Anodic oxides on 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 (ß-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 × 1017 cm-3. ß-Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer alpha-Ti compound. Its application, however, requires a passivation behaviour comparable or better than alpha-Ti which in fact is found.