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CORROSION BEHAVIOUR OF MILD STEEL COATED WITH EPOXY-GRAPHENE FILM IN 3.5 WT.% NaCl SOLUTION
AHMAD RIDHWAN BIN AHMAD FADZI B051410047
UNIVERSITI TEKNIKAL MALAYSIA MELAKA 2017
CORROSION BEHAVIOUR OF MILD STEEL COATED WITH
EPOXY-GRAPHENE FILM IN 3.5 WT.% NaCl SOLUTION
This report is submitted in accordance with requirement of the University Teknikal
Malaysia Melaka (UTeM) for Bachelor Degree of Manufacturing Engineering
(Engineering Materials) (Hons.)
by
AHMAD RIDHWAN BIN AHMAD FADZI
B051410047
930713-06-5519
FACULTY OF MANUFACTURING ENGINEERING
2017
Disahkan oleh:
_____________________________ ______________________________
Alamat Tetap: Cop Rasmi: _____________________________ _____________________________ _____________________________
Tarikh: _______________________ Tarikh: _______________________
*Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh laporan PSM ini perlu dikelaskan sebagai SULIT atau TERHAD.
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA
Tajuk: CORROSION BEHAVIOUR OF MILD STEEL COATED WITH EPOXY-GRAPHENE FILM IN 3.5 WT.% NaCL SOLUTION Sesi Pengajian: 2016/2017 Semester 2 Saya AHMAD RIDHWAN BIN AHMAD FADZI (930713-06-5519) mengaku membenarkan Laporan Projek Sarjana Muda (PSM) ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut: 1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan
untuk tujuan pengajian sahaja dengan izin penulis. 3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan
pertukaran antara institusi pengajian tinggi. 4. *Sila tandakan (√)
(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysiasebagaimana yang termaktub dalam AKTA RAHSIA RASMI 1972)
(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/ badan di mana penyelidikan dijalankan)
SULIT
TERHAD
TIDAK TERHAD
DECLARATION
I hereby, declared this report entitled “Corrosion Behaviour of Mild Steel Coated with
Epoxy-Graphene Film in 3.5 wt.% NaCL Solution” is the result of my own research except
as cited in references.
Signature : ……………………………………
Author’s Name : Ahmad Ridhwan Bin Ahmad Fadzi
Date : 22 June 2017
APPROVAL
This report is submitted to the Faculty of Manufacturing Engineering of Universiti
Teknikal Malaysia Melaka as a partial fulfilment of the requirement for Degree of
Manufacturing Engineering (Engineering Materials) (Hons). The member of the
supervisory committee are as follow:
…………………….................................
(Dr. Mohd Shahadan Bin Mohd Suan)
i
ABSTRAK
Kakisan boleh menjadi masalah utama bagi kebanyakan bahan pada masa kini dan
bagi mengatasi masalah hakisan, sistem salutan telah dibangunkan. Sistem penyalutan boleh
datang dalam pelbagai cara yang sesuai dengan aplikasi yang berbeza. Salutan cat
merupakan salah satu langkah asas dalam membendung masalah kakisan yang boleh
didapati. Penambahan pengisi dalam sistem salutan cat epoksi boleh membantu
meningkatkan perlindungan kakisan bagi sistem salutan. Jenis dan kuantiti pengisi yang
dipilih dalam meningkatkan salutan epoksi perlu dikaji terlebih dahulu sebelum ia boleh
digunakan. Kajian ini akan memberi tumpuan kepada mengkaji tingkah laku kakisan sistem
salutan epoksi-graphene ke atas keluli lembut sebagai substrate. Penyediaan bahan bagi
kajian ini akan melibatkan dengan resin epoksi, pengeras dan graphene sebagai pengisi
dalam sistem salutan. Lapisan cat epoksi-graphene telah digunakan pada substrat keluli
lembut dengan menggunakan kaedah berusan tangan dan dibiarkan ia mengering untuk
tempoh satu hari pada suhu bilik. Pengisi graphene telah ditambah ke dalam sistem salutan
cat epoxy dengan berbeza rumusan kandungan yang bermula dengan 0%, 2%, 4%, 6%, 8%
dan 10% mengikut berat graphene. Kelakuan kakisan pada salutan epoxy-graphene telah
dijalankan dengan menggunakan ujian Litar Terbuka Potensi (OCP) dan ujian polarisasi.
