TRIVALENT CHROMIUM CONVERSION COATINGS ON Al and Al-Cu ALLOYS

330 Pages·2015·16.75 MB·English

by  Jiantao Qi

TRIVALENT CHROMIUM CONVERSION

COATINGS ON Al and Al-Cu ALLOYS

A thesis submitted to The University of Manchester for the degree of Doctor of

Philosophy in the Faculty of Engineering and Physical Sciences

2015

Jiantao Qi

School of Materials / Corrosion and Protection Centre
Table of Contents

Chapter 1 INTRODUCTION ......................................................... 28

Chapter 2 LITERATURE REVIEW ............................................. 30

2.1 Background of Aluminium and AA2024-T3 Aluminium Alloy ................. 30
2.1.1 Composition and microstructure ................................................................................. 30
2.1.2 Surface morphology and pre-treatment ....................................................................... 31
2.2 Corrosion Behaviour of Aluminium and AA 2024-T3 Alloy ..................... 36
2.2.1 Corrosion of pure aluminium ...................................................................................... 36
2.2.2 Corrosion of AA2024-T3 alloy ................................................................................... 37
2.3 Chromate Conversion Coating (CCC) ........................................................ 42
2.3.1 Background ................................................................................................................ 42
2.3.2 Nature and coating growth .......................................................................................... 43
2.3.3 Composition and structure .......................................................................................... 44
2.3.4 Factors influencing CCC formation ............................................................................ 46
2.3.5 Corrosion resistance performance ............................................................................... 50
2.3.6 Promising alternatives to CCC .................................................................................... 51
2.4 Zirconium-based Conversion Coating ........................................................ 52
2.4.1 Background ................................................................................................................ 52
2.4.2 Formation and structure .............................................................................................. 53
2.4.3 Chemistry of Zr (VI) complex .................................................................................... 54
2.4.4 Factors influencing Zr-based conversion coating ........................................................ 55
2.4.5 Corrosion protection ................................................................................................... 58
2.5 Anodic Coating .......................................................................................... 59
2.5.1 Background ................................................................................................................ 59
2.5.2 Formation and morphology......................................................................................... 59
2.5.3 Electrochemical behaviour ......................................................................................... 61
2.6 Trivalent Chromium Conversion (TCC) Coating ....................................... 63
2.6.1 Background ................................................................................................................ 63
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2.6.2 Formation and structure .............................................................................................. 63
2.6.3 Effect of pre- and post-treatment ................................................................................ 65
2.6.4 Evidence of Cr (VI) .................................................................................................... 66
2.6.5 Corrosion protection ................................................................................................... 68
2.7 Key Unsolved Issues .................................................................................. 71
2.8 Introduction to Present Work ..................................................................... 72
Figure captions 2.1-2.19................................................................................... 73

Chapter 3 EXPERIMENTAL PROCEDURE .............................. 95

3.1 Introduction ............................................................................................... 95
3.2 Specimen and Solution Pre-treatment ........................................................ 95
3.2.1 Materials .................................................................................................................... 95
3.2.2 Solutions and reagents ................................................................................................ 96
3.2.3 Superpure aluminium preparation ............................................................................... 96
3.2.4 AA2024-T351 aluminium alloy preparation ............................................................... 97
3.2.5 Ultramicrotomy .......................................................................................................... 98
3.3 Trivalent Chromate Conversion Coating Process ....................................... 98
3.4 Characterization Technologies ................................................................... 99
3.4.1 Scanning electron microscopy (SEM) ......................................................................... 99
3.4.2 Transmission electron microscopy (TEM) .................................................................. 99
3.4.3 Glow discharge optical emission spectroscopy (GDOES) ........................................... 99
3.4.4 Atomic force microscopy and scanning Kelvin probe force microscopy ................... 100
3.4.5 Rutherford backscattering spectroscopy and Nuclear Reaction Analysis ................... 100
3.4.6 X-ray Photoelectron Spectroscopy (XPS) ................................................................. 100
3.4.7 Raman spectroscopy ................................................................................................. 101
3.5 Electrochemical Examination ................................................................... 101
3.5.1 Cyclic voltammetry .................................................................................................. 102
3.5.2 Polarization and electrochemical impedance spectroscopy (EIS) .............................. 102
3.5.3 Electrochemical noise analysis (ENA) ...................................................................... 103
Figure captions 3.1-3.2 .................................................................................. 103

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Chapter 4 TRIVALENT CHROMIUM CONVERSION
COATINGS ON ALUMINIUM ................................................... 105

