A recently developed methodology for predicting the long-term occurrence of localized corrosion has been applied to systems containing chlorides and various inorganic inhibitors. The key element of this methodology is the computation of the repassivation potential using a mechanistic model that considers competitive processes at the metal ? salt film ? solution interface in the limit of repassivation. A database of repassivation potentials has been established for type 316L stainless steel in contact with a large number of solutions that combine chlorides with hydroxides, molybdates, vanadates, nitrates, and sulfates at selected temperatures. 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. The model is capable of predicting the repassivation potential and, hence, the occurrence of localized corrosion over wide ranges of experimental conditions using parameters that can be generated from a limited number of experimental data.
Quantitative modeling of localized corrosion is extremely challenging because of the large number of factors that influence the nucleation, growth and repassivation of pits or crevices. Among the key factors, properties of chemical species in an aqueous environment, concentrations of components, alloy composition, and temperature are of particular importance. In the last three decades, considerable progress was made in understanding the mechanisms of localized corrosion of various metallic materials. At the same time, various modeling methodologies have been developed by considering atomic or molecular processes, microstructural features, and transport processes in macroscopic cavities.
In our previous papers, a comprehensive computational model has been proposed to predict the tendency of metals to undergo localized corrosion as a function of environmental conditions. An important feature of this model is its capability of relating the key parameters that determine localized corrosion to the chemistry of aqueous environments, including complex media encountered in chemical processes. To predict the occurrence of localized corrosion, this approach relies on calculating two characteristic parameters as functions of solution chemistry, i.e., (1) the corrosion potential and (2) the repassivation potential, also called the protection potential. The repassivation potential (Erp, or Ercrev, depending on whether one measures the potential of an open or crevice specimen) is a measure of the tendency of an alloy to undergo localized corrosion in a given environment. In this paper, the two repassivation potentials are referred to by the common notation of Erp, because the two potentials coincide at high pit depths. The underlying justification for the use of Erp is the fact that, for engineering applications, only the fate of stable pits or crevice corrosion is important. Pits that nucleate, but do not grow beyond an embryonic stage (metastable pits) do not adversely affect the performance of engineering structures. It has been shown in previous papers that Erp is the potential below which stable pitting or crevice corrosion does not occur. Also, Erp is relatively insensitive to prior pit depth greater than a certain value and surface finish. The predicted repassivation potential is then compared to the corrosion potential (Ecorr) in the same environment to determine the alloy?s susceptibility to localized corrosion. The separation of localized corrosion modeling into two steps is valid as long as the initiation of stable localized corrosion is being consider
