Jean Louis Crolet

Role of Conductive Corrosion Products in the Protectiveness of Corrosion Layers

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.

Method for Studying Stabilization of Localized Corrosion on Carbon Steel by Sulfate-Reducing Bacteria

Initiation of a corrosion pit in plain carbon steel (CS) was simulated by preconditioning a small area and a larger concentric area as an anode and cathode, respectively. Preconditioning was achieved by applying a galvanic current between the electrodes. During this initial step, the effect of sulfate-reducing bacteria (SRB) on resulting polarization of this artificial galvanic cell was investigated. The previously conditioned electrodes then were left short-circuited, and the naturally flowing galvanic current was measured using a zero-resistance ammeter. In the presence of SRB biofilms, an active and naturally stable galvanic cell was achieved. In sterile conditions, the coupling current either was unstable or very low. This technique was aimed at studying risk factors linking bacterial contamination to the onset of rapid pitting corrosion.