Electrochemical and impedance characterization of passive films on niobium in alkaline and acidic solutions

In this work electrochemical characterization of the anodic passive films formed on Nb in alkaline (0.1-5 M KOH) and acidic solution (H(2)SO(4)) was carried out by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Potentiodynamic measurements yielded a classical electrochemical passivation curve for both alkaline (KOH) and acidic (H(2)SO(4)) solution. In addition ETS-measurements were performed at,several potentials in order to characterize the Nb/solution, and/or Nb/Nb(2)O(5)/solution interfaces. The impedance behaviour of the passive films on Nb can be adequately described with a simple EEC; R(cl) in series with a parallel circuit CPE (Q) - R(ct(ox)). The calculate values of the passive oxide (Nb(2)O(5)) films resistivity ranged between 10(11)-10(10) Omega cm with relative dielectric constant (epsilon(f) approximate to 14) for KOH-solutions and approx. 2.10(12) Omega cm for 1M H(2)SO(4), indicating that the Nb-passive films, generated in acidic solutions, are better corrosion protectors then those formed in alkaline Solutions.