Effects of CO₂ and H₂S on Corrosion of Martensitic Steels in Brines at Low Temperature Available to Purchase

Corrosion studies were conducted for martensitic carbon steels in 5 wt% NaCl brine solutions at 4°C and 10 MPa (1,450 psi). These studies simulated different subsurface environments relevant to Arctic drilling. Here, two high-strength martensitic carbon steels, S-135 and UD-165, were studied in three different environments: (1) a CO₂-NaCl-H₂O solution with a CO₂:H₂O molar ratio of 0.312 in the whole system, (2) an H₂S-NaCl-H₂O solution with an H₂S:H₂O molar ratio of 3.12 × 10⁻⁴, and (3) a CO₂-H₂S-NaCl-H₂O solution with the same acid gas to water ratios as environments 1 and 2. Results from the CO₂+H₂S mixed environment indicated that sour corrosion mechanism was dominant when the CO₂:H₂S molar ratio was 1,000. This impact of a small amount of H₂S on the corrosion mechanism could be attributed to the specific adsorption of H₂S on the steel surface. Electrochemical and mass loss measurements showed a distinct drop in the corrosion rate (CR) by more than one order of magnitude when transitioning from sweet to sour corrosion. This inhibiting effect on CR was attributed to the formation of a protective sulfide thin film. Tafel analyses of the anodic reaction showed that the Bockris mechanism was unlikely in the conditions tested. When comparisons were made between modeled and experimental CRs, good agreement was found in the CO₂-only and H₂S-only environments, but not in the CO₂+H₂S environment.