On the Anodic Dissolution of Tantalum and Niobium in Hydrofluoric Acid

Abstract

© The Author(s) 2018. Ta and Nb are group V valve metals which resist corrosion. Anodic dissolution of Ta and Nb in acidic fluoride media of varying H concentration is investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. Polarization curve showed a clear active and passive region in all the solutions employed in this study. At a given HF concentration, Nb anodi polarization currents are larger compared to those of Ta. EIS of Ta and Nb exhibited a low frequency capacitive loop in activ region, and a low frequency negative differential resistance in the passive region. The surface of Ta and Nb was characterized usin XPS, and it reveals the presence of both sub oxide and pentoxide. The electrochemical results could be explained by a four ste mechanism involving sub-oxide and pentoxide intermediates. The analysis shows that direct dissolution of sub-oxide occurs via a electrochemical pathway and is facilitated by HF − 2 species and HF in undissociated form. On the other hand, chemical dissolutio of pentoxide which occurs in parallel is facilitated by the HF species in undissociated form.