Unexpected cathodic role of Mg41Sm5 phase in mitigating localized corrosion of extruded Mg-Sm-Zn-Zr alloy in NaCl solution

Effect of Mg41Sm5 second-phase on corrosion of as-cast and extruded Mg-Sm-Zn-Zr alloys in NaCl solution was investigated. The net-like Mg41Sm5 phase played a cathodic role in the as-cast alloy and induced obvious localized corrosion of adjacent anodic α-Mg grains. However, hot-extrusion deformed the relatively large net-like Mg41Sm5 phase into uniformly dispersed finer particles, which effectively mitigated localized corrosion and formed uniform protective film on the whole alloy surface. It enhanced the corrosion resistance of the extruded alloy for three times compared with the as-cast counterpart and outperformed that of benchmark Mg alloy AZ91.