Effect of FeCO₃ Supersaturation and Carbide Exposure on the CO₂ Corrosion Rate of Carbon Steel

The effect on corrosion of carbon steel of varying bicarbonate (HCO₃⁻) and ferrous ion (Fe²⁺) concentrations in carbon dioxide (CO₂) purged in 1 wt% sodium chloride (NaCl) and 50 wt% monoethylene glycol (MEG, C₂H₆O₂) solutions was studied. The iron carbide (Fe₃C) in the steel was exposed by pre-corrosion to explore its role in the iron carbonate (FeCO₃) film formation process at pH-stabilized conditions. The corrosion layers formed ranged from being protective and showing passive behavior (corrosion potential approximately −0.5 V vs. silver/silver chloride [Ag/AgCl]) to being non-protective despite highly supersaturated solutions and long exposure times. The corrosion rate and potential development are discussed based on thermodynamic, kinetic, and electrochemical principles. The corrosion potential increased sharply after a protective FeCO₃ film was established, indicating passivation. New reaction mechanisms are proposed at these high potentials, which are more anodic than that obtainable by H⁺ reduction. Dissolved Fe₃C and magnetite (Fe₃O₄) are important factors in this passive potential range.