Sampel keluli lembut yang bersalut dengan 0% dan 2% kandungan graphene dapat
memberikan perlindungan yang lebih baik sebagai penghalang terhadap aktiviti kakisan
manakala lapisan sampel dengan kandungan 4%, 6%, 8% dan 10% graphene tidak dapat
memberikan perlindungan yang terbaik terhadap kakisan ke atas keluli lembut. Sampel
dengan tambahan sebanyak 10% kandungan graphene menunjukkan aktiviti kakisan teruk
dan dapat dibuktikan melalui pemerhatian makroskopik dan mikroskopik selepas semua
sampel telah menjalani ujian rendaman dalam 3.5% larutan NaCl selama 30 hari.
ii
ABSTRACT
Corrosion can be a major problem for most of the material nowadays and as the
countermeasure for the corrosion problem, a coating system were developed. Coating system
can come in many ways suitable with different application. Paint coating can be say as the
one of the most basic type of coating available. Addition of filler inside the epoxy paint
coating system can help improving the corrosion protection of the coating system. Type and
quantity of the filler selected in improving the epoxy coating need to be studies first before
it is can be applied. This study will be focus on the study of the corrosion behaviour of the
epoxy-graphene coating system. The preparation of the material with involve with epoxy
resin, hardener and graphene as the filler in the coating system. The epoxy-graphene paint
coating was applied on the mild steel substrate by using hand-brush method and let it cure
for one day in room temperature. Graphene filler was added into the epoxy paint coating
system with different formulate of the graphene content started with 0%, 2%, 4%, 6%, 8%
and 10% by weight of the graphene. The corrosion behaviour of these epoxy-graphene
coating was determined using Open Circuit Potential (OCP) test and Polarization test. The
mild steel sample coated addition of 0% and 2% of graphene able to provide better protection
as a barrier against the corrosion activities while coating sample with addition of 4%, 6%,
8% and 10% of graphene content are not able to provide proper corrosion barrier. Sample
with addition of 10% of the graphene shows the worst corrosion activities proven by both
macroscopic and microscopic observation after all samples were undergoes with immersion
in 3.5% of NaCl solution for 30 days. Both OCP and Polarization test conducted also showed
that the addition of 10% of graphene into the epoxy coating system are not able to provide
better corrosion barrier and causes the mild steel substrate to corrode.
iii
DEDICATION
To my beloved parents
Ahmad Fadzi Bin Othman and Alisa Binti Mohamed
To my siblings
Nur Najwa Atikah Binti Ahmad Fadzi, Anis Amiera Binti Ahmad Fadzi, Anis Hanani
Binti Ahmad Fadzi and Ahmad Aniq Syazwan Bin Ahmad Fadzi
To all my friends, lecturers and supervisor
for giving me moral support, money, cooperation, encouragement and also understandings
Thank You So Much
iv
ACKNOWLEDGEMENT
First and foremost, praise to Allah the Almighty for the strength and hope given to
me throughout doing this research project. I would like to express my gratitude to my
supervisor Dr. Mohd Shahadan Bin Mohd Suan for the continuous support, guidance and
knowledge with my FYP project. His guidance helped me all time of the project and writing
this report.
I would like to give my gratitude to my fellow colleagues from Bachelor of
Manufacturing Engineering (Material Engineering) for all the ups and downs we have been
through all these semesters and all this time executed this project together.
Last but not least, I would like to express my deepest gratitude to my beloved parents,
Ahmad Fadzi Bin Othman and Alisa Binti Mohamed and siblings for their supports and
believe towards me throughout my studies in UTeM. Their love, care and prayers are greatly
appreciated. I am highly indebted to them and may Allah repay their kindness.
