In the CO2 corrosion of steels, the bicarbonate ion HCO3- is simultaneously the buffer for carbonic acid, the source of FeCO3 precipitation and the product of the cathodic reaction, In addition to the spatial separation of the production of Fe++ and HCO3-, the galvanic coupling between the steel and cementite layers is also the principal cause of internal acidification in these layers, since the HCO3- ions are then removed from the steel surface by electromigration. This can facilitate the initiation of localized corrosion, by lateral galvanic coupling.
This mechanism also explains the role of traces of free acetic acid and the existence of multiple steady states. Finally, transposition to the corrosion of iron by H2S or to the corrosion of copper is also discussed,
Keywords : CO2 corrosion, cementite, protectiveness of corrosion layers, galvanic coupling, H2S corrosion, copper corrosion