Effect of impurity interaction on the corrosion film characteristics and corrosion morphology evolution of X65 steel in water-saturated supercritical CO2 system

The corrosion behavior of X65 steel in water-saturated supercritical CO2 system containing NO2 with or without O2, H2S and SO2 impurities was investigated using weight loss measurement and surface analysis techniques. The interactions between NO2 and O2, SO2 or H2S promoted the formation of extra corrosive species and subsequently changed the corrosion film characteristics. In water-saturated supercritical CO2-NO2-O2-SO2-H2S system, FeSO4 and S preferentially formed in the early corrosion stage due to the impurity interactions. As the corrosion time prolonged, (NH4)Fe3(SO4)2(OH)6 and FeOOH formed due to the subsequent reactions of the impurity interaction products, and S disappeared in the corrosion scale via participating in the subsequent corrosion reactions during long-term corrosion. Localized corrosion was initiated by the inhomogeneous precipitation of acid-rich water in the presence of NO2. However, the propagation rate of the corrosion pit in depth direction greatly reduced with time, and the corrosion morphology of X65 steel shifted from localized corrosion towards general corrosion during long-term corrosion.