v
TABLE OF CONTENT
Abstrak ……………………………………………………………………………………. i
Abstract. …………………………………………………………………………………... ii
Dedication ………………………………………………………………………………... iii
Acknowledgement ………………………………………………………………………... iv
Table of Content ………………………………………………………………………….. v
List of Figures ……………………………………………………………………………viii
List of Charts and Tables …………………………………………………………………. x
List of Abbreviations, Symbols and Nomenclature ……………………………………….xi
CHAPTER 1: INTRODUCTION
1.1 Introduction ……………………………………………………………………… 1
1.2 Problem Statement ……………………………………………………………..... 3
1.3 Objectives ………………………………………………………………………...4
1.4 Scope of Project …………………………………………………………………..4
CHAPTER 2: LITERATURE REVIEW
2.1 Corrosion ....………………………………………………………………………5
2.2 Corrosion Mechanism ……………………………………………………………5
2.3 Forms of Corrosion ...……………………...……………………………………..7
2.3.1 General Corrosion …………….………………………………………….7
2.3.2 Galvanic Corrosion………………………..……………………………...7
2.3.3 Crevice Corrosion ……………………..…………………………………8
2.3.4 Pitting Corrosion …………………………………………………………8
2.3.5 Intergranular Corrosion …………………………………………………..9
2.3.6 Erosion Corrosion ………………………………………………………..9
2.3.7 Stress Corrosion Cracking ………………………………………………10
2.3.8 Selective Leaching Corrosion ………………………………………….. 11
2.4 Corrosion Control ……………………………………………………………….11
vi
2.4.1 Design Consideration ……………………………………………………..11
2.4.2 Material Selection for Corrosion-Resistant ……………………………….11
2.4.3 Protective Coating ………………………………………………………...12
2.4.5 Cathodic Protection ……………………………………………………….12
2.4.6 Inhibitor …………………………………………………………………...13
2.5 Film Forming Mechanism ………………………………….……………………..14
2.5.1 Oxidation ………………………………………………………………….14
2.5.2 Solvent Evaporation ……………………………………………………....15
2.5.3 Polymerization ……………………………………………………………15
2.6 Composition of Coating …………………………………………………………..15
2.6.1 Binders and Resin ………………………………………………………....16
2.6.2 Pigments …………………………………………………………………..16
2.6.3 Additives ……………………………………………………………….....16
2.7 Epoxy Resin Coating ……………………………………………………….……..17
2.7.1 Curing Mechanism ………………………………………………………..18
2.7.2 Epoxy Coating Adhesion ………………………………………………….18
2.7.3 Adhesion Mechanism of the Coating ………………………………...…...19
2.7.4 Finding Gap of Adhesion of Epoxy with Others Substrate ………….........21
2.8 Corrosion on Mild Steel ……………………………………………………...…...22
2.8.1 Introduction ………………………………………………………...…......22
2.8.2 Mild Steel Corrosion Behaviour With Hydrochloric Acid …………….....22
2.8.3 Corrosion Behaviour of Mild Steel with Seawater ……………………….26
CHAPTER 3: METHODOLOGY
3.1 Introduction ……………………………………………………………………….28
3.1.1 Process Flowchart …………………………………………………….......29
3.2 Preparation and Characterization of Raw Material ………………………………30
3.3 Samples Preparation ……………………………………..…………………….....30
3.3.1 Epoxy-Graphene Preparations …………………………………………...31
3.3.2 Preparation of Mild Steel Panel ……………………………………….....31
3.3.3 Preparation of Sodium Chloride Solution ………………………………..32
3.4 Coating on Mild Steel Panel Preparation …………………..………………….....32
3.4.1 Coating Sample Preparation ………………………….…………………..33
3.4.2 Applying Coating on the Mild Steel ……………………………………..34
vii
3.5 Corrosion Test ………………………..………………………………………….35
3.5.1 Open Circuit Potential (OCP) …………………………………………...35
3.5.2 Polarization Test ………………………………………………………...36
3.6 Surface Observation…….………………………………………………………..36
3.6.1 Optical Microscope ……………………………………………………...36
CHAPTER 4: RESULT AND DISCUSSION
4.1 Introduction……………………………………………………………...………38
4.1.1 Coating Sample………………………………………………………….38
4.1.2 Coating Sample Formulation……………………………………………39
4.2 Thermogravimetric Analysis…………………………………………………….40
4.3 Corrosion Analysis……………………………………………………………....42
4.3.1 Open Circuit Potential (OCP) Test……………………………………....42
4.3.2 Polarization Analysis…………………………………………………….45
4.4 Morphology of Coated Sample Analysis………………………………………...47
4.4.1 Macroscopic Observation………………………………………………..47
4.4.2 Microscopic Observation………………………………………………...49
CHAPTER 5: CONCLUSION AND RECOMMENDATION
5.1 Conclusion………………………………………………………………………51
5.2 Recommendation for Future Works…………………………………………….52
REFERENCES ……………………………………………………………...………...53
viii
LIST OF FIGURES
2.1 (a) Uniform corrosion 8
