Microstructure and Corrosion Behavior of the As-Extruded Mg–4Zn–2Gd–0.5Ca Alloy

In order to study the corrosion resistance of extruded magnesium alloys, the Mg–4Zn–2Gd–0.5Ca alloy was extruded at the speed of 0.01–0.1 mm/s with the temperature of 280–360 °C in present study. Hot extrusion results show that the volume fraction of precipitates (Vpre), VDRX (the dynamic recrystallization rate) and the average size of DRXed grain (dDRX) decrease with the decrease in extrusion speed, and the corrosion rate of the alloy also shows a downward trend. On the contrary, the values of Vpre, VDRX and dDRX increase with the increase in extrusion temperature, and the corrosion resistance of Mg–4Zn–2Gd–0.5Ca alloy decreases. When the extrusion speed is 0.01 mm/s and the extrusion temperature is 280 °C, the alloy has the best corrosion resistance. The corrosion of extruded Mg–4Zn–2Gd–0.5Ca alloy occurs preferentially on the magnesium matrix around W and I phases in the DRXed zone. With the further corrosion, the corrosion continues to spread along the phase, and the corrosion area gradually increases. Galvanic corrosion plays a leading role in the corrosion process. Moreover, there are a large number of basal plane textures in the unDRXed region, which is conducive to improving the corrosion resistance of magnesium alloys. In addition, the decrease in grain size also makes the corrosion of magnesium alloy more uniform.