Corrosion resistance and microstructure evolution correlation in Zn-6%Mg-x%Al coatings

In this research, Zn-6%Mg-x%Al (x = 15∼22) coatings were prepared using a laboratory-scale simulator. Their properties were characterized by NSS tests, electrochemical measurements, microstructural analysis, and SKPFM. The NSS test data revealed that the corrosion resistance of the Zn-6%Mg-x%Al coatings did not improve continuously with increase aluminum content. The Zn-6%Mg-19%Al coating exhibited the best corrosion resistance, showing no red rust after 2400 h of testing. To further identify the coating with the optimal corrosion resistance, electrochemical measurements were performed on four selected coatings. The results showed that Zn-6%Mg-19%Al coating exhibited the lowest corrosion current density (915 μA/cm²). While previous studies have primarily focused on corrosion products, the key distinction of this work is that it elucidates the corrosion mechanism from the perspective of microstructural constituency. Microstructural analysis revealed that the dendrites consist of an α-Al core and a shell of Zn-Al eutectoid microstructure. The volume fraction of this core-shell dendritic structure was found to determine the corrosion resistance of the Zn-6%Mg-x%Al coatings. SKPFM measurements demonstrated that the Volta potential of α-Al phase in the dendritic core was −567 mV, which is higher than that of Zn (−653 mV). In contrast, the Zn-Al eutectoid structure in the dendritic shell exhibited the lowest Volta potential of −711 mV. The Zn-6%Mg-19%Al coating achieves its optimal corrosion resistance by balancing the area ratios of the α-Al dendritic core and the Zn-Al eutectoid shell.