Non-chromate conversion coating treatments for zinc-alloy electrodeposits PDF
Preview Non-chromate conversion coating treatments for zinc-alloy electrodeposits
Loughborough University Institutional Repository Non-chromate conversion coating treatments for zinc-alloy electrodeposits ThisitemwassubmittedtoLoughboroughUniversity’sInstitutionalRepository by the/an author. Additional Information: • A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University. Metadata Record: https://dspace.lboro.ac.uk/2134/7611 Publisher: (cid:13)c J.A. Wharton Please cite the published version. This item is held in Loughborough University’s Institutional Repository (https://dspace.lboro.ac.uk/) and was harvested from the British Library’s EThOS service (http://www.ethos.bl.uk/). It is made available under the following Creative Commons Licence conditions. For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/ Non-Chromate Conversion Coating Treatments for Zinc-Alloy Electrodeposits by J. A. Wharton A Doctoral in fulfilment for thesis the submitted partial of the requirements award of Doctor Philosophy of Loughborough University of September 1997 Supervisor: Dr G. D. Wilcox IPTNE, LoughbgTough, Utiiversity, Loughborough, Leics,. LEI I 3TU. j', Industrial Supervisor: Dr K. R. Baldwin Structural, Materials Centre (SMC), DERA, Famborough, O, '"dUl4 '--"'Ha-nt-s. OLX. 'ý , 0 by J. A. Wharton, 1997. Abstract In the hexavalent have recent years, suspected carcinogenic and toxic effects of chromium caused great in imposition in the the concern, resulting of stringent environmental and workplace regulations metal finishing industry. This has lead to the for to the formulations. In this search suitable alternatives chromate investigation have been to their molybdate, cerium and permanganate-based solutions examined as for suitability as possible replacements chromate conversion coatings on zinc-alloy electrodeposits. Investigations have from immersion primarily concentrated on producing passive surfaces molybdate-based The have been to processes. novel conversion coatings examined using scanning electron microscopy the Analysis has been to elucidate coating morphologies. of the molybdate-based coatings undertaken determine K-edge EXAFS (extended X-ray characterise and the composition using molybdenum absorption fine 'Me behaviours have been structure) studies. corrosion of the treated surfaces assessed using neutral (fog) linear technique. salt spray corrosion tests and electrochemical methods utilising the polarisation Findings indicate that simple molybdate, cerium and permanganate-based coating treatments only achieve, best, by barrier hindering dissolution thus at corrosion protection means of a effect, as a result the processes, improvements formation the providing modest to the of white and red corrosion products compared with 'as-plated' deposits. However, zinc-alloy certain molybdate-based treatments, utilising a permolybdate inhibition species, appeared to possess additional corrosion resistant behaviours, possibly associated with an for or retarding of the cathodic reactions. Consequently, superior corrosion performances were apparent a number of the permolybdate coatings in comparison with the simple molybdate treatments, achieving level of performance characteristics to still at a reduced more comparable a chromate coating, although Specific deemed be protection. permolybdate coatings were to capable of providing enhanced corrosion by barriers performances, other than to acting solely as corrosion- i Acknowledgments I like to thank the both fiiends, have helped in would all people colleagues and who me any way during for the this thesis. research In I indebted Dr G. D. Wilcox for his help to particular, arn my supervisor patience, and guidance. Similarly, I like to thank Dr K. R. Baldwin, industrial for his help would my supervisor, and informative discussions, DERA for its financial (Agreement No FRNIcU773). the and also support : I'lianks due fellow for informal informative to are also my colleagues and research students, and discussions, Dr D. H. Ross for his invaluable help principally and advice. I like would to thank Dr M. Gilbert for the provision of laboratory facilities and to acknowledge EPSRC/Daresbury the Laboratory for beatntime. I to the allocation of synchrotron would also wish thank Dr JR Mosselmans for his help during Daresbury Laboratory. to the and guidance my visits Tbanks are also due to Mr F. Page Mr J. S. Bates for their help the SEM. and on ii Contents Table of i ABSTRACT .................................................................................................................... ii ACKNOWLEDGMENTS .............................................................................................. TABLE OF CONTENTS ............................................................................................... ABBREVIATIONS viii ...................................................................................................... SYMBOLS viii .................................................................................................................. 1 I INTRODUCTION ....................................................................................................... I 1.1 Definition Conversion Coating of a .............................................................................................................. 2 1.2 Terminology ................................................................................................................................................. 3 2 LITERATURE SURVEY ............................................................................................. 3 2.1 Chromate Conversion Coatings .................................................................................................................. 3 2.1.1 Coating Growth Mechanism .. ................................................................................................................. 7 2.1.2 Corrosion Resistance Chromate Coatings of ........................................................................................... 10 2.2 Molybdate-based treatments **** ... * ............................................................................................................. 13 2.2.1 Iso- Heteropolymolybdate Species and ................................................................................................. 14 2.2.2 Molybdate/Phosphoric Coatings acid .................................................................................................... 15 2.2.3 'MolyPhos' Treatments .......................................................................................................................... 16 2.2.4 Molybdate Species Incorporated into Aluminiurn Films ......................................................................... 17 2.2.5 Other Patented Molybdate Treatments .................................................................................................. 19 2.2.6 Peroxomolybdate Compounds .............................................................................................................. 20 2 3 Ce i dt t t r um ase rea men s -b . .......................................................................................................................... 20 2.3.1 Cerium Rare Earth Metal (REM) Inhibitors and .................................................................................... 