Surface characterization of cast Ti-6Al-4V in hydrofluoric-nitric pickling solutions

Hydrofluoric acid offers a fast etching rate for removing 200 μm-thick alpha-cases from the surfaces of cast Ti-6Al-4V. The acid picks up hydrogen gas into substrate of the cast, which can be limited by introducing an oxidizing acid. This study proposes the optimal ratios of hydrofluoric/nitric acid as the pickling solutions. The cast Ti-6Al-4V in these solutions is monitored with chemical etching and electrochemical polarization. The mechanisms of etching procedures are initiated by generating oxygen vacancies that lead to several products from a series of quasi-chemical steps. The final product of the reactions may deduce as the form of H2TiF6 that is peeled out by the evolution of produced gases. Polarized curves are performed to illustrate passive behaviors of the alpha-case removed Ti-6Al-4V. The amounts of charge transference increase with increasing hydrofluoric acid concentrations. The newly formed passive film was estimated to be 20 nm in thickness with non-stoichiometric arrangements in the 4% HF electrolyte.