The objective of this study is to evaluate the effect of low-level hydrogen sulfide (H2S) on carbon dioxide (CO2) corrosion of carbon steel in acidic solutions, and to investigate the mechanism of iron sulfide scale formation in CO2/H2S environments. Corrosion tests were conducted using 1018 carbon steel in 1 wt.% NaCl solution (25 degrees C) at pH of 3 and 4, and under atmospheric pressure. The test solution was saturated with flowing gases that change with increasing time from CO2 (stage 1) to CO2/100 ppm H2S (stage 2) and back to CO2 (stage 3). Corrosion rate and behavior were investigated using linear polarization resistance (LPR) technique. Electrochemical impedance spectroscopy (EIS) and potentiodynamic tests were performed at the end of each stage. The morphology and compositions of surface corrosion products were analyzed using scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the addition of 100 ppm H2S to CO2 induced rapid reduction in the corrosion rate at both pHs 3 and 4. This H2S inhibition effect is attributed to the formation of thin FeS film (tarnish) on the steel surface that suppressed the anodic dissolution reaction. The study results suggested that the precipitation of iron sulfide as well as iron carbonate film is possible in the acidic solutions due to the local supersaturation in regions immediately above the steel surface, and these films provide corrosion protection in the acidic solutions. (C) 2010 Elsevier Ltd. All rights reserved.