Polarization Behavior of High-Alloy OCTG in CO₂ Environment as Affected by Chlorides and Sulfides

Pitting corrosion resistance of 13% Cr steel, duplex stainless steel, and high Ni austenitic stainless alloys for Oil Country Tubular Goods (OCTG) has been examined by electrochemical measurements in a high temperature, high pressure autoclave, simulating well conditions. Each steel shows the pitting potential in the CO₂-NaCl-H₂O system, the value of which becomes less noble with increased Cl⁻ concentration and temperature. The presence of H₂S is detrimental. High alloying additions of Cr and Mo enhance pitting resistance whether H₂S is present or not. The corrosion potentials of these alloys in the simulated well conditions, in which oxygen is absent, do not exceed the pitting potential. Therefore, pitting corrosion can not occur by the passivity breakthrough mechanism, and may occur only when some other factor causes breakdown.