Effects of Sulfates on Corrosion of 430 Ferritic Steel in CO2-Saturated Water Containing Oxygen

Abstract

Aqueous corrosion can be an issue for structural materials in direct supercritical CO2 power cycles, especially in low temperature components such as water separators and coolers where aqueous condensation appears as supercritical CO2 working fluid containing impurities (SOx, O2, H2O) cools down. This study aims at testing 430 ferritic steel as a material of construction for the low temperature components. Immersion experiments were carried out in an aqueous solution with and without H2SO4 (pH was 3 and 4, respectively) in an autoclave pressurized to 8 MPa with 99% CO2 and 1% O2 gas mixture. Temperature of each solution was 50°C. Immersion tests lasted up to 500 hours. Post-corrosion surface characterization techniques included X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The characterization indicated that general corrosion took place in both cases with and without H2SO4. The corrosion rate of samples was calculated based on the mass loss. The corrosion rate values for 430 steel in solutions with and without H2SO4 were very low and similar after 96 hours of immersion. These initial results indicated that the presence of SO4 2- in CO2 -saturated water containing O2 did not have a detrimental effect on the corrosion resistance of 430 steel.