Tantalum ͑Ta͒ electrochemistry is studied in aqueous HF and NH 4 F by cyclic voltammetry and electrochemical impedance spectroscopy. The much greater hydrogen evolution current and the much lower charge-transfer resistance for the high-frequency impedance loop in HF relative to NH 4 F demonstrate that the native Ta 2 O 5 film dissolves in HF, but not in NH 4 F. Weight loss experiments and cyclic voltammetry demonstrate that at cathodic potentials in aqueous HF, dissolution of Ta oxide allows electrochemical dissolution of the exposed surface into Ta 5+. In addition, unusual low frequency impedance behavior is seen for Ta in HF analogous to that reported by Bojinov for several different metal surfaces at anodic potentials in concentrated acids. This suggests the presence of an ultrathin, passive film in HF and is consistent with the poor adhesion often obtained for Cu electrodeposition onto Ta. This passive film may correspond to the Ta suboxide ͑TaO͒ previously observed by X-ray photoelectron spectroscopy at the Ta/Ta 2 O 5 interface.
Ta and Nb are group V valve metals which resist corrosion. Anodic dissolution of Ta and Nb in acidic fluoride media of varying HF concentration is investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. Polarization curves showed a clear active and passive region in all the solutions employed in this study. At a given HF concentration, Nb anodic polarization currents are larger compared to those of Ta. EIS of Ta and Nb exhibited a low frequency capacitive loop in active region, and a low frequency negative differential resistance in the passive region. The surface of Ta and Nb was characterized using XPS, and it reveals the presence of both sub oxide and pentoxide. The electrochemical results could be explained by a four step mechanism involving sub-oxide and pentoxide intermediates. The analysis shows that direct dissolution of sub-oxide occurs via an electrochemical pathway and is facilitated by HF − 2 species and HF in undissociated form. On the other hand, chemical dissolution of pentoxide, which occurs in parallel, is facilitated by the HF species in undissociated form.
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