literature

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45 www.ijasrjournal.org

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Effect of Surface Roughness on Corrosion behavior of Aluminum

Alloy 6061 in Salt Solution (3.5%NaCl)

Ahmad Almansour 1 , Mazen Azizi

2 , Abdul Munem Jesri

3 , Sami Entakly

4

1,2 (Department of Material Engineering, Aleppo University, Syria)

3 (Department of Production Engineering Aleppo University, Syria)

4 (PhD Candidate, Department of Production Engineering Aleppo University, Syria)

Abstract: The corrosion of AA6061 after changing the roughness at different degrees was investigated. The aim of

this work was to determine the effect of roughness on corrosion behavior of AA6061.

The roughness of samples were (Ra= 0.64, 1.83, 3.48, 7.04 μm). The mechanical properties were investigated by

hardness tests and tension tests. The corrosion behavior was investigated by immersion tests in 3.5% NaCl salt

solutions. The microstructure was investigated by optical microscope.

The results showed that the corrosion rate deceased gradually in alkaline salt solution with decreasing the

roughness. However in acidic and neutral salt solutions, with decreasing roughness the corrosion rates decreased

gradually with the existence of a sharp steps in decreasing.

Keywords: Aluminum Alloys, AA6061, Corrosion of Aluminum, Surface Roughness.

1. Introduction

Corrosion is the transformation of a metal through a chemical or electrochemical reactions,

starting at its surface. Although all metals have a tendency to be oxidized, some will oxidize more easily than others.

That will be done as a result of contact with an electrolyte like water or moist air. Aluminum is one of these metals,

aluminum surface will react spontaneously to form aluminum oxide. This oxide layer is tightly bonded with surface

of metal and doesn’t have defects.This natural, stable oxide layer is an integral part of the aluminum surface, thus

protecting it from further oxidation [1].

There are two factors affecting the corrosion resistance of wrought aluminum alloys, related with

surface, the first is the near surface that will be full of dislocation and deformations, and this will induce the

corrosion. In addition the boundaries of grains will be active, and any pre-heating will cause precipitate of secondary

phases that affect corrosion, so a subsequent high shear finishing processes like grinding are applied to remove

these layers.

This is can be noted in automotive applications, where alloys of the Al-Mg-Si-(Cu) family (AA6xxx)

are used for external closure panels. Materials of these sheet are generally supplied in a cleaned state, and therefore

resist corrosion. However, mechanical grinding is frequently applied as process prior to final cleaning, pre-treatment

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

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and painting at the automotive manufacturer and this process typically produces fine surface layers of several

micrometers thickness. The relatively thick layers were not removed by cleaning and pre-treatment [2].

Studies had been achieved in Innoval Technology about shear processing of aluminum alloy surfaces

and its influence on corrosion. The mechanical processes were hot or cold rolling, grinding, machining or cutting

and these process will change the surface structure. The surface microstructure is locally transformed and if the

surface shear is high enough, a fine grain size layer will result. This layer has very different optical, mechanical and

electrochemical properties differ of bulk microstructure. These properties can be used to understand why and how

aluminum alloys corrode either as a result of mechanical abrasion, or during preparing samples for electrochemical

testing, or in service [3].

In 2006, a research about precipitation and corrosion behavior of Nano-Structured Near-Surface

Layers on an AA6111 Aluminium Alloy was performed. This alloy 6061-T6 with magnesium and silicon has high

strength, excellent formability, good weldability and good corrosion resistance. The most application of this alloy

application is in the ship building and transport industries where welding often forms part of the manufacturing

process. Al6061-T651 is, however, prone to corrosion in chloride-containing environments. A nano-structured,

near-surface layer has been generated by mechanically grinding an AA6111 alloy. After heat treatment at 180°C for

30 minutes, Q phase particles, ~20 nm diameter, were precipitated preferentially at grain boundaries within the

nano-structured near surface layer. Other precipitates were not observed in the bulk alloy after this heat treatment.

This preferential precipitation results in the near-surface layers having increased corrosion susceptibility than the

whole microstructure, due to the micro-galvanic coupling between the precipitates at grain boundary and the grain

matrix. It is expected that localized attack is intergranular [4].

The second factor is the shape of this layer that will be the same of shape base metal, curvatures,

meanders…etc. and this presents important role in corrosion resistance, where some sites will be weak and may

deteriorate and initiate local corrosion attacks. Especially for aluminum used in exterior architectural applications,

aggressive elements such as chloride ions (Cl - ), sulphates (SO4

-- ) or others, may be potential causes of corrosion

depending on the local environment. For this reason, an effective lifetime protective surface treatment is essential

for architectural applications [1].

Fig.1. Shape of oxide film that will form on surface of Aluminum Alloy [1].

By grinding and polishing process it is possible to remove the deformed layer and get finer surface,

and this will reflect on corrosion behavior, so in this research we will study the effect of surface roughness on

corrosion behavior of wrought aluminum alloy 6061.

It is obvious that research on the relationship between surface roughness and corrosion rate is not

taking much concern from scientists. The present work is a contribution to this field to shed the light on the

importance of surface roughness and its influence on corrosion rate.

