Insights into the effect of H2S on the corrosion behavior of N80 steel in supercritical CO2 environment

In this study, the corrosion behavior of N80 steel exposed to 80 bar supercritical CO2 and 0∼15 bar H2S environments was investigated by weight loss test, surface analysis, water chemistry simulation, corrosion thermodynamic and kinetic calculations. The results show that a very low H2S pressure of 0.004 bar in supercritical CO2 environment boosts the corrosion rate of N80 steel, whereas the rate is considerably reduced with further increasing the H2S pressure. The variation of corrosion rate with H2S pressure largely depends on the changes in cathode process and overall protectiveness of corrosion product film caused by H2S. As the H2S pressure rises, the evolution of corrosion product film on the steel is as follows: FeCO3 dominated film below 0.16 bar H2S, FeCO3 and FeS mixed film within 0.16∼4 bar H2S, and FeS dominated film over 4 bar H2S. Benefiting from the integrity of film structure and the uniformity of film composition, N80 steel experiences uniform corrosion in the formation environment of FeCO3 or FeS dominated film. However, the localized corrosion easily occurs in the formation environment of FeCO3 and FeS mixed film owing to the loss of film integrity induced by uneven precipitation of FeCO3 inside the corrosion product film.