Corrosion and Hydrogen Permeation in H2S Environments with O2 Contamination, Part 2: Impact of H2S Partial Pressure

Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen contamination should be avoided during materials testing in H2S-containing media. In this second paper, as part of a series of articles that evaluates how traces of oxygen modify the corrosion of pure iron and hydrogen permeation across iron membranes in H2S-containing solutions, the impact of changing the H2S partial pressure from 100 kPa to 0.1 kPa was investigated. It was found that bulk solution chemistry for all H2S partial pressures changes with time, due to the formation of H2S-O-2 reaction products (sulfates, sulfites, and thiosulfates), which results in bulk solution acidification. Electrochemical and weight-loss measurements confirm that Fe corrosion rates in baseline well-deaerated H2S-containing solutions decrease with decreasing H2S partial pressure, although these are observed to be much higher under continuous oxygen contamination. With decreasing H2S partial pressure, hydrogen uptake in Fe also decreases, due to lower and lower concentrations of dissolved H2S and the associated increase in pH. However, even at 1 kPa and 0.1 kPa H2S, permeation effciencies remain close to 100% when no O-2 contamination is present. The hydrogen uptake is always relatively lower in Fe exposed to oxygen-polluted H2S solutions. Permeation efficiencies decrease continuously. From electrochemical data and surface characterization, these observations at lower H2S partial pressures are attributed to the disruptive effect of oxygen on the nature of sulfide corrosion products, and hydrogen entry promotion, along with the contribution of an additional cathodic reaction that does not result in hydrogen entry into the metal.