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Title: Improved corrosion resistance in zinc and zinc aluminium alloy galvanised strip steels
Author: Challis, M. R.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2002
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The research detailed in this thesis addresses two major challenges facing the performance of organically coated galvanised steels (OCS); namely, improving corrosion and environmental performance. The project aimed to address the viability of chromium free inhibitor chemistries and new metallic coating chemistries with a view to producing organic coated steel products for construction with superior performance. The future of OCS materials depends on the removal of hexavalent chromium from conversion coatings and inhibitor systems. Scanning electrochemical techniques have been developed to provide spatially resolved current intensity measurement and by a series of mathematical treatments semi-quantitative estimates of corrosion. The studies have shown that whilst anodic inhibitors (Cr2O42-, MoO42-, PO42-) can elevate corrosion intensity on zinc galvanised steel they appear far more efficient on the more corrosion resistant Galfan (4.2% Al). By contrast cathodic inhibitors based on REM cations (Ce3+, Y3+, La3+) are efficient on both substrates. The difference is related to the heterogeneous microstructure of the Galfan coating. Despite improved corrosion performance the heterogeneous microstructure of the Galfan coating leads to corrosion localisation at zinc rich dendrites. Galfan samples (2.4% Al) differing only in the spelter addition of trace magnesium levels (0.01% to 0.05%) were studied using the SVET. For the bare substrate reduced surface cracking and dendrite breakthrough reduced surface corrosion activity as magnesium levels increased. By contrast increasingly large sub surface dendrites caused a rapid increase in cut edge corrosion intensity. Chromium free OCS samples were prepared using novel primer pigments based on clay minerals ion exchanged with REM cations incorporated into a PVC plastisol primer and applied to a Galfan substrate pre-treated with a phosphate coating. The REM exchanged clay materials performed equivalent to or better than the best available chrome free pigment, calcium exchanged silica. In combination with improved metallic coating chemistry these materials are likely to provide the high performance coated products of tomorrow.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available