Formation of Sol-gel Coatings on Aluminium Alloys A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences 2011 Zuwei Feng Corrosion and Protection Centre School of Materials Contents Contents LIST OF FIGURES AND TABLES………………………………….……………9 ABSTRACT………………………………………………………………………...20 DECLARATION…………………………………………………………………..21 COPYRIGHT STATEMENT…………………………………………………….22 ACKNOWLEDGEMENT…………………………………………………………23 Chapter 1 …………………………………………………………………………...24 1 INTRODUCTION ................................................................................................25 Chapter 2 …………………………………………………………………………..29 2 LITRATURE SURVEY…………………………………………………………30 2.1 Introduction…………………………………………………………………….30 2.2 Surface Pre-treatment of Aluminium and Its Alloys………………………...30 2.2.1 Electropolising of aluminium and its alloys………………………………..31 2.2.2 Chemical polishing of aluminium and its alloys…………………………...32 2.2.3 Alkaline etching/acid desmutting…………………………………………...33 2.2.4 Mechanical polishing………………………………………………………...34 2.3 Chromate Conversion Coatings……………………………………………….34 2.3.1 Chromate conversion coating…………………………………………….....35 2.3.1.1 Application of chromate conversion coatings…………………………….....35 2.3.1.2 Chemistry of chromate conversion coatings………………………………...38 2.3.1.3 Protection of aluminium by chromate/fluoride conversion coating………...40 2.4 Non-chromate Conversion Coatings……………………………………….....40 2.4.1 Zirconium conversion coatings……………………………………………...40 2.4.2 Cerium conversion coatings…………………………………………………43 2.5 Sol-gel Processes and Sol-gel Coatings………………………………………..44 2.5.1 Basic terms and concepts used in the sol-gel process………………………45 2.5.1.1 Colloid………………………………………………………………………45 2.5.1.2 Sol……………………………………………………………………….......45 2.5.1.3 Ceramic……………………………………………………………………...45 2.5.1.4 Precursors……………………………………………………………………45 2.5.1.5 Alkane, alkyl, alcohol, alkoxy, and alkoxide……………………………......46 2.5.2 Key features of conventional sol-gel processes……………………………..46 2 Contents 2.5.2.1 Hydrolysis and condensation………………………………………………..46 2.5.2.2 Gelation and polymerisation………………………………………………...47 2.5.2.3 Drying/curing process…………………………………………………….....48 2.5.3 Sol-gel coating application…………………………………………………..50 2.5.3.1 Dip coating process……………………………………………………….....51 2.5.3.2 Spin coating…………………………………………………………………52 2.5.3.3 Spray deposition……………………………………………………………..53 2.5.3.4. Electrodeposition…………………………………………………………...53 2.5.4 The general development of the sol-gel process……………………………53 2.5.5 The development of sol-gel process with organic modification…………...55 2.5.6 Advantages of sol-gel process with organic modification………………….57 2.5.7 Sol-gel coatings for corrosion protection of aluminium and its alloys……58 2.5.7.1 Inorganic sol-gel coatings…………………………………………………...58 2.5.7.2 Organically modified hybrid sol-gel coatings on aluminium…………….....59 2.5.7.3 Formation of nanostructures and nanoparticles in the sol-gel coatings during sol-gel process……….........................................................................61 2.5.7.4 Organic modified coatings with direct addition of nanoparticles…………...63 2.5.7.5 Organic modified coatings with additions of corrosion inhibitors……….....64 2.6 Influence of Environmental Restrictions for Sol-gel Development…………66 2.7 Introduction to Present Work…………………………………………………66 Chapter 3……………………………………………………………………………69 3 FORMATION OF CRACK-FREE SOL-GEL COATINGS ON AA1050 ALUMINIUM ALLOY………………………………………………...70 3.1 Introduction………………………………………………………………….....70 3.2 Experimental…………………………………………………………………..71 3.3 Results…………………………………………………………………………..72 3.3.1 Surface of AA1050 aluminium alloy after etching and desmutting……………………………………………………………………72 3.3.2 Topography of the coating………………………………………………......73 3.3.3 Surface cracks and roughness changes observed by AFM and SKPM…..75 3.3.4 Morphology, microstructure and compositions of the coatings…………..76 3.3.5 Corrosion protection of the AA1050 aluminium alloy………………….....77 3.4 Discussion………………………………………………………………………78 3.4.1 Performance of the coating……………………………………………….....78 3 Contents 3.4.1.1 Influence of organic/inorganic ratios………………………………………..78 3.4.1.2 Influence of withdrawal speed………………………………………………79 3.4.1.3 Influence of curing temperature……………………………………………..79 3.4.2 Structure and composition of the coatings…………………………………80 3.4.3 Mechanism of crack generation and crack-free coating formation………80 3.5 Conclusions……………………………………………………………………..81 Chapter 4…………………………………………………………………………..100 4 INFLUENCE OF WATER CONCENTRATION ON THE THICKNESS AND PERFORMANCE OF SOL-GEL COATINGS………………………..101 4.1 Introduction…………………………………………………………………...101 . 