In vitro degradation and biomineralization ability of hydroxyapatite coated Mg-9Li-7Al-1Sn and Mg-9Li-5Al-3Sn-1Zn alloys

A biocompatible hydroxyapatite (HA) coating has been deposited on novel Mg-9Li-7Al-1Sn (LAT971) and Mg-9Li-5Al-3Sn-1Zn (LATZ9531) alloys to control their corrosion and rapid degradation. HA coating was applied by a simple two-step conversion coating process, where phosphate conversion coating (PCC) is alkali treated to form HA coating. A ~ 34 μm and ~ 26 μm thick coating was observed on LAT971 and LATZ9531 alloys, respectively, and phase analysis confirmed it to be HA via X-ray diffraction technique. Potentiodynamic polarization test reveals that the HA-coated LAT971 alloys showed the protection efficiency of ̴ 96% with the lower corrosion rate of 23 μm/y than the uncoated one (525 μm/y). The HA-coated LATZ9531 alloy showed the protection efficiency of ~ 12% with a relatively lower corrosion rate of 567 μm/y than that of uncoated one (647 μm/y). Moreover, the polarization resistance of HA-coated LAT971 and LATZ9531 was measured to be ~ 113 times and 1.2 times of their uncoated alloys, respectively. Electrochemical impedance spectroscopy (EIS) results showed the lower degradation rate of HA-coated alloys with the higher charge transfer resistance for the coated LAT971 (3.91 MΩ·cm2) and LATZ9531 (1.13 kΩ·cm2) than that of uncoated one (686 Ω·cm2 and 204 Ω·cm2, respectively). Immersion test in simulated body fluid (SBF) revealed rapid degradation of the uncoated alloys whereas HA-coated alloys showed a limited degradation with the biomineralization ability. Thus, the limited degradation and biomineralization ability of the HA-coated alloys meets the specific requirement for the biodegradable implants for load bearing applications.