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Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels / PETER MORGAN

Swansea University Author: PETER MORGAN

DOI (Published version): 10.23889/SUThesis.69269

Abstract

This thesis aimed to enhance the understanding of how bare and organically coated Zn-Al galvanised steels, specifically HDG, Galfan, and Zn55Al, corrode and develop corrosion protection using alternative inhibitors to hexavalent chromium. Hexavalent chromium poses significant health and environmental...

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Published: Swansea University, Wales, UK 2025
Institution: Swansea University
Degree level: Doctoral
Degree name: EngD
Supervisor: Williams, G., Penney, D.
URI: https://cronfa.swan.ac.uk/Record/cronfa69269
first_indexed 2025-04-10T14:23:25Z
last_indexed 2025-04-12T04:36:42Z
id cronfa69269
recordtype RisThesis
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spelling 2025-04-11T10:44:24.8216247 v2 69269 2025-04-10 Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels fed902f23e2b884159e8297ae3884560 PETER MORGAN PETER MORGAN true false 2025-04-10 This thesis aimed to enhance the understanding of how bare and organically coated Zn-Al galvanised steels, specifically HDG, Galfan, and Zn55Al, corrode and develop corrosion protection using alternative inhibitors to hexavalent chromium. Hexavalent chromium poses significant health and environmental hazards.The aqueous localised corrosion behaviour of Zn55Al, relative to HDG and Galfan, is poorly understood. Therefore, the corrosion behaviour of bare Zn55Al coated steel immersed in aqueous chloride was investigated using Scanning Vibrating Electrode Technique (SVET), Time-Lapse Microscopy (TLM), and Scanning Kelvin Probe Force Mi- croscopy (SKPFM). It was found upon immersion that Zn55Al’s zinc-rich interdendritic phase initially undergoes preferential anodic attack, but over time, the attack extended to aluminium-rich dendrites, causing more severe localised corrosion. High chloride concentrations yielded extensive local corrosion.The inhibitory performance of the chromate-free corrosion inhibitor, 2-(1,3-benzothiazol- 2-ylthio)succinic acid (BTSA), was evaluated against the aqueous localised corrosion of HDG, Galfan, and Zn55Al surfaces. Determined by SVET and Linear Polarisation Resistance (LPR), 10 mM of BTSA dosed in aqueous chloride electrolyte provided inhibitor efficiencies upwards of 90% for each substrate. BTSA provided mixed inhibition, but the magnitude of cathodic inhibition decreased as galvanised coating aluminium content increased.The ability of in-coating BTSA to mitigate corrosion-driven organic coating failure of HDG steel was investigated. Time-lapse microscopy and SKP experiments showed that in-coating BTSA, added directly to a model coating or as a smart release exchangeable anion reduced the rate of cathodic delamination of HDG by up to a factor of 20. This performance was likely due to strong adsorption on the intact metal surface, stifling cathodic reduction of oxygen.Finally, a novel accelerated test cell was developed to assess in-coating inhibitors against the anodic undermining of ZnAl galvanised substrates. The preliminary design showed a measurable anodic undermining of unpigmented PVB-coated Zn55Al within 20 hours of initiation. E-Thesis Swansea University, Wales, UK Corrosion, coating failure, corrosion inhibitor, galvanized steel, Zinc alloy. 10 3 2025 2025-03-10 10.23889/SUThesis.69269 A selection of content is redacted or is partially redacted from this thesis to protect sensitive and personal information. COLLEGE NANME COLLEGE CODE Swansea University Williams, G., Penney, D. Doctoral EngD EPSRC EPSRC 2025-04-11T10:44:24.8216247 2025-04-10T15:14:13.0204253 Faculty of Science and Engineering School of Engineering and Applied Sciences - Materials Science and Engineering PETER MORGAN 1 69269__34008__d5c05147700d490f84635a997a97fbc3.pdf 2025_Morgan_P.final.69269.pdf 2025-04-10T15:20:49.7008693 Output 25031753 application/pdf E-Thesis – open access true Copyright: The Author, Peter Morgan, 2025 true eng
title Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
spellingShingle Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
PETER MORGAN
title_short Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
title_full Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
title_fullStr Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
title_full_unstemmed Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
title_sort Chromate-Free Corrosion Protection of Zn-Al Alloy Coated Steels
author_id_str_mv fed902f23e2b884159e8297ae3884560
author_id_fullname_str_mv fed902f23e2b884159e8297ae3884560_***_PETER MORGAN
author PETER MORGAN
author2 PETER MORGAN
format E-Thesis
publishDate 2025
institution Swansea University
doi_str_mv 10.23889/SUThesis.69269
college_str Faculty of Science and Engineering
hierarchytype
hierarchy_top_id facultyofscienceandengineering
hierarchy_top_title Faculty of Science and Engineering
hierarchy_parent_id facultyofscienceandengineering
hierarchy_parent_title Faculty of Science and Engineering
department_str School of Engineering and Applied Sciences - Materials Science and Engineering{{{_:::_}}}Faculty of Science and Engineering{{{_:::_}}}School of Engineering and Applied Sciences - Materials Science and Engineering
document_store_str 1
active_str 0
description This thesis aimed to enhance the understanding of how bare and organically coated Zn-Al galvanised steels, specifically HDG, Galfan, and Zn55Al, corrode and develop corrosion protection using alternative inhibitors to hexavalent chromium. Hexavalent chromium poses significant health and environmental hazards.The aqueous localised corrosion behaviour of Zn55Al, relative to HDG and Galfan, is poorly understood. Therefore, the corrosion behaviour of bare Zn55Al coated steel immersed in aqueous chloride was investigated using Scanning Vibrating Electrode Technique (SVET), Time-Lapse Microscopy (TLM), and Scanning Kelvin Probe Force Mi- croscopy (SKPFM). It was found upon immersion that Zn55Al’s zinc-rich interdendritic phase initially undergoes preferential anodic attack, but over time, the attack extended to aluminium-rich dendrites, causing more severe localised corrosion. High chloride concentrations yielded extensive local corrosion.The inhibitory performance of the chromate-free corrosion inhibitor, 2-(1,3-benzothiazol- 2-ylthio)succinic acid (BTSA), was evaluated against the aqueous localised corrosion of HDG, Galfan, and Zn55Al surfaces. Determined by SVET and Linear Polarisation Resistance (LPR), 10 mM of BTSA dosed in aqueous chloride electrolyte provided inhibitor efficiencies upwards of 90% for each substrate. BTSA provided mixed inhibition, but the magnitude of cathodic inhibition decreased as galvanised coating aluminium content increased.The ability of in-coating BTSA to mitigate corrosion-driven organic coating failure of HDG steel was investigated. Time-lapse microscopy and SKP experiments showed that in-coating BTSA, added directly to a model coating or as a smart release exchangeable anion reduced the rate of cathodic delamination of HDG by up to a factor of 20. This performance was likely due to strong adsorption on the intact metal surface, stifling cathodic reduction of oxygen.Finally, a novel accelerated test cell was developed to assess in-coating inhibitors against the anodic undermining of ZnAl galvanised substrates. The preliminary design showed a measurable anodic undermining of unpigmented PVB-coated Zn55Al within 20 hours of initiation.
published_date 2025-03-10T05:27:44Z
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score 11.089407

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