Effect of H2S on Fe corrosion in CO2-saturated brine

The effect of H2S at ppm level concentrations on iron corrosion in 3 wt% NaCl solutions saturated with CO2 in the temperature range of 25–85 °C is examined using electrochemical and surface science techniques. Small H2S concentrations (5 ppm) have an inhibiting effect on corrosion in the presence of CO2 at temperatures from 25 to 55 °C. At 85 °C, however, 50 ppm H2S is needed to provide significant corrosion inhibition. At higher H2S concentrations, the corrosion rate increases rapidly, while still remaining below the rate for the H2S-free solution. Characterization of the iron surfaces after corrosion was carried out using X-ray photoelectron spectroscopy and X-ray diffraction. A sulfur peak (S2p) is observed at a binding energy of 161.8 eV in all cases, attributable to disulfide $$ ({\text{S}}₂^2-) $$ formation. Corrosion protection in the temperature range 25–55 °C can be attributed to Fe(II) bonded to S and O. At 85 °C, protection of the iron surface is most likely due to FeS2 formation. Morphological changes on the iron surface after exposure to H2S containing solutions were observed by SEM. A thin protective film was seen after exposure to solutions containing 5 ppm H2S at 25 °C, while at 85 °C, with the addition of 50 ppm H2S to CO2-saturated brine solution, a dense protective film was formed on the iron surface.