2.1 (b) Galvanic corrosion 8
2.2 (a) Crevice corrosion 9
2.2 (b) Pitting corrosion 9
2.3 (a) Intergranular corrosion 10
2.3 (b) Erosion corrosion 10
2.4 Stress corrosion cracking 10
2.5 Molecular structure of epoxy resin 17
2.6 (a) Corrosion rate of mild steel 23
2.6 (b) Corrosion rate of high carbon steel 23
2.7 (a) Micrographs for mild steel surface in before immersion 24
2.7 (b) Micrographs for mild steel surface in after immersion for
90 min at 25o C in HCL solutions: (1) 0.25; (2) 1.00,
(3) 1.50 and (4) 2.50 mol dm-3 24
2.8 The relation between Ecorr. for mild steel and log cHCl at 25 oC. 25
2.9 Percentage of weight loss in respect of duration of immersion. 26
2.10 (a) Icorr value against immersion periods 27
2.10 (b) Ecorr value against immersion periods 27
3.1 Overall flowchart of epoxy-graphene coating
preparation and analyses 29
3.2 Flowchart of epoxy-graphene coating preparations 33
ix
3.3 Illustration of mixing for coating material 33
3.4 Spreading technique for the coating on the mild steel panel
surface 34
3.5 Sample for open circuit potential test 35
3.6 Experimental setup for OCP test 35
3.7 Experimental setup for polarization 36
4.1 Thermogravimetric analysis (TGA) of the coating system with
different graphene filler contents 41
4.2 OCP of coated mild steel subjected to 720 h of immersion
in 3.5% NaCl solution prepared at room temperature 44
4.3 Graphene filler particle arrangement in the epoxy coating;
(a) 2% sample and (b) 10% sample 45
4.4 Polarization curves of samples prepared at room temperature 46
4.5 Morphology of coated mild steel before and after 30 days
of immersion in 3.5% NaCl prepared using different
formulations under OM observations; (a) 2%. (b) 4%,
(c) 6%, (d) 8%, (e)10% of Graphene filler 50
x
LIST OF TABLES
2.1 Corrosion rate of iron exposed to 3.5% NaCl solutions containing various
concentrations of dissolved Oxygen 6
2.2 Past studies on the Epoxy resin adhesion with substrates 21
2.3 Potentiodynamic polarization parameters for mild steel corrosion in
HCl solutions of different concentrations at 25 oC 25
3.1 Materials and chemicals use for epoxy-graphene coating preparation 30
3.2 Compositions of Epoxy-graphene samples 31
4.1 Finished sample of mild steel coated epoxy-graphene coating 38
4.2 Weight formulation of epoxy, hardener and epoxy 39
4.3 Physical evaluation of the paint system after immersion test 47
4.4 Surface morphology of coated mild steel after immersion 47
xi
LIST OF ABBREVIATIONS, SYMBOLS AND
NOMENCLATURE
NACE - Nation Association of Corrosion Engineers
Fe - Iron
H2O - Water
SACP - Sacrificial Anode Cathodic Protection
DC - Direct Current
UV - Ultra Violet
SrCrO4 - Strontium Chromate
HNO3 - Nitric Acid
HCIO4 - Perchloric Acid
HCL - Hydrochloric Acid
NaCl - Sodium Chloride
KCL - Potassium Chloride
Pt - Platinum
SCE - Standard Calomel Electrode
OCP - Open Circuit Potential
EIS - Electrochemical Impedance Spectroscopy
Rct - Coating Resistant
Cdl - Capacitance Double Layer
Ecorr - Corrosion Potential
Icorr - Corrosion Current
% - Percentage
Atm - Atmosphere
Wt.% - Weight Percentage
SiC - Silicon Carbide
FESEM - Field Emission Scanning Electron Microsopy
PVC - Polyvinyl Chloride
1
CHAPTER 1
INTRODUCTION
1.1 Introduction
Protection of the surfaces especially on the metalwork application need more
attention particularly in projects which involved with large areas of exposed steel. The
exposed surfaces may deteriorate due to the chemical reaction of the surfaces with the
ambient surrounding which also can be known as corrosion attack (Alhumade et al.,
2015). Corrosion in steel can be an expensive problem when it is happened which it also
can contributed to significantly high portion of loss and can causes lost production,
inefficient operation and high maintenance (Karthik et al., 2015). Corrosion also
becomes the major causes of energy and material loss. It has been reported that 1/5 of
energy globally and average 4.2% of gross national product (GNP) is lost each year due
to corrosion (Nguyen et al., 1991). And it is estimated that the impact of the corrosion
greater than $100,000,000,000 per year in the United State alone (Mentroke et al., 2001).
