Modeling Localized Corrosion of Corrosion-Resistant Alloys in Oil and Gas Production Environments: Part I. Repassivation Potential

A model has been developed for predicting the localized corrosion repassivation potential (Erp) for alloys in environments containing chloride ions and hydrogen sulfide. The model has been combined with Erp measurements for a 13-Cr supermartensitic stainless steel (UNS S41425) at various concentrations of Cl− and H2S. The model accounts for competitive adsorption at the interface between the metal and the occluded site environment, the effect of adsorbed species on anodic dissolution, and the formation of solid phases in the process of repassivation. The effect of H2S is complex, as it may give rise to a strong enhancement of anodic dissolution in the occluded environment and may lead to the formation of solid metal sulfide phases, which compete with the formation of metal oxides. H2S can substantially reduce the repassivation potential, thus indicating a strongly enhanced tendency for localized corrosion and stress corrosion cracking. However, exceptions exist at lower H2S and Cl− concentrations, at which H2S may lead to the inhibition of localized corrosion. The model accurately reproduces the measured repassivation potentials for Alloy S41425 and the limited literature data for Alloy CA6NM (UNS J91574), thus elucidating the conditions at which H2S increases the propensity for localized corrosion and those at which it does not. Because the repassivation potential defines the threshold condition for the existence of stable pits or crevice corrosion, the model provides the foundation for predicting localized corrosion and stress corrosion cracking in environments that are relevant to oil and gas production.