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

A recently developed framework for predicting localized corrosion and stress corrosion cracking of corrosion-resistant alloys (CRAs) in oil and gas production environments relies on the computation of the repassivation potential and corrosion potential. While the repassivation potential defines the threshold condition for the existence of stable pits or crevice corrosion, the corrosion potential quantifies the driving force for localized corrosion. Localized corrosion can occur if the corrosion potential exceeds the repassivation potential. In a previous study, a model was developed for predicting the repassivation potential of CRAs in H₂S-containing environments. In this work, a mixed potential model has been developed for calculating the corrosion potential of passive alloys in wide ranges of temperature, pressure, salinity, and H₂S concentration. The model simulates passive dissolution of CRAs and incorporates the main cathodic reactions including the reduction of water and H₂S molecules. The mixed potential model is integrated with a speciation-based thermodynamic model for calculating phase and chemical equilibria in the environment. The model has been parameterized using long-term corrosion potential measurements for Alloys 2535 (UNS N08535) and S13Cr (UNS S41425). The measurements have been performed at temperatures ranging from 20°C to 232°C with NaCl concentrations of ∼0.3 molal and 5.7 molal in the liquid phase and in the presence of N₂, H₂S, and N₂−H₂S mixtures in the gas phase. The model accurately represents the experimental corrosion potential data and can be used to elucidate the environmental conditions at which CRAs are susceptible to localized corrosion.