i
CORROSION INHIBITION OF 6061 ALUMINIUM ALLOY IN MARINE
ENVIRONMENTS BY MILK
MOHD ZULKARNAIN BIN ZULKIFLI
Thesis submitted in fulfilment of the requirement
for the award of the degree of
Bachelor of Mechanical Engineering
Faculty of Mechanical Engineering
UNIVERSITI MALAYSIA PAHANG
DECEMBER 2010
ii
UNIVERSITI MALAYSIA PAHANG
FACULTY OF MECHANICAL ENGINEERING
We certify that the project entitled “Corrosion Inhibition of 6061 Aluminium Alloy in
Marine Environments by Milk” is written by Mohd Zulkarnain Bin Zulkifli. We have
examined the final copy of this project and in our opinion; it is fully adequate in terms
of scope and quality for the award of the degree of Bachelor of Engineering. We
herewith recommend that it be accepted in partial fulfilment of the requirements for the
degree of Bachelor of Mechanical Engineering.
Luqman Hakim Bin Ahmad Shah
Examiner Signature
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SUPERVISOR’S DECLARATION
I hereby declare that I have checked this project and in my opinion this project is
adequate in terms of scope and quality for the award of the degree of Bachelor of
Mechanical Engineering.
Signature
Name of Supervisor : JULIAWATI BINTI ALIAS
Position : LECTURER
Date :
iv
STUDENT’S DECLARATION
I hereby declare that the work in this thesis is my own except for quotations and
summaries which have been duly acknowledged. The project has not been accepted for
any degree and is not concurrently submitted for award of other degree.
Signature
Name : MOHD ZULKARNAIN BIN ZULKIFLI
ID Number : MA07067
Date :
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ACKNOWLEDGEMENTS
Praise be to Allah S.W.T, the Most Gracious, the Most Merciful for all the
blessings and guidance upon me through my study. Thank you so much dear Allah for
giving me strength and answering my prayers.
This thesis would not have been possible without the guidance and the help of
several individuals who in one way or another contributed and extended their valuable
assistance in the preparation and completion of this research.
First and foremost, I would like to record my sincere gratitude to my supervisor,
Madam Juliawati Binti Alias for her supervision, advice and guidance from the very
early stage of this research as well as giving me extraordinary experiences throughout
the work. Above all and the most needed, she provides me unflinching encouragement
and support in various ways. I am indebted to her more than she knows. One simply
could not wish for a better or friendlier supervisor.
Many thanks go to all FKM laboratory instructor and technicians who are
willingly landing their hands in laboratory and experimental work and all these staff of
the Mechanical Engineering Department, UMP, who helped me in many ways and
made my stay in UMP pleasant and unforgettable. Special thanks from me also go to
my friends who are under same supervisor. I would like to acknowledge their comments
and suggestions which were crucial for the successful completion of this study.
Where would I be without my family? My parents deserve special mention for
their inseparable support and prayers. My father, Zulkifli Bin Adlan, in the first place is
the person who put the fundament by learning character, showing me the joy of
intellectual pursuit ever since I was a child. My mother, Jusnani Binti Hj Abdul Ghani,
is my special one who sincerely raised me with her caring and gentle love. Angah and
adik, thanks for being supportive and caring siblings.
Finally, I would like to thank everybody who was important to the successful
realization of this thesis, as well as expressing my apology that I couldn’t mention
personally one by one.
vii
ABSTRACT
6061 aluminium alloy is an important material with wide ranges of industrial
applications and marine technology. This study examines the use of milk for
improvement of corrosion resistance of 6061 aluminium alloy in sodium chloride.
Weight loss method and potentiodynamic polarization measurement were employed to
study the corrosion behavior of 6061 aluminium alloy in sodium chloride. The weight
loss method showed that the presence of milk significantly decreases the corrosion rates
of 6061 aluminium alloy 6061 in the test solutions. Pitting corrosion take places in all
the specimens during immersion test and the pitting can be seen through surface
analysis by using optical microscope 200x magnifications. With the presence of milk in
the sodium chloride, the pitting growth is smaller and lesser compare to no milk added.
