Corrosion behaviors in ZX-series Mg alloys: The interplay of Mn solute, grain structure, and phase evolution after laser surface melting

This study investigates the microstructural and functional evolution of hot-extruded ZX21 and ZXM211 magnesium alloys subjected to laser surface melting (LSM). The effects of grain size, crystallographic texture, solute enrichment, and secondary phase characteristics on corrosion resistance and microhardness are systematically examined. LSM homogenizes the microstructure, weakens basal texture, and enables a uniform distribution of secondary phases, shifts corrosion toward less localized corrosion attack. Mn’s high melting point and low diffusivity favored solute retention in α-Mg during LSM, limiting second phase precipitations and promoting grain coarsening. Although potentiodynamic polarization indicated a higher average corrosion rate for LSM-treated ZXM211, the corrosion mode was more uniform, consistent with a lower second-phase fraction and a topology in which Ca₂Mg₆Zn₃ embedded within Mg₂Ca limits effective cathode exposure and mitigates microgalvanic intensification. Despite these favorable microstructural changes, secondary precipitation strengthening remains negligible, and no significant improvement in hardness is observed post-LSM, though both as-extruded and LSM-treated states retained high microhardness.