Changes in Corrosion of X70 Steel in Water-Saturated Supercritical CO2 System Caused by Impurity

During the process of Carbon Capture and Storage, some impurity may be found in the supercritical CO2 fluid. The changes in corrosion of X70 Steel in water-saturated supercritical CO2 system caused by impurity has been studied by the simulation experiment. The average corrosion rate was got from weight loss measurement. The morphologies and compositions of corrosion products were analyzed by SEM, EDS and XRD. It was observed that the existence of separate impurity would aggravate the corrosion of X70 steel in the water-saturated supercritical CO2 system. The average corrosion rate was the highest when NO2 or SO2 was included, followed by H2S, and the average corrosion rate was the smallest when O2 was contained. Among them, X70 steel suffered local corrosion when NO2 was contained. The corrosion scale was monolayer structure of FeCO3 under the supercritical CO2-H2O system. After the addition of O2, Fe2O3 appeared in the corrosion scale, indicating that the oxygen corrosion process occurred. While with the addition of H2S, corrosion scales were double layers. FeS was mainly present in the outer layer. The corrosion process was controlled by CO2 and H2S. The corrosion products were mostly FeSO3·xH2O and a spot of FeCO3 when SO2 was contained. SO2 mainly governed the corrosion process. Besides, corrosion product was Fe2O3 without FeCO3 in the supercritical CO2-H2O-NO2 system, NO2 completely controlled the corrosion process.

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