THE FREQUENCY-DEPENDENCE OF THE NOISE RESISTANCE FOR POLYMER-COATED METALS

Electrochemical noise and impedance data for polymer-coated steel expo sed to seawater have been analyzed in the time and frequency domains. Good agreement between spectral noise plots and impedance spectra was observed provided that accurate collection of current noise data was n ot affected by instrument limitations. The frequency range in which va lid spectral noise plots for polymer-coated steel can be obtained is q uite limited compared to impedance plots. The noise resistance R-n equ als the polarization resistance R-p only if both the potential and cur rent power spectral density plots have the same slope resulting in fre quency independent spectral noise plots. This means that the impedance spectra have to display a dc limit in the frequency region in which n oise data are collected, i.e., the sampling frequency f(s) for collect ion of noise data must be less than the breakpoint frequency f(b) rela ted to the dc limit of the impedance spectrum