The Influence of Solution Anion on the Mechanism of Transpassive Dissolution of Ferrous- and Nickel-Based Alloys

Abstract

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.

Transpassivity characterization of the alloy UNS N08367 in a chloride-containing solution

“…The interface becomes active due to the charge transfer and the two different adsorption processes occurring at the interface. The appearance of an inductive loop at low frequencies at the transpassive potential range of some metals in acidic solutions has been reported previously [9][10][11][12][13][14][15][16].…”

“…At potentials of 1.15 and 1.25 V versus SCE, it is clear that two positive inductive loops are formed. The capacitive loop appearing at high to medium frequencies is attributable to the parallel combination of the charge-transfer resistance and interfacial capacitance; the inductive loop at low frequencies reflects the relaxation of the surface coverage of an intermediate adsorption reaction [11][12][13][14][15][16]. In this study, we found that two lowfrequency inductive loops exist, indicating the adsorption of two intermediates.…”

EIS Investigations of Transpassive Dissolution of Ferritic Steels in Aqueous and Molten Electrolytes

“…The experiments were carried out at 20±1 °C in naturally aerated solutions. The compositions of the electrolyte selected on the basis of its electropolishing and surface levelling ability was 13.5M H 3 PO 4 -0.5 M H 2 SO 4 -2 M C 3 H 5 (OH) 3 .The high-temperature measurements have been performed in molten p.a. KH 2 PO 4 at a temperature of 270 ± 5 °С.…”

Photoelectrochemical analysis of the effects of pH and sulfate ions on the structure and the composition of the passive film formed on Fe-20Cr-15Ni alloy

“…The Cr-transpassive potential was increased by decreasing the pH. In buffer solutions, secondary passivation [10,13] occurred at potentials noble to 0.6 V SCE (pH 8.5) or 0.9 V SCE (pH 4.0), presumably due to the enrichment of Fe and Ni that exhibit passivity at this high potential region [11,12]. However, the secondary passivation feature almost disappeared in sulfate solutions, especially at a pH of 2.0.…”

“…A borate solution is beneficial for the passivation of alloys, while chloride is detrimental for the formation of stable passive film [9]. Bojinov et al [10] reported that Fe-Cr and Ni-Cr alloys show a stable secondary passivation feature in a 1 M H 3 PO 4 solution but not in a 1 M H 2 SO 4 solution. He also found that the transpassive dissolution rate of Ni is significantly lower in phosphoric acid solutions than in sulfuric acid solutions, whereas that of Cr is faster in phosphoric acid solutions than in the other.…”