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

Industries transporting CO 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 FeCO 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 FeCO involved the presence of an amorphous phase, the intermediate formation of FeCl 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 FeC. Partially broken α-Fe and FeC 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 FeCO, with the lattice [102̅] from the FeCO 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 CO transport industries that use cleaning procedures (e.g., acid treatment, pigging) as part of their operational activities.