Investigation on corrosion behaviors of Mg-Zn-Al-Sn-Mn alloy treated by solution and artificial aging

Hot extrusion, solution, and artificial aging were conducted on the designed Mg-Zn-Al-Sn-Mn alloy. The microstructure evolution and corrosion behaviors of the alloy were studied. A multimodal structure composing of both deformed and recrystallized grains is observed in as-extruded alloy, while a fully recrystallized grain structure forms in solutionized alloy. Moreover, the majority of second phases dissolved into Mg matrix during solution. In aged state, the highest hardness of HV84 was realized after 24 h aging, and the densely distributed tau-Mg32(Al, Zn)49 was the main strengthening precipitates. With increasing aging time, some amounts of precipitate had evidently coarsened by the dissolution of smaller ones, leading to a bimodal distribution of large (150-200 nm) and fine (similar to 50 nm) precipitates in over-aging state. The solutionized alloy owned the optimal corrosion resistance because of its large grain size and dissolution of second phase. However, the corrosion resistance was degraded again by aging treatment attributed to a mass of tau-Mg32(Al, Zn)49. Moreover, the intergranular corrosion took the place of pitting and filiform corrosion, and became a dominant corrosion mechanism in aged alloy.