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.