Microstructure and degradation behaviour of Mg-Zn(-Ca) alloys

Magnesium alloys are promising candidates for biomedical applications because of their advantageous properties. However, a too high degradation rate can be a problem. It is not desirable for the material to degrade completely before tissue healing has taken place. In this study, the relationship between the microstructure and bio-corrosion behaviour of Mg-3Zn(-0.3Ca) were analysed. It was found that nano-scale precipitates formed during ageing of Mg-Zn decrease the corrosion resistance. The increase in bio-corrosion rate with ageing time appears to be monotonic. The parallel filament-like corrosion trenches mirror the growth direction of the precipitates. The grain size and volume fraction of second phases are both key factors for controlling the bio-corrosion rate of Mg-Zn-Ca. The minimum corrosion rate was observed in the Mg-Zn-Ca solution treated at 420 \(^o\)C for 24 hours which has a balanced second phase volume fraction and grain size. 3D FIB and MicroCT tomography were used to show the internal structure of as-cast Mg-Zn and Mg-Zn-Ca. 3D reconstructions of corroded as-cast MZ3 and MZX30 were also shown. An HA (Ca5(PO4)3\(^.\)(OH)) coating was formed on Mg alloys. It was found that the pitting corrosion on the HA-coated alloys occurs and HA coating cannot provide sufficient protection to the Mg substrate.