Understanding the influence of SO2 and O2 on the Corrosion of Carbon Steel in Water-Saturated Supercritical CO2

The general and localized corrosion behaviour of X65 carbon steel in water-saturated supercritical CO2 conditions containing O2 and SO2 at 35 °C and 80 bar is evaluated. The results indicate that crystalline FeCO3 forms in the presence of solely water and CO2. However, the combined introduction of small concentrations of O2 and SO2 (as low as 20 and 2 ppm, respectively) changed its crystal morphology. Increasing the concentration of SO2 to 50 and 100 ppm whilst maintaining O2 content at 20 ppm resulted in the formation of FeSO3 • 3 H2O. General corrosion rates increased significantly from 0.1 mm/year to 0.7 mm/year as a result of the increase in SO2 content from 0 to 100 ppm based on 48 hour experiments, whilst localized corrosion rates rose from 0.9 to 1.7 mm/year. Additional tests involving solution replenishment over 48 hours indicated that the higher corrosion rates observed in the presence of SO2 did not present the worst case scenario corrosion rates and highlight the importance of having a system where the process fluid is continuously replenished. The corrosion product morphology and chemistry was identified through a combination of scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and surface profilometry measurements.