The stability of the passive state of iron-chromium alloys in sulphuric acid solution

The passivation and the transpassive dissolution of Fe–Cr alloys (12% and 25% Cr) was studied with a combination of electrochemical techniques—conventional and rotating ring–disk voltammetry, impedance spectroscopy and the contact electric resistance (CER) technique developed to measure the dc resistance of surface films. Rotating ring–disk studies indicated that both soluble Cr (VI) and Fe (III) are released from the alloys in the transpassive region. The electronic resistance of the transpassive anodic film was found to decrease as Cr (VI) is released from the outermost layers adjacent to the interface and to increase subsequently due to the formation of a Fe (III) rich secondary passive film. Impedance spectra of the Fe–25% Cr alloy were found to include contributions from both the film growth and transpassive dissolution reactions, whereas the corresponding spectra of the Fe–12% Cr alloy reflected mainly the contribution of the film. On the basis of the experimental results, a generalized model of the transpassivity of Fe–Cr alloys is proposed. 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. It alters the non-stationary film growth rate controlled by the transport of positive defects (oxygen vacancies). 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.