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. (C) 2002 The Electrochemical Society.