The electrochemical measurements also showed the similar finding that the presence of
milk reduces the corrosion rates, and that corrosion current densities (icorr)
simultaneously increases the values of polarization resistance Rp. The inhibition
efficiencies increase with increasing of milk concentration. The nature of adsorption of
milk on the metal surface has also been examined. Scanning electron microscope (SEM)
and energy dispersive x-ray spectroscopy (EDS) confirmed the formation of thin film
on the metal surface, which reduces the overall corrosion reaction.
viii
ABSTRAK
Aloi aluminium 6061 merupakan bahan penting yang diaplikasikan secara meluas
dalam industri dan teknologi marin. Kajian ini meneliti penggunaan susu untuk
peningkatan ketahanan pengaratan aloi aluminium 6061 di dalam larutan natrium
klorida. Kaedah kehilangan berat dan pengukuran polarisasi potensiodinamik digunakan
untuk mempelajari perilaku pengaratan aloi aluminium 6061 di dalam natrium klorida.
Kaedah kehilangan berat menunjukkan bahawa kehadiran susu secara signifikan
mengurangkan kadar pengaratan aloi aluminium 6061 di dalam larutan uji. Pengaratan
jenis lubang mengambil tempat di semua spesimen selama ujian perendaman dan
lubang dapat dilihat melalui analisis permukaan dengan menggunakan mikroskop 200x
pembesaran. Dengan kehadiran susu dalam larutan natrium klorida, pertumbuhan
lubang adalah kurang dan saiz lubang lebih kecil berbanding dengan tanpa susu
ditambah ke dalam larutan uji. Pengukuran elektrokimia juga menunjukkan penemuan
yang sama bahawa kehadiran susu mengurangkan kadar pengaratan, arus kakisan (icorr),
secara bersamaan meningkatkan nilai rintangan polarisasi Rp. Kecekapan penghalangan
meningkat seiring dengan peningkatan kepekatan susu. Sifat serapan susu pada
permukaan logam juga telah diperiksa dengan menggunakan mikroskop pengimbas
electron (SEM) dan spektroskopi x-ray pemancar tenaga (EDS) mengesahkan
pembentukan lapisan tipis di permukaan logam, yang mengurangkan proses pengaratan
secara keseluruhan.
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TABLE OF CONTENTS
Page
EXAMINER’S DECLARATION ii
SUPERVISOR’S DECLARATION iii
STUDENT’S DECLARATION iv
DEDICATION v
ACKNOWLEDGEMENTS vi
ABSTRACT vii
ABSTRAK viii
TABLE OF CONTENTS ix
LIST OF TABLES xii
LIST OF FIGURES xiii
LIST OF SYMBOLS xvi
LIST OF ABBREVIATIONS xvii
CHAPTER 1 INTRODUCTION
1.1 Background of Study 1
1.2 Problem Statement 2
1.3 Objectives of Study 3
1.4 Scopes of Project 3
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction 4
2.2 Forms of Corrosion 5
2.3 Marine Corrosion 8
2.4 Corrosion Control 9
2.5 Corrosion Inhibitor
2.5.1 Inhibitor Classification
9
11
2.6 Passivity 12
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CHAPTER 3 METHODOLOGY
3.1 Introduction
3.1.1 General Procedure for Experiment
21
22
3.2 Material Preparation
3.2.1 Specimen Preparation
3.2.2 Solution Preparation
3.2.2.1 Procedure for NaCl Solution Preparation
3.2.2.2 Procedure for Inhibitor Preparation
23
23
27
27
27
3.3 Weighing Before and After Testing
3.3.1 Before Immersion
3.3.2 After Immersion
28
28
28
3.4
3.5
3.6
3.7
3.8
3.9
Immersion Test in Sodium Chloride Solutions
Optical Microscope Analysis
SEM-EDS Analysis
Cleaning Specimen After Immersion
Corrosion Rate Analysis
Electrochemical Test
3.9.1 Electrochemical Cell Set-up
3.9.2 General Parameters
3.9.3 Inhibition Efficiency
29
30
31
32
33
34
34
36
37
2.7 Corrosion Rate Measurement 14
2.7.1 Tafel extrapolation 14
2.8
Aluminium and Its Alloy
2.8.1 Properties of Aluminium
2.8.2 Classification of Aluminium Alloy
2.8.3 6061 Aluminium Alloy
15
15
16
16
2.9 Potential of Milk as Inhibitor
2.9.1 Physical and Chemical Status of Milk
2.9.2 Milk Constituents
2.9.3 Milk Fat
2.9.4 Milk Carbohydrates
17
17
18
19
20
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CHAPTER 4 RESULT AND DISCUSSIONS
4.1
4.2
4.3
4.4
4.5
4.6
4.7
Introduction
Visual Inspection After Exposure of Specimens
Surface Analysis