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

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2. Experimental procedure

2.1. Sample Preparation

AA6061 with base composition by weight % is 1Mg, 0.56Si, 0.28Fe, 0.08Cu, 0.23Mn, 0.61Mg,

0.05Zi, 0.02Cr, and 0.04Ti was used in this study. The composition was measured by spectrometer XMF 104 that

manufactured by Unsisantis Europe company in Germany. The condition of alloy was as received from

manufacturing operation “hot rolled” and thus the specimens was full of dislocations.

Specimens with dimensions 19×15×7 mm were used. The emery papers and Al2O3 powder and diamond

powder were used to obtain specimens with different surface roughness. The degree of roughness was measured by

roughness machine tester TR110, produced by TIME HOLLND Company, China. Table 1 shows the different

samples related with roughness.

Table 1. Samples of Specimens related with Roughness

Samples Ra (μm) 1 0.64

2 1.83

3 2.06

4 3.48

5 7.04

2.2. Mechanical tests

2.2.1. Hardness test

Brinell test was applied to determine the hardness with ERNSL apparatus provided from ERNST

Company, Italy. The ball steel diameter was 5 mm, the applied force was 125 kg.

2.3. Optical Microscopy Observation

The samples of alloy AA6061 were examined using an optical microscope B-353 provided by

Optica Company, Germany.

2.4. Corrosion test

The corrosion tests were carried out in salt solutions 3.5wt% NaCl with different values of

PH(2,7,12) which was prepared using standard procedures, by adding highly pure NaCl to reagent water (3.5% NaCl

with 96.5% H2O), and the salt was dissolved in the water by using the magnetic mixer GD503, manufactured by

Sartorius company, Germany. This concentration is approximate to salts concentrations in sea water and this

percentage causes the higher corrosion of aluminum because of the quantity of dissolute Oxygen and ion

conductivity at higher values. This solution was divided into three groups. HCl was added to the first group to obtain

acidic solution, NaOH was added to the second group to obtain alkaline solution, and the third group was left

without additions to keep neutral solution. The value of PH for three groups were controlled by PH Meter P11,

manufactured by Sartorius Company, Germany.

The samples were degreased with acetone and then rinsed in distilled water before immersion in test

solutions. The electro-chemical experiment was monitored for 8 days. The corrosion test results were evaluated

using weight loss. The weight loss (mg) for each sample was evaluated by finding the difference in weight “final

weight initial weight” considering the total surface area of the specimen in accordance with ASTM G311 standard

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

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recommended practice ASTM, 1994. For this purpose weighting apparatus, M-power, produced by Satorius

company, Germany was used.

Corrosion rate for each specimen was evaluated from the weight loss measurement following standard

procedures as relation down.

CR: corrosion rate (mm/year).

W: reduction in weight (gr).

A: area (mm 2 ).

T: time of immersion (hours).

D: density of AA6061 (gr/cm 3 )

3. Results

3.1. Mechanical Properties:

3.1.1. Hardness Test:

The average value of hardness was 275 BHN. This high value was a result of dislocations and defects

during manufacturing process.

3.1.2. Tension Test:

Table.2 shows the average values that results from tension test, the strength of specimen in addition to

elongation. The value of elongation was low because of defects in specimen.

Table.2 Results of tension test

Strength (Mpa) Yield Strength (Mpa) Elongation ΔL/L% 242 195 9

3.2. Optical Microscopy observation:

Figure.2 shows the microstructure of the Specimens, the microstructure is consist of aluminum matrix and

precipitates of Mg2Si dispread on it.

Fig.2. Microstructure of Aluminum Alloy 6061

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

www.ijasrjournal.org 41 | Page

3.3. Corrosion Test:

3.3.1. Corrosion Rate:

Table 2 shows results of immersion test in acidic salt solution, the corrosion rate of as received sample

of AA6061 is gradually decreasing with decreasing of Ra and the sharp decreasing get when the Ra has the value

(0.64μm).

Table 2: Corrosion rates of samples of AA6061 in acidic salt solutions, immersion time was eight days

Sample Ra (μm) Corrosion rate (mm/y)

1 0.64 0.002223639 2 1.83 0.007792235 3 2.06 0.00845617 4 3.48 0.008821884 5 7.04 0.008911285

The results showed in table 2 was represented in Fig 3.

Fig 3. Corrosion rates of AA6061 samples in acidic salt solution.

Table 3 shows results of immersion test in neutral salt solution, the corrosion rate of as received sample of

AA6061 is sharply decreasing with decreasing of Ra between values (2.06-7.04 μm), then decreasing gradually up to value (0.64μm).

Table 3: Corrosion rates of samples of AA6061 in neutral salt solutions, immersion time was eight days

Sample Ra (μm) Corrosion rate (mm/y)

1 0.64 0.001067847 2 1.83 0.00115001 3 2.06 0.001337646 4 3.48 0.001749653 5 7.04 0.002265203

The results showed in table 3 was represented in Fig 4.

