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.