Corrosion Resistance and Mechanism of Four 6Mg Zinc-Based Coatings: A Comparative Study of 15Al, 17Al, 19Al, and 22Al

High-magnesium Zn-Al-Mg coatings with different Al contents were prepared using a laboratory hot-dipped galvanizing simulator. The planar corrosion resistance and its mechanism were systematically investigated by analyzing the microstructure and corrosion products at different corrosion cycles. Cyclic corrosion test results show that the planar corrosion resistance does not increase monotonically with higher Al content; the 19Al6Mg coating exhibits the best planar corrosion resistance. Among the four alloy coatings, the 15Al6Mg coating demonstrates the best cut-edge corrosion resistance in both cut-edge and punched Hole corrosion tests. Furthermore, the corrosion mechanism of coatings with different Al contents was analyzed. The types of corrosion products formed on these Zn-Al-Mg coatings are essentially identical. However, as the Al content increases, the proportion of the Al phase and the resulting Al-Zn eutectoid structure increases. During corrosion, the Al-Zn eutectoid structure, which has a lower corrosion potential, corrodes preferentially. This explains why increasing the Al content to 22% leads to a reduction in the overall corrosion resistance of the coating.