Study of Sensitization in AA5083

Aluminium Alloy

A thesis submitted to The University of Manchester for the degree

of

Doctor of Philosophy

in the Faculty of Science and Engineering

2017

WU WEI

SCHOOL OF MATERIALS

Corrosion and Protection Centre

Contents

Contents

Contents ...... 2

List of Figures ...... 7

List of Tables ...... 15

List of Abbreviations ...... 16

Abstract ...... 17

Declaration ...... 18

Copyright ...... 19

Publications ...... 20

Acknowledgements ...... 21

Chapter 1 Introduction ...... 22

1.1 Background...... 22

1.2 Overall Research Objectives ...... 23

1.3 Structure of Thesis ...... 24

Chapter 2 Literature Review ...... 25

2.1 and Its Alloys ...... 25

2.1.1 Introduction ...... 25

2.1.2 Elements in Aluminium Alloys ...... 27

Page 2

2.1.3 Classification of Aluminium Alloys ...... 28

2.1.4 Families ...... 29

2.1.5 Properties of Aluminium Alloys ...... 30

2.2 Aluminium- Alloys ...... 33

2.2.1 Introduction ...... 33

2.2.2 Segregation in Al-Mg Alloys ...... 35

2.2.3 Precipitation in Al-Mg Alloys ...... 35

2.3 The Corrosion of Aluminium Alloys ...... 39

2.3.1 Introduction ...... 39

2.3.2 Effect of Alloying on Corrosion Behaviour ...... 41

2.3.3 Pitting Corrosion ...... 41

2.3.4 Intergranular Corrosion ...... 43

2.3.5 Stress Corrosion Cracking ...... 45

2.4 Sensitization in Aluminium-Magnesium Alloys...... 47

2.4.1 Introduction ...... 47

2.4.2 Sensitization in AA5083 Alloy ...... 48

2.5 Effect of Sensitization on Corrosion Behaviour of AA5083 Alloy ...... 51

2.5.1 Effect of Sensitization on Structure of AA5083 Alloy ...... 51

2.5.2 Effect of Sensitization on the Susceptibility to Intergranular Corrosion ...... 54

Page 3 2.6 Effect of Sensitization on Mechanical Properties of AA5083 Alloy ...... 57

