Corrosion behavior of Mg-1%Ca-x%Zn alloys in simulated body fluid under plastic strain loading

As-cast Mg alloy specimens were prepared by melting with induction furnace under CO2-HCF-134a protective gas and pouring into permanent steel mold. Three types of chemical composition were resulted in this research e.g. Mg-1.2Ca, Mg-1.17Ca-4.75Zn and Mg-1.39Ca-7.89Zn, and this is meant to have strengthening effect of Zn addition. Based on weight reduction of unloaded specimens during immersion test, the average corrosion rates were observed increase as 8.0, 17.2 and 28.3 mm/year with increasing Zn addition. In contrast, the addition of Zn decreases the average corrosion rates of loaded specimens as 118.7, 107.3 and 93.6 mm/year. Zn addition increase the yield strength of Mg alloys due to the formation of second phase MgZn, Ca2Mg6Zn3 and Ca2Mg6Zn13 detected by X-ray diffraction. The micro-galvanic coupling between α-Mg matrix with the second phases accelerated corrosion rates of Zn-added Mg alloys. Meanwhile in plastically loaded specimens, the corrosion rate is more significantly governed by the stress difference between the working stress and the yield strength of the specimens, and the higher the difference the higher the corrosion rate. From this simple experiment, it can be deduced that corrosion behavior of implant Mg alloys is complex phenomena, the corrosion kinetics must be carefully determined from the beginning until the end of implant application.