Corrosion mechanisms of Al-alloyed hot-dipped zinc coatings in wet supercritical carbon dioxide

Commercial hot-dip galvanized zinc (Zn-0.2% Al), galfan (Zn-5% Al), and galvalume (Zn-55% Al) coatings were exposed to wet scCO₂, followed by an assessment of microstructural changes in the coating structure. Zinc and galfan coatings showed similar surface activity with abundant zinc dissolution and uniform corrosion product deposition. The Al-rich phases of the galfan coating were intact in the studied atmosphere and eventually became embedded within the deposits. The zinc dissolution ended for zinc and galfan coatings when the free metal surface was blocked from the surrounding medium by a uniform corrosion product layer. Galvalume showed scarce reactivity: dissolution of zinc in the Zn-rich interdendritic areas proceeded slowly compared to zinc and galfan, and the Al-rich dendrites (with nm-scale Zn inclusions) remained intact. The ion beam milling technique combined with modern nm-resolution energy-dispersive spectroscopy analysis enabled previously unseen visualization of the discussed corrosion processes.