Corrosion Behaviour of X65 Steels in Water-Containing Supercritical CO₂ Environments With NO₂/O₂

Corrosion experiments were performed on X65 carbon steel in water-saturated supercritical CO₂ (SC-CO₂) and under-saturated SC-CO₂ conditions at 80 bar and 35°C in the absence and presence of various combinations of NO₂ and O₂ ranging from 0-100 ppm and 0-1000 ppm, respectively. The purpose of the experiments was to understand the implications of the presence of these two species on both general and localized corrosion in likely conditions encountered during CO₂ transport.

The presence of 100 ppm NO₂ and 1000 ppm O₂ in water-saturated SC-CO₂ resulted in general corrosion rates reaching 0.3 mm/year after 48 hours and a localized corrosion rate of 6.8 mm/year, compared to 0.1 and 0.92 mm/year, respectively in the absence of both species. The final stages of testing considered corrosion rates in under-saturated SC-CO₂ with 100 ppm NO₂ and 1000 ppm O₂ at water contents between 0 and 1770 ppm. Although no corrosion occurred in dry conditions, increasing water concentration from 300 to 1770 ppm resulted in the general corrosion rates rising from 0.05 to 0.68 mm/year. However, there was no significant difference in the extent of localized corrosion observed with changing water content over this range, with values ranging between 0.5 and 0.6 mm/year.