Formation mechanism and prediction method of pitting corrosion of X65 steel in H2S/CO2 coexistent environment

Carbon steel pipes are widely used in oil and gas production systems, and they are susceptible to corrosion, perforation and leakage in multiphase flow environments containing various corrosive media such as CO2, H2S, and Cl-. This may cause pipeline and equipment failure, thus leading to safety hazards and economic losses. At present, researches on the metal corrosion in H2S/CO2 coexistent environments mainly focus on the formation laws of various FeS products, the protective performances of product films, and the formation mechanisms of pitting corrosion under high H2S partial pressures, but seldom on the possibility and occurrence conditions of pitting corrosion induced by extremely trace amount of H2S. Taking X65 steel as an example, this paper carries out 4 sets of testing experiments and 2 sets of verification experiments by using the large-scale H2S multiphase flow corrosion loop experimental system, studies the influence of different H2S partial pressures on the pitting corrosion behavior of low carbon steel under 40 ℃ and constant CO2 partial pressure, and proposes a prediction method based on the thermodynamic theory. The following results are obtained. First, when the H2S partial pressure is higher than 50 Pa, X65 steel undergoes general corrosion with a low corrosion rate. When it is lower than 24 Pa, with its gradual decreasing, pitting corrosion of X65 steel intensifies, with the pitting corrosion rate exceeding 11 mm/a at 2.4 Pa of H2S partial pressure. Second, when it is extremely low and the supersaturation of Mackinawite type FeS is less than 1, the rapidly formed FeS film cannot fully cover the metal surface. Thereby under the action of galvanic corrosion, the exposed surface is rapidly corroded by saturated CO2 solution, and ultimately local pitting corrosion occurs on low carbon steel. Third, the supersaturation boundary control curve of the Mackinawite type FeS on the surface of low carbon steel under different H2S partial pressures is helpful in the accurate prediction of pitting corrosion behaviors of carbon steel under different conditions. In conclusion, within the lower limit of H2S partial pressure specified in relevant industry standards, even trace amount of H2S in H2S/CO2 coexistent environments may still induce pitting corrosion on carbon steel. This understanding is of important theoretical and practical significance for the prediction and prevention of local pitting corrosion on oil and gas pipelines in H2S/CO2 coexistent environments. © 2024 Natural Gas Industry Journal Agency. All rights reserved.