Degradation Kinetics of As-Cast and Solution-Treated (T4) Magnesium-Based Alloys for Biodegradable Orthopedic Implants

Abstract

Mg-0.5Ca alloy is a potential candidate for biodegradable implants. The addition of 2 wt pct Zn and 0.5 wt pct Sn was shown to improve the degradation resistance in Hank's balanced salt solution (HBSS) at room temperature. However, the alloy exhibited excessive pitting due to the formation of numerous second phase precipitates namely Mg2Zn11 and Mg2Sn. Hence, T4 heat treatment was performed that resulted in complete dissolution of the Mg2Sn phase. This resulted in a significant reduction in the volta-potential difference between the matrix and Mg2Zn11 phase of the T4-treated alloy, as confirmed by scanning Kelvin probe force microscopy (SKPFM). The T4-treated alloy encountered less micro-galvanic attack during immersion periods in simulated body fluid owing to the smaller grains, reduced volume fraction of second phases and decreased potential difference between the phases. As a result, the T4-treated XZT020 alloy demonstrated improved corrosion resistance with fewer and smaller shallower pits on the corroded surface than the as-cast XZT020 alloy. Mg2O and Mg(OH)2 are the corrosion byproducts identified on both alloy surfaces, according to XRD investigation of the corroded surface.