21 2.3.2 Cerium Coating Treatments .................................................................................................................. 22 2.3.3 Cerium-based Treatment Patent ............................................................................................................ 23 2.3.4 'Stainless Aluminium! Treatment ........................................................................................................... 25 2.3.5 XANES Cerium Coating of a ................................................................................................................ 26 2 4 Zi i b dt t t rcon um- ase rea men s . ........ . ........................................................................................................... 27 2.5 Miscellaneous Systems ............................................................................................................................... 27 2.5.1 Tungstate-based coatings ...................................................................................................................... 28 2.5.2 Cobalt complexes ................................................................................................................................. 28 2.5.3 Trivalent Chromium Processes .............................................................................................................. 29 3 BACKGROUND TO COATED SUBSTRATE MATERIALS ..................................... 29 3.1 Zinc Coatings ...................................................... . ..................................................................................... iii 3.2 Zinc Coatings 30 alloy .................................................................................................................................... 3.2.1 Mechanism Deposition 30 of ..................................................................................................................... 3.2,2 Zinc-Nickel Alloy Coatings 31 ................................................................................................................... 3.2.3 Zinc-Cobalt Alloy Coatings 32 .................................................................................................................. 3.2.4 Coffosion Mechanism Zinc Alloys 32 of .................................................................................................... 3.3 Passivation Zinc-based Coatings 33 of ........................................................................................................... 4 ELECTRODE KINETICS 35 ......................................................................................... 4.1 Fundamental Concepts 35 ......... . ....... . .......................................................................................................... 4.1.1 ChargeTransfer 35 .................................................................................................................................... 4.1.2 Equilibrium Potentials 35 ........................................................................................................................... 4.1.3 Non-Equilibrium Potentials 36 ................................................................................................................... 4.2 The kinetics Corrosion Reactions 38 of .......................................................................................................... 5 CORROSION TESTING 41 .......................................................................................... 5.1 Neutral Salt Spray (Fog) Corrosion Test 41 .................................................................................................. 5.2 Electrochemical Techniques 42 ...................................................................................................................... 5.2.1 Electrochemical Measurements 42 ............................................................................................................. 43 5.2.2 Linear (LPR) polarisation resistance ..................................................................................................... 6 COATING CHARACTERISATION 47 ........................................................................... 6.1 X-ray Absorption Spectroscopy 47 .................................................................................. 47 6.1.1 Extended X-ray Absorption Fine Structure (EXAFS) ............................................................................ 6.1.2 Interpretation EXAFS Data 49 of .............................................................................................................. 7 EXPERIMENTAL 50 ...................................................................................................... 7.1 Coupon Preparation 50 .............................................................................................. .............. 7.2 Preparation Zinc-Nickel Alloy Electrodeposits 50 of ...... ............ . ................................................ 7.3 Passivation Treatments 51 ............... . ......................................................... . .................................................. 7.3.1 Chromate Treatment 51 ......................................................................... .................................................... 7.3.2 Simple Molybdate Treatment 51 ............................................................ .................................................... 7.3.3 HeteroPolymolybdate Treatments 51 ..................................................... .................................................... 7.3.4 Cerium-based Treatments 52 ................................................................. .................................................... 7.3.4.1 Simple Cerium-based Treatments 52 .............................................. .................................................... 7.3.4.2 Dual Immersion Treatments 52 ...................................................... .................................................... 7.3.4.3 Cerium-based Immersion / Anodic Surface Modification 52 ........... .................................................... 7.3.4.4 Cerium Chloride / Hydrogen Peroxide 53 ....................................... .................................................... 7.3.5 Permolybdate Treatments 53 ................................................................. .................................................... 7.3.6 Permanganate Treatment 54 .................................................................. .................................................... 7.3.7 Molybdate-based / Permanganate 'Single Solution! Treatments 54 .......... .................................................... 7.3.8 Molybdate-based / Permanganate 7wo Solution! Treatments 55 ............ .................................................... iv 7.4 Corrosion Tests 55 .......................................................................................................................................... 7.4.1 Neutral Salt Spray (Fog) Corrosion Test 55 ............................................................................................... 7.4.2 Electrochemical Measurements 56 ............................................................................................................. 7.4.2.1 Potentiodynamic Polarisation Measurements 56 ................................................................................. 