Long-term corrosion and stress corrosion cracking of X65 steel in H2O-saturated supercritical CO2 with SO2 and O2 impurities

Abstract

In this study, four testing cycles (total 480 h) were conducted to investigate corrosion and stress corrosion cracking (SCC) of X65 steel in H2O-saturated supercritical CO2 (s-CO2) containing SO2 and O2 impurities. The results showed that X65 steels experienced severe corrosion and formed a porous corrosion layer composed of iron sulfates, carbonates and oxy-hydroxides. Increasing time from 120 to 480 h resulted in the shift of dominant corrosion products from iron carbonates to iron oxy-hydroxides due to acid regeneration cycle. The mass loss of X65 steel almost linearly increased with exposure time, indicating a nearly constant corrosion rate in the s-CO2 environment. In comparison with previous studies, timely replenishing s-CO2 fluid in a static autoclave is crucial to accurately determine the long-term corrosion kinetic of pipeline steel. Based on literature survey and current study, several factors (SO2 content, H2O content, addition of NO2) affect the long-term corrosion rate in s-CO2 environments, and a power-function-based empirical model is purposed to depict the long-term corrosion rate of X65 steel in H2O-saturated s-CO2 + O2 + 100/2000 ppm SO2. The long-term SCC assessments showed that the SCC susceptibility of X65 would be still quite low, which might be related to the O2-depressed hydrogen cracking and/or the lack of localized stress concentration.