Corrosion Rate Determination
Energy Dispersive X-Ray Spectroscopy (EDS)
Inhibition Mechanism
Potentiodynamic Test
38
38
40
44
46
48
50
CHAPTER 5 CONCLUSION AND RECOMMENDATION
5.0 Introduction 57
5.1 Conclusion 57
5.2 Recommendations 58
REFERENCES
59
APPENDICES 61
A Figure of Specimens for Immersion Test
Figure of Specimens for Electrochemical Test
61
61
Figure of Sodium Chloride as Test Solution
Figure of Sodium Milk used as Inhibitor
62
62
B Example Calculation for Corrosion Rate
Example Calculation for Inhibition Efficiency
63
69
C Gantt Chart /Project Schedule FYP 1
Gantt Chart /Project Schedule FYP 1
70
71
xii
LIST OF TABLES
Table No. Title Page
2.1 Typical composition of 6061 aluminium alloy 17
2.2 Form presence in milk 18
2.3 Composition of cow’s milk 18
3.1 Procedure for prepare inhibitor 28
3.2 Procedure for corrosion product removal 32
4.1 Corrosion rate result 44
4.2 Electrochemical parameters of 6061 aluminium alloy in 3.5%
NaCl solution at various concentration of milk
53
4.3 Value of inhibition efficiency at various concentration of milk 55
xiii
LIST OF FIGURES
Figure No. Title Page
2.1 Forms of corrosion 7
2.2 Pourbaix diagram for aluminium in aqueous environment 13
2.3 Tafel slope calculations 15
2.4 Molecular structure of β Glycol bond 17
2.5 Fat globules in milk 19
2.6 Structure of a lactose molecule 20
3.1 Experimental procedures 22
3.2 Dimension of specimen for immersion test 23
3.3 Dimension of specimen for electrochemical test 24
3.4 Specimen preparation process 25-26
3.5 Four decimal micro weighing scales 29
3.6 Immersion test of specimen 30
3.7 Optical microscope 30
3.8 SEM-EDS device 31
3.9 Corrosion products cleaning 33
3.10 Electrochemical cell interface with WPG 100 potentiostat
and computer
34
3.11
Electrochemical cell 36
4.1 Specimen after being immersed 39
4.2 Surface morphology of aluminium immersed in sodium
chloride without inhibitor
40
4.3 Surface morphology of aluminium immersed in sodium
chloride with 20% concentration inhibitor
41
xiv
4.4 Surface morphology of aluminium immersed in sodium
chloride with 40% concentration inhibitor
41
4.5 Surface morphology of aluminium immersed in sodium
chloride with 60% concentration inhibitor
42
4.6 Surface morphology of aluminium immersed in sodium
chloride with 80% concentration inhibitor
42
4.7 Surface morphology of aluminium immersed in sodium
chloride with 100% concentration inhibitor
43
4.8 Relation between corrosion rate and inhibitor
concentration for 6061 aluminium alloy in NaCl for 28
days immersion period
45
4.9(a) SEM image obtained from 6061 aluminium alloy sample
after being immersed 28 days in 3.5% NaCl with the
presence of 20% concentration milk
46
4.9(b) EDS spectrum 2 in well away from pitting 46
4.9(c) EDS spectrum 5 acquired on the pitting 46
4.10(a)
Compositional features on the pitting 47
4.10(b) Compositional features away from pitting 47
4.11 Surface morphology of 6061 aluminium alloy after 28
days immersion in sodium chloride with the presence of
milk
48
4.12 Schematic diagrams of surface film 48
4.13 Tafel extrapolation plot obtained in 3.5% sodium chloride
for 6061 aluminium alloy
50
4.14 Tafel extrapolation plot obtained in 3.5% sodium chloride
mixed with 20% inhibitor for 6061 aluminium alloy
50
4.15 Tafel extrapolation plot obtained in 3.5% sodium chloride
mixed with 40% inhibitor for 6061 aluminium alloy
51
4.16 Tafel extrapolation plot obtained in 3.5% sodium chloride
mixed with 60% inhibitor for 6061 aluminium alloy
51
4.17
Tafel extrapolation plot obtained in 3.5% sodium chloride
mixed with 80% inhibitor for 6061 aluminium alloy
52
xv
4.18
Tafel extrapolation plot obtained in 3.5% sodium chloride
mixed with 100% inhibitor for 6061 aluminium alloy
52
4.19 Polarization curves of 6061 aluminium alloy in 3.5%
NaCl solution at various concentrations of milk
53
4.20 Effect of milk concentration on corrosion rate 54
4.21 Effect of inhibitor concentration on the inhibition
efficiency
55
