Corrosion behavior and mechanisms of 3Cr steel in a high-salinity O2–CO2 environment

To investigate the corrosion behavior and mechanisms of 3Cr steel under high-salinity, high-Ca 2+ , and O 2 –CO 2 conditions, we simulated the service environment of a western China oilfield (total salinity 250,022 mg/L, Ca 2+ > 15,400 mg/L). Immersion and electrochemical tests were conducted at 120 °C under 0.5 MPa CO 2 with O 2 partial pressures from 0 to 0.4 MPa. The results showed that the uniform corrosion rate increased sharply from 0.6492 mm/a in pure CO 2 to 4.8551 mm/a at 0.4 MPa O 2 , with localized corrosion notably aggravated. It was found that the high Cl − concentration acted as a crystallization inhibitor, forcing the FeCO 3 corrosion product into an amorphous state. In pure CO 2 , the corrosion film displayed a multilayered architecture: a surface crystalline CaCO 3 layer with micropores exhibiting inward/outward growth, an intermediate amorphous FeCO 3 layer, and an inner amorphous Cr-rich oxide layer. This structure was governed by the kinetic dominance of Ca 2+ , which preferentially precipitated as CaCO 3 , thereby suppressing FeCO 3 growth through interfacial competition and localized acidification. With O 2 introduction, the synergistic effect of O 2 , Ca 2+ , and Cl − triggered the formation of a loose, porous Fe 2 O 3 /CaCO 3 composite scale. Notably, 3Cr steel exhibited a weak deposition behavior of CaCO 3 because of both the lower deposition tendency and adhesion strength of CaCO 3 on the amorphous Cr-rich layer. This structural deficiency led to non-uniform film coverage and generated a “small-anode/large-cathode” configuration, accelerating lateral pitting propagation. Although the dense Cr-rich layer effectively hindered Cl − penetration into the substrate, it cannot prevent the expansion of acidified occluded cells. These findings elucidate the synergistic effects of O 2 –CO 2 –Cl − –Ca 2+ on 3Cr steel corrosion and provide a theoretical basis for material selection and protection of tubing steels in oxygenated, high-salinity oil and gas wells.