Abstract
Magnesium (Mg) alloys are promising candidates for biodegradable orthopedic implants due to their similar mechanical properties to human bones, biocompatibility, and inherent biodegradability. However, their rapid degradation, especially during the early stages of implantation, leads to premature implant failure and inferior osseointegration. Surface modification via biocompatible and bioresorbable coatings offers an effective approach to mitigate the degradation reaction of the Mg implants. In this study, calcium phosphate (Ca-P) coatings prepared by chemical conversion coating processes are investigated for their potential to control the degradation rate of a biodegradable WE43 (Mg-4 wt.% yttrium (Y)-3 wt.% rare earth elements) alloy.Pretreatment processes, including phosphoric acid (H3PO4) pre-activation and sodium fluoride (NaF) solution immersion, were performed before the Ca-P chemical conversion coating. Electrochemical analysis in Hanks’ balanced salt solution (HBSS) at 37°C revealed that all coated samples exhibited improved corrosion resistance compared to the uncoated alloy substrate. Notably, Ca-P coating following H3PO4 pre-activation showed the most substantial improvement in corrosion resistance, reducing the degradation rate to less than half that of the uncoated WE43 alloy. Cross-sectional transmission electron microscopy (TEM) analysis showed that the improved corrosion resistance can be attributed to the amorphous magnesium phosphate layer formed during the H₃PO₄ pre-activation, which refined the Ca-P coating structure on top and improved the adhesion of the coating to the alloy substrate.Additionally, the addition of NaF to the chemical conversion coating solution (comprising calcium nitrate (Ca(NO3)2) and potassium dihydrogen phosphate (KH2PO4)) refined the Ca-P coating structure and enhanced its chemical stability. The coating formation mechanisms and the effects of different process parameters on the Ca-P chemical conversion coatings will be discussed based on electrochemical measurements and microstructure characterizations, and the possibility of controlling the degradation rates of biodegradable WE43 Mg alloy will be explored.