Study on the Corrosion Behavior of L360Q Steel in a Supercritical CO2 Environment with Impurities

Pure CO₂ is non-corrosive, but it can become corrosive in humid environments, thereby threatening pipeline integrity. This study focused on L360Q steel and conducted high-pressure static corrosion simulation experiments in a supercritical CO₂ environment at 40°C and 14 MPa. Using the weight loss method in conjunction with characterization techniques such as SEM, EDS, and ultra-deep-field 3D microscopy, the study systematically compared the independent effects of aqueous-phase precipitation and trace single impurities (0.02 mol% H₂S, SO₂, or O₂) on the uniform corrosion and localized pitting behavior of the pipe material. The results indicate that higher water content corresponding to the reported aqueous-phase precipitation threshold was an important factor promoting severe corrosion under the investigated conditions. Under the non-aqueous-phase precipitation, the material exhibited only slight corrosion (0.0034 mm/a) in a pure CO₂ environment. In supercritical CO₂ containing water, the severity of corrosion damage caused by the three impurities was, in descending order: SO₂ > H₂S > O₂. Among these, the hydrolysis of SO₂ created a strongly acidic environment, promoting the formation of loose and discontinuous corrosion product layers, as inferred from SEM/EDS observations, resulting in a pitting corrosion rate as high as 1.232 mm/a; H₂S promoted localized corrosion, which may be associated with the formation of a discontinuous S-containing corrosion product layer and the resulting local electrochemical heterogeneity, resulting in a pitting corrosion rate of 0.802 mm/a; although O₂ has a lower uniform corrosion rate, it causes localized rupture of the oxide film in an acidic environment, thereby inducing pitting corrosion. The results suggest that, under the present experimental conditions, SO₂-induced acidification and H₂S-associated local electrochemical heterogeneity, potentially related to discontinuous S-containing corrosion products, may contribute to the enhanced localized corrosion of L360Q steel.