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The aim of this work is to generalize the model for the transpassive dissolution of ferrous-and nickel-based alloys in acidic solutions by assessing the effect of the solution anion on the individual steps of the process. First, experimental voltammetric and impedance spectroscopic data on transpassive dissolution of pure Cr, Fe-12%Cr, Fe-12%Cr-5%Mo, Fe-25%Cr, Fe-25%Cr-10%Mo, pure Ni, Ni-10%Cr, and Ni-20%Cr in phosphoric acid solution are compared with earlier data for the same materials in sulfuric acid solution. It is found that phosphate affects mainly the dissolution reaction of the main element (Fe and Ni) significantly enhancing the rate of secondary passivation in comparison to sulfate. In addition, the accelerating effect of Mo on transpassive dissolution of Cr is much less pronounced in phosphate solutions. Second, the physicochemical basis of the generalized kinetic model is described. The additional assumptions and simplifications that are necessary to transform the model into a set of kinetic equations are listed, and the individual models for the transpassive dissolution of ferrous-and nickel-based alloys are presented. The kinetic parameters of the transpassive dissolution process in 1 M H 3 PO 4 are determined and compared to those in 1 M H 2 SO 4 published earlier. Finally, some conclusions on the effect of solution anion on transpassive dissolution are drawn, and the limitations of the models are outlined.

The present paper compares the kinetics of transpassive dissolution of industrial ferritic steels in a phosphoric acid-based electropolishing solution to that for the same materials in molten KH 2 PO 4 . Both steady state and transient techniques point to two parallel pathways of the overall process, the first corresponding to oxidative dissolution of Cr as Cr(VI). The first and the second path is identified as the isovalent dissolution of Fe as Fe(III) mediated by a surface chemical step involving interaction with an electrolyte-originating species. Kinetic models of the process involving only surface reaction steps have been found to reproduce successfully the steady-state and the small amplitude ac response of the studied materials in both electrolytes. The kinetic parameters are estimated by a calculation procedure that involves simultaneous fitting of the impedance spectra at several polarisation potentials and the steady state current vs. potential curve for a particular material-electrolyte combination to the model equations. The ability of the kinetic models proposed for aqueous solutions to account for the behavior of materials in molten salt medium is discussed.