A comprehensive computational system has been developed for predicting long-term general and localized corrosion of Fe-Ni-Cr-Mo-W alloys in complex aqueous environments. The system relies on the computation of the corrosion and repassivation potentials as functions of solution chemistry and temperature. The corrosion potential is calculated from a mixed-potential model that combines comprehensive thermodynamic speciation calculations with a detailed treatment of partial electrochemical processes that may occur on the metal surface. The mixed-potential model has been verified by calculating corrosion rates in mixed acids and corrosion potential as a function of pH and concentration of oxidizing species. The repassivation potential is calculated from a separate model that quantitatively considers competitive processes at metal/salt film/solution interfaces in the limit of repassivation. This model has been shown to be accurate for reproducing the repassivation potential for mixtures containing both aggressive and inhibitive ions. Furthermore, a generalized correlation has been established to relate the repassivation potential to alloy composition. The combined predictive methodology has been validated by calculating the critical crevice temperature for a number of nickelbase alloys.
Localized corrosion is an extremely complex phenomenon that is influenced by diverse factors including the properties of chemical species in an aqueous environment, concentrations of components, alloy composition and temperature. In the last three decades, considerable progress was made in understanding the initiation, growth and repassivation of localized corrosion of various metallic materials.1-6 Several modeling approaches to localized corrosion have been developed by considering atomic/molecular processes7,8, microstructural features1,9 and transport processes in macroscopic cavities.10-13 These models have successfully contributed to our understanding of various aspects of pitting and crevice corrosion.
In previous papers,14,15 a different computational model has been proposed to predict the tendency of metals to undergo localized corrosion as a function of environmental conditions. This approach essentially divides the task of predicting localized corrosion into two parts, i.e., (1) calculating the corrosion potential and (2) predicting the repassivation potential, also called the protection potential. The repassivation potential (Erp) is a measure of the tendency of an alloy to undergo localized corrosion in a given environment. 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 papers15-17 that (i) Erp is the potential below which stable pitting or crevice corrosion does not occur and (ii) it is relatively insensitive to prior pit depth and surface finish. The predicted repassivation 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 considered because the corrosion potential is not affected at this stage by the localized corrosion processes and the interaction between pits can be ignored. However, such a separation is not valid once significant pit or crevice corrosion growth occurs.
