A general model for the repassivation potential as a function of multiple aqueous species. 2. Effect of oxyanions on localized corrosion of Fe-Ni-Cr-Mo-W-N alloys

A recently developed model for predicting the repassivation potential has been applied to stainless steels and nickel-base alloys in aqueous environments containing chlorides and various inhibiting anions. The model accounts for the effects of solution chemistry and temperature on the repassivation of localized corrosion by considering competitive dissolution, adsorption, and oxide formation processes at the interface between the metal and the occluded site solution. An extensive database of repassivation potentials has been established for six alloys (UNS 31603, N06600, N06690, S31254, S32205, and UNS S41425) in contact with solutions that combine chlorides with hydroxides, molybdates, vanadates, sulfates, nitrates, and nitrites at various concentrations and temperatures. Also, repassivation potentials are reported for four alloys (UNS N08367, N08800, N06625, and N10276) in chloride solutions. The database has been used to establish the parameters of the model and verify its accuracy. The model quantitatively predicts the transition between concentrations at which localized corrosion is possible and those at which inhibition is expected. It is capable of predicting the repassivation potential over wide ranges of experimental conditions using parameters that can be generated from a limited number of experimental data. The parameters of the model have been generalized as a function of alloy composition, thus making it possible to predict the repassivation potential for alloys that have not been experimentally investigated. (C) 2008 Elsevier Ltd. All rights reserved.