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.