THE DETERMINATION OF COATING PERFORMANCE WITH IMPEDANCE MEASUREMENTS .3. IN-SITU DETERMINATION OF LOSS OF ADHESION

The increased anti-corrosion performance of modern coatings is among others achieved by the (optimal) barrier properties. This implies a high D.C. resistance of the coating, which hinders the common way of interpreting measurements on these coatings carried out with Electrochemical Impedance Spectroscopy (EIS). Results of EIS measurements use to be evaluated in terms of capacitive behaviour and/or D.C. resistance of the coatings. It was shown in earlier papers that coatings can take up water from the environment while the D. C. resistance remained very high (> 100 mOMEGA cm2). In this paper it is shown that loss of adhesion could take place while the D.C. resistance remained very high. The loss of adhesion of epoxy barrier coatings is preceded by an excessive swelling of a part of the coating, which is reflected by a change of the dielectrical properties of the epoxy coating. In order to detect these changes it is necessary to use a Constant Phase Element (CPE) in the equivalent of the impedance. When using a capacitor the changes of the dielectrical properties cannot be clearly detected. A simulation with a model for the impedance of a coating with a part that swells excessively describes the measured response and explains the differences between the results when a capacitor and a CPE is used in the equivalent circuit.