Abstract
Recently, it has become necessary to improve the corrosion resistance of zinciferous coated steel sheets, especially 55% Al-Zn alloy coated steel sheets. However, it is known that cracks are easily generated on the surface of 55% Al-Zn alloy coated steel sheet during production processes. This is due to the hardness of the metallic coating. We studied non-chromate corrosion-preventive compounds for organic-inorganic composite coatings and found that the composite coating containing both Zn3(PO4)2 and CaSiO3 had the highest corrosion resistance among all the additives tested. The corrosion mechanism was investigated by the analysis of the specimens after corrosion test using SEM, AFM, TEM and EDX. It was found that the amorphous surface layer, which was approximately 30 nm in thickness and contained Ca, Cr, P, Si, C, and O, was newly formed on the steel surface exposed after deformation of the coating. It is presumed that Ca2+ and HPO42- are formed by dissolution of the CaO component in CaSiO3 and hydrolysis of ZnO3(PO4)2, respectively, and they react each other to form an insoluble Ca-phosphoric compound, which might inhibit the corrosion of metals exposed in the cracks.
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