BASIC ASPECTS OF ELECTROCHEMICAL IMPEDANCE APPLICATION FOR THE LIFE PREDICTION OF ORGANIC COATINGS ON METALS

Electrochemical impedance spectroscopy (EIS) applies well to surfaces having a high electrical or electrochemical impedance. The method is, particularly suitable for the early detection of the degradation of highly resistive protective organic coatings on metals. Initial application of electrical methods for evaluating organic coatings on steel involved measurement of DC resistance or determination of an equivalent capacitance measured at a single frequency. Changes in the capacitance relate to the uptake of water while changes in the resistance of the coating have been analyzed in terms of penetration by ionic species from the environment. Measurement of the frequency-dependent impedance of a coating has allowed a more detailed evaluation based on the deviation of the coating impedance from a capacitive response. Several models specify readily determined parameters that correlate with long-term behavior. This paper reviews the fundamental concepts and models used to predict long-term coating behavior from electrochemical impedance spectra. Coated steel panels have been tested by this method.