The initial corrosion mechanism of hot-dipping Zn–Al–Mg alloy coating in chloride-containing solution

Abstract

In this study, the initial corrosion behavior of surface and cut-edge structures of hot-dip Zn-1.5Al-1.1 Mg alloy coatings in a 3.5wt.% NaCl solution was investigated. Micro-electrochemical experiments were conducted to explore the influence mechanism of coating structure on corrosion initiation. The results indicate that different microstructures significantly affect the corrosion resistance of the coating due to their potential differences, highlighting varied roles in this process. Preferential corrosion of the ternary eutectic phase occurs, leading to the gradual formation of corrosion products that cover the coating surface. This protective layer hinders further erosion by corrosive media, thereby improving the coating’s corrosion resistance. Furthermore, the galvanic difference between each phase of the coating is concealed by the coating and the substrate, and the corrosion products can accumulate on the substrate, illustrating the dual protection of the coating on the steel substrate at the cut-edge: cathodic protection due to its negative potential and long-term protection of the corrosion product film.