The novel Cr-containing low alloy steels have exhibited good corrosion resistance in CO₂ environment, mainly owing to the formation of Cr-enriched corrosion film. In order to evaluate whether it is applicable to the CO₂ and H₂S coexistence conditions, the corrosion behavior of low-chromium steels in CO₂-H₂S environment with high pressure and high temperature was investigated using weight loss measurement and surface characterization. The results showed that P110 steel suffered localized corrosion and both 3Cr-P110 and 5Cr-P110 steels exhibited general corrosion. However, the corrosion rate of 5Cr-P110 was the highest among them. The corrosion process of the steels was simultaneously governed by CO₂ and H₂S. The outer scales on the three steels mainly consisted of FeS1-x crystals, whereas the inner scales on Cr-containing steels comprised of amorphous FeS1-x, Cr(OH)₃ and FeCO₃, in contrast with the amorphous FeS1-x and FeCO₃ mixture film of P110 steel. The more chromium the steel contains, the more chromium compounds the corrosion products contain. The addition of chromium in steels increases the uniformity of the Cr-enriched corrosion scales, eliminates the localized corrosion, but cannot decrease the general corrosion rates. The formation of FeS1-x may interfere with Cr-enriched corrosion scales and lowering the corrosion performance of 3Cr-P110 and 5Cr-P110 steels.
Keywords: CO2/H2S corrosion; X-ray photoelectron spectroscopy; corrosion scale; low-chromium steel; scanning electron microscope; weight loss.