Comparison of electrochemical impedance and noise data in the frequency domain

Impedance and noise data obtained for two polymer coated steel samples in sea water have been compared in the frequency domain (Bode plots). Potential and current noise data were collected simultaneously and transformed into power spectral density (PSD) plots from which spectral noise plots were obtained. Excellent agreement between impedance and noise spectra obtained with two different sets of instrumentation was observed. The noise resistance Rn is frequency-independent only if the slopes of the potential and current PSD plots are equal. In all other cases Rn is frequency dependent and therefore not a systems parameter. A limitation of the electrochemical noise technique applied to the study of polymer coatings is the fact that meaningful data can usually be collected only at frequencies below 1 Hz. Therefore features of impedance spectra indicating coating degradation and water uptake which occur at higher frequencies cannot be detected.