate MIO HB QD Polyurethane

Aluminium Aluminium

HB QD epoxy epoxy (2 pack) finish or

II D metal spray epoxy

(2 pack) primer (2 pack) MC polyurethane 300 39

(100µm) sealer

undercoat finish

Item 159 111 112 168 or 164

Polyurethane Zinc phosphate HB glass

(2 pack) finish or II

D - epoxy flake epoxy

MC polyurethane 475 28(Alternative) (2 pack) (2 pack) Finish

Item 110 123 168 or 164

Zinc phosphate MIO HB QD

III R or D - HB QD epoxy epoxy (2 pack)

200 20 primer finish

Item 111 112

Zinc phosphate Epoxy MIO

epoxy sealer or (2 pack) Polyurethane

IV R or D HDG ‘T’ wash extended cure HB QD or (2 pack) finish 175 35

epoxy (2 pack) extended cure

MIO primer

Item 155 110 or 121 112 or 121 168 or 169

Key: R = Ready D = Difficult HB = High Build MC = Moisture Cured MIO = Micaceous Iron Oxide QD = Quick Drying = To Site

Figure 9. Summary Table of the Highways Agency Table 19/2B from 1900 Series, May 2008 Amendment.

* Zinc rich epoxy primer (provisionally BD35 Item 109) + Coating applied over HDG

I 109* 112 168/164 275

111 112 168/164 325

II 109* 112 112 168/164 400

110 123 168/164 550

110 112 112 168/164 375

111 112 112 168/164 400

III 109* 112 200

111 112 225

IV+ 155 110/121 112/121 168/169 200

Figure 10. Draft revision to Highways Agency systems (October 2009)

Type 1st Coat 2nd Coat 3rd Coat 4th Coat Min. total d.f.t. (µm)

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Figure 11. Summary Table – Network Rail NR/L3/GN/039 (Issue 5)

Reference Title Surface Coats (stripe coats omitted)

Estimated number preparation and cost £/m2

profile A B C D (2008) Intermediate Coat Top Coat

High solids epoxy Thermally Aluminium Epoxy sealer primer (Item 7.1.4)

N1 sprayed Sa 3 or zinc (Item 7.1.1) 150µm min. or 35 metal/ 70 to 100µm 100µm min. 25µm max. Epoxy intermediate epoxy coat (Item 7.2.2)

150µm min.

Epoxy Epoxy blast Primer Epoxy

N2 glass Sa 21⁄2 (Item 7.1.2) – glass flake 28

flake 70 to 100µm 25µm min. (Item 7.2.3)

400µm min.

Epoxy blast primer High solids epoxy (Item 7.1.2) primer (Item 7.1.4)

Epoxy intermediate N4 Epoxy MIO

Sa 21⁄2 50µm min. or 100µm min. or coat (Item 7.2.2) 2570 to 100µm Zinc rich epoxy blast Epoxy intermediate

125µm min.Primer (Item 7.1.3) coat (Item 7.2.2) 50µm min. 125µm min.

Anti-graffiti paint: Polyurethane

coloured finish (Item 5.6.3) 50µm min.

Acrylic urethane (Item 7.3.1) 50µm min.

Polysiloxane (Item 7.3.4) 50µm min.

7.3 Network Rail specifications Network Rail’s requirements for protective treatments to be used on bridges are given in the following documents.

• NR/L3/CIV/039: Specification for the assessment and certification of protective coatings and sealants (Issue 5)

• NR/L3/CIV/040: Specification for the use of protective coating systems (Issue 1)

• NR/GN/CIV/002: The use of protective coatings and sealants (Issue 5).

NR/L3/CIV/039 defines the requirements for the assessment, certification, registration and specification of protective coatings and sealants for use on Network Rail’s infrastructure. NR/L3/CIV/040 defines the requirements for the selection and use of protective coating systems, and NR/GN/CIV/002 supports NR/L3/CIV/040 by providing guidance and information on the selection, and application of such systems.

