PITTING CORROSION BEHAVIOUR OF ALUMINIUM ALLOYS LEON MEI CHEN Report submitted in partial fulfilment of the requirements for the award of the degree of Bachelor of Mechanical Engineering Faculty of Mechanical Engineering UNIVERSITI MALAYSIA PAHANG JUNE 2013
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PITTING CORROSION BEHAVIOUR OF ALUMINIUM ALLOYS
LEON MEI CHEN
Report submitted in partial fulfilment of the requirements for the award of the degree of
Bachelor of Mechanical Engineering
Faculty of Mechanical Engineering
UNIVERSITI MALAYSIA PAHANG
JUNE 2013
vii
ABSTRACT
The present work is aimed to investigate the effect of temperature and concentration of
solution on the pitting corrosion of AA6061 – T6 aluminium alloy and study its
electrochemical behaviour and physical behaviour in sodium chloride (NaCl) solution
using the polarization technique. The experiments were carried out under static
conditions at different NaCl concentration solutions (3.5, 4.5 and 5.5) wt% and different
temperatures (25, 35, 45, 55 and 65) oC. This experiment started with different NaCl
concentration solutions at room temperature condition by using potentiostat/galvometer
instrument. Water bath machine had been used to control the solution temperature in this
experiment. Natural pitting corrosion experiment had been tested for 2 months in
different NaCl concentration solutions. Comparison between two methods which were
tested in different concentration was discussed. It was found experimentally that
increasing in NaCl concentration and temperatures lead to decrease in the breakdown
potential (Ecorr) and increase in corrosion rate of as-received materials. Based on the
results obtained, the corrosion rate increased from 0.1529 mmpy to 0.3650 mmpy for the
electrochemical experiment and 0.2517 mmpy to 0.4692 mmpy for natural pitting when
concentration of the solutions increased from 3.5 wt% to 5.5 wt%. The influence of
solutions’ temperature (25 – 65 oC) on the pitting corrosion of AA6061-T6, showed the
changes of the corrosion rate from 0.1529 mmpy to 1.205mmpy. In conclusion, the
highest corrosion rate obtained at the highest solution temperature. The increased in
concentration and temperature lead to the increasing of corrosion rate of AA6061-T6.
viii
ABSTRAK
Satu eksperimen yang bertujuan mengkaji kesan-kesan beberapa pembolehubah
terhadap hakisan bopeng pada AA 6061-T6 aloi aluminium dan mengkaji fizikal
elektrokimia dan fizikal dalam larutan batrium kloida (NaCl) dengan menggunakan
teknik polarisasi telah dijalankan. Kajian ini telah dijalankan dengan menggunakan
pelbagai kepekatan NaCl ((3.5, 4.5, 5.5) wt% dan suhu yang berbeza (25, 35, 45, 55 and
65) oC. Eksperimen ini bermula dengan larutan yang berbeza kepekatan NaCl pada
keadaan suhu bilik. “Water bath” yang boleh mengawal suhu larutan NaCl telah
digunakan dalam eksperimen. Terdapat satu eksperimen semulajadi bagi hakisan bopeng
telah dikaji dalam dua bulan dengan menggunakan larutan kepekatan NaCl yang
berbeza. Perbandingan dua kaedah yang berbeza digunakan untuk menguji kehakisan
dalam kepekatan yang berbeza telah dinyatakan. Eksperimen peningkatan dalam
kepekatan dan suhu larutan NaCl mengakibatkan penurunan dalam potensi kerosakan
(Ecorr) dan meningkatkan kadar hakisan sampel yang diujikan. Berdasarkan keputusan
yang diperolehi, kadar hakisan bagi eksperimen elektrokimia menambah dari 0.1529
mmpy sehingga 0.3650 mmpy. Bagi bopeng semula jadi, kadar hakisan telan meningkat
dari 0.2517 mmpy sehingga 0.4692 mmpy semasa kepekatan larutan NaCl meningkat
dari 3.5wt% sehingga 5.5wt%. Kadar hakisan menunjukkan perubahan dari 0.1529
mmpy dan meningkat sehingga 1.205 mmpy bagi suhu larutan NaCl yang berbeza (25 –
65) oC. Kesimpulannya, pada suhu 65
oC mendapat kadar hakisan yang paling tinggi.