Application of coating on the metals surfaces has long been developed as the
countermeasure for the corrosion problem. In principle, application of coating can help
in prevention of the deterioration process of the metal surfaces which involves with the
chemical reactions of the metal with the surrounding atmosphere. As an oil-based (alkyd)
coating had been applied regardless of the surface characteristics and exposure
conditions in the early 1990s, corrosion prevention in the industry has become popular
(Chew et al., 2000). Coating by epoxy resin are one of the coating material used in
coating application.
2
Epoxy resins as the coating agents has been widely used in coating industry which
epoxy can be classified as one of the most versatile classes of polymer to be used as the
coating as their excellent in overall properties over other (Balgude et al., 2016). Epoxy
resins have excellent adhesions, good in mechanical properties, and their notable
chemical resistance under different aggressive environment such as wet and high
humidity conditions make it suitable as the coating materials (Svendsen et al., 200).
Other than being used in coating application, epoxy resins have make their ways in wide
range of applications including in use in electronic/electrical components, high tension
electrical insulators, fiber-reinforced plastic material, structural and engineering
adhesives etc (Malshe et al., 2008).
Graphene is a new generation material which it is an allotrope of carbon element first
isolated by simple mechanical exfoliation in 2004 by Novoselov and Geim by using
simple adhesive tape (Novoselov et al., 2004). Graphene is a two-dimensional honey
comb which contain of single layer lattice formed by the tightly packed sp2 bonded
carbon. Graphene forms an excellent electrons carrier space which graphene can provide
high electron mobility at room temperature (250,000 cm2/V) which makes them has
extraordinary electrical properties (Singh et al., 2001). Besides, graphene also excellent
in both optical and mechanical properties. Due to the excellent properties graphene have,
it is believed that graphene also could be used in coating application as the enhancement
material. Graphene also has been regards as the highly water and oil repellent material
while graphene oxide is hydrophilic material (Hsieh et al., 2011). This properties makes
graphene suitable as coating which can provides resistance to water and oil.
Epoxy coating can suffer from brittleness and poor resistance to crack propagation
and due to contains of hydrophilic hydroxyl group and free volumes in their structure,
they are permeable to an aggressive agents such as water which can cause the corrosion
rate of the metal accelerate over time (Pourhashem et al., 2016). As for improving the
corrosion resistance of the epoxy as the coating material, nanocomposite coating seems
to be promising in enhancement of the epoxy coating. There are recent studies on
graphene improvise epoxy coating to the corrosion resistance (Chang et al., 2014).
Monetta et al., (2015) stated in their report that the addition of graphene nanoflakes
significantly changes the barrier properties of epoxy resin and it also improves the barrier
properties of neat epoxy resin in water contact angle measurement.
3
This combination of epoxy-graphene coating also can be known as composite coating
can provide protection to the metal effectively due to the mechanical properties that
graphene has which enhance the weaknesses from the epoxy coating. Graphene can
provide strong repellent of water and oil in the coating application. Besides that,
graphene also has good mechanical properties which can improvise the epoxy coating to
become conductive type coating. Electrically conductive coating are new classes of
coating systems which developed for a variety of applications such as static charge
dissipation, electromagnetic/radio frequency interference shielding, antistatic coatings
and space heating (Azim et al., 2006). This conductive type coating provided low surface
resistance which effectively bleed-off the electric charge build up from the frictional
contact between the dielectric substrates and other materials which also can help prevent
dangerous discharge sparks.
