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New functional coatings on steel sheet by a metallic pre-coating followed by galvannealing
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New functional coatings on steel sheet by a metallic pre-coating followed by galvannealing

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It was the aim of this project to investigate new zinc alloy coatings on steel sheet obtained by means of galvannealing steel sheet with metallic pre-coatings. The investigations focused on three 'model-systems' (pre-coatings with Ni, Mn and Fe obtained either by electrolytical deposition (Ni, Fe) or magnetron sputtering (Ni, Fe, Mn). These pre-coated sheets were galvanised in a hot-dip simulator and then subjected to heat treatment (galvannealing). The microstructure of the test sheets was characterised by metallographic methods, GDOS and SNMS depth profile analysis, X-ray diffractometry and wet-chemical analysis. In addition, some application-related tests (powdering behaviour, phosphatability, paintability and corrosion resistance in the painted condition) were carried out. Particular regard was given to the electrochemical characterisation and corrosion behaviour of the alloy coatings. The electrochemical characterisation was performed in particular by means of electrical impedance spectrometry (EIS). Further on, accelerated corrosion tests (Kesternich and salt spray test) as well as outdoor exposure tests were carried out. In the case of Ni pre-coatings, formation of relatively brittle binary Zn-Ni and ternary Zn-Ni-Fe alloy coatings with very complex microstructures was observed. The corrosion behaviour of the coatings was in all somewhat inferior as compared to commercial Zn-Ni alloy coatings. With the use of Fe pre-coatings, formation of very homogeneous Zn-Fe coatings was achieved through galvannealing when the pre-coating thickness was above 1 μm. No alloy coatings could be produced with Mn pre-coated samples due to oxidising of the Mn-coating and uncomplete wetting of the samples during galvanising.
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ISBN 92-894-1976-8
Katalognummer KI-NA-20-044-EN-S
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ISSN ISBN 92-894-1976-8 DOI Katalognummer KI-NA-20-044-EN-S
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