Influence of Zinc on Surface Treatments of Aluminium-Zinc Alloys

248 Pages·2010·7.62 MB·English

Influence of Zinc on Surface
Treatments of Aluminium-Zinc
Alloys

A thesis submitted to the University of Manchester for the degree of Doctor
of Philosophy in the Faculty of Engineering and Physical Sciences

2010
Marialuisa Gentile

Corrosion and Protection Centre
School of Materials

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List of Contents

List of Contents

List of Figures................................................................................................................7
List of Tables................................................................................................................16
Abstract........................................................................................................................17
Declaration...................................................................................................................18
Copyright Statement.....................................................................................................19
Acknowledgements......................................................................................................20

Chapter 1
Introduction…………………………………………………………………............21
Overview......................................................................................................................21
Objectives………………….........................................................................................23

Chapter 2
Properties, Corrosion and Surface Treatments of Aluminium and Aluminium
Alloys...........................................................................................................................25
Introduction..................................................................................................................25
Properties of Aluminium..............................................................................................26
The Al-H O Pourbaix Diagram....................................................................................27
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Properties of Zinc.........................................................................................................29
The Zn-H O Pourbaix Diagram...................................................................................31
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Aluminium Alloys........................................................................................................32
Classification System of Aluminium Alloys................................................................32
Metallurgy of Aluminium Alloys.................................................................................34
Effect of Composition and Microstructure on Corrosion Resistance of Aluminium
Alloys...………………………………………………………………………………35
Properties of Al-Zn Alloy…..………………………………………………………..36
The Aluminium-Zinc Equilibrium Diagram…………………………………………36
Corrosion of Aluminium..............................................................................................37
Pitting Corrosion..........................................................................................................37
Theories of the Mechanism of Pitting Corrosion.........................................................41
Pitting Corrosion of Aluminium and its Alloys……………………………………...43
Surface Treatments of Aluminium…………………………..….................................45
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List of Contents
Mechanical Polishing...................................................................................................45
Electropolishing............................................................................................................45
Relationship Between Current and Voltage.................................................................46
Alkaline Etching...........................................................................................................48

Chapter 3
Oxides of Aluminium and Aluminium Alloys..........................................................61
Introduction..................................................................................................................61
Properties of Aluminium Hydroxide/Oxyhydroxides and Oxides …..........................61
Aluminium Trihydroxides……………………………………………........................62
Aluminium Oxide Hydroxides.....................................................................................63
Aluminium Oxide.........................................................................................................64
Anodic Oxides Formed by Anodic Polarization of Aluminium........... .......................65
Barrier Anodic Films....................................................................................................66

Porous-type Anodic Films........................................................................................... 67
Mechanisms of Ionic Transport through Oxide Films……………………………….69
The High Field Conduction Theory………………………………………………….69
Oxidation of Aluminium Alloys……………………………………………………...74
Enrichment of Aluminium Alloys…………………………………………………....75

Chapter 4
Experimental Procedure............................................................................................90
Introduction..................................................................................................................90
Materials.......................................................................................................................90
Surface Treatments.......................................................................................................91
Mechanical Polishing...................................................................................................91
Electropolishing...........................................................................................................91
Etching and Desmuting................................................................................................91
Anodizing……………………………………………………………………...……..92
Open Circuit Potential Responses………………………………………...…………92
Weight Loss Measurements………………………………………………………….93
Analytical Techniques...................................................................................,..............94
Rutherford Backscattering Spectroscopy (RBS)..........................................................94
Medium Energy Ion Scattering (MEIS)……………………………………………...94
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List of Contents
Glow Discharge Optical Emission Spectroscopy (GDOES)………………………....95
Scanning Electron Microscopy (SEM)…………………………………………….....95
Atomic Force Microscopy (AFM) and Interferometry…...……………………….....95
Electron Backscattered Diffraction (EBSD)………………………………………....97
Transmission Electron Microscopy (TEM)..................................................................97

Chapter 5
Influence of Surface Treatments on Zinc Enrichment in Al-Zn Alloys………..104
Introduction................................................................................................................104
Results........................................................................................................................105
RBS and MEIS Spectra…...………………………………………...........................105
GDOES Spectra……….…………………………….................................................107
Transmission Electron Microscopy............................................................................107
Etching Rate…………………………............................................................................108
Discussion..................................................................................................................108
Conclusions................................................................................................................111

Chapter 6
Influence of Zinc on the Surface Morphologies of Al-Zn Alloy...........................128
Introduction................................................................................................................128
Results........................................................................................................................129
Scanning Electron Microscopy (SEM) of Al-Zn Alloys ………..…….......................129
Potential-time Behaviour during Anodic Alkaline Etching………………………...131
Potential-time Behaviour during Etching in Alkaline Solution………………….....131
Discussion..................................................................................................................132
Conclusions................................................................................................................137

