Nb electrodissolution in Acid Fluoride Medium. Steady-state and Impedance Investigations

The electrochemical behavior of Nb in acid fluoride media has been investigated by voltammetric, steady-state, and impedance techniques as a function of variables like c(F) (the total fluoride concentration, varied between 0 and 2 M!, pH (0-7), and potential (-1 to 7 V vs. SCE). An active/passive transition is observed, followed by a wide current pseudoplateau where current depends on c(F) and pH in a way indicating that HF and related species dissolve the passivating oxide whereas F- is inactive. Impedance diagrams obtained on this plateau can be reproduced by a literature model (surface charge approach), and the derived parameters may be used to calculate structural and kinetic quantities, as well as the formation ratio of the surface oxide.