Combined Effect of Carbon Dioxide, Hydrogen Sulfide, and Acetic Acid on Bottom-of-the-Line Corrosion

This research work presents a study of the combined influence of the partial pressure of hydrogen sulfide (H₂S) and the concentration of free acetic acid (CH₃COOH) on the general and localized carbon dioxide (CO₂) corrosion at the bottom of the line. Experiments were carried out for up to 21 days in three 4-in (101.6-mm) internal diameter flow loops at 70°C with 2 bars of CO₂. The flow regime was stratified for all of the experiments. It was found that trace amounts of H₂S (from 0.004 bar to 0.13 bar) greatly retards the CO₂ corrosion with general corrosion rates usually 10 to 100 times lower than their pure CO₂ equivalent. However, the most protective conditions were observed at the lowest partial pressure of H₂S, while corrosion increased when more H₂S was added. The presence of a mackinawite film on the coupon surface seems to be the origin of this protectiveness. When 1,000 ppm of free acetic acid was added to the system, the general corrosion rate doubled compared with a baseline CO₂ environment and by between one and two orders of magnitude compared to baseline H₂S/CO₂ mixtures.