Comparison of electrochemical impedance and noise data for polymer coated steel in the frequency domain

The degradation of different polymer coating systems on steel exposed to natural (NS) and artificial (AS) seawater has been monitored using electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA). The aim of this project is to determine the impact of microorganisms on degradation of the protective properties of polymer coatings on steel during exposure to NS. The marine test sites were Port Hueneme, California and Key West, Florida. A new approach has been developed to obtain electrochemical impedance and noise data via modem from the laboratory using equipment placed at the remote test sites. Potential and current noise data were determined simultaneously. The experimental approach is described and representative data from exposure to NS and AS are presented for coating systems with different combinations of primers and topcoats. Electrochemical noise data have been analyzed in the time and frequency domains. Noise data presented as noise impedance plots have been compared to impedance spectra in Bode plots. In most cases excellent agreement between the impedance curves obtained with the two different techniques has been observed in the limited bandwidth available for ENA. The noise resistance R-n was found to be independent of frequency and equal to the polarization resistance R-p only for frequency-independent noise impedance plots.