Role of Conductive Corrosion Products
in the Protectiveness of Corrosion Layers |
by |
J.L. Crolet,* N. Thevenot, and S. Nesic** |
Abstract |
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. |