Corrosion of Carbon and Alloy Steels in Aqueous CO₂ Environment

Corrosion resistance to aqueous CO₂ environment of three high alloy OCTG, a 13% Cr stainless (CR13), a ferritic-austenitic stainless (CR22) and a high Ni austenitic stainless (NIC42) steels has been examined in autoclaves simulating deep gas well environments. Experimental factors involved were temperature ambient to 200°C, CO₂ partial pressure 1 to 60 atm. and NaCl concentration 0 to 20%. The 13% Cr steel shows high corrosion resistance in CO₂ bearing waters within certain critical conditions in terms of temperature, partial pressure of CO₂ and concentration of NaCl. The presence of NaCl above some critical concentration in the environment causes pitting on the 13% Cr steel. The high Ni austenitic and the duplex stainless steels are very resistant to weight loss and pitting corrosion in such environments. Polarization curves obtained in the autoclave indicate that the high Ni austenitic and the duplex stainless steels are resistant to pitting corrosion in 3% NaCl at 150°C, because their pitting potentials are much more noble than the respective corrosion potentials. The corrosion rate of carbon steel is controlled by the stability of FeCO₃ film that forms on the surface of corroding steel and can be very low when environmental factors are favorable.