New insights into corrosion initiation and propagation in a hot-dip Al-Zn-Mg-Si alloy coating via multiscale analytical microscopy

Multiscale analytical microscopy provides new mechanistic insights into the corrosion processes within a pre- painted hot-dip Zn-55Al-2Mg-1.5Si coated steel exposed to a coastal environment. This study is the first to conclusively show that corrosion initiation is not only affected by the electrochemical activities of constituent phases but also by their morphology. Corrosion initiates when sub-micron Zn particles within the Zn-Al binary eutectic dissolve. Thereafter, corrosion propagates by the dissolution of large Zn particles and by Mg dealloying from MgZn2 and Mg2Si intermetallic phases. The late stages record the corrosion of primary Al-rich dendrites, leading to a volumetric expansion in corrosion product.