4.1 Introduction ............................................................................................. 105
4.2 Results ..................................................................................................... 108
4.2.1 OCP measurements .................................................................................................. 108
4.2.2 Coating morphology and growth kinetics .................................................................. 108
4.2.3 Coating composition by GDOES, TEM/EDX, RBS and NRA .................................. 110
4.2.4 Chemical states by XPS and Raman spectroscopy .................................................... 112
4.2.5 Electrochemical impedance spectroscopy ................................................................. 114
4.2.6 Effect of air-ageing treatment on Cr(VI) ................................................................... 115
4.2.7 Effect of sodium sulphite on Cr(VI) .......................................................................... 117
4.3 Discussion ................................................................................................ 119
4.3.1 Formation and composition of TCC coatings ............................................................ 119
4.3.2 Growth kinetic mechanism of TCC coatings ............................................................. 120
4.3.3 Cr (VI) formation mechanism ................................................................................... 121
4.3.4 Effect of air ageing and sodium sulphite on Cr(VI) reduction ................................... 122
4.4 Summary .................................................................................................. 122
Figure Captions 4.1-4.22 ................................................................................ 124

Chapter 5 TRIVALENT CHROMIUM CONVERSION
COATINGS ON AA2024-T351 ALLOYS ................................... 151

5.1 Introduction ............................................................................................. 151
5.2 Results ..................................................................................................... 152
5.2.1 Alloying component on surface and in 3-D space ..................................................... 152
5.2.2 Coating initiation on the second-phase particles ........................................................ 153
5.2.3 Coating morphology and growth kinetics .................................................................. 155
5.2.4 Coating composition by TEM/EDX .......................................................................... 157
5.2.5 Quantitative composition by RBS and NRA ............................................................. 158
5.2.6 Chemical states by XPS ............................................................................................ 159
5.2.7 Electrochemical behaviour ....................................................................................... 160

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5.3 Discussion ................................................................................................ 163
5.3.1 Coating formation..................................................................................................... 163
5.3.2 Influence of the second phase particles ..................................................................... 164
5.3.3 Compositions of TCC coatings on alloys .................................................................. 165
5.3.4 Cr(VI) formation mechanism .................................................................................... 166
5.3.5 Electrochemical behaviour ....................................................................................... 167
5.4 Summary .................................................................................................. 168
Figure Captions 5.1-5.25 ................................................................................ 170

Chapter 6 INFLUENCE OF SURFACE PRE-TREATMENTS 197

6.1. Introduction............................................................................................. 197
6.2 Results ..................................................................................................... 199
6.2.1 Surface morphology and composition ....................................................................... 199
6.2.2 Coating formation and composition .......................................................................... 201
6.2.3 Coating growth kinetics ............................................................................................ 203
6.2.4 Electrochemical properties ....................................................................................... 204
6.2.5 Pre-treatments on as-received alloys ......................................................................... 207
6.3 Discussion ................................................................................................ 209
6.3.1 Coating morphologies and compositions................................................................... 209
6.3.2 Coating formation and growth kinetics ..................................................................... 211
6.3.3 Electrochemical characterization .............................................................................. 212
6.3.4 Mechanism of the improvement of TCC coating process .......................................... 213
6.4. Summary ................................................................................................. 213
Figure Captions 6.1-6.23 ................................................................................ 215

Chapter 7 INFLUENCE OF COATING POST-TREATMENTS
........................................................................................................ 237

7.1. Introduction............................................................................................. 237
7.2 Results ..................................................................................................... 239
7.2.1 Water temperature influence on morphologies .......................................................... 239

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7.2.2 Water temperature influence on compositions ........................................................... 239
7.2.3 Water pH influence .................................................................................................. 240
7.2.4 Electrochemical behaviours ...................................................................................... 241
7.2.5 Influence of sodium sulphite in the water bath .......................................................... 243
7.3 Discussion ................................................................................................ 244
7.3.1 Influence of the water temperature ............................................................................ 244
7.3.2 Influence of pH values of the water bath ................................................................... 245
7.3.3 Influence of sodium sulphite ..................................................................................... 246
7.4. Summary ................................................................................................. 247
Figure Captions 7.1-7.12 ................................................................................ 248

Chapter 8 INFLUENCE OF COPPER CONCENTRATION IN
Al-Cu ALLOYS............................................................................. 264

8.1 Introduction ............................................................................................. 264
8.2 Results ..................................................................................................... 265
8.2.1 Topographic characteristic ........................................................................................ 266
8.2.2 OCP behaviour and cross-sectional characteristic ..................................................... 266
8.2.3 Coating composition by RBS and Raman spectroscopy ............................................ 267
8.2.4 Copper concentration effect on coating growth ......................................................... 269
8.2.5 Effect of copper sulphate addition into the bath ........................................................ 270
8.2.6 Influence on transient formation of Cr (VI) ............................................................... 271
8.2.7 Influence on electrochemical behaviour .................................................................... 272
8.3 Discussion ................................................................................................ 274
8.3.1 Influence of copper in alloys ..................................................................................... 274
8.3.2 Influence of copper sulphate addition into the bath ................................................... 276
8.3.3 Copper and Cr (VI) chemistry .................................................................................. 276
8.4 Summary .................................................................................................. 277
Figure Captions 8.1-8.19 ................................................................................ 279

