Hydration and Structural Transformations during Titanium Anodization under Alkaline Conditions

Abstract

Ti anodic films were potentiostatically grown in 0.1 M NaOH aqueous solutions, in a potential range in which the voltammetric characterization showed an anodic shoulder related to the oxidation of TixOy (with y/x < 2) oxides. A variation in the density of donors and the flat band potential obtained by EIS measurements was related to changes in Ti suboxides proportion, confirmed by AR-XPS measurements. A modified Point Defect Model (PDM) was fitted to EIS experimental data in order to estimate the kinetic parameters describing the passivity of the Ti films. The model accounts for the formation and diffusion of oxygen and hydroxyl vacancies during film growth. The kinetic parameters related to film dehydration (e.g. hydroxyl vacancies) decrease as the formation potential becomes more positive, suggesting that the film hydration is crucial in the structure of the oxide grown during Ti anodization.