Hydrogen Sulfide Concentration Influence Toward Uniform and Localized Corrosion Rate of API 5L X65 Steel at Elevated Temperature

Trace amounts of hydrogen sulfide (H₂S) in a carbon dioxide (CO₂) dominant environment reduce general corrosion rates by forming thin iron sulfide layers but increase localized corrosion. This study investigates how varying H₂S concentrations affect the corrosion rates of API 5L X65 steel at 70°C. Using a 2 L glass cell setup, steel coupons were immersed in a solution containing 1,000 ppm NaCl and 500 ppm HAc for 7 d. Corrosion rates were determined through weight loss measurements, and surface morphology was analyzed using scanning electron microscopy and energy dispersive x-ray analysis. H₂S concentrations of 0 ppm, 30 ppm, 60 ppm, and 100 ppm were tested. At 0 ppm H₂S, the uniform corrosion rate was 2.53 mm/y. The addition of 30 ppm H₂S reduced the rate to 1.00 mm/y, showing a 60% inhibition efficiency. However, the highest pitting corrosion rate of 6.30 mm/y occurred at 30 ppm H₂S, with a maximum pit depth of 123 μm. The results suggest that lower H₂S concentrations lead to localized corrosion, while higher concentrations form a protective film, reducing corrosion rates.