A summary table of the main systems for new works is reproduced in Figure 11. Other systems are available, e.g. systems suitable for the interior of box girders. Refer to NR/L3/CIV/039 for full details.

The choice of protective treatment depends upon the life requirement of the structure, the environment, and maintenance requirements.

7.3.1 Service life Ideally, the performance data for a protective coating product/system (usually obtained from the results of accelerated ageing tests) will indicate a nominal 25 year service life to first maintenance. However, products and systems that have previously been tested under the 3000 hours (15 years) criteria in accordance with Issue 4 (RT/CE/S/039 (RT98) 17) may continue to be used. The time to first major maintenance assumes that the protective treatment is applied by suitably competent operatives such as those certificated under the ICATS scheme.

7.3.2 Environment The environment is classified in accordance with BS EN ISO 12944-2 2. The corrosivity categories (C grades) for exterior environments are designated as C2- Low, C3-Medium, C4-High and C5 Very High. Generic descriptions of these exterior environments are provided in NR/GN/CIV/002.

7.3.3 Colour Top coats are normally required to have a Class A match to BS 4800 16 or BS 381C 18 shades.

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8. Inspection and quality control

Inspection forms an integral part of quality control. Its purpose is to check that the requirements of the specification are being complied with and to provide a report with proper records to the client. One of the greatest assets to the coating inspector is a clear written specification that he can refer to without doubt.

The appointment of an appropriately qualified third party inspector should be seen as an investment in quality and not just an additional cost. Inspection of the processes, procedures and materials required for the protective coating of steel structures is vital, since a major error in even one operation cannot be easily detected after the next operation has been carried out, and if not rectified immediately can significantly reduce the expected life to first maintenance.

Inspection should commence at the initial stages, for without control to ensure a properly treated substrate, the value of subsequent operations is reduced. Good surface preparation is the foundation for durability and should not be underestimated.

Before abrasive blast cleaning to remove millscale and rust, the surface should be visually free from oil, grease and other contaminants. After the removal of millscale and rust, it is necessary to check that the required level of cleanliness and surface profile have been achieved by reference to the relevant standards.

Similarly, the treatment of specific surfaces e.g. welds, edges and holes, etc. should be inspected to a suitable condition to receive the protective coating. Immediately before the application of the protective coating, a final check should be carried out to ensure that the surface is free from dust and loose particulate matter and has not been re-contaminated.

Paint types and qualities should be checked and there is normally a set procedure required by the Highways Agency and Network Rail to test the quality of ‘as delivered’ material and material being applied to the structure. The application process and prevailing conditions for both shop and site applications need to be checked along with the specified wet and dry film thicknesses. There is sometimes a requirement to check adhesion especially of thermally sprayed coatings.

Metal coatings, including thermal sprayed and hot-dip galvanizing, should be checked for adequate preparation if they are to be over-painted.

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Above: Bell’s Bridge, Glasgow

Thermally sprayed coatings should be sealed immediately after application to avoid the formation of corrosion products from the coating material or, especially in the case of aluminium, the appearance of red rust from the substrate.

The use of mordant ('T-wash') treatments to hot-dip galvanized coatings should be monitored to ensure that the application is consistent with the manufacturers’ data sheets and that the treatment has been effective with the treated surface free from residual solution and made thoroughly dry.

Records of all aspects of inspection along with any problems should be made and any departure from the specified process noted.

The appointment of a suitably qualified inspector should be regarded as an important part of the job. Those involved with such duties should have been trained in internationally recognised practices and standards and be in possession of the appropriate certification. Such a scheme is available through The Institute of Corrosion in the UK. This international scheme is available for the qualification and certification of industrial painting and coating inspectors and operates in accordance with EN ISO/IEC 17024 19.

For further information on the inspection of surface preparation and coating treatments, refer to Steel Bridge Group Guidance Note GN 8.06 20.

