In carbon dioxide (CO2) corrosion of steels, the bicarbonate ion (HCO3 –) is simultaneously the buffer for carbonic acid (H2CO3), the source of iron carbonate (FeCO3) precipitation, and the product of the cathodic reaction. In addition to spatial separation of the production of Fe2+ and HCO3 –, galvanic coupling between the steel and cementite (Fe3C) layers is the principal cause of internal acidification in these layers, since the HCO3 – ions are removed from the steel surface by electromigration. This can facilitate localized corrosion by lateral galvanic coupling. This mechanism explains the role of traces of free acetic acid (CH3COOH, or HAc) and the existence of multiple steady states. Transposition to corrosion of iron by hydrogen sulfide (H2S) or to corrosion of copper is discussed.