Abstract
The general and localized corrosion behaviour of carbon steel (UNS G15130) in water-saturated supercritical CO2 conditions containing O2 and SO2 at 35 0C and 8 MPa is evaluated with a view to the effect this may have on pipeline integrity during dense-phase CO2 transport. 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 (mole), respectively) change FeCO3 crystal morphology. Increasing the concentration of SO2 to 50 and 100 ppm whilst maintaining O2 content at 20 ppm resulted in the formation of FeSO3·3H2O as well as FeCO3. In conjunction with the change in corrosion product chemistry and morphology, general corrosion rates of samples increased from 0.1 mm/year to 0.7 mm/year as a result of the rise in SO2 content from 0 to 100 ppm (based on 48 hour experiments), whilst localized corrosion rates (determined from surface profilometry) rose from 0.9 to 1.7 mm/year. The research demonstrates that localized corrosion measurements are a fundamental requirement when determining the threat posed to carbon steel pipelines during dense-phase CO2 transport, exceeding the uniform corrosion rate by nearly one order of magnitude under certain conditions. 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) and X-ray diffraction (XRD).
