Effects of Alloying Elements on Carbon Dioxide Corrosion in 13% to 20% Chromium-Containing Steels Available to Purchase

Effects of alloying elements on corrosion rates were investigated for 13% to 20% chromium-containing steels in wet carbon dioxide (CO₂) environments without wet hydrogen sulfide (H₂S) gas at 150°C to 200°C. Results showed that a reduction in carbon content and an increase in chromium, molybdenum, and nickel content improved CO₂ corrosion resistance. However, corrosion rate was independent of nitrogen content. The combined additions of nickel and copper greatly improved CO₂ corrosion resistance. To satisfy the criteria in which the corrosion rates became < 0.1 mm/y, the nickel content should have been > 1% and copper > 0.5% at 180°C. This level of alloying corresponded to the reduction of the Cr + 1.6% Mo index value by ~ 6%. In the case at 200°C, nickel content was required at > 4%, and copper content at > 1%. The reason for the improvement of CO₂ corrosion resistance is thought to be the combined additions of nickel and copper that made the corrosion film more stable and more protective. The equation to be satisfied with a corrosion rate < 0.1 mm/y at 180°C in 0.02% C-containing steels was indicated as Cr + 1.6% Mo ≥ 19% (copper-free steels) and Cr + 1.6% Mo ≥ 13% (combined additions of nickel and copper).