EIS characterization of the barrier layer formed over Ti during its potentiostatic anodization in 0.1 M HClO ₄/x mM HF (1 mM ≤ x ≤ 500mM)

The effect of HF concentration on the morphology and growth of barrier layer during anodization of Ti electrodes has been studied. The wide range of concentrations selected, rarely used for the formation of self-organized porous films, corresponds to the formation of films with different morphologies. The voltammetric behavior showed a high potential range in which anodic films were potentiostatically grown (6.5 V). As the HF concentration increased, SEM images evidenced two transitions in the morphology of the outer layer formed at the oxideelectrolyte interface, which match with the transition observed in the steady state current densities. However, EIS spectra only showed one transition. Fitting EIS spectra with an electrical equivalent circuit, commonly employed to study passive films in aggressive media, showed a monotonic change in the growth and properties of the barrier layer formed, evidencing that fluoride ions are inserted into the oxide through oxygen vacancies playing an important role during self organized anodic film formation.