Chapter 9 GENERAL SUMMARY AND CONCLUSIONS...... 303

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9.1 Conclusions ............................................................................................. 303
9.1.1 TCC coatings on Al .................................................................................................. 303
9.1.2 TCC coatings on AA2024-T351 alloys ..................................................................... 304
9.1.3 Influence of pre-treatments ....................................................................................... 305
9.1.4 Influence of coating post-treatments ......................................................................... 306
9.1.5 Influence of copper concentration in Al-Cu alloys .................................................... 306
9.2 Suggestions for Future Work ................................................................... 307
9.2.1 Effect of Cr(III) component on Cr(VI) examination ................................................. 307
9.2.2 Influence of organic acid addition into the bath......................................................... 308
9.2.3 Influence of sodium sulphite ..................................................................................... 309
9.2.4 Polymeric structure of the Zr-/Cr-based component .................................................. 310
9.2.5 Coating growth kinetics by in-situ AFM ................................................................... 310
Figure Captions:............................................................................................. 312

REFERENCES.............................................................................. 313

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LIST OF FIGURES

Figure 2. 1 The copper assay by the cyclic voltammetry method (a) in deaerated borate
buffer over 40 min NaOH pre-treated, unpretreaed, and 40 min NaOH+30 s HNO -pre-
3
treated AA2024-T3 alloy, and (b) the linear correlation of Cu(0)/Cu(I) oxidation peak area
against Cu oxidation peak height from NaOH-treated AA2024-T3 alloys after the indicated
etching times ....................................................................................................................... 76
Figure 2. 2 The potential-pH Pourbaix diagram and the graphical presentation of the various
equations for aluminium-water system at 25°C [39]. All ionic activities and gas pressures
have been set arbitrarily at 10-6 g ion/l and 1 atm respectively. ............................................ 77
Figure 2. 3 The schematic illustration of a process for copper redeposition by the dissolution
of large Al CuMg and Al Cu intermetallic particles in Al alloys. The particle dealloying
2 2
process included (i) the anodic polarization by the adjacent matrix, (ii) the formation of
detached metallic Cu clusters along the particle remnant coarsens, (iii) the oxidation of the
detached clusters, and (iv) the precipitation or reduction of copper ions that stimulated the
secondary pitting. ................................................................................................................ 78
Figure 2. 4 The schematic illustration of a typical coating system on aluminium matrix. ..... 79
Figure 2. 5 (a) the schematic representation of the hydrolysis-polymerization-precipitation
mechanism for Cr(OH) backbone formation, and (b) the condensation of Cr(VI) on the
3
Cr(III) backbone by nucleophilic attack of hydroxyl ligands in the backbone. ..................... 80
Figure 2. 6 Illustration model for the chromate conversion coating formed on AA2024-T3. 81
Figure 2. 7 The refined model for the chromate conversion coatings formed on AA2024-T3
alloy. ................................................................................................................................... 82
Figure 2. 8 The periodic table of the shaded elements, which compounds were considered as
alternatives for the oxo-Cr6+ inhibitors. ............................................................................... 83
Figure 2. 9 Mechanism of deposition of Zr/Ti layer on AA6016 by SKPFM and SEM. ....... 84
Figure 2. 10 The structure of Zr/Ti-based conversion coating over aluminium alloy. ........... 85
Figure 2. 11 The equivalent circuit model for the physical significance of the anodic film. .. 86
Figure 2. 12 The hydrolysis process of trivalent chromium chloride, which accelerated by the
local pH increasing. ............................................................................................................. 87
Figure 2. 13 The proposed biphasic chemical structure of the TCC coatings on AA2024. The
thickness values listed were those estimated from measurements made in vacuum. ............. 88

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Figure 2. 14 Aging effect on TCC coating structure............................................................. 89
Figure 2. 15 The schematic illustration of the mechanism for the transient formation of
Cr(VI) in the TCC coating during immersion in the air-saturated solution. .......................... 90
Figure 2. 16 The schematic drawing of the artificial scratch cell. ......................................... 91
Figure 2. 17 The plan-view macrophotography of the artificial scratch cell sheets in 0.5 M
NaCl, (a) TCC-treated surface exposed for 21 days, (b) nonTCC surface exposed near TCC-
treated surface for 21 days, (c) bottom nonTCC surface exposed in control cell for 14 days,
and (d) top nonTCC surface exposed in control cell for 14 days. ......................................... 92
Figure 2. 18 The equivalent circuit model used for TCC and nonTCC surfaces in the artificial
scratch cells and for control cells in the Harrison’s solution. ................................................ 93
Figure 2. 19 The equivalent circuit model used for TCC coated alloy surface. ..................... 94

