Microstructure and corrosion resistance of bone-implanted Mg–Zn–Ca–Sr alloy under different cooling methods

The cooling gradient of Mg–3Zn–1Ca–0.5Sr alloy in cast ingots under different cooling methods (air cooling, warm-water cooling and ice–water-mixture cooling) was examined and the effect of cooling rate on the structure and corrosion properties was studied. The microstructure of the alloy was composed of α-Mg, Ca2Mg6Zn3 and Mg17Sr2 phases. As the solidification cooling rate increased, the grain was refined, Zn and Sr were less segregated, the distributions of Zn and Sr were more uniform, and corrosion rate was found to first increase and then decrease; this contradicts the findings of recent research. With cooling rate increasing, the number of corroded microcouples comprising second phase and α-Mg increases. More α-Mg participates in corrosion, leading to a layered and deep corrosion pit and an increased corrosion rate. However, as the microstructure became sufficiently dense, the corroded structure protected the deep α-Mg from participating in corrosion, thus reducing the corrosion rate.