7.4.2.2 Electrochemical Corrosion Tests 56 ................................................................................................... 7.4.2.3 Analysis Errors 56 of ......................................................................................................................... 7.5 Coating Characterisation /Elucidation 57 ................... o ................................. o ............. o .................... o ........... 7.5.1 Scanning Electron Mcroscopy 57 ....................................... 0 ..................................................................... 7.5.2 EXAFS Analysis 57 ................................................................................................................................... 7.5.2.1 Standards 57 ...................................................................................................................................... 7.5.2.1.1 Sodium Heptamolybdate 57 ............................................................................................................ 7.5.2.1.2 Sodium Peroxomolybdate 57 .......................................................................................................... 7.5.2.1.3 Dizinc Trimolybdenum(IV) Oxide 57 .............................................................................................. 58 7.5.2.1.4 Trizinc Dimolybdenum(VI) Oxide .............................................................................................. 7.5.2.2 Coated Surfaces 58 ............................................................................................................................ 7.5.3 EXAFS Experimental 58 ........................................................................................................................... 7.5.3.1 EXAFS Data Analysis 58 ................................................................................................................... 8 RESULTS AND DISCUSSION 60 ................................................................................. 8.1 Coating Morphology ............................................................................................................................. 8 1 1 Chromate Coatin 60 g s . . .................. ............................................................................................................. 8.1.2 Simple Molybdate Coatings 60 ..... ............................................................................................................. 8.1.3 Heteropolymolybdate 61 Coatinas ............................................................................................................. 8.1.4 Cerium Based Coatings 61 ........................................................................................................................ 61 8.1.4.1 Simple Cerium-based Treatments .................................................................................................. 61 8.1.4.2 Dual Immersion Treatments .......................................................................................................... 62 8.1.4.3 Immersion / Anodic Surface Modification Treatments ................................................................... 8.1.4.4 Cerium Chloride / Hydrogen Peroxide Treatments 62 ......................................................................... 62 8.1.5 Permolybdate Coatings ......................................................................................................................... 63 8.1.6 Permanganate Coatings ........................................................................................................................ 63 8.1.7 Combined Molybdate Permanganate Treatments and ............................................................................ 63 8.1.7.1 Molybdate / Permanganate Single Solution Treatments . ................................................................ 63 8.1.7.2 Molybdate / Permanganate Two Solution Treatment . .................................................................... 8.2 Neutral Salt Spray (Fog) Corrosion Test 64 . ................................................................................................ 8.2.1 Chromate Coatings 64 ............................................................................................................................... 8.2.2 Simple Molybdate, Coatings 65 .................................................................................................................. 8.2.3 Heteropolymolybdate Coatings 66 ............................................................................................................. 8.2.4 Cerium-based Coatings 66 ......................................................................................................................... 8.2.4.1 Simple Cerium-based Treatments 66 .................................................................................................. 8.2.4.2 Dual Immersion Treatments 66 .......................................................................................................... 8.2.4.3 Immersion / Anodic Surface Modification Treatment 67 ..................................................................... 8.2.4.4 Cerium Chloride / Hydrogen Peroxide 67 ........................................................................................... 8.2.5 Permolybdate Coatings 67 ......................................................................................................................... 8.2.6 Permanganate Coatings 68 ........................................................................................................................ 8.2.7 Combined Molybdate 68 Permanganate Treatments and ............................................................................ 8.2.7.1 Molybdate / Permanganate Single Solution Treatment 68 ................................................................... 8.2-7.2 Molybdate / Permanganate Two Solution Treatment 68 ..................................................................... 8.3 Potentiodynamic Polarisation Curves 69 ....................................................................................................... 8.3.1 Zinc-Nickel Alloy Electrodeposit 69 .......................................................................................................... V 69 8.3.2 Chromate Coating ....................................... ..................................................... 70 8.3.3 Simple Molybdate Coating ........................... ..................................................................................... 70 8.3.4 Permolybdate Coatings ................................ ..................................................................................... 73 8.4 Short Term Electrochemical Corrosion Tests (24 hours) ......................................................................... 73 8.4.1 Zinc-Nickel Alloy Electrodeposit .......................................................................................................... 73 8.4.2 Chromate Coating ................................................................................................................................ 73 8.4.3 Simple Molybdate Coatings .................................................................................................................. 74 8.4.4 Permolybdate Coatings ......................................................................................................................... 