Effects of ZnAl-LDH on the corrosion resistance of heat-treated Al-Zn-Si coatings with a new elemental composition for hot stamping

This study proposes a method to significantly enhance the corrosion resistance of Al-Si coatings for protecting press hardening steel by alloying them with Zn. The AZS coating containing Al as the balance, 20±0.5 wt.% Zn, 9.5 wt.% Si, was produced with low Zn content. Despite the modest amount of Zn, it effectively served as a sacrificial anode to protect the substrate under austenitization conditions. The ±30 °C variation in austenitizing temperature around 900 °C caused structural deformation of the Al-dendrite phase within the coating, which in turn affected the galvanic corrosion rate at the Al-Zn interface. These differences significantly altered the electrochemical behavior and shielding properties of ZnAl-LDH, resulting in variations in the corrosion resistance of the coating. Additionally, when the heat treatment duration was increased by 2 min based on the 5 min heat treatment condition at 900 °C, the corrosion resistance of the AZS coating significantly decreased. The difference in corrosion resistance characteristics under these heat treatment conditions is attributed to the duration of nucleation growth of ZnAl-LDH on the coating surface and the insertion timing of guest anions such as CO₃^2- and Cl^-.