Far left: A66 Overbridge, Longnewton Left: Merrals Shaw Underbridge, M2 Junction 2,

Rochester

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9. References and further reading

1. Steel bridges – Material matters. Weathering steel, 2009, Corus.

2. BS EN ISO 12944-2: 1998, Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 2: Classification of environments, British Standards Institution.

3. ISO 9223: 1992, Corrosion of metals and alloys – Corrosivity of atmospheres – Classification, International Standards Organisation.

4. BS EN ISO 12944-3: 1998, Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 3: Design considerations, British Standards Institution.

5. BS EN ISO 8501-1, 2007, Preparation of steel substrates before application of paints and related products. Visual assessment of surface cleanliness. Rust grades and preparation grades of uncoated steel substrates and of substrates after overall removal of previous coatings, British Standards Institution.

6. BS EN ISO 8503-1:1995, (BS 7079-C1:1989) Preparation of steel substrates before application of paints and related products. Surface roughness characteristics of blast cleaned steel substrates. Specifications and definitions for ISO surface profile comparators for the assessment of abrasive blast-cleaned surfaces, British Standards Institution.

7. BS EN ISO 8503-5:2004, (BS 7079-C5:2004) Preparation of steel substrates before application of paints and related products. Surface roughness characteristics of blast cleaned steel substrates. Replica tape method for the determination of the surface profile, British Standards Institution.

8. Manual of Contract Documents for Highway Works: Specification for Highway Works, series 1900; Notes for Guidance on the Specification for Highway Works, series NG1900, The Stationery Office, May 2008.

9. GN 5.06, Flame cutting of structural steels, Guidance Notes on Best Practice in Steel Bridge Construction, SCI-P-185, The Steel Bridge Group, The Steel Construction Institute, 2009.

10. BS EN 1011-2, 2001, Welding. Recommendations for welding of metallic materials. Arc welding of ferritic steels. British Standards Institution.

11. GN 8.04, Thermally sprayed metal coatings, Guidance Notes on Best Practice in Steel Bridge Construction, SCI-P-185, The Steel Bridge Group, The Steel Construction Institute, 2009.

12. BS EN ISO 14713: 1999, Protection against corrosion of iron and steel in structures - Zinc and aluminium coatings – Guidelines. British Standards Institution.

13. BS EN 10025: 2004, Hot rolled products of non-alloy structural steels - Part 1, General technical delivery conditions. British Standards Institution.

14. GN 8.03, Hot-dip galvanizing, Guidance Notes on Best Practice in Steel Bridge Construction, SCI-P-185, The Steel Bridge Group, The Steel Construction Institute, 2009.

15. GN 8.02, Protective treatment of bolts, Guidance Notes on Best Practice in Steel Bridge Construction, SCI-P-185, The Steel Bridge Group, The Steel Construction Institute, 2009.

16. BS 4800: 1989, Schedule of paint colours for building purposes. British Standards Institution.

17. RT/CE/S/039: Specification RT98 – Protective treatment for Railtrack infrastructure, Issue 4, February 2002.

18. BS 381C: 1996, Specification for colours for identification, coding and special purposes, British Standards Institution.

19. BS EN ISO/IEC 17024: 2003 Conformity Assessment. General requirements for bodies operating certification of persons. British Standards Institution.

20. GN 8.06, The inspection of surface preparation and coating treatments, Guidance Notes on Best Practice in Steel Bridge Construction, SCI-P-185, The Steel Bridge Group, The Steel Construction Institute, 2009.

References and further reading

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This page: Whatman's Field Bridge, Maidstone

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www.corusgroup.com

Care has been taken to ensure that this information is accurate, but Tata Steel Europe Limited, and its subsidiaries, does not accept responsibility or liability for errors or information which is found to be misleading.

Corus cares about the environment – this brochure is printed with biodegradable vegetable inks and using material with at least 80% recycled content.

Corus Construction Services & Development PO Box 1 Brigg Road Scunthorpe North Lincolnshire DN16 1BP T: +44 (0) 1724 405060 F: +44 (0) 1724 404224 E: [email protected] www.corusconstruction.com CD1000:UK:01/2010

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