Corrosion performance of aluminum-containing zinc coatings

Abstract

Corrosion performance of zinc coatings containing aluminum in excess of 5 % was assessed in modified salt-spray tests. X-ray diffraction studies indicated that the corrosion products consisted mainly of simonkolleite Zn 5(OH)8Cl2- H2O, hydrozincite Zn 5(OH)6(CO3)2, and zinc aluminum carbonate hydroxide hydrate Zn6Al2(OH)16CO 3·(H2O). The content of Zn6Al 2(OH)16CO3·4(H2O) in the corrosion product increased as the aluminum content in the coatings increased. SEM-EDX analyses revealed that the microstructural features formed by the primary aluminum-rich a-phase frequently corroded first and at a faster pace than the zincrich β-phase in these coatings. The volume fraction and morphology of the zinc-rich β-phase existing in the coatings as degenerated eutectic are the two main factors which determine the corrosion resistance of Zn-Al coatings under development. The corrosion resistance of coatings peaked at about 12% Al in this study. © 2010 ISIJ.