Figure 3. 1 The schematic illustration for the grinding process of the specimen embedded in
the resin in specific directions and steps by 800 grit papers of (a) the side-view of the ground
tip, and (b) the top-view of the ground tip ......................................................................... 104
Figure 3. 2 The schematic illustration for the trimming process of well-grinded specimens,
where B letter means the distance finish between 1 to half screen square in ultramitrotomy.
......................................................................................................................................... 104

Figure 4. 1 The dependence of the open circuit potential of electropolished aluminium on the
time of immersion in a dilute SurTec 650 bath at 40 °C under the usual naturally-aerated
condition(named O ) and under the de-oxygenated condition (named N ). ........................ 129
2 2
Figure 4. 2 Scanning electron micrographs (3 kV SE2 signal) of TCC coatings formed on
electropolished aluminium in a dilute SurTec 650 bath at 40 °C for different times: 15 s; (b)
120 s; (c) and (d) 600 s. ..................................................................................................... 130
Figure 4. 3 Atomic force microscopy topography images of TCC coatings formed on
electropolished aluminium in a dilute SurTec 650 bath at 40 °C for different times: (a) the
electropolished aluminium; (b)15 s; and (c) and (d) 300 s. ................................................. 131
Figure 4. 4 Bright field transmission electron micrographs of ultramicrotomed cross-sections
of TCC coatings formed on electropolished aluminium in a dilute SurTec 650 bath at 40 °C
for different times: (a) 15 s; (b) 60 s; (c) 120 s; and (d) 300 s. ........................................... 132

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Figure 4. 5 Variation of the thickness of TCC coating on aluminium with time of immersion
in a dilute SurTec 650 bath at 40°C. The thicknesses of the coatings were determined from
TEM of ultramicrotomed sections. .................................................................................... 133
Figure 4. 6 GDOES elemental depth profiles of electropolished aluminium following
immersion for 300 s in a dilute SurTec 650 bath at 40 °C: (a) Zr, Cr, C, and (b) S, O, Si, Cu.
Both are smoothed by 10 neighboured points. The dashed line marks the aluminium/coating
interface. The insert shows the Al signal. .......................................................................... 134
Figure 4. 7 High angular annular dark field (HAADF) transmission electron micrograph and
EDX maps of the coating formed on electropolished aluminium for 300 s in a dilute SurTec
650 bath at 40°C................................................................................................................ 135
Figure 4. 8 Experimental and simulated (solid line) RBS spectra for the trivalent chromium
coatings formed for 300 s in a dilute SurTec 650 bath at 40°C. .......................................... 136
Figure 4. 9 Possible hydrolysis mechanism and structure of chromium sulphates. ............. 137
Figure 4. 10 High resolution XPS spectrum for (a) Al 2p and (b) Zr 3d photoelectron regions
and curve fitting for the 300 s TCC coated aluminium ....................................................... 138
Figure 4. 11 (a) High resolution XPS spectrum for C 1s and (b) O 1s photoelectron regions
and curve fitting for the 300 s TCC coated aluminium ....................................................... 139
Figure 4. 12 High resolution XPS spectrum and curve fitting for the Cr 2p photoelectron
region: (a) with CrF ; and (b) without CrF . The aluminium was coated for 300 s. ............ 140
3 3
Figure 4. 13 (a) Raman spectra ranging from 200 to 1200 cm-1 for (I) electropolished
aluminium and the TCC coatings formed after 1200 s of immersion in the SurTec 650 bath at
40 °C which were under (II) open-air condition and (III) the de-oxygenated conditions. The
fresh specimens were analysed immediately after coating formation. All data were smoothed
over five neighbouring points. (b) Fittings of Raman spectra ranging from 700 to 1050 cm-1
of the fresh TCC coatings formed for 1200 s of immersion in the SurTec 650 bath at 40 °C
which was under naturally-aerated condition. .................................................................... 141
Figure 4. 14 Dependence of the modulus of the impedance on (a, c) frequency and (b, d)
phase angle for the bare aluminium in the non-coated condition and following coating for 60,
300 and 600 s. The specimens were post-treated in water at 40 °C. The impedance
measurements were carried out in 0.1 M Na SO solution (a, b) and in the mixture solution of
2 4
0.1 Na SO and 0.1 M NaCl (c, d); equivalent circuit models for (e) the bare Al and (f) the
2 4
coatings formed for 60, 300 and 600 seconds. R , R and R represent the resistances of the
e coat p

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