Peningkatan kepekatan dan suhu menyebabkan peningkatan kadar kakisan AA6061-T6.
ix
TABLE OF CONTENTS
Page
EXAMINER 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 xii
LIST OF SYMBOLS xvi
LIST OF ABBREVIATIONS xvii
CHAPTER 1 INTRODUCTION
1.1 Introduction 1
1.2 Background of Study 1
1.3 Problem Statement 3
1.4 Objectives 3
1.5 Scopes 3
1.6 Thesis Outline 4
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction 5
2.2 Pitting Corrosion 5
2.2.1 Stage of Pitting 10
2.2.1.1 Pit Initiation and Passive Film Breakdown 11
2.2.1.2 Metastable Pitting 13
2.2.1.3 Stable Pitting and Pit Growth 14
2.2.2 Pitting Potential 16
x
2.3 Factors Influencing Pitting Corrosion 19
2.3.1 Effect of Temperature on Pitting 19
2.3.2 Effect of Concentration 20
2.4 Electrochemical Corrosion Measurement 22
2.5 Material 23
2.5.1 Types of Aluminium Alloys 23
2.5.2. 1 Effect of Alloying Elements 24
2.5.2 Aluminium Alloy 6061-T6 26
CHAPTER 3 METHODOLOGY
3.1 Introduction 29
3.2 Sample Preparation 31
3.3 Metallographic Analysis 32
3.4 Compositional Analysis 38
3.5 Electrochemical Test 40
3.6 Weight Loss Method 43
3.7 Scanning Electron Microscope (SEM) 44
CHAPTER 4 RESULTS AND DISCUSSION
4.1 Introduction 46
4.2 Polarization Results of Electrochemical Test 47
4.3 Weight Loss Method Results 50
4.4 Effect of Solution Concentration on Corrosion Rate 52
4.4.1 Comparison of Corrosion Rate between Electrochemical
Test and Natural Pitting
53
4.5 Effect of Temperature on Corrosion Rate 54
4.6 ScanningElecton Microscope (SEM) Results 57
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS
5.1 Introduction 63
5.2 Conclusions 64
xi
5.3 Recommendations 65
REFERENCE 66
APPENDICES
A1 Compositional Analysis of Sample Material 69
A2 Aluminium Alloys: Chemical Composition Limits 70
A3 Equivalent Values for Variety of Aluminium Alloys 71
B1 Visual Inspection of Different Concentrations for Electrochemical
Test
72
B2 Visual Inspection of Different Temperatures for Electrochemical
Test
74
B3 Visual Inspection of Different Concentration for Weight Loss
Method
76
C1 Gantt Chart PSM 1 78
C2 Gantt Chart PSM 2 79
xii
LIST OF TABLES
Table No. Page
2.1 Main Alloying Elements in Wrought Alloy Designation System 24
2.2 Properties of selected Aluminium Alloys 27
2.3 The following table gives main features of aluminium and AA 6061-
T6
28
3.1 A typical ceramographic grinding and polishing procedure for the
grinding and polishing machine
35
3.2 Composition analysis of as-received material 39
3.3 Parameter setup in electrochemical test 40
3.4 Manipulated Parameter. 41
3.5 Manipulated Parameter 43
4.1 Tafel polarization data of different concentration of solution NaCl 47
4.2 Tafel polarization data of different temperature of solution NaCl 49
4.3 Data of samples of weight loss test 51
xiii
LIST OF FIGURES
Figure No. Page
2.1 Microphotograph of piting corrosion on a un-clad 2024 aluminium
alloy
6
2.2 Typical pit shapes 6
2.3 Autocatalytic process occurring in a corrosion pit 7
2.4 SEM micrographs (x1.000) of samples grained with 960 C dm-2
at (a)
40 A dm-2
and (b) 120 A dm-2.
