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 films on Fe, Cr and Fe-Cr alloys. It emphasises the coupling between the ionic defect structure and electronic conduction. According to the model, the passive him can be represented as a heavily doped n-type semiconductor-insulator-p-type semiconductor junction. At low potentials in the passive state, the positive defects injected at the metal/film interface play the role of electron donors. At high positive potentials, the negative defects injected at the film/solution interface play the role of electron accepters. At sufficiently 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 and oxygen evolution on the film surface. Equations for the electronic conductivity of the passive film, as depending on the concentration of point defects, are derived. The proposed model is compared with experimental data obtained 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, photocurrent and impedance spectroscopy and in situ de resistance measurements by the contact electric resistance (CER) technique. (C) 2000 Elsevier Science Ltd; All rights reserved.