Influence of elemental sulfur on the corrosion mechanism of X80 steel in supercritical CO2-saturated aqueous phase environment

•Elemental sulfur can inhibit uniform corrosion rate and induce pitting on X80 steel.•There were two polymorphs of FeOOH and Fe2O3 present in product scales.•Sulfur was more likely to cause pitting in supercritical CO2-saturated water.•FeS protected matrix in presence of sulfur in supercritical CO2-saturated water. [Display omitted] The corrosion mechanisms of X80 steel under conditions containing elemental sulfur (S) were investigated in a supercritical CO2-saturated aqueous phase environment. The uniform corrosion rate slightly increased with immersion time in the absence of S, while it decreased with immersion time in the presence of S, and pitting was induced. There were two polymorphs of FeOOH in this study: α-FeOOH, which formed in an S-free environment by conversion of Fe(NO3)3; γ-FeOOH, which formed in the presence of S by conversion of FeSO4. There were also two polymorphs of Fe2O3: α-Fe2O3 could be formed by Fe(OH)3 in both environments, while γ-Fe2O3 could be formed from α-FeOOH in an S-free environment or γ-FeOOH in the presence of S. The S could cause pitting through the acid regeneration cycle reaction and occluded corrosion defects. Moreover, FeS gradually replaced FeCO3 to protect the substrate in the S-containing environment.