Coupling between dissolution and passivation revisited – Kinetic parameters of anodic oxidation of titanium alloys in a fluoride-containing electrolyte

• Ti alloys in fluoride-containing solution studied by electrochemical methods.
• Current-potential and impedance data point to two parallel reaction pathways.
• Composition of the oxides on the studied alloys estimated by XPS.
• Kinetic model parameters estimated from both electrochemical and XPS data.

In the present paper, the anodic film growth on Ti alloys in water- and fluoride-containing ethylene glycol electrolyte were investigated using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential and fluoride content pointed to two parallel reaction pathways – film growth/dissolution and titanium dissolution through the film. The chemical composition of the anodic films was estimated by X-ray photoelectron spectroscopy (XPS). XPS analyses revealed the presence of a non-stoichiometric oxide containing mainly Ti4+ and a certain amount of Ti3+, with a certain degree of hydroxylation. Mo and Al were found to be enriched in the oxides, and evidence of transpassive oxidation/dissolution of Mo was found. A kinetic model of the process is proposed and its parameters are estimated by quantitative comparison with the current–potential and EIS data. The main features of the XPS data, namely film thickness, concentration of Ti(III) and fluoride as depending on potential are also reproduced by the model.