The influence of solution anion on the mechanism of transpassive dissolution of ferrous and nickel-based alloys

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 H₃PO₄ are determined and compared to those in 1 M H₂SO₄ published earlier. Finally, some conclusions on the effect of solution anion on transpassive dissolution are drawn, and the limitations of the models are outlined.