Influence of fluoride content on the barrier layer formation and titanium dissolution in ethylene glycol–water electrolytes

In the present paper, the initial stages of anodic film growth on Ti in water-containing ethylene glycol electrolyte with the addition of fluoride (0.015–0.17 M) were investigated using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential and fluoride content point to two parallel reaction pathways – film growth/dissolution and titanium dissolution through the film. The chemical composition of the anodic films in electrolytes with different fluoride content 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. A kinetic model of the process is proposed and its parameters are estimated by quantitative comparison with the current–potential and EIS data. The apparent reaction orders of the individual steps with respect to fluoride are also estimated. The main features of the XPS data are also reproduced by the model.

► Initial stages of Ti oxidation in ethylene glycol containing water and fluoride investigated.
► Electrochemical techniques point to two parallel reactions – film growth and Ti dissolution.
► A partly hydroxylated oxide containing mainly Ti4+ and Ti3+ is formed.
► A kinetic model of the process reproduces well both electrochemical and XPS data.
► Apparent reaction orders of individual steps with respect to fluoride estimated.