The CO2 stream in a CCUS application generally includes impurities which 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, 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 0-2000 ppm of O2; instead, a maximum was measured at 1000 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, 25 °C and 35 °C both in the water-saturated CO2 and the CO2-saturated water environments. Localized corrosion or 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 2000 ppm O2 and 60% and 80% relative humidity (RH) in 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, 4 MPa and 35 °C.

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