Anodic oxides on a beta type Nb–Ti alloy and their characterization by electrochemical impedance spectroscopy

Anodic oxides were grown on the surface of an electropolished (Ti–30 at% Nb) beta‐titanium (β‐Ti) alloy by cyclic voltammetry. The scan rate was 100 mV s −1 between 0 and 8 V in increments of l V in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was carried out right after each anodic oxide growth increment to study the electronic properties of the oxide/electrolyte interface in a wide frequency range from 100 kHz to 10 MHz with an AC perturbation voltage of 10 mV. A film formation factor of 2.4 nm V −1 was found and a relative permittivity number (dielectric constant) of 42.4 was determined for the oxide film formed. Mott–Schottky analysis on a potentiostatically formed 7 nm thick oxide film was performed to assess the semiconducting properties of the mixed anodic oxide grown on the alloy. A flat band potential of −0.47 V (standard hydrogen electrode, SHE) was determined, connected to a donor density of 8.2 × 10 17  cm −3 . β‐Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer α‐Ti compound. Its application, however, requires a passivation behaviour comparable or better than α‐Ti which in fact is found.