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

Abstract 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·cm 2 ) and LATZ9531 (1.13 kΩ·cm 2 ) than that of uncoated one (686 Ω·cm 2 and 204 Ω·cm 2 , 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.