Ti anodization in alkaline electrolyte: The relationship between transport of defects, film hydration and composition

The hydration state and structural transformations of Ti films potentiostatically grown in 0.1 M NaOH are characterized utilizing EIS, Mott-Schottky and XPS techniques. Variations presented in the density of donors, the flatband potential and the fraction of species measured by XPS, in the region of characterization of these films are associated with changes in the ratio of different Ti sub-oxides within the passive film. Oxygen and hydroxyl vacancies and their respective formation rate constants are estimated from EIS spectra fits with a modified Point Defect Model. A consistent behavior is observed for the hydration state and structural transformations of the films as a function of potential. Films anodized at less positive potentials than 1.17 V vs SCE present higher hydroxyl vacancy diffusivities (e.g higher film hydration) and a heterogeneous mixture of titanium sub-oxides, whereas the opposite behavior is observed for films formed at more positive potentials than 1.17 V vs SCE, where the hydroxyl vacancy diffusivities decrease by one order of magnitude and the oxygen vacancy diffusivities increase three times. Thus, films anodized at more positive potentials present a higher dehydration, are more homogeneous and mainly composed of TiO₂.