Role of Conductive Corrosion Products in the Protectiveness of Corrosion Layers

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.