Corrosion fatigue and stress corrosion cracking of magnesium alloys in a simulated physiological environment

Magnesium (Mg) alloys have attracted great attention as potential materials for temporary implants in uses such as pins, screws, plates and stents. The usage of Mg alloys is appealing as it avoids the need for a follow-up surgery commonly undertaken when implants are constructed out of traditional materials such as titanium alloys, stainless steels and cobalt-chromium alloys. This reduces health care costs and inconvenience for patients. However, the poor corrosion resistance of Mg alloys in the physiological environment presents a major challenge in their use as biodegradable temporary implants. The complex interaction of mechanical loadings and aggressive physiological environment also poses a considerable risk of premature failure of implants. The most critical mechanisms by which implants may fail are corrosion fatigue (CF) and stress corrosion cracking (SCC). It is therefore essential to fully characterize the CF and SCC resistance of Mg alloys as potential implants before putting them into actual use. Accordingly, this PhD thesis has attempted to evaluate CF and SCC resistance of one of the most common Mg alloys (i.e., AZ91D), and an extruded Mg alloy specifically designed for implant application (i.e., ZX10), in a simulated physiological environment. CF behaviour of Mg alloys was evaluated using tension-compression cyclic testing under simulated physiological conditions (i.e., modified-simulated body fluid (m-SBF) maintained at 37 °C at a frequency of 5 or 10 Hz). SCC of Mg alloys was investigated using slow strain rate tensile (SSRT) testing in the m-SBF at 37 °C. A substantial decrease in the mechanical integrity of tested alloy was observed under cyclic and monotonic loadings when tested in m-SBF. Fractographic evidence further confirmed susceptibility of Mg alloy to SCC and CF. ZX10 showed superior resistance to CF and SCC compared to AZ91D Mg alloy suggesting it is a more suitable material for temporary implant applications