Enhanced multi-environment corrosion resistance of HVOF-sprayed hydroxyapatite coatings on Ti-6Al-6Mo alloy for dental implants

Titanium dental implants require exceptional corrosion resistance in aggressive oral environments, where fluoride and antibiotics compromise long-term integrity. This study investigates a vanadium-free Ti-6Al-6Mo alloy coated with hydroxyapatite (HA) using high-velocity oxygen fuel (HVOF) spraying. Surface characterization using scanning electron microscopy and X-ray diffractometry confirmed a dense and adherent HA layer (Ca/P = 1.79) with favorable crystallinity. Electrochemical performance was evaluated in three different environments, namely artificial saliva (AS), AS with gentamicin (2 g/L), and AS with NaF (225 ppm F⁻) at 37 °C using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The HA-coated Ti-6Al-6Mo samples show a lower contact angle than the base Ti-6Al-6Mo, which indicates the hydrophilic properties in AS, AS+gentamicin, and AS+NaF solutions. The HA-coated specimens exhibit significant enhanced corrosion resistance compared to the uncoated alloy: the corrosion current density (icorr) decreased from 3.68 to 0.18 µA/cm², while polarization resistance (Rp) increased from 41.7 to 2660 kΩ in the AS+gentamicin solution. Corrosion rates dropped by 70% across all media, with the lowest rate of 0.003 mm/year and highest Rp of 2660 kΩ observed for the HA-coated alloy in gentamicin-containing AS solution, indicating a synergistic protection. The HVOF-sprayed HA coating in Ti-6Al-6Mo alloy markedly improves electrochemical stability in diverse oral environments, supporting its application for long-lasting dental implants.