Abstract
A modeling framework has been developed for rationalizing and predicting localized corrosion of corrosion-resistant alloys in environments that contain hydrogen sulfide and chlorides. The framework relies on the computation of the repassivation potential and corrosion potential as a function of the composition of the aqueous environment, temperature, and pressure. It has been applied to four alloys that are used in oil and gas production, i.e., a 13Cr supermartensitic stainless steel (UNS S41425), duplex stainless steel 2507 (S32750) and two nickel-base alloys, 2535 (N08535) and 29 (N08029). For alloys S32750, N08535, and N08029, repassivation potentials have been measured at temperatures ranging from 358 K to 505 K as a function of Cl- and H2S. These measurements have been used to parameterize a recently developed mechanistic model for calculating the repassivation potential. 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 both oxide and sulfide solid phases in the process of repassivation. H2S can lead to an increase in the propensity for localized corrosion as evidenced by a decrease in the repassivation potential. However, exceptions exist at low dissolved H2S and Cl concentrations, at which inhibition of localized corrosion may be observed. The model accurately represents the experimental data and elucidates the interplay of the effects of Cl, H2S and temperature on localized corrosion. Furthermore, it has been validated by predicting the critical crevice temperature.