4.2 Experimental………………………………………………………………….103 4.3 Results…………………………………………………………………………104 4.3.1 Surface observation of the coated AA1050 aluminium alloy…………….104 4.3.2 Formation of sol-gel coatings with total GPTMS/TPOZ to water molar ratio of 0.003…………………………………………………………104 4.3.3 Formation of sol-gel coatings with total GPTMS/TPOZ to water molar ratio of 0.03………………………………………………………….104 4.3.4 Formation of sol-gel coatings with total GPTMS and TPOZ to water molar ratio of 0.06………………………………………………………….105 4.3.4.1 In the absence of cerium oxide nanoparticles……………………………...105 4.3.4.2 In the presence of cerium oxide nanoparticles……………………………..106 . 4.3.5 Anodic polarisation behaviour of coated alloy……………………………106 4.3.5.1 Anodic polarisation behaviour of thin sol-gel coatings or coatings in the presence of cracks…………………………………………………...106 4.3.5.2 Anodic polarisation behaviour of the alloy coated with total GPTMS and TPOZ to water molar ratio of 0.06…………………………..107 4.4 Discussion……………………………………………………………………..107 4.4.1 Influence of water concentration to the thickness of the coatings……….107 4.4.2 Influence of water contents on the crack formation……………………...107 4.4.3 Influence of water contents on the barrier-type protection of the alloy in aggressive environment……………………………………………108 4.4.4 Influence of cerium oxide nanoparticles on the barrier-type protection of sol-gel coatings………………………………………………109 4 Contents 4.5 Conclusions……………………………………………………………………109 Chapter 5…………………………………………………………………………..123 5 INFLUENCES OF ALLOY SUBSTRATES AND SURFACE PRETREATMENTS ON SOL-GEL COATING FORAMTION ……………………………...…………………………………..124 5.1 Introduction…………………………………………………………………...124 5.2 Experimental………………………………………………………………….125 5.3 Results…………………………………………………………………………127 5.3.1 Formation of sol-gel coatings with a GPTMS/TPOZ ratio of 0.7 on magnetron sputtered superpure aluminium…………………………..127 5.3.2 Formation of sol-gel coatings with a GPTMS/TPOZ ratio of 2.7 on magnetron sputtered superpure aluminium and aluminium-copper alloys…………………………………………………...129 5.3.2.1 SEM examination…………………………………………………………..129 5.3.2.2 TEM examinations…………………………………………………………130 5.3.3 Formation of sol-gel coatings on electropolished superpure aluminium with different water contents…………………………………130 5.3.3.1 Surface of electropolished superpure aluminium………………………….130 5.3.3.2 Sol-gel coating formation with GPTMS/TPOZ ratio of 2.7 on electropolished superpure aluminium……………………………………...130 5.3.3.3 Sol-gel coating formation with GPTMS/TPOZ ratio of 5.5 on electropolished superpure aluminium……………………………………..131 5.4 Discussion……………………………………………………………………..131 5.5 Conclusions……………………………………………………………………132 Chapter 6…………………………………………………………………………..149 6 CHEMISTRY OF SOL-GEL COATINGS…………………………………...150 6.1 Introduction…………………………………………………………………...150 6.2 Experimental………………………………………………………………….151 6.3 Results…………………………………………………………………………153 6.3.1 FTIR and Raman Spectra………………………………………………….153 6.3.1.1 FTIR spectra from GPTMS alkoxide, acetic acid and water………………153 5 Contents 6.3.1.2 FTIR and Raman spectra from hydrolysed GPTMS sol-solution and GPTMS coating……………………………………………………………154 6.3.1.3 FTIR and Raman spectra from TPOZ alkoxide, TPOZ sol solution and TPOZ coating………………………………………………………………155 6.3.1.4 FTIR and Raman spectra from the coatings formed with GPTMS/TPOZ ratio of 0.7………………………………………………………………...157 6.3.1.5 FTIR and Raman spectra from the coatings formed with GPTMS/TPOZ ratio of 2.7…………………………………………………………………158 6.3.2 X-ray photoelectron