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)