Corrosion Resistance of Martensitic Stainless Steels in Environments Simulating Carbon Dioxide Gas Wells

Martensitic stainless steels (SS) have been used widely for oil-country tubular goods (OCTG) because of their high strength and excellent corrosion resistance in carbon dioxide (CO₂) gas wells. Cr-rich precipitates form after tempering heat treatments of martensitic SS. These precipitates can affect the corrosion resistance of martensitic SS in environments simulating CO₂ gas wells because Cr is considered to be an important element for improving corrosion resistance. The effects of chemical composition and tempering heat treatments on corrosion rates and pitting potentials were investigated to clarify the effect of precipitates on corrosion resistance of martensitic SS in environments simulating CO₂ gas wells. The general corrosion index (GCI = Cr – 12 C + 0.77 Ni + 10 N [mass %]) was confirmed as a good index for estimating corrosion rates of martensitic SS in environments simulating CO₂ gas wells. The term “C coefficient/Cr coefficient” (–12), was defined as the amount of Cr as carbide precipitates (M₂₃C₆) in martensitic SS from the analysis of precipitates. The term “N coefficient/Cr coefficient” was positive because soluble N was effective in improving corrosion resistance, as demonstrated by surface analysis. From electrochemical analysis, the anode reaction of martensitic SS was controlled by a high GCI. Pitting potentials in environments simulating CO₂ gas wells depended on Cr – 12 C + Mo + 1.1 Ni + 6 N (mass %). From the surface analysis, Mo corroded in solution to form molybdate (MoO₄^2–), which inhibited corrosion. Corrosion rates and pitting potentials in environments simulating CO₂ gas wells were governed mainly by the amount of Cr in the matrix. This amount could be estimated by deducting the amount of precipitated Cr from the total amount of Cr in the steel, regardless of the tempering heat treatment.