The effect of asymmetric electrodes on the analysis of electrochemical impedance and noise data

Collection of electrochemical potential and current noise data for determination of the noise resistance and evaluation of the frequency dependence of characteristic noise parameters requires the use of two identical electrodes, i.e. electrodes with the same electrode kinetics. When the requirement of the use of identical electrodes is not fulfilled, complications can arise in the interpretation of noise data. Similar problems can occur in the. measurement and interpretation of impedance spectra as has been demonstrated for polymer coated steel, where the properties of the electrode with the more protective coating dominate the experimental results. An equivalent circuit for asymmetric electrodes is being proposed which allows monitoring of coating degradation even if only one electrode has developed coating damage. Experimental data for three different coating systems on steel exposed to artificial and natural seawater for several months are presented. A significant decrease of the noise resistance was observed only when damage to the coating on both electrodes had occurred.