spectroscopy…………………………………………159 6.3.2.1 C ls core level………………………………………………………………159 6.3.2.2 O ls core level……………………………………………………………..160 6.3.2.3 Zr 3d core level…………………………………………………………….160 6.3.2.4 Si 2p core level……………………………………………………………..160 6.4 Discussion……………………………………………………………………..161 6.4.1 Structure of GPTMS sol solution after hydrolysis and GPTMS coating…………………………………………………………….161 6.4.1.1 Changes of GPTMS alkoxide after hydrolysis for 2h……………………..161 6.4.1.2 Aqueous acetic acid species prior and acetic acid in GPTMS sol solution…………………………………………………………………161 6.4.1.3 Structure of GPTMS coating………………………………………………162 6.4.2 Structures of TPOZ sol solution and TPOZ coating……………………..162 6.4.2.1 Changes of TPOZ alkoxide after hydrolysis for 2 h……………………….162 6.4.2.2 Structure of TPOZ coating…………………………………………………163 6.4.3 Structure of sol-gel coating with the GPTMS/TPOZ ratio of 0.7 and 2.7……………………………………………………………………….164 6.5 Conclusions……………………………………………………………………164 Chapter 7…………………………………………………………………………..183 7 FORMATION OF SOL-GEL COATINGS ON AA2024 ALUMINIUM ALLOY………………………………………………………………………….184 7.1 Introduction…………………………………………………………………...184 7.2 Experiments…………………………………………………………………...185 7.3 Results…………………………………………………………………………187 6 Contents 7.3.1 Formation of sol-gel coating on etched and desmutted AA2024 aluminium alloy……………………………………………………………..187 7.3.2 Formation of sol-gel coating on HNO cleaned AA2024 aluminium 3 Alloy………………………………………………………………………...188 7.3.3 Formation of sol-gel coatings with cerium oxide nanoparticles on nitric acid cleaned AA2024 aluminium alloy……………………………..189 7.3.4 Formation of double layer sol-gel coatings on nitric acid cleaned AA2024 aluminium alloy…………………………………………………...191 7.3.5 Corrosion test……………………………………………………………….192 7.4 Discussion……………………………………………………………………..193 7.4.1 Influence of surface pretreatment on surface morphologies of AA2024 aluminium alloy…………………………………………………...193 7.4.2 Influence of surface pretreatment to the sol-gel coating formation on AA2024 aluminium alloy………………………………………………..195 7.4.3 Influence of cerium oxide nanoparticles…………………………………..196 7.4.4 Performance of double-layer sol-gel coating……………………………...197 7.5 Conclusions……………………………………………………………………197 Chapter 8…………………………………………………………………………..225 8 GENERAL SUMMARY, CONCLUSIONS AND FUTURE WORK……….226 8.1 General Summary…………………………………………………………….226 8.1.1 Introduction…………………………………………………………………226 8.1.2 Influence of organic/inorganic moieties and thickness to crack formation……………………………………………………………..226 8.1.3 Factors that influence the coating thickness………………………………227 8.1.4 Incorporation of cerium oxide nanoparticles……………………………..227 8.1.5 Influence of aluminium substrates………………………………………...228 8.1.6 Adhesion of sol-gel coatings to aluminium substrates……………………228 8.1.7 Influence of surface pretreatment for AA1050 and AA2024 aluminium alloy…………………………………………………………….229 8.1.8 Morphologies, microstructures, and chemical structures of the sol-gel coatings……………………………………………………………...230 8.1.9 Performance of coated aluminium alloys in aggressive chloride solution……………………………………………………………………...230 8.1.10 Sol-gel coating system to replace a chromate-containing coating system……………………………………………………………………....231 8.2 Conclusions……………………………………………………………………232 8.2.1 Crack-free sol-gel coating formation……………………………………...232 8.2.2 Interaction between aluminium and aluminium alloy substrates and sol-gel coatings……………………………………………………………..233 7 Contents 8.2.3 Corrosion protection of sol-gel coated AA1050 and AA2024 aluminium alloys…………………………………………………………...235 8.3 Suggestions for the Future Work….