6.1 Figure of specimens for immersion test 61
6.2 Figure of specimens for electrochemical cell 61
6.3 Figure of sodium chloride as test solution 62
6.4 Figure of milk used as inhibitor 62
xvi
LIST OF SYMBOLS
A Area
Al Aluminium
Al3+
Aluminium dissolves to 3 electron
Al2O3 Aluminium oxide
Al(OH)3 Hydroxide
AlOOH Oxyhydroxide
ba Anodic Tafel slopes
bc Cathodic Tafel slopes
C Carbon
-COOH Carboxyl
d Diameter
e electron
Ecorr Corrosion potential
H2 Hydrogen gas
H+ Hydrogen ion
H2O Water
icorr Corrosion current density
K Corrosion constant
Si silicon
W Weight loss
& And
°C Celcius
xvii
LIST OF ABBREVIATIONS
ASTM American Standard Testing Method
EDS Energy Dispersive X-Ray Spectroscopy
FYP
NaCl
Final year project
Sodium Chloride
SEM Scanning Electron Microscope
UMP University Malaysia Pahang
1
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
The corrosion of aluminium and its alloy is the subject of critical technological
importance due to the increasing industrial application of these materials. Aluminium
and its alloy represent an important category of materials due to their high technological
value and wide range of industrial applications, especially in aerospace, household
industries, automotive, transportation and marine technology. Mainly is because of their
good specific strength, excellent formability and corrosion resistance. Therefore the
understanding of the corrosion resistance and electrochemical behavior of aluminium
for the future industrial applications and development is vital.
Corrosion is defined as destruction or deterioration of a material because of its
chemical reaction with its environment. One example of aggressive environments is
seawater. Seawater systems are used by many industries such as shipping, offshore oil
and gas production, power plants and coastal industrial plants. Exposure of these
structures in marine environments will cause corrosion that finally leads to total
damage. Therefore, it is very important to study on corrosion prevention in this
environment.
With the fact of corrosion represent a tremendous economic loss and much can be
done to reduce it. There are many ways to reduce corrosion rate and one of the most
popular and acceptable practice is the use of inhibitors. Large numbers of organic
compound was studied and are being studied to investigate their corrosion inhibition
2
potential. All these studies reveal that organic compounds especially those with N, S,
and O showed significant inhibition efficiency.
An inhibitors is a substance that, when added in small concentrations to an
environment, decreases the corrosion rate. In a sense, an inhibitor can be considered as a
retarding catalyst. There are numerous inhibitor types and compositions. Most inhibitors
have been developed by empirical experimentation, and many inhibitors are proprietary
in nature and thus their composition is not disclosed. Inhibition is not completely
understood because of these reasons, but it is possible to classify inhibitors according to
their mechanism and composition.
The safety and environmental issues of corrosion inhibitors arisen in industries
has always been global concern. In recent days, many alternative eco-friendly corrosion
inhibitors have been developed. The growing needs for the corrosion inhibition
becomes increasingly necessary to delay or stop the attack of metal in aggressive
solution. Many efforts made to find suitable natural source to be used as corrosion
inhibitor in various corrosion media. This study considered this particular issue when
applying selected 6061 aluminium alloy to its application which would be suitable with
our natural environment for instance tropical seawater. Milk offers interesting
possibilities for corrosion inhibitor due to its safe use, low cost, availability and the
most important is the potential usages of milk discussed in this research are in line with
the recent trend of environment-friendly concept (Rosliza et. al., 2009).
