Flow loop corrosion experiments with carbon steel have been carried out in CO2 environments with H2S levels below the NACE sour limit of 3 mbar H2S (“semi-sour” environments). The presence of 0.5 – 1.5 mbar H2S in loop experiments with up to 2 bar CO2 and temperatures up to
80°C resulted in slightly higher corrosion rates than in corresponding experiments without H2S. The corrosion rates were in the range of 0.1-2 mm/y. In an experiment with 0.5 mbar H2S at a CO2/H2S partial pressure ratio of 4500 both iron sulfides (FeS) and iron carbonates (FeCO3) were detected on the steel surface. The mixed sulfide/carbonate films were 30-80 µm thick. Thermodynamic solubility data are clearly in favour of iron sulfide formation under these conditions, thus it is likely that the coexistence of FeS and FeCO3 is caused by local depletion of sulfide ions within the corrosion product films. Experiments with CO2/H2S ratios of 1200-1500 resulted in formation of thin iron sulfide films (1- 10 µm) on the corroding surfaces. No iron carbonates were found in corrosion product films formed at CO2/H2S ratios below 1500.
Keywords: CO2 corrosion, H2S corrosion, corrosion product films, iron sulfide, iron carbonate