Investigation of pH ₂ S influence on 3% Cr tubing steel corrosion behaviours in CO ₂ –H ₂ S–Cl ⁻ environment

A series of experiments were conducted in autoclave to investigate corrosion properties of low Cr oil country tubular goods in CO2/H2S saturated brine solution (3 wt-%NaCl) at 90°C. Test material is 3Cr-80, which is gradually accepted in oil and gas industry, especially for downhole tubing and casing. The CO2 partial pressure was fixed at 4·0 bar in all experiments, and the H2S partial pressure was varied from 0, 2, 4, 8, 20, 40 and 800 mbar to 4000 mbar (4 bar). The results show that when the pH2S varied at a lower range (2–40 mbar), 3Cr-80 materials’ corrosion rates were restricted, and coupons’ surface morphologies became glossy compared to the pure CO2 corrosion. Under the condition of pH2S = 800 mbar, H2S no longer have a retardation effect on CO2 corrosion but acceleration. The average corrosion rate was higher than pure CO2 corrosion. When the pH2S rise to 4000 mbar (equal to the CO2 partial pressure), 3Cr corroded much more severe and serious mesa attack was observed.