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.
In the general paradigm of the study of carbon dioxide (CO2) corrosion of steel, recent developments have revealed the decisive influence of variations in the protectiveness of corrosion layers.1 Thus, corrosion layers containing the same solid components can be extremely protective,2 very little so, or even corrosive. 3 For a carbon-manganese steel, such as the St52 grade(1) used in some studies,2-3 the corrosion layers are composed of an insoluble corrosion product, iron carbonate (FeCO3), and/or an undissolved component from the steel, namely cementite (Fe3C). Like the metal, Fe3C is an electronic conductor. The cathodic corrosion reaction, therefore, can occur as readily on Fe3C as on the surface of the steel itself. This leads to the possibility of galvanic coupling between the steel substrate and the layer of undissolved Fe3C.
Based upon the general electrochemical characteristics of the reduction of hydrogen (H+) ions and the anodic dissolution of iron, the first study on the aggravation of corrosion by layers of undissolved Fe3C led to the conclusion that galvanic coupling between the steel and the Fe3C was only of a marginal nature and suggested that local acidification must occur in the aqueous medium trapped within the pores of the solid layer.3 However, it has since been realized that these basic electrochemical properties, which had been observed in strong acid solutions and assumed to be general, cannot be transposed to CO2 media.4 Under specific conditions of this sort, different orders of reaction for the reduction of H+ and different slopes of the individual Tafel lines leads to a situation where it is impossible to distinguish between the effect of galvanic coupling and that of internal acidification.4 It is now necessary to consider that one or the other may occur, or even both together.
