XPS and EIS study of the passive film formed on orthopaedic Ti–6Al–7Nb alloy in Hank's physiological solution

The composition and structure of the passive film formed on Ti–6Al–7Nb alloy by electrochemical oxidation in Hank's physiological solution were studied using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The oxide layer was predominantly TiO2, but contained small amounts of suboxides TiO and Ti2O3 at potentials more negative than 0.75 V. At more positive potentials, TiO2 was the only form. The formation of suboxides in the lower potential range is less pronounced than in Ti–6Al–4V alloy. The passive range in Hank's physiological solution is broad and extends up to 6.0 V. Aluminium oxide Al2O3, and niobium oxides, Nb2O5, and NbO and/or NbO2, are incorporated in the passive layer. Angular resolved XPS analysis confirmed that they are located mainly at the outer oxide/solution interface of the TiO2 matrix. The thickness of the oxide layer was dependent on the oxidation potential and, after oxidation at 5.75 V, it reached 9.4 nm. EIS measurements correlate well with the XPS data. The incorporation of the oxides of alloying elements into the TiO2 layer is reflected in the increase in the outer layer resistance at high anodic potentials and longer immersion times. The consequences of this process are beneficial for the overall stability and high corrosion resistance of the Ti–6Al–7Nb alloy under physiological conditions.