Coupling between ionic defect structure and electronic conduction in passive films on iron, chromium and iron-chromium alloys

A quantitative kinetic model is presented for the steady-state passive film s on Fe, Cr and Fe-Cr alloys. It emphasises the coupling between the ionic defect structure and electronic conduction. According to the model, the pas sive him can be represented as a heavily doped n-type semiconductor-insulat or-p-type semiconductor junction. At low potentials in the passive state, t he positive defects injected at the metal/film interface play the role of e lectron donors. At high positive potentials, the negative defects injected at the film/solution interface play the role of electron accepters. At suff iciently high positive potentials the concentration of these ionic defects and corresponding electron holes reaches high enough values for the film to transform into a conductor. This enables transpassive dissolution of Cr an d oxygen evolution on the film surface. Equations for the electronic conduc tivity of the passive film, as depending on the concentration of point defe cts, are derived. The proposed model is compared with experimental data obt ained for pure Fe, pure Cr, Fe-12%Cr alloy and Fe-25%Cr alloy passivated in 0.1 M berate solution (pH 9.2) using rotating ring-disk voltammetry, photo current and impedance spectroscopy and in situ de resistance measurements b y the contact electric resistance (CER) technique. (C) 2000 Elsevier Scienc e Ltd; All rights reserved.