Electrochemical impedance spectroscopy of epoxy-vinyl coating in aqueous medium analyzed by dipolar relaxation of polymer

The performance of electrochemical devices, namely, the potentiostat, has increased considerably lately, and some commercial devices determine impedance on the order of teraohms (10(12) Omega). At the same time, the protective effectiveness of paint films has also improved markedly, and the paint films exhibit capacitance behavior only in the millihertz range. Up to now, electrochemical impedance spectroscopy (EIS) spectra of polymer-coated specimens were interpreted with a mechanism involving only the film resistance due to flaws through the polymer layer and the film capacitance. However, since almost no flaw should be present at huge impedance, a new insight in the modeling of EIS data for polymer coated metal electrode is needed. Therefore, the concept of dipolar relaxation developed in polymer science is introduced to interpret EIS data. To compare the dipolar relaxation and the electrode impedance, the Cole-Cole capacitance determined straightforwardly from impedance spectra was found to be a very powerful tool. It is shown that the water uptake of a paint film could be evaluated from the Cole-Cole capacitance at infinite frequency C(infinity), because the plasticizing effect induced by absorbed water in lower frequencies may lead to an overestimation of water uptake. (C) 2002 The Electrochemical Society.