Chapter 7
Influence of Surface Pretreatments on Anodizing of Al-Zn Alloys.....................149
Introduction................................................................................................................149
Results…………………………………………………………………………...….149
Voltage-time Response of the Al-Zn Alloy...............................................................149
Scanning Electron Microscopy (SEM) of the Anodized Al-Zn Alloys………….…151
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List of Contents
Transmission Electron Microscopy of Anodized Al-Zn Alloys................................153
Discussion…………………………………………………………..........................153
Conclusions……………………………………………………………………........156

Chapter 8
Influence of Grain Orientation on Zinc Enrichment and Surface Morphologies
of Al-Zn Alloys……………………………………..…….......................................165
Introduction................................................................................................................165
Results……………………………………................................................................166
FEG-SEM-EBSD.......................................................................................................166
RBS…………………………………………………………………………............167
MEIS………………………………………………………………………………..167
AFM and Trace Analysis………….……………………………….............................168
Interferometry………………………………………………………….....................170
Discussion…………………………………………………………………………..171
Dissolution of Al-Zn Alloys……………….………………………………………..171
Influence of Grain Orientation on Zinc Enrichment………………….....………….174
Conclusions………………………………………………………………………....176

Appendix A-RBS, MEIS and GDOES Spectra of Al-Zn Alloys………..………193
Appendix B-Surface Morphologies of Al-Zn Alloys in Rolled Conditions and
Surface Pretreated....................................................................................................214
Appendix C-Potential Measurements during Alkaline Etching and Anodic
Alkaline Etching of Al-Zn Alloys...……………….................................................224
Appendix D-RBS and MEIS Spectra of (225), (314) and (113)...........................226
Appendix E - Inverse Pole Figure Representing the Grain Analysed in the Step
Height Measurements...................................................................………………...230

General Conclusions and Future Work.................................................................235

Bibliography.............................................................................................................238
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List of Figures and Tables
List of Figures
Chapter 2

Figure 2.1/pag.50 FFC crystal structure of aluminium.
Figure 2.2/pag.51 Al-H O Pourbaix diagram.
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Figure 2.3/pag.52 Rages of aluminium hydroxide complexes in water.
Figure 2.4/pag.53 Hexagonal close-packing structure of zinc.
Figure 2.5/pag.54 Zn-H O Pourbaix diagram.
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Figure 2.6/pag.55 Aluminium-Zinc equilibrium diagram.
Figure 2.7/pag.56 Influence of temperature on pitting of aluminium.
Figure 2.8/pag.57 Scheme of the autocatalytic process occurring in a corrosion pit.
Figure 2.9/pag.58 Schematic illustration of surface wave and mound morphologies of
electropolished aluminium surface.
Figure 2.10/pag.59 Typical relationship between the current and the anode potential
electropolishing of aluminium.
Figure 2.11/pag.60 Rate of etching 99.5% aluminium caustic soda.

Chapter 3

Figure 3.1/pag78. Structure of gibbsite.
Figure 3.2/pag.79 Structures of boehmite and diaspore.
Figure 3.3/pag.80 Schematic diagrams of growth of anodic alumina at high Faradic
efficiency.
Figure 3.4/pag.81 Schematic diagrams illustrating the incorporation of foreign
species, derived from electrolyte ions, into anodic alumina films formed at high
Faradic efficiency.
Figure 3.5/pag.82 Schematic diagrams showing the development of a porous anodic
film on aluminium.
Figure 3.6/pag.83 Schematic representation of field assisted dissolution.
Figure 3.7/pag.84 Structure of a porous film on pure aluminium.
Figure 3.8/pag.85 Schematic diagrams showing the plan and cross-sectional views of
pore and its adjacent cell formed in acid solution.
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List of Figures and Tables
Figure 3.9/pag.86 Potential energy (P.E.) of mobile ions vs distance with and without
an applied field.
Figure 3.10/pag.87 Schematic diagram of the mechanism of formation of the
enriched layer.
Figure 3.11/pag.88 Linear relationship between the amount of enrichment and the
Gibbs free energy change for oxide formation per equivalent.
Figure 3.12/pag.90 Schematic diagram of different model of enrichment of copper in
aluminium copper alloys.

Chapter 4

Figure 4.1/pag.98 Schematic diagram of the experimental arrangement for the
electropolishing cell.
Figure 4.2/pag.99 Schematic diagram of the apparatus used for alkaline etching.
Figure 4.3/pag.100 Schematic diagram of the apparatus used for anodizing at
constant current.
Figure 4.4/pag.101 Schematic diagram of the apparatus used for corrosion potential
measurements.
Figure 4.5/pag.102 Schematic representation of the procedure employed in the
measurement of the step heights.

