ZINC-CHROMIUM COATED STEEL SHEET: PROPERTIES AND PRODUCTION(162)

摘要

The automotive industry has interest on more tailorable corrosion protective coatings based on zinc.Sometimes thinner coatings, providing at least comparable cathodic corrosion protection as standardzinc coatings with 7.5 to 10 μm thickness, are of interest to ensure higher robustness during formingand welding. Especially the LASER- and arc welding processes as well as brazing and solderingbenefit from thinner coatings. On the other hand for special applications a higher corrosionprotection is wanted to open cost saving opportunities in the paint shop or to overcome constructivelimitations. As the electro galvanizing process is particularly suited for producing steel sheet withlow zinc coating thickness (< 5 μm), we searched for alloying elements enhancing the corrosionresistance of zinc.Metallic zinc-chromium (Zn-Cr) alloys with a chromium content of 3 – 7 % show excellentcorrosion behavior, forming capability (coating adhesion, powdering, friction behavior, gallingbehavior), joining properties (spot weldability, laser weldability, adhesive bondability) and fit allother specific demands of the automotive industry. This paper deals with the specific applicationand processing properties of Zn-Cr coatings compared to pure zinc coatings. An additional benefitin respect to perforation corrosion can be gained when a very thin protective layer, e. g. phosphatelayer or corrosion protective primer, is applied on the zinc-chromium coating resulting in anextraordinarily resistance against flange corrosion.Metallic Zn-Cr alloys can be deposited in a chromium(VI)- free process, from a sulfate electrolytecontaining zinc, chromium(III) and sodium with a current efficiency of 50 - 70 %. Polyethyleneglycol is used as an additive to enhance the co-deposition of chromium. Chromium content andcurrent efficiency both depend on Zn:Cr-proportion, current density, temperature, pH-value andflow velocity. Line trials showed, that a steel sheet with features presented above can be producedon a conventional electro galvanizing line. Compared to pure zinc, production of zinc-chromium ismore challenging because plating parameters must be kept within closer limits and additionalprocess restrictions have to be taken into account.