Effects of Water Content on the Pitting Behavior of X65 Steel in Supercritical and Dense-Phase Carbon Dioxide Environments

In the study, the corrosion behavior of X65 pipeline steel was investigated under the pressure of 8 MPa in both supercritical (SC) and dense-phase (DP) CO2 environments with different water contents from 18% to 1000% relative humidity (RH). The results revealed the significant phase-dependent shift in the critical water content required for corrosion initiation. At 18% RH, no corrosion was detected in SC CO2 after 120 h of exposure, whereas DP CO2 environment led to a low corrosion rate (1.3 × 10−3 mm/a). At 60% RH, corrosion was detected in the environments of both phases (1.7 × 10−2 mm/a for SC and 2.0 × 10−2 mm/a for DP). These results indicated that the critical water content required for corrosion initiation in DP CO2 was lower than that in SC CO2. This difference might be attributed to the substantially higher density and viscosity of DP CO2, which promoted water condensation and stabilized the liquid film on the steel surface. Corrosion rate gradually increased with the increase in water content and peaked at of 0.57 mm/a in SC and 0.68 mm/a in DP CO2. Pitting occurred in DP CO2 environment with different water contents, but pitting was absent in SC CO2 environment with the water content below 80% RH. Surface analysis and electrochemical characterization indicated that the corrosion product was exclusively FeCO3 under all conditions. However, its microstructure and protective properties evolved differently in the two phases due to the differences in ion transport and crystal growth kinetics. These findings demonstrated that the critical water content for CO2 pipeline corrosion was a phase-dependent variable rather than a fixed value. Therefore, differentiated moisture-control standards should be implemented based on the operational CO2 phase state.