Chapter 5

Figure 5.1/pag.115 RBS spectrum of the Al-1at.%Zn alloy in the rolled condition.
Figure 5.2/pag.116 RBS spectrum of the Al-1at.%Zn alloy after mechanical
polishing.
Figure 5.3/pag.117 RBS spectra of the Al-1at.%Zn alloy after alkaline etching in 0.5
M NaOH at 313K for 300 s.
Figure 5.4/pag118. RBS spectrum of the Al-1at.%Zn alloy after etching under the
anodic condition at 5 mA/cm2 in a solution of 0.5 M NaOH for 70 s at room
temperature
Figure 5.5/pag.119 RBS spectrum of Al-1at.%Zn alloy after electropolishing at 20V
in a solution of perchloric/ethanol (1:4 v/v) for 180 s.
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List of Figures and Tables
Figure 5.6/pag.120 MEIS spectrum for Al-1at.%Zn alloy after etching under open
circuit condition at 313 K in a solution of 0.5 M NaOH for 300 s.
Figure 5.7/pag.121 MEIS spectrum for Al-1at.%Zn alloy after electropolishing at
20V in a solution of perchloric/ethanol (1:4 v/v) for 180 s.
Figure 5.8/pag.122 Enrichment of zinc of the binary alloys after the following
treatments: electropolishing at 20V for 180 s in perchloric acid/ethanol (1:4 v/v),
etching in open circuit condition for 300 s in 20g/l NaOH solution, etching in anodic
condition at 5 mA/cm2 for 70 s in 20 g/l NaOH solution.
Figure 5.9/pag.123 GDOES spectrum for Al-1at.% Zn alloy after electropolishing at
20 V in a solution of perchloric/ethanol (1:4 v/v) for 180 s.
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solution of perchloric/ethanol (1:4 v/v) aatt 2200VV ffoorr 118800ss..
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solution of perchloric/ethanol (1:4 v/v) aatt 2200VV ffoorr 118800ss..

Chapter 6

Figure 6.1/pag.141 Secondary electron micrographs at different magnifications of the
Al-1at.%Zn alloy specimen in the rolled condition.
Figure 6.2/pag.142 Secondary electron micrographs at different magnification of the
Al-1at.%Zn alloy after mechanical polishing.
Figure 6.3/pag.143 Secondary electron micrographs at diverse magnification of Al-
1at.%Zn alloy of the specimen electropolished at 20V in a solution of
perchloric/ethanol (1:4 v/v) for 180 s.
Figure 6.4/pag.144 Secondary electron micrographs at diverse magnification of Al-
1at.%Zn alloy of the specimen anodic alkaline etched at 5 mA/cm2 in a solution of 0.5
M NaOH for 70 s at room temperature.
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List of Figures and Tables
Figure 6.5/pag.145 Secondary electron micrographs at diverse magnification of Al-
1at.%Zn alloy of the specimen specimen etched in open circuit condition at 313 K in a
solution of 0.5 M NaOH for 300 s.
Figure 6.6/pag.146 Energy dispersive X-ray spectroscopy of a particle on the surface
of the alkaline etched specimens.
Figure 6.7/pag.148 Potential-time behaviour during anodic alkaline etching of Al-
1at.%Zn alloy in alkaline solution 0.5 M NaOH at T = 298 K for 70 s.
Figure 6.8/pag.147 Potential-time behaviour during anodic alkaline etching of
aluminium (99.99wt.%) in alkaline solution 0.5 M NaOH at T = 298 K for 70 s.
Figure 6.9/pag.149 Open circuit potential transient following alkaline etching of Al-
1at.%Zn alloy in 0.5 M NaOH at T = 313 K for 300 s.
Figure 6.10/pag.150 Secondary electron micrographs of a etching of pure aluminium
and Al-1at.%Zn alloy in alkaline solution 0.5 M NaOH at 313 K for 300 s.

Chapter 7

Figure 7.1/pag.158 Voltage-time response of Al-1at.%Zn alloy anodized in 0.1 M
ammonium pentaborate at 5 mA/cm2 until the final potential of 200V. The alloy
before anodizing was in rolled condition and surface treated with the following
treatments: electropolishing at 20V for 180 s in perchloric acid/ethanol (1:4 v/v),
alkaline etching in 0.5 M NaOH solution for 300s, anodic alkaline etching at 5
mA/cm2 in 0.5 M NaOH solution for 70s.
Figure 7.2/pag.159 Secondary electron micrograph at different magnification of the
Al-1at.%Zn alloy in rolled condition and anodized in 0.1M ammonium pentaborate at
5 mA/cm2 to 200V.
Figure 7.3/pag.156 Secondary electron micrograph at different magnifications of Al-
1at.%Zn alloy mechanically polished and subsequently anodized in 0.1M ammonium
pentaborate at 5 mA/cm2 to 200V.
Figure 7.4/pag.157 Secondary electron micrograph at different magnifications of the
Al- 1at.%Zn alloy electropolished in at 20V for 180 s in perchloric acid/ethanol (1:4
v/v) and anodized in 0.1M ammonium pentaborate at 5 mA/cm2 to 200V.
Figure 7.5/pag.158 Secondary electron micrograph at different magnifications of the
Al-1at.%Zn alloy alkaline etched in 0.5 M NaOH solution for 300s and anodized in
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