Comparison of microstructure and corrosion behavior among Zn-0. 2%Al, Zn-6%Al-3%Mg, and Zn-19%Al-6%Mg

In recent years, the development of coating alloys has become increasingly diversified, with new types such as zinc, aluminum-zinc, and zinc-aluminum-magnesium coatings emerging successively. The current research indicates that the co-addition of magnesium and aluminum in zinc-based coatings can significantly enhance their corrosion resistance. Magnesium can compensate for the limitations of aluminum in protecting cut edges, while also improving coating hardness and wear resistance. Its application potential still requires further in-depth exploration. GI（Zn-0. 2%Al）, Zn-6%Al-3%Mg, and Zn-19%Al-6%Mg（mass fraction） were prepared by hot-dip coating simulator. These coatings were systematically analyzed using methods such as X-ray diffraction（XRD）, scanning electron microscopy equipped with energy-dispersive spectroscopy（SEM-EDS）, microhardness testing, salt spray testing, and electrochemical corrosion testing to examine their surface morphology, cross-sectional microstructure, microhardness, and corrosion behavior. Pandat simulation results reveal that in the Zn-Al-Mg ternary system, the distance between coating composition point and ternary eutectic point directly determines its melting point, the farther the distance, the higher the melting point. Microstructural analysis shows that as the aluminum and magnesium content increases, the coating grains refine, and the zinc-rich phase decreases, effectively hindering dislocation movement and thereby improving hardness and strength of coating. Among them, the average microhardness of Zn-19%Al-6%Mg coating is 260. 077 HV, approximately twice that of Zn-6%Al-3%Mg and four times that of the GI pure zinc coating. Electrochemical corrosion test results indicate that the corrosion current density of Zn-19%Al-6%Mg coating is 915 μA/cm², lower than that of Zn-6%Al-3%Mg（1 570 μA） and GI coating（1 940 μA）, demonstrating the best corrosion resistance. The improved corrosion resistance of this coating is mainly attributed to the formation of stable Mg₆ Al₂（OH）₁₆CO₃·4H₂O corrosion product on the surface. The higher Mg²⁺ mass fraction effectively delays the formation of Zn₆ Al₂（OH）₁₆CO₃·4H₂O, enhancing the stability of the rust layer.