Thermal Oxidation of Ti-6Al-4V for Bone Implants: A Narrow Temperature–Time Window Minimises Ti, Al and V Release in Accelerated Hydrochloric Acid Testing

Peri-implant acidification during inflammation can break down the passive film of titanium implants and release Ti, Al and, in particular, V from Ti-6Al-4V. This study aimed to identify an air thermal-oxidation regime that makes the Ti-6Al-4V surface resistant to a severe acid challenge. Polished discs oxidised at 150–800 °C for 15–60 min were characterised by optical metallography, XRD and SEM/EDS and tested for 240 h in 30% HCl, with ICP-MS analysis of the spent solutions. Protection depended strongly and non-monotonically on the temperature–time combination. Oxidation at 150 °C produced only Ti2O and slightly reduced the resistance. At 650 °C for 30 min, an ordered Ti6O layer (30.0 wt.%) with incipient rutile (1.3 wt.%) formed, the mass loss fell below the gravimetric detection limit, and dissolved Ti, Al and V decreased from 674, 52 and 32 mg L−1 to 1, 1.5 and 0.29 mg L−1. At 750 °C, the rutile content increased to about 20 wt.%, but the boundary oxide network broke up into Al- and V-rich islands and only 15–18% protection was retained. Protection was therefore governed by the continuity and chemical uniformity of the oxide layer rather than by the amount of rutile formed. These accelerated-test results require confirmation in simulated body fluids and biological studies.