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 Zn₅(OH)₈Cl₂·H₂O, hydrozincite Zn₅(OH)₆(CO₃)₂, and zinc aluminum carbonate hydroxide hydrate Zn₆Al₂(OH)₁₆CO₃·4(H₂O). The content of Zn₆Al₂(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 α-phase frequently corroded first and at a faster pace than the zinc-rich 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.