A. P. Nazarov and D. Thierry

French Corrosion Institute, 220 Rue Pierre Rivoalon, Brest, F-29200 France

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Received October 30, 2008

Abstract—A mechanism of the atmospheric corrosion of a carbon steel under polymer (paint and varnish) coatings is studied. The potential distribution around an artificial coating defect was measured in situ with a scanning Kelvin probe (SKP). The potential gradient between the steel surface at the defect and under the coating determines the mechanism of the subfilm corrosion. Measuring the galvanic currents in a steel under coating–steel at defect model system revealed the spatial separation of partial electrochemical reactions. The separation was additionally confirmed by the elemental analysis of the metal surface upon removing the coating. The mechanism of corrosion exfoliation is shown to differ depending on the kind of rate, which determines the electrochemical reaction at the coating defect. In the presence of aqueous NaCl electrolyte, the defect acts as an anode, which results in the cathodic exfoliation of the coating. In the case of drier corrosion in an atmosphere at a humidity of 95%, the steel at a defect acts as a cathode with respect to the surrounding interface, which leads to the development of anodic zones around the defect.

DOI: 10.1134/S2070205109060173