IN-SITU DETERMINATION OF THE LOSS OF ADHESION OF BARRIER EPOXY COATINGS USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Model epoxy coatings on a steel substrate have been investigated whilst immersed in a 3% NaCl solution using electrochemical impedance spectroscopy (EIS). The impedance of these barrier coatings remained high for a very long time during exposure and a superficial evaluation of the Nyquist plots of the impedance gave no indication of any change. However, a more accurate study matching the impedance to equivalent circuits containing a constant phase element (CPE) has revealed changes in the dielectrical properties of the coatings during long exposure times. In this way it was possible to detect localised loss of adhesion and the start of the corrosion process at the interface. These results have been compared with those of an investigation of the impedance of a dielectric sensor coated with the same coatings. With the dielectric sensor, a qualitatively similar response was observed as for the coatings on the steel panels and a separate water phase has been detected under the coating after immersion. With these additional results, the response of the coating/steel system may best be explained in terms of areas in the coating with different dielectrical behaviour due to localised loss of adhesion and the formation of corrosion products under the coating.