Effects of Al-Mg on the Microstructure and Phase Distribution of Zn-Al-Mg Coatings

In this work, the composition of the zinc–aluminum–magnesium alloy coating was designed to have a fixed aluminum–magnesium ratio of 1:1, while the content of aluminum and magnesium elements increases gradually within the range of 1–2 wt.%. The micro-morphology of the coating with different compositions was observed by a scanning electron microscope (SEM). Combined with the surface distribution results of energy dispersive spectrometer (EDS) analysis elements and the phase analysis results of diffraction of X-rays (XRD), the phase distribution of the coating is understood. The statistical calculation of the phase distribution was carried out after staining the SEM image by ImageJ, This is consistent with the solidification simulation results of the thermodynamic simulation software (PADAT). The influence of magnesium and aluminum elements on the microscopic morphology and phase distribution of the zinc–aluminum–magnesium (ZnAlMg) coating was studied, and the mechanism of action was analyzed. The results show that the volume ratio of binary eutectic phase (Zn/MgZn2) and ternary eutectic phase (Zn/Al/MgZn2) in the coating tends to increase as the contents of the two elements elevate. The quantity of MgZn2 is the critical factor for the corrosion resistance of the coating; the more MgZn2, the better the corrosion resistance.