51316-7708-Localized Corrosion of Corrosion-Resistant Alloys in Oil and Gas Production Environments: II. Corrosion Potential

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Product Number: 51316-7708-SG

ISBN: 7708 2016 CP

Author: Andrzej Anderko

Publication Date: 2016

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A recently developed framework for predicting localized corrosion 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 in H2S-containing environments for various alloys including supermartensitic stainless steels (S13Cr and S15Cr) a duplex alloy (2507) and austenitic alloys (2535 28 and 29). In this study a comprehensive 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 and active-passive transition of corrosion-resistant alloys. It incorporates the main cathodic reactions including the reduction of water protons and H2S molecules. The mixed-potential model is integrated with a speciation-based thermodynamic model for phase and chemical equilibria in the environment. Thus it reflects the effects of acid gas partitioning between the aqueous gas and possibly liquid hydrocarbon phases. The model has been parameterized using long-term corrosion potential measurements for alloys 2535 and S13Cr. The measurements have been performed at temperatures ranging from 293 to 505 K with NaCl concentrations of 0.287 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 data and can be used to elucidate the environmental conditions at which CRAs are susceptible to localized corrosion.