2.6.1 Effect of Sensitization on Hardness Properties ...... 57

2.6.2 Effect of Sensitization on Strength Properties ...... 59

2.7 Effects of Sensitization on the Stress Corrosion Cracking of AA5083 Alloy 62

2.7.1 Testing Methods of Stress Corrosion Cracking ...... 62

2.7.2 Roles of Precipitation in Stress Corrosion Cracking ...... 64

Chapter 3 Experimental Method ...... 69

3.1 Materials ...... 69

3.2 Experimental Procedure ...... 71

3.2.1 Alkaline Etching ...... 71

3.2.2 Sensitization Process ...... 71

3.2.3 Nitric Acid Mass Loss Test (NAMLT) ...... 72

3.2.4 Constant Displacement Stress Corrosion Cracking (SCC) Test ...... 73

3.3 Analytical Techniques ...... 74

3.3.1 Optical Microscopy (OM) ...... 74

3.3.2 X-ray Diffraction (XRD) ...... 75

3.3.3 Scanning Electron Microscopy (SEM) ...... 75

3.3.4 Energy Dispersive X-Ray Analysis (EDX) ...... 76

3.3.5 Electron Probe Microanalysis (EPMA) ...... 77

3.3.6 Transmission Electron Microscopy (TEM) ...... 77

Page 4 3.3.7 3D Volumetric Reconstruction ...... 78

Chapter 4 In-service Sensitization of AA5083 Aluminium Alloy ...... 80

4.1 Introduction ...... 80

4.2 General observation of the in-service sensitized AA5083 alloy...... 80

4.3 The Distribution of Constituent Intermetallic Particles ...... 83

4.4 Compositional Variation through the Plate Thickness ...... 90

4.5 Failure Analysis of the in Service Cracked Component ...... 96

4.6 Susceptibility to Intergranular Corrosion ...... 99

4.7 Structure of the Grain Boundaries in the In-service Sensitized AA5083 Alloy ...... 102

4.8 Conclusions ...... 114

Chapter 5 Sensitization of AA5083 Aluminium Alloy at Low Temperature ...... 116

5.1 Introduction ...... 116

5.2 Experimental Set-up ...... 118

5.3. General Observation of the O Temper AA5083 Alloy...... 119

5.4 Structure of the Grain Boundaries in the Low Temperature Sensitized AA5083 Alloy ...... 125

5.5 Electrochemical Behaviour of the Low Temperature Sensitized AA5083 Alloy ...... 132

5.6 Susceptibility to Intergranular Corrosion ...... 134

5.7 Propagation of Intergranular Corrosion in the Sensitized AA5083 Alloy ... 137

Page 5 5.8 Evolution of Precipitates along Grain Boundaries ...... 144

5.9 Conclusions ...... 154

Chapter 6 Sensitization and Its Effect on Stress Corrosion Cracking ...... 156

6.1 Introduction ...... 156

6.2 Experimental Set-up ...... 156

6.2.1 Specimens Fabrication ...... 157

6.2.2 Stress Corrosion Cracking Constant Displacement Loading Test .... 160

6.3 Susceptibility to Intergranular Corrosion of the Sensitized AA5083-H116 Alloy ...... 162

6.4 General Observations of Stress Corrosion Cracking ...... 164

6.5 The Effect of Sensitization on Crack Propagation ...... 170

6.6 Conclusions ...... 176

Chapter 7 Conclusions and Suggestions for Future Works ...... 178

7.1 Summary ...... 178

7.2 Conclusions ...... 183

7.3 Future Works ...... 187

References ...... 188

Word count: 35,300

Page 6 List of Figures

List of Figures

Figure 2-1: Aluminium-magnesium binary equilibrium phase diagram [16] ...... 34

Figure 2-2: Theoretical domains of corrosion, passivation and immunity of aluminium at 25°C [42] ...... 40

Figure 2-3: Optical micrographs showing the effect of sensitization heat treatment on the microstructure of as-received sample and specimens sensitized at various temperature for 168 hours: (a) as-received (b) 100°C, (c) 150°C, (d) 175°C, (e) 200°C. [79] ...... 52

Figure 2-4: TEM observations of β precipitates: (a) a bright-field TEM image of AA5083 alloy sensitized at 150ºC for 189 hours showing a continuous film of β precipitates along grain boundary; (b) a dark-field TEM image illustrating the continuous nature of β precipitation in this condition [67]. .53

Figure 2-5: Effect of sensitization temperature and sensitization time on IGC susceptibility of AA5083-H116 [79] ...... 54

Figure 2-6: Minimum sensitization time for AA5083-H116 to become susceptible to IGC at various temperatures [79] ...... 56

Figure 2-7: Age hardening curves for Al-Mg based alloys aged at 423K [70] ....57

Figure 2-8: Variations of the hardness with sensitization temperature and sensitization time for AA5083-H116 alloy [79] ...... 58

Figure 2-9: Mechanical properties measured in air at a displacement rate of 10-6 s-1 for AA5083 alloy as a function of sensitization time [67]...... 59

Figure 2-10: Normalized strain to failure vs sensitization time. Insets show TEM micrographs of the degree of grain-boundary precipitation at selected times

Page 7

[67]...... 60

Figure 2-11: Effect of sensitization heat treatment on (a) tensile strength and (b) yield strength of AA5083-H116 [79] ...... 61

Figure 2-12: Schematic diagram of typical crack propagation rate as a function of crack tip stress intensity behaviour illustrating the regions of stage 1, 2 and 3 crack propagation [90]...... 63

Figure 2-13: Simulation of the evolution in microstructure of grain boundaries in AA5083 alloy: (a) non-precipitate; (b) individual precipitates formed by β phase; (c) near-continuous β phase and (d) continuous film of β phase [91]...... 65

Figure 2-14: Free corrosion potential of samples sensitized in different condition at 175°C in 3.5 wt. % NaCl solution [91]...... 66

Figure 2-15: Initial SCC crack growth rate and incubation time of sensitized AA5083 alloys [91]...... 68

Figure 3-1: Images of materials used in this research: (a) in-service sensitized AA 5083 aluminium alloy; (b) as-received O temper AA 5083 aluminium