1.2 PROBLEM STATEMENT
This organic inhibitor can be applied in some practical areas such as:
(i) The main uses of seawater are for cooling purpose, fire fighting, oil field
water injection and desalination plants. 6061 aluminium alloy can be used as
container for these applications and the organic inhibitor may add to the
container to retard the corrosion cause by the seawater.
(ii) Marine corrosion includes the immersion of components in seawater,
equipment and piping that use seawater or brackish water, and corrosion in
3
marine atmospheres. Exposure of components can be continuous or
intermittent. Ships, marinas, pipelines, offshore structures, desalination
plants, and heat exchangers are some examples of system that experience
marine corrosion.
The corrosion problems in these systems have been well studied over many years
despite several published information on materials behavior in seawater, failures still
occur. Therefore, more investigations need to carry out to obtain better understanding
on material corrosion behavior.
1.3 OBJECTIVES OF STUDY
The objectives of this study are:
(i) To study the corrosion behavior of 6061 aluminium alloy in NaCl.
(ii) To investigate the effect of variation concentration of milk as inhibitor, on
the corrosion rate of 6061 aluminium alloy in NaCl.
1.4 SCOPES OF PROJECT
The scope of this study includes:
(i) Preparation for specimen and inhibitor.
(ii) Exposure of specimen in sodium chloride (NaCl).
(iii) Cleaning process of corrosion product.
(iv) Weighing sample by digital weighing scale.
(v) Analysis corrosion rate by using weight loss method and electrochemical
technique.
(vi) Surface morphology examination using scanning electron microscope.
(vii) Compositional features characterization using energy dispersive x-ray
spectroscopic
4
CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
Corrosion is a very serious problem. There are several examples that show how
corrosion cost is very high when doing the maintenance. Corrosion of bridge is a major
problem as they age and require replacement, which costs billions. One large chemical
company have to spent a lot of budget for corrosion maintenance in its sulfuric acid
plants, the petroleum industry spends a million dollar per day to protect underground
pipelines and another spends on painting steel to prevent rusting by a marine
atmosphere. Corrosion engineering is the application of science and art to prevent or
control corrosion damage economically and safely. In solving corrosion problems, the
corrosion engineer must select the method that will maximize profit.
Corrosion is the chemical transformation of metal due to chemical reactions. The
most common form of corrosion is oxidation, where metal atoms combine with oxygen
atoms to form oxides. Iron rust is the most recognizable form of corrosion, and appears
when iron oxide forms on iron or steel components that are exposed to air or water,
however, virtually all metals and alloys are susceptible to corrosion. Technically,
corrosion can occur in other types of materials, such as ceramics or polymers, but the
process is either rare or different enough that the term ―corrosion‖ is generally not used.
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2.2 FORMS OF CORROSION
Engineers often take their specific environment into effect and try to understand
the types of possible corrosion when designing metal components and structures. There
are several types of corrosion, depending on the metal, corrosive agent, geometry, and
environment: (Jones et. al., 1982)
(i) General Corrosion: Uniform corrosion is characterized by corrosive attack
proceeding evenly over the entire surface area, or a large fraction of the total
area. General thinning takes place until failure. On the basis of tonnage
wasted, this is the most important form of corrosion. However, uniform
corrosion is relatively easily measured and predicted, making disastrous
failures relatively rare. In many cases, it is objectionable only from an
appearance standpoint. As corrosion occurs uniformly over the entire surface
of the metal component, it can be practically controlled by cathodic
protection, use of coatings or paints, or simply by specifying a corrosion
allowance.
(ii) Pitting Corrosion: Pitting corrosion is a localized form of corrosion by which
cavities or holes are produced in the material. Pitting is considered to be
more dangerous than uniform corrosion damage because it is more difficult
to detect, predict and design against. Corrosion products often cover the pits.
A small, narrow pit with minimal overall metal loss can lead to the failure of
an entire engineering system. Pitting corrosion, which, for example, is
almost a common denominator of all types of localized corrosion attack.
Pitting is initiated by:
a. Localized chemical or mechanical damage to the protective oxide film;
water chemistry factors which can cause breakdown of a passive film are
acidity, low dissolved oxygen concentrations which tend to render a
protective oxide film less stable and high concentrations of chloride such
as in seawater.
b. Localized damage to, or poor application of, a protective coating.