0

0,001

0,002

0,003

0,004

0,005

0,006

0,007

0,008

0,009

0,01

0,64 1,83 2,06 3,48 7,04

C o

rr o

si o

n r

a te

( m

m /y

)

Roughness Ra (μm)

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

www.ijasrjournal.org 42 | Page

Fig 4. Corrosion rates of AA6061 samples in neutral salt solution.

Table 4 shows results of immersion test in alkaline salt solution, the corrosion rate of as received sample of

AA6061 is gradually decreasing with decreasing of Ra between values.

Table 4: Corrosion rates of samples of AA6061 in alkaline salt solutions, immersion time was eight days

Sample Ra (μm) Corrosion rate (mm/y)

1 0.64 0.016949 2 1.83 0.019374 3 2.06 0.020567 4 3.48 0.021979 5 7.04 0.022121

The results showed in table 4 was represented in Fig 5.

Fig 5. Corrosion rates of AA6061 samples in alkaline salt solution.

0

0,0005

0,001

0,0015

0,002

0,0025

0,64 1,83 2,06 3,48 7,04

C

o rr

o si

o n

r a

te (

m m

/y )

Roughness Ra (μm)

0

0,005

0,01

0,015

0,02

0,025

0,64 1,83 2,06 3,48 7,04

C o

rr o

si o

n r

a te

( m

m /y

)

Roughness Ra (μm)

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

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In case of alkaline solutions PH=12, the values of Corrosion rates were observed to be more than acidic and

neutral solution.

One of the main corrosive reactions is:

NaOH+Al+H2O→NaAlO3+H2O ………(1) [5].

With decreasing the roughness, the contact surface with solution is reducing, and as a result the quantity of reaction

is reduced, consequently the corrosion rate is reduced.

However, the mechanism of corrosion of the Al matrix in neutral media is related with the formation o f protective

layer of aluminum hydroxides Al(OH)3.

Al → Al+3 + 3e - ……. (2)

Al+3 + 3H2O → Al(OH)3 + 3H+ ……….. (3) [6] The Al(OH)3 layer becomes more protector with decreasing the roughness, because the defects is lowered.

In acidic solution, the solubility of Al 3+

facilitates the dissolution of the Al matrix and further accelerates

the chloride attack.

The cl - ions will initiate at weak sites in the oxide film by chloride attack, The resulting HCl formation

inside the pit causes accelerated pit propagation, This product was considered to be AlCl3 or Al(OH)2Cl [7].The

decreasing of roughness reduce the weak sites on surface and consequently reduce the corrosion rate.

3.3.2. Shape of Corrosion:

Fig.6 shows the optical micrograph of AA6061 surfaces after removing from immersion solutions. In the

alkaline solutions, the surfaces of specimens for different roughness are covered with Al(OH)3 and the general

corrosion is controlled. In acidic and neutral solutions, pits were observed on surfaces and with increasing in

roughness the pits increased.

4. Conclusions

This paper studied effect of degree of roughness on corrosion behavior of aluminum alloy AA6061. The

roughness was achieved on specimens by grinding in different emery paper in addition to use Al2O3 powder, and

diamond powder, to obtain on five surfaces.

The immersion corrosion tests in 3.5% salt solutions with different values of PH were applied. With the

decrease in the roughness, the corrosion rate decrease, because of, in alkaline solution, the contact surface between

solution and specimen was reduced and by this the corrosion reactions became less.

In neutral solution, the corrosion depend on Al(OH)3 layer that formed, and with decreasing the roughness

this layer became more protecting.

In acidic solution, with decreasing the roughness, the sites of collection the HCl were reduced, and by this

the chloride aggressive became less.

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

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Roughness

Ra (μm) PH =2 PH =7 PH=12

0.64

1.83

2.06

3.48

7.04

Scale

Fig.6. Shape of corroded specimens

REFERENCES

[1] Christian Vargel, Corrosion of Aluminum, (Elsevier Journal, Paris, France, 2004).

[2] Scamans G M, Afseth A, Thompson G E and Zhou X 2000 Proceeding of 2nd International Conference on Aluminium Surface

Science and Technology (Manchester) p9.

[3] Geoff Scamans, Shear processing of aluminium alloy surfaces and its influence on corrosion, (Innoval Technology Limited).

[4] X Zhou, Precipitation and Corrosion Behaviour of Nano-Structured Near-Surface Layers on an AA6111 Aluminium Alloy, Journal of

Physics: Conference Series 26 (2006) 103–106.

International Journal of Academic Scientific Research

ISSN: 2272-6446 Volume 3, Issue 4 (November-December 2015), PP 37-45

www.ijasrjournal.org 45 | Page

[5] Marcos D. Navarro, Stress Assisted Corrosion of Aluminum 6061 in Base Chemical Solution, California State University, Sacramento

-Ronald E. McNair Scholar.

[6] Hani Aziz Ameen, Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5% NaCl solution with range of temperature

(100-500)°C, AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL STRIAL RESEARCH,2011.

[7] B. Zaid, D. Saidi, A. Benzaid, S. Hadji, Effects of PH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

,(Corrosion Science 50 (2008) 1841–1847).