1.2 Problem Statement
Corrosion is a natural process which the process of something such as metal
deteriorates because of oxidation which a chemical reaction that created oxides that flake
away from the base of the materials (Alhumade et al., 2015). This process can contribute
to the destructions of the material which become a serious problem faced by industries
especially industries that involving with metalwork application. Corrosion can come in
many types such as uniform corrosion, galvanic corrosion, concentration cell corrosion,
pitting corrosion and etc. By applying a coating, the corrosion can be controlled.
The used of epoxy as the coating material has widely been used on the various of
application in preventing the corrosion problem but there are some weaknesses in epoxy
coating such as poor resistance of crack propagation and permeable to aggressive agent
which can increased the corrosion rate (Hsieh et al., 2011).
There are been studies on improving epoxy coating with various means possible
such as natural renewable resources which has been done by El-Fattah et al., (2006).
His studies in improvement of the epoxy coating corrosion resistance, antimicrobial
activity, mechanical and chemical properties by using chitosan which is also known as
marine polymer as the enhancement material. The incorporation of chitosan into epoxy
coating led to an improvement in alkali, acid and solvent resistance of epoxy coating
4
based on the result obtained by El-Fattah et al., (2006). Enhancement of the epoxy
coating with other materials can bring out the capability of the coating itself into
another level which it is can provide better protection for the applications in future.
1.3 Project Objective
The objectives of this work are as follows:
I. To develop epoxy-graphene film as coating material on mild steel.
II. To study the effect of graphene + epoxy as the coating material against corrosion.
1.4 Scope of Study
In this study, an epoxy-graphene coating system is develop by using 2.0, 4.0, 6.0,
8.0, and 10.0 wt.% of combination of epoxy and graphene, as these formula will later
provide different result of the corrosion behaviour on the mild steel as which the follow
one of the stated objective of this study. The corrosion behaviour of these five different
sample later will be evaluate by using corrosion test which is open circuit potential
(OCP), and polarization test on the sample. Macro and microscopy observations on the
morphology and cross section of each coated sample will be perform to confirm the
occurrence of corrosion.
5
CHAPTER 2
LITERATURE REVIEW
2.1 Corrosion
Corrosion can be defined as the deterioration of a material, which usually metal due
to the interaction of the metal with the surrounding environment. Common examples of
metal corrosion are rusting of irons, tarnishing of silver, dissolution of metals in acid
solutions and growth of patina in copper. Corrosion has become one of the serious threads
to the worldwide industries significantly. The annual cost of corrosions and corrosions
protections in the United States is estimated by the Nation Association of Corrosion
Engineers (NACE) to be excess of 10 billion dollars (Roberge et al., 1990).
2.2 Corrosion Mechanisms
Corrosions occurs by the electrochemical process which is consists of anode which
acts as the negative electrode and cathode that act as the positive electrode, electrolyte which
can be the present water or soil, and a circuit that connected both anode and cathode as the
requirement for the corrosion to occurs (Scully et al., 2000). The dissolution of metal occurs
at the anode which the corrosions current entering the electrolyte and flowing to the cathode.
Most of the metallic material commonly exposed to the water or aqueous corrosion will
involve on the interactions of iron (Fe) with water (H2O).
During the interactions occurs, the metal ions will go into solution at anodic areas in
an amount which chemically equivalent to the reaction at cathodic areas. For iron-based
6
alloys case, following reaction of the iron usually takes place at the anodic areas (Roberge
et al., 1990).
Fe Fe2+ + 2e- Equation 2.1
The loss of electrons leaves positively charged ions at the anode which travel from
the anode to cathode via the electrolyte such as water and carry the positive current. The
electrons which are released on the anode travel from the anode to the cathode via the
metallic circuit. These electrons are utilized in the reduction of oxygen present in water
which is in contact with the cathode. Thus, following reactions may occur at the cathode.
