Synergistic effect of rare-earth and transition metal conversion coatings on bioactive hydroxyapatite mineralization and electrochemical behavior of AZ31 magnesium alloys.

Cerium-vanadium conversion coatings were investigated as pretreatment layers to improve hydroxyapatite (HA) deposition on AZ31 magnesium alloy for enhanced corrosion protection and in-vitro bioactivity. Cerium-only (CeCC), vanadium-only (VCC), and a mixed Ce/V conversion coating (CeVCC) were prepared, and the optimized mixed conversion layer was subsequently used as the pretreatment for HA deposition to form a CeVCC + HA hybrid coating. Surface morphology and elemental composition were characterized by SEM/EDS, while wettability and surface functional groups were evaluated using static water contact-angle measurements and ATR-FTIR, respectively. Corrosion behavior in simulated body fluid (SBF) was assessed by electrochemical impedance spectroscopy (EIS) during prolonged immersion, complemented by hydrogen evolution measurements as kinetic indicators of Mg degradation. The CeVCC pretreatment produced a more compact and uniform surface and promoted homogeneous HA growth with improved interfacial integrity. Compared with bare AZ31 and single-layer coatings, the CeVCC + HA hybrid exhibited a higher impedance response and lower hydrogen evolution, indicating a markedly lower corrosion rate, while retaining Ca-P surface characteristics associated with bioactive behavior in SBF. These results demonstrate that Ce/V conversion pretreatment is an effective strategy for stabilizing HA coatings on Mg alloys and improving their performance in physiological environments.