Mechanistic Insights of Dissolution and Mechanical Breakdown of FeCO3 Corrosion Films

Study of a Local Structure at the Interface between Corrosion Films and Carbon Steel Surface in Undersaturated CO2 Environments

Industries transporting CO₂ gas-saturated fluids have infrastructures made of carbon steel. This is a good material with great mechanical properties but prone to corrosion and potential failure. Corrosion in sweet environments involves the formation of FeCO₃ as a corrosion film, which is recognized to play a protective role under certain conditions. This work on the dissolution of corrosion films in sweet environments, under acidic and undersaturated conditions, demonstrates that the effects on the integrity of steel are far more significant than the damage observed on the surface of the corrosion film. Our results prove that dissolution of FeCO₃ involved the presence of an amorphous phase, the intermediate formation of FeCl₂ or FeCl⁺, and the presence of a phase with short distance atom-atom correlations. The amorphous phase was identified as a mixture of retained γ-Fe and Fe₃C. Partially broken α-Fe and Fe₃C structures were identified to prove the damage on the material, confirming the interface zone without evident damage on the corrosion film. Dissolution affected both the α-Fe and FeCO₃, with the lattice [102̅] from the FeCO₃ crystalline structure being the fastest to dissolve. The damage of steel at the molecular scale was evident at the macroscale with pit depths of up to 250 μm. The impact on the integrity of steel can be, therefore, more drastic than frequently reported in industrial operations of CO₂ transport industries that use cleaning procedures (e.g., acid treatment, pigging) as part of their operational activities.