Investigating the Effect of H₂S and Corrosion Inhibitor on the Corrosion of Mild Steel under High Pressure CO₂ Conditions

The effect of H₂S and corrosion inhibitor on the aqueous corrosion behavior of mild steel was evaluated at high CO₂ partial pressure conditions. The experiments were performed in a 7.5 L autoclave with different temperatures (25°C and 80°C) and different H₂S concentrations (1000 ppm_v and 2000 ppm_v) at 12 MPa CO₂. The corrosion rate of steel samples was determined by electrochemical and weight loss measurements. The surface and cross-sectional morphology and the composition of the corrosion product layers were analyzed by using surface analytical techniques (SEM, EDS and XRD). Results showed that the presence of 1000 ppm_v and 2000 ppm_v H₂S decreased the corrosion rate of mild steel compared with pure CO₂ condition. However, the final corrosion rates were still higher than the targeted threshold (< 0.1 mm/y). Surface and cross-sectional analyses revealed the formation of FeS in the presence of H₂S and no localized corrosion was observed. The addition of 400 ppm_v of an imidazoline-based corrosion inhibitor reduced the corrosion rate below 0.1 mm/y in high pressure CO₂ conditions with 2000 ppm_v H₂S.