Impact of O₂ Content on the Corrosion Behavior of X65 Mild Steel in the Gaseous, Liquid, and Supercritical CO₂ Streams

The CO₂ stream in a carbon capture utilization and storage application generally includes impurities that could cause internal corrosion of CO₂ pipelines. The general and localized corrosion behavior with a variety of O₂ concentrations for X65 mild steel both in water-saturated CO₂ and CO₂-saturated water environments was evaluated using an autoclave. Corrosion tests were performed at 8 MPa and 25°C, 8 MPa and 35°C, and 4 MPa and 35°C to simulate the liquid, supercritical, and gaseous CO₂ transportation. Results indicate that notably higher general corrosion rates were recorded at each O₂ concentration in the CO₂-saturated water phase than those in the water-saturated CO₂ environment. The general corrosion rates did not show gradual rise at 0 ppm to 2,000 ppm of O₂; instead, a maximum was measured at 1,000 ppm of O₂ at 8 MPa and 25°C, and 50 ppm O₂ at 8 MPa and 35°C in the water-saturated CO₂ environment and 50 ppm at 8 MPa and 25°C, and 100 ppm at 8 MPa and 35°C in the CO₂-saturated water environment. The general corrosion rate at 4 MPa and 35°C followed a different changing trend with O₂ content from that in 8 MPa, and 25°C and 35°C both in the water-saturated CO₂ and the CO₂-saturated water environments. Localized corrosion or an average corrosion rate of beyond 0.1 mm/y was identified in each test in the CO₂-saturated water environment. When O₂ was introduced, a more porous corrosion product scale was detected on the coupon surfaces. A final series of corrosion tests with 100 ppm and 2,000 ppm O₂ and 60% and 80% relative humidity in a CO₂ environment did not show any sign of localized corrosion attack, and the average corrosion rates were below 0.1 mm/y at 8 MPa, 25°C and 35°C, and 4 MPa and 35°C.