Corrosion mechanism in marginal sour (CO₂/H₂S) environment with correlation of H₂S in aqueous state

Marginal sour environments, where H₂S concentrations less than 100 ppm coexist with CO₂, present unique corrosion challenges in oil and gas transportation. Unlike fully sweet or sour environment this condition is poorly understood and studied. Existing standards like NACE MR0175 rely on gas-phase concentration at 25 ℃, which may not reflect aqueous-phase corrosion where gas solubility is temperature-dependent. This study investigates the corrosion behavior of API 5L X65 carbon steel at 1 bar and varying temperatures using gas mixtures containing 0, 30, 60, and 100 ppm H₂S balanced with CO₂. Uniform corrosion rates were determined by ASTM G1-03, pitting corrosion by ASTM G46-21, and surface morphology by SEM/EDX. Findings revealed at a constant temperature of 50 ℃, the uniform corrosion rate decreased with increasing H₂S concentration, from 3.36 mm/yr at 0 ppm to 0.40 mm/yr at 100 ppm, due to the formation of protective FeS layers. This trend was consistent across all tested temperatures. However, at constant H₂S concentration, the uniform corrosion rate increased with temperature. At 100 ppm H₂S, the corrosion rate increased from 0.22 mm/yr at 30 ℃ to 0.71 mm/yr at 70 ℃. SEM analysis revealed a distinct local breakdown of the corrosion product layer at 30 ppm H2S for both 50 ℃ and 70 ℃. Localized corrosion was most severe at 30 ppm H₂S and 50 ℃, where the highest pitting rate of 8.44 mm/yr was recorded, indicating the formation of an unstable and non-protective scale. A threshold of aqueous H₂S concentration (2.78 × 10-6 M) was identified, when exceed this threshold the pitting rates remained below 2.00 mm/yr. This study highlights low H₂S levels drive severe localized corrosion, while H₂S(aq) serves as a key predictor of FeS protectiveness for improved risk mitigation in marginal sour environment.