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.