Electrode potential, inflammatory solution chemistry and temperature alter Ti-6Al-4V oxide film properties

Abstract

Post implantation, a thin TiO2 film protects Ti-6Al-4V in vivo. Mechanically and chemically destructive conditions may disrupt this oxide film and promote corrosion at the device interface. While tribocorrosion has been explored in vitro, pre-clinical tests fail to fully reproduce the damage documented on retrievals. A gap persists in our fundamental knowledge of the oxide film and its response to adverse electrochemical events. In this study, we first characterized the TiO2 oxide film structure after inducing β phase selective dissolution. Next, we used electrochemical impedance spectroscopy and Mott-Schottky analysis to investigate the oxide film on freshly prepared surfaces following negative potential excursions. We systematically increased electrode potential from -1 V vs. Ag/AgCl in 0.1 V steps to +0.5 V. Various H2O2 concentrations (0.1 M to 0.001 M) in phosphate buffered saline and temperatures ranging from 24 °C to 55 °C were investigated. Atomic force microscopy showed morphological changes to oxide domes over the α phase following β phase selective dissolution. Below the Ti-6Al-4V open circuit potential (ca. -0.3 V), we identified systematic variations in polarization resistance (Rp) and capacitance (C) with potential, solution chemistry, and temperature. The combined effect of cathodic activation and H2O2 solution concentration synergistically and systematically decreased Rp by orders of magnitude (p = 0.000). Increased solution temperatures decreased Rp (p = 0.000). At high concentrations of ROS (0.1 M), we documented changes in constant phase element capacitance, from n-type to p-type. Finally, we developed empirical equations to predict Rp for a given concentration, temperature, and potential. In totality, these experiments help to further elucidate the complex interactions at the oxide-biology interface in vivo and provide guidance for in vitro testing methods development.