Evaluating the Electrochemical and In Vitro Degradation of an HA-Titania Nano-Channeled Coating for Effective Corrosion Resistance of Biodegradable Mg Alloy

Extensive research into magnesium (Mg) alloys highlights their possible applications in the field of biodegradable implants. As magnesium alloys are highly electronegative, it is imperative to tailor their degradation rate for clinical safety. Surface coatings have been widely used for the corrosion protection of Mg alloys, but the presence of spatial defects limits their effectiveness. An innovative and near-defect-free hydroxyapatite (HA)-TiO2 nano-channeled (TNC) coating architecture has been developed on ZM21 Mg alloy in the present study by combining anodization and the sol-gel dip coating technique. The HA-TNC coating positively shifted the Ecorr of ZM21 Mg alloy from −1.38 to −0.61 V. Accordingly, the corrosion current density (Icorr, 5.8 × 10−6 A/cm2) was suppressed by 53.4 times compared to uncoated ZM21 Mg alloy. The polarization resistance (Rp) and charge transfer resistance (Rct) values are the highest among all other samples, indicating the superior shielding ability of the coating. During in vitro immersion for up to 28 days in simulated body fluid (SBF), the HA−TNC coating maintained the lowest degradation rate and hydrogen evolution rate (HER) of 1.10 ± 0.22 mg/cm2/day and 1.83 ± 0.41 mL/cm2/day, respectively. A compact and structurally stable 2D plate-like HA (Ca/P:1.55), mineralized on HA-TNC-coated ZM21, provides effective shielding against the penetration of aggressive ions with prolonged SBF immersion. The findings of the present study provide a rational design for the development of bioactive ceramic coatings on Mg-based bioimplants.