9
2.5 Distribution of the pits size for AC-graining with 480 C dm-2
at 40
and 120 A dm-2
9
2.6 Schematic representation of shapes of pit initiation and propagation 10
2.7 Stage of penetration of passive film leading to corrosion pit
formation. (a) Initial stage of pit formation on pit
(b) Partially perforated passive film
(c) Fragment of passive film on edge pit
11
2.8 Phase diagram of a passive metal demonstrating the processes
leading to pit nucleation.
(a) Penetration mechanism and phase diagram of a passive layer with
related processes of ion and electron transfer within the oxide and at
its phase boundaries including schematic potential diagram (Φ).
(b) Film breaking mechanism and related competing processes.
(c) Adsorption mechanism with increased local transfer of metal ions
and related corrosion current density, ic caused by complex
aggressive anions leading to thinning of the passive layer and
increases layer field strength and final free corrosion current density
ic,h within the pit
12
2.9 Typical metastable pit transients observed on 302 stainless steel
polarized at 420mV SCE in 0.1M NaCl solution
13
2.10 The limitation to pit growth shows in each Evan diagrams:
(a) diffusion limitation at cathode,
(b) salt film formation at anode, and
(c) IR limitation between anode and cathode.
15
2.11 Schematic of polarization curve showing critical potentials and
metastable pitting region. Ep, pitting potential; ER, repassive
potential; Ecorr, corrosion potential
17
xiv
2.12 Typical anodic dissolution behaviour of an active – passive metal 17
2.13 Schematic illustrations of the crevice corrosion attack on the crevice
wall (left), and the IR-produced E(x) distribution and resulting i(x)
current densities (skewed polarization curve) on the crevice wall
(right).
18
2.14 Schematic anodic overvoltage curves for an active-passive metal or
alloy.
20
2.15 Potentiodynamic polarization curves for various alloys at pH 6.0 in
NaC1 solution of different concentrations.
21
2.16 Classic Tafel Analysis 22
3.1 A flow chart showing a summary of the research methodology 30
3.2 Shearing machine (MSV-C 31/6) 31
3.3 Sample after shearing process and cutting process 31
3.4 Sectional cut-off machine 32
3.5 Sample connected with copper wire by using insulation tape 32
3.6 Voltmeter 33
3.7 Mounting cup 33
3.8 LECOSET 7007 (resin and liquid) 34
3.9 Cold Mounting Machine 34
3.10 Finishing sample (a) bottom view (b) top view 34
3.11 Manual grinding machine 35
3.12 Polishing machine 36
3.13 Microid extender and (b) 6 micron diamond suspension for red felt
cloth
36
3.14 0.05 micron colloidal silica for imperial cloth (watted) 37
3.15 Etching solution 37
3.16 Optical microscope 38
xv
3.17 Spectrometer Foundry-Master UV machine 38
3.18 Sample as-received material 39
3.19 WPG-100 Potentiostat equipment 42
3.20 Electrochemical measurement setup 42
3.21 Water Bath 42
3.22 Natural pitting experiment 43
3.23 Experimental flow chart for weight loss method 44
3.24 PHENOMWORLD Scanning electron microscope 45
3.25 Preparation before analyzed by using SEM 45
4.1 Polarization graph of different concentration solution 48
4.2 Polarization graph of different temperature solution 50
4.3 Effect of the concentration of NaCl solutions on the corrosion
potential of AA6061-T6 at room temperature
52
4.4 Comparison of corrosion rate between 2 methods 53
4.5 Comparison of corrosion rate between electrochemical test and
natural pitting test
54
56
4.6 Corrosion potential versus temperature of solution
56
4.7 Corrosion rate versus temperature of solution
4.8 Microstructure of each sample after experiment (1500x