Magnesium alloys for temporary implant applications: stress corrosion cracking and biocompatible coating

Magnesium (Mg) alloys have emerged as potential candidate materials for construction of biodegradable temporary implant devices particularly due to advantages of favourable mechanical properties, biodegradability and biocompatibility. However, the poor corrosion resistance of Mg alloys in the physiological environment presents a major challenge to their use as biodegradable temporary implants. Furthermore, complex interaction of mechanical loading and aggressive physiological environment may often pose a considerable risk of premature failure of implant devices before expected service life due to the phenomenon of environmentally assisted cracking, i.e. stress corrosion cracking (SCC) and corrosion fatigue. Therefore, it is essential to evaluate the mechanical integrity of Mg alloys before they can be put to actual use. Accordingly, this study has attempted to establish a mechanistic understanding of SCC of one of the most common magnesium alloys, AZ91D and a few novel aluminium-free magnesium alloys, ZX50, WZ21 and WE43 in the simulated body environment. The SCC susceptibility of the alloys in a simulated human body fluid was established by slow strain rate tensile (SSRT) testing using smooth specimens under different environmental conditions that could mechanistically confirm the occurrence of SCC. However, to assess the life of the implant devices that often possess fine micro-cracks, SCC susceptibility of notched specimens of AZ91D was investigated by circumferential notch tensile (CNT) testing to generate important design data (i.e. threshold stress intensity for SCC (KISCC) and SCC crack growth rate). The SCC susceptibility of the alloys was confirmed by fractographic features of transgranular and/or intergranular cracking. This study also investigated the role of novel biocompatible graphene-calcium carbonate coating in improving the corrosion/SCC resistance of AZ91D alloy as well as in promoting the in-situ biomineralization of bone apatite in the physiological environment.