In the present study, various processes have been carried out on the extruded Mg-4Zn-4Al-0.6Ca-0.5Mn (wt%) alloy to achieve a low corrosion rate Magnesium (Mg) alloy. Microstructure studies revealed that the extruded alloy contained the needle-like Al-Mn second phases and the globular and elongated (Mg, Al)2Ca + ϕ(AlMgZn) phases. After solution treatment, the area fraction of the second phase decreased drastically from 5.1 % to 2.4 %. The later multi-directional forging (MDF) formed a modified ultrafine grain structure with an average grain size of 0.9 µm. The post-annealing process reduced the dislocation density of the heavily-deformed grains of the MDFed alloy, resulting in a grain size of 5.89 µm and an area fraction of 2.8 %. The annihilation of dislocations and homogeneous distribution of broken second-phase particles after MDF and post-annealing processes improved the ultimate compressive strength and compressive strain from 352 MPa and 4.2 % for the extruded to 376 MPa and 5.1 % for the post-annealed alloy. Moreover, by applying solution treatment, MDF, and post-annealing, the corrosion rate of the extruded alloy drastically decreased from 2.13 mm/y to 0.76 mm/y due to grain refinement and reduced area fraction of phases and their random distribution. According to the corrosion results, the area fraction of second-phase particles and the grain size obtained after different processes significantly influenced the corrosion properties, which was discussed in detail in the present study. On the one hand, the post-annealed alloy contained a significantly lower area fraction of particles than the extruded alloy, reducing micro-galvanic corrosion. On the other hand, significant grain refinement after the MDF process lowered the corrosion rate compared to the extruded alloy. However, the short-time annealing process annihilated the dislocations and high-energy sites while not significantly increasing the grain size and deteriorating the mechanical properties. Therefore, the combination of MDF and short-time annealing could be a remarkable route to obtaining the low corrosion rate and high strength Mg alloy.
