Effect of O2 and Temperature on Sour Corrosion

The effects of O2 and, implicitly, of oxidized sulfur species on sour corrosion were studied in a series of two-stage glass cell experiments at different temperatures (25, 40, 60, 80°C). In stage I, aqueous solutions were purged with a mixed gas of hydrogen sulfide (H2S) and O2 at a volumetric ratio of 5:1 for 1 day. The oxidized sulfur specie sulfate (SO42−) formed at all temperature conditions. The amounts of SO42− increased with increasing temperature. Sulfite (SO32−) was only found at 25°C. Thiosulfate (S2O32−) was detected at 25, 40, and 60°C. In addition, considerable amounts of elemental sulfur were formed at 40, 60, and 80°C. In stage II, corrosion coupons were exposed in the brines resulting from stage I. The highest corrosion rates were observed at 60°C. The presence of reaction products of H2S and O2 had a more significant effect with regard to localized corrosion at high temperatures. Small pits were observed at lower temperatures (25°C, 40°C), while at 60°C and 80°C large pits and deep localized attacks occurred. With the simultaneous presence of O2 and H2S, high corrosion rates generated severe localized attacks. In the cells with oxidized sulfur species present from stage I, a consistent increase in the severity of localized attacks was observed in the subsequent exposure periods, i.e., N2, H2S, mixed gas (H2S and O2 with a volumetric ratio of 5:1) exposure period, when compared to the blank.