Transpassivity mechanism of iron-chromium-molybdenum alloys studies by AC impedance, DC resistance and RRDE measurements

The transpassive dissolution of Fe-25%Cr, Fe-25%Cr-5%Mo and Fe-25%Cr-10%Mo alloys in 1 M H₂SO₄ was studied with a combination of electrochemical techniques—conventional and rotating ring-disk voltammetry, impedance spectroscopy and the d.c. resistance measurements with the contact electric resistance (CER) technique. Rotating ring-disk studies indicated that the amount of soluble Cr^VI released from the alloys in the transpassive region increases significantly with the Mo addition in the substrate. The electronic resistance of the anodic film was found to decrease as Cr^VI is released and to stabilise subsequently due to the formation of a secondary film depleted in Cr. Impedance spectra of the Fe-25%Cr alloy were found to include contributions from both the film growth and transpassive dissolution reactions. Mo addition was shown to have a catalytic effect on the transpassive dissolution of Cr. The experimental results were compared to a generalized model of the transpassivity of Fe–Cr alloys. The model represents the anodic film as a highly doped n-type semiconductor—insulator—p-type semiconductor (n-i-p) structure. Injection of negative defects at the film/solution interface results in their accumulation as a negative surface charge. The transpassive dissolution reaction is assumed to be a two-stage process featuring a Cr^IV intermediate. The relaxation of the Fe fraction in the outermost cation layer of the film is taken into account as well. Fitting of the experimental data on the basis of equations derived for the steady state and impedance response enable the determination of the kinetic parameters of transpassive dissolution as depending on the Mo content in the alloy.