Self-healing effect by zinc phosphate and calcium silicate included in organic-inorganic composite coating on 55%Al-Zn coated steel sheet

Abstract

Improved corrosion resistance of steel sheets plated with zinciferous metal, and particularly 55% Al-Zn alloy coated steel sheets, has become necessary in recent years. However, 55% Al-Zn alloy coated steel sheets are susceptible to surface cracks during forming due to the hardness of the metallic coating. This research examined various non-chromate corrosion-preventive compounds for organic-inorganic composite coatings and found that a composite coating containing a mixture of Zn3(PO4)2 and CaSiO3 provided the highest corrosion resistance among all the additives tested. The corrosion mechanism was investigated by post-test analysis of the specimens by SEM, AFM, TEM and EDX, revealing that an amorphous surface layer, which was approximately 30 nm thick and contained Ca, Cr, P, Si, C, and O, had newly formed on the steel surface exposed after deformation of the coating. It is presumed that Ca2+ and HPO42- were formed by dissolution of the CaO component in CaSiO3 and hydrolysis of ZnO3(PO4)2, respectively, and reacted to form an insoluble Ca-phosphoric acid compound which inhibited corrosion of the metal exposed by cracks. © 2011 The Japan Institute of Metals.