Analysis of electrochemical impedance and noise data for polymer coated metals

Electrochemical impedance and noise data have been collected for three coating systems applied on steel during exposure to natural seawater at Key West, Florida for 9 months. Potential and current noise data have been determined simultaneously by remote control via modem from CEEL. Analyses of the electrochemical noise data have been performed in the time and in the frequency domains. The mean potential E(coup) and the mean coupling current I-coup between two coupled electrodes of the same material, the standard deviations of potential sigma{V(t)} and current noise sigma{I(t)} fluctuations and the noise resistance R(n) obtained as the ratio of these two values have been calculated for each measurement which was carried out twice per week. The current index I-d = sigma{I(t)}/rms{I(t)} has also been determined. After Fast Fourier Transform (FFT) of the experimental data into the frequency domain, spectral noise plots have been obtained in a frequency range between 1 Hz and 1 mHz from which the slope m at the highest frequencies, the intercept B at 1 Hz and the spectral noise resistance R(sn)degrees defined as the DC limit of spectral noise plots have been determined. Significant chances of I-coup,I- sigma{I(t)}, R(n) and R(sn)degrees were observed when coating deterioration had occurred on both electrodes as indicated by changes of the impedance spectra due to increasing contributions from charge transfer and mass transport reactions occurring in pores filled with corrosion products and at delaminated areas of the coating. Spectral noise plots have been compared to impedance spectra and certain limitations of the electrochemical noise analysis (ENA) technique due to instrumental problems have been noted. A unique relationship of R(n) with protective properties of polymer coatings remains questionable. (C) 1997 Elsevier Science Ltd.