6
c. The presence of non-uniformities in the metal structure of the
component, for instance nonmetallic inclusions.
(iii) Galvanic Corrosion: Galvanic corrosion is refers to corrosion damage
induced when two dissimilar materials are coupled in a corrosive electrolyte.
When a galvanic couple forms, one of the metals in the couple becomes the
anode and corrodes faster than it would all by itself, while the other becomes
the cathode and corrodes slower than it would alone. For galvanic corrosion
to occur, three conditions must be present:
a. Electrochemically dissimilar metals must be present.
b. These metals must be in electrical contact.
c. The metals must be exposed to an electrolyte.
The relative nobility of a material can be predicted by measuring its
corrosion potential. The well known galvanic series lists the relative
nobility of certain materials in sea water. A small anode or cathode area
ratio is highly undesirable. In this case, the galvanic current is concentrated
onto a small anodic area. Rapid thickness loss of the dissolving anode tends
to occur under these conditions. Galvanic corrosion problems should be
solved by designing to avoid these problems in the first place.
(iv) Stress Corrosion: Stress corrosion cracking is the cracking induced from the
combined influence of tensile stress and a corrosive environment. The
impact of this type of corrosion on a material usually falls between dry
cracking and the fatigue threshold of that material. The required tensile
stresses may be in the form of directly applied stresses or in the form of
residual stresses.
(v) Crevice Corrosion: Crevice corrosion is a localized form of corrosion
usually associated with a stagnant solution on the micro-environmental level.
Such stagnant microenvironments tend to occur in crevices shielded areas
such as those formed under gaskets, washers, insulation material, and
clamps.
7
(vi) Intergranular Corrosion: The microstructure of metals and alloys is made up
of grains, separated by grain boundaries. Intergranular corrosion is localized
attack along the grain boundaries, or immediately adjacent to grain
boundaries, while the bulk of the grains remain largely unaffected. This form
of corrosion is usually associated with chemical segregation effects which
means impurities have a tendency to be enriched at grain boundaries or
specific phases precipitated on the grain boundaries. Corrosion then occurs
by preferential attack on the grain-boundary phase, or in a zone adjacent to it
that has lost an element necessary for adequate corrosion resistance. In any
case the mechanical properties of the structure will be seriously affected.
Figure 2.1 Forms of corrosion
Source: Roberge (1999)
8
2.3 MARINE CORROSION
The corrosion of metals and alloys in chlorinated seawater has long presented
challenges for those responsible for materials selection and has been much studied.
Marine corrosion is of particular interest to designers of ships and shoreline facilities
because most metals used in these structures are vulnerable to damage from seawater.
Maintenance cost for ships, offshore structures and other related equipment are
dependent on how marine corrosion issues and failures are managed.
In addition to the salt (NaCl) in seawater, there are other commonly occurring
constituents, dissolved gases, living organism, and various other materials found in
seawater. Rives, temperature, dissolved oxygen and pollutants are some examples of
issues that may affect the corrosion of a given component in seawater. Marine
atmospheric corrosion is generally considered to be one of the more aggressive
atmospheric corrosion environments. Some factors that affect corrosion rates in marine
atmosphere are humidity, wind, temperature, location, airborne contaminants and
biological organism. Alloy selection, metallic coatings, organic coatings (inhibitor) and
cathodic protection are commonly used methods for providing proper corrosion
protection to various components.
The choice of an appropriate material for seawater service is a difficult decision
that has to be mad by a designer prior to specification of the system. A number of alloys
have been successfully used in seawater services. Marine grade aluminium can form an
oxide on the surface that excludes contaminants and prevents corrosion. Marine grade
here mean 5000 or 6000 series aluminium alloys, such as 5058 and 6061. Aluminium
forms an oxide on the surface thus it will not corrode unless the oxide is damage or
washed away. In marine environments, the presence of aggressive anions which is
chloride leads to pit formations and film breakdown. Pitting is one of extreme localized
attack that results in holes in the metal. These holes may be small or large in diameter,
but most cases they are relatively small. Pitting is particularly vicious because it is a
localized and intense form of corrosion, and failures often occur with extreme
suddenness (Fontana, 1986).