PITTING CORROSION BEHAVIOUR OF ALUMINIUM ALLOYS

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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

    SUPERVISORS DECLARATION iii

    STUDENTS 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 ().

    (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