Role of surface reactions in the transpassive dissolution of ferrous alloys in concentrated H₃PO₄

The transpassive dissolution of a range of ferrous alloys (Fe–12% Cr, Fe–12% Cr–5% Mo, Fe–25% Cr, Fe–25% Cr–10% Mo) in concentrated H₃PO₄ was studied within the frames of an investigation of electropolishing mechanisms. Measurements by the contact electric resistance (CER) technique have demonstrated that in the transpassive region, the resistance of the anodic film first decreases with increasing potential due to enhanced conductivity of that film and then increases at higher potentials, indicating transpassive film growth. The release of soluble Cr(VI) as detected by rotating ring–disc measurements is the higher, the higher the Cr content in the alloy, and further increases upon addition of 10% Mo to the Fe–Cr alloys. Impedance spectra in the transpassive region have been found to include contributions both from the anodic film and a multistep transpassive dissolution reaction at the film/solution interface. The results were interpreted using a generalised model of transpassive dissolution. The kinetic parameters of the process were determined and the influence of alloy composition on their values is discussed in relation to the processes of anodic surface treatment.