O2 + 4H+ + 4e 2H2O (if the water is acidic) Equation 2.2
O2 + 2H2O + 4e 4H+ (if the water is basic) Equation 2.3
In neutral solutions, the reduction of oxygen is of considerable importance for many
metals and in such cases which the amount of oxygen dissolved in the water will determine
the corrosion rate. Table 2.1 shows the corrosions rate of ions in an aqueous solution is
affected by the amount of oxygen content of the water.
Table 2.1: Corrosion rate of iron exposed to 3.5% NaCl solutions containing various
concentrations of dissolved Oxygen (Scully et al., 2000).
Oxygen Pressure (atm) Corrosion Rate (mm/year)
0.2 2.2
1 4.5
10 86.4
61 300
Dissolved oxygen reacts with hydrogen atoms adsorbed at random on the iron
surface, independent of the presence or absence of impurities in the metal. The oxidation
reaction proceeds as rapidly as oxygen reaches the metal surface. These reaction as shown
in Equation 2.4 (Roberge et al., 1990)
2Fe+ + 2H2O + O2 2Fe2+ 4(OH-) Equation 2.4
7
After dissolution, ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++) and will
combine with hydroxide ions (OH-) formed at the cathode to give a corrosion product called
rust (Fe2O3).
2.3 Forms of Corrosion
Classification of the corrosions form can be identified by the which it manifested
itself which is the classification on the appearance of the corroded metal. Each corrosion
form can also be identified by observation with naked eye. There are eight basic forms of
the corrosions forms which is general corrosion, galvanic corrosion, crevice corrosion,
pitting, intergranular corrosion, erosion corrosion, stress corrosion cracking, and selective
leaching corrosion (Fontana et al., 1967).
2.3.1 General Corrosion
General corrosion or also can be known as uniform corrosion is the most common
form of corrosion which it is normally being characterized by a chemical or electrochemical
reaction that proceeds in uniform matter on the exposed surface area (Fontana et al., 1967).
The material decreasing in the thickness constantly and finally fails or destructions due to
the corrosions occurs as shown in Figure 2.1a. Microscopic anodes and cathodes are
continuously changing their electrochemical behaviour from anode to cathode in uniform
attack. This general corrosion can be prevented or reduced with proper materials including
coatings, inhibitor or cathodic protection.
2.3.2 Galvanic Corrosion
Galvanic corrosion which occurs due to the dissimilar metals metallically connected
with each other and exposed to the corrosion environment, this differences in potential
produce electron flow between them (Fontana et al., 1967). The less noble metal will act as
anode due to its less corrosion resistant and suffers accelerated attack while more noble metal
act as cathode because of their more corrosion resistant material (Figure 2.1b) were cathodic
protected by the galvanic current. The tendency of a metal to corrode in a galvanic cell is
determined by its positions in the “galvanic series” of metals and alloys (Sastri et al., 2007).
8
The order may vary with both temperature and composition of the electrolyte. As for
example, the galvanic corrosion cell of the combination of copper lines and galvanized steel
water mains which the soil as the electrolyte, copper line as the cathode and the water main
is the anode.
Figure 2.1: Corrosion types; (a) Uniform corrosion, and (b) Galvanic corrosion
2.3.3 Crevice Corrosion
Crevice corrosion also can be called as concentration-cell corrosion occurs because
of the environment differences surrounding the metal. Also, can be referred as the deposit or
gasket corrosion (Figure 2.2a). This type of corrosion attack commonly occurs in localized
areas which associated with small volumes of stagnant solutions (Evans et al., 1976). Normal
mechanical construction can create crevices at sharp corners, spot welds, lap joint, fasteners
and tube sheet supports. There are at least five type of concentration cell exist and the most
common of the fives are “oxygen” and “metal ion” cells. The area become cathodic when
there are high oxygen concentration on the areas on the surface which in contact with the
electrolyte compare to area which have less concentration of oxygen. Same applies to the
metal ions concentration cell which area on surface with higher concentration of metal ions
in the electrolyte, it tend to become cathodic.
2.3.4 Pitting Corrosion
Pitting corrosion is form of corrosion which extremely localized attack resulted holes
on the metal surface and randomly occur that the holes may be small or large in diameter
which the depth penetration may greater than the diameter of the holes (McKay et al., 1936).
(a) (b)