………………………………………...237 REFERENCE……………………………………………………………………..240 Number of words: 49803 8 List of figures and tables List of Figures and Tables Chapter 2 Figure 2.1, page 68. Schematic diagrams of stages of the dip coating process Chapter 3 Figures 3.01, page 83. Scanning electron micrograph of the surface of the etched and desmutted AA1050 aluminium alloy Figures 3.02, page 83. Atomic force micrograph of the surface of the etched and desmutted AA1050 aluminium alloy Figures 3.03, page 84. EDX analyses of the second phase particles present in the AA1050 aluminium alloy: (a) Al Fe particle; (b) AlFeSi particle 6 Figures 3.04, page 85. Scanning electron micrographs of the etched and desmutted AA1050 aluminium alloy after coating with GPTMS /TPOZ ratio of 0.7: (a) curing at 60 C, withdrawal speed of 60 mm/min; (b) curing at 75 C, withdrawal speed of 60 mm/min; (c) curing at 60 C, withdrawal speed of 180 mm/min; (d) curing at 75 C, withdrawal speed of 180 mm/min; (e) curing at 110 C, withdrawal speed of 180 mm/min; (f) curing at room temperature, withdrawal speed of 180 mm/min Figures 3.05, page 86. Scanning electron micrographs of the etched and desmutted AA1050 aluminium alloy after coating with GPTMS /TPOZ ratio of 1.5 and curing at 75 °C: (a) withdrawal speed of 60 mm/min; (b) withdrawal speed of 180 mm/min Figures 3.06, page 87. Surface appearance of AA1050 aluminium alloy after sol- gel coating with GPTMS/TPOZ ratios of 2.7 and 5.5: (a) GPTMS/TPOZ ratios of 2.7, withdrawal speed of 60 mm/min, and curing at 60 C; (b) GPTMS/TPOZ ratios of 2.7, withdrawal speed of 180 mm/min, and curing at 60 °C; (c) GPTMS/TPOZ ratios of 2.7, withdrawal speed of 180 mm/min and curing at 110 °C; (d)GPTMS/TPOZ ratios of 5.5, withdrawal speed of 180 mm/min and curing at 110 ° Figure 3.07, page 88. Scanning electron micrographs of cross-sections of sol-gel coated AA1050 aluminium alloy: (a) and (b) GPTMS/TPOZ ratio of 0.7 at a withdrawal speed of 60 mm/min and curing at 75 °C; (c) and (d) GPTMS/TPOZ ratio of 0.7 at a withdrawal speed of 180 mm/min and curing at 75 °C; (e) GPTMS/TPOZ ratio of 0.7 at a withdrawal speed of 180 mm/min and curing at 110 °C 9 List of figures and tables Figure 3.08, page 89. Scanning electron micrographs of cross sections of the etched and desmutted AA1050 aluminium alloy after coating with different GPTMS/TPOZ ratios: (a) 1.5, with cracks present and protrusion of a second phase particle; (c) 2.7, displaying a smooth surface and good adhesion at the locations of second phase particles; (d) 5.5, with a smooth surface developed. A withdrawal speed of 180 mm/min and a curing temperature of 110°C were employed. Figure 3.09, page 90. AFM height images and corresponding surface potential maps of the etched and desmutted AA1050 aluminium alloy after coating with different GPTMS/TPOZ ratios: (a) height image of the surface after coating with a GPTMS/TPOZ ratio of 0.7, showing the presence of a crack; (b) corresponding surface potential map of (a), showing reduced surface potential at the site of the crack. (c) height image of the surface after coating with a GPTMS/TPOZ ratio of 2.7, showing no cracks in the surface; (d) corresponding surface potential map of (c), showing a relatively uniform surface potential Figure 3.10, page 91. AFM images of the etched and desmutted AA1050 aluminium alloy after coating with different GPTMS/TPOZ ratios: (a) 0.7; (b) 1.5; (c) 2.7; (d) 5.5. A withdrawal speed of 180 mm/min and a curing temperature of 110°C were employed Figure 3.11, page 92. TEM images of the etched and desmutted AA1050 aluminium alloy after coating under various conditions: (a) resin penetration through a crack for a coating formed with a GPTMS/TPOZ ratio of 0.7 at a withdrawal speed of 60 mm/min and curing at 75 °C; (b) crack-free coating formed with a GPTMS/TPOZ ratio of 2.7 at a withdrawal speed of 180 mm/min and curing at 110 °C; (c) and (d) minor defects and Al Fe particles in a coating formed with a 3 GPTMS/TPOZ ratio of 5.5 at a withdrawal speed of 180 mm/min and curing at 60 °C Figure 3.12, page 93. Selected area diffraction from the sol-gel coating with a GPTMS/TPOZ ratio of 0.7: (a) initial pattern indicating an amorphous structure; (b) pattern after prolonged exposure under the electron beam, indicating monoclinic ZrO . A withdrawal speed of 60 mm/min and a curing temperature of 60 °C were 2 employed Figure 3.13, page 94. Experimental (dotted) and simulated (solid line) RBS spectra of etched and desmutted AA1050 aluminium alloy after coating with different 10
Description: