Corrosion Performance of Aluminum-containing Zinc Coatings

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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.