E-Thesis 63 views 35 downloads
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...
| 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 |
| 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 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. |
|---|---|
| Item Description: |
A selection of content is redacted or is partially redacted from this thesis to protect sensitive and personal information. |
| Keywords: |
Corrosion, coating failure, corrosion inhibitor, galvanized steel, Zinc alloy. |
| College: |
Faculty of Science and Engineering |
| Funders: |
EPSRC |