76 8.5 Long Term Electrochemical Corrosion Tests ........................................................................................... 76 8.5.1 Zinc-Nickel Alloy Electrodeposit .......................................................................................................... 77 8.5.2 Chromate Coating ................................................................................................................................ 79 8.5.3 Simple Molybdate Coating .................................................................................................................... 79 8.5.4 Comparison in Long Term Corrosion Behaviour ................................................................................... 80 8.5.5 Heteropolymolybdate Coatings ............................................................................................................. 81 8.5.6 Cerium-based Coatings ......................................................................................................................... 81 8.5.6.1 Cerium Chloride/ Cerium Nitrate Coating Treatment .................................................................... 82 8.5.6.2 Immersion / Anodic Surface Modification Treatment ..................................................................... 83 8.5.6.3 Cerium Chloride / Hydrogen Peroxide Treatment .......................................................................... 8.5.7 Permolybdate Coatings 84 ......................................................................................................................... 86 8.5.7.1 Comparison Corrosion Behaviour of ............................................................................................. 86 8.5.8. Permanganate Coating ......................................................................................................................... 88 8.5.9 Molybdate-based / Permanganate'Single Solution! Coating Treatments ................................................. 88 8.5.9.1 Molybdate / Permanganate'Single Solution! Coating ...................................................................... 89 S. 5.9.2 Permolybdate / Permanganate 'Single Solution! Coating ................................................................. 90 8.5.10 Molybdate-based / Perinanganate 'Two Solution' Coating Treatments ................................................. 90 8.5.10.1 (I)Molybdate(2)Permanganate'Two Solution! Coating ............................................................. 91 8.5.10.2 (1) Permanganate (2) Molybdate'Two Solutioif Coating I ........ .................................................... 91 8.5.10.3 (1) Permolybdate (2) PermanganateTwo Solution! Coating ........................................................ 8.5.10.4 (1) Permanganate (2) Permolybdate'Two Solution! Coating 92 ........................................................ 8.6 Summary the Electrochemical Tests 93 of ............. . ........................ . .......... . ................................................ 8.6.1 Chromate Coatings 93 ............................................................................................................................... 8.6.2 Simple Molybdate Coatings 93 .................................................................................................................. 8.6.3 Cerium-based Coatings 94 ......................................................................................................................... 8.6.4 Permolybdate Coatings 94 ......................................................................................................................... 8.6.5 Permanganate Coatings 95 ........................................................................................................................ 8.7 EXAFS Coating Characterisation 96 ............................................................................................................. 8.7.1 Standards 96 ............................................................................................................................................. 8.7.1.1 Sodium Molybdate Dihydrate 96 ........................................................................................................ 8.7.1.2 Sodium Molybdate 97 ........................................................................................................................ 8.7.1.3 Potassium Molybdate 97 .................................................................................................................... 8.7.1.4 Molybdenum Trioxide 97 .................................................................................................................... 8.7.1.5 Sodium Heptamolybdate 97 ............................................................................................................... 8.7.1.6 Sodium Peroxomolybdate 98 ............................................................................................................. 8.7.1.7 Molybdenum Dioxide 98 .................................................................................................................... 8.7-1.8 Molybdenum Disulfide 99 .................................................................................................................. 8.7.1.9 Zinc Molybdate 99 ............................................................................................................................. 8.7.1.10 Dizinc Trimolybdenum. (IV) Oxide 99 .............................................................................................. 8.7.1.11 TrizincDimolybdenum(VI) Oxide 100 ............................................................................................ 8.7.2 Coated Surfaces 100 ................................................................................................................................. 8.7.2.1 Simple Molybdate Coating 100 .......................................................................................................... vi 101 8.7.2.2 Permolybdate Coating ................................................................................................................. 102 8.7.3 Determination the Hexavalent Molybdenum Content of ...................................................................... 103 8.9 The Mechanism for Costing Formation ................................................................................................. 105 9 CONCLUSIONS .................................................................................................... 10 REFERENCES .................................................................................................... 11 MICROGRAPHS ........................................................................................................ 12 TABLES .................................................................................................................... 13 FIGURES ................................................................................................................... 14 APPENDICES ............................................................................................................ PATENT APPLICATION 1997. K. R. Baldwin, UK Patent Application 9718642.3 J. A. Wharton, G. D. Wilcox and number PUBLICATIONS 10. J. A. Wharton, G. D. Wilcox K. R. Baldwin, Trans. Inst. Metal Finish., 1996,74,2 and vii
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