The corrosion behavior of 3% Cr steel is tested by a high-temperature/high-pressure autoclave. The corrosion environment is categorized into CO2–alone and CO2/H2S conditions. At 90°C, with the addition of H2S to the CO2, the surface corrosion condition improved greatly, and the corrosion rates declined compared with the CO2–alone condition. Under CO2/H2S condition, with an increasing CO2/H2S partial-pressure ratio, the corrosion rate reached a peak value at pCO2/pH2S = 100, and then declined. Through the analysis of the corrosion products of the samples in different conditions by scanning electron microscope (SEM), EDS, and X-ray-diffraction (XRD) methods, it was found that the inner film is finer and denser than the outer scale. The partial-pressure ratio of H2S corrosion regime should be between 10 and 100 and is not the previous 200.
Corrosion has wide-ranging implications for the integrity of materials used in the petroleum industry. The implication of CO2 corrosion can be viewed in terms of its effect of capital and operational expenditures and health, safety, and the environment (Kermani and Harrop 1996), which brings huge losses and poses security threats to the development of oil and gas (Minxu et al. 2002; Kinsella at al. 1998). Medium Cr-containing steels showed improved corrosion resistance compared with carbon steel and lower cost than 13% Cr steel (Ueda and Takabe 2002; Muraki et al 2002; Nose et al. 2001). The effect of medium chromium-additions was attributed to the formation of chromium enriched corrosion products (e.g., observed on 3% Cr-steel). Steels alloyed with 3% CR or more were found to show improved corrosion performance over CR-free steel by a factor of two or more (Nose et al. 2001). The corrosion becomes complicated when H2S is added into the CO2 corrosion system. The current research should focus on the formation mechanism of the corrosion product film, properties of corrosion product of different structures and protection to the matrix. The corrosion experiment that has been carried out with 3% Cr tubular steel in pure CO2 and CO2/H2S environments. The results would be to supplement the CO2/H2S corrosion theory of 3% CR steel and provide theoretical basis for the proper selection of the tubing and casing in oil and gas fields.