Study on Influence of Microstructure and Localized Electrochemical Properties of Zn-6.8%Al-2.3%Mg Coating on Corrosion Resistance

The aim of the present investigation is a comprehensive study of microstructure and localized electrochemical properties of Zn-6.8%Al-2.3%Mg coating with a mild steel substrate by incorporating thermodynamic calculations, corrosion thermodynamics computations, x-ray diffraction (XRD), field emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FESEM + EDS), scanning Kelvin probe force microscopy (SKPFM), scanning vibrating electrode technique (SVET), and time-lapse photography, the findings revealed that: (1)The Zn-6.8%Al-2.3%Mg coating structure is primarily composed of extensive ternary eutectic phases, dendritic Al-rich phases, and island-like pure Zn phases. And the area occupied by the pure Zn phase is relatively small, while the areas covered by the Al-rich phase and the ternary eutectic phase are larger; (2) In most regions of the Zn-6.8%Al-2.3%Mg coating, corrosion begins predominantly from the ternary eutectic phase. A smaller portion initiates at the Al-rich phase, with the final corrosion occurring at the pure Zn phase; (3) During the initial stage of corrosion, the Zn-6.8%Al-2.3%Mg coating produces corrosion products that cover a wide area with high density. This is related to the increased magnesium content which leads to an increase in the area of the ternary eutectic phase, thereby making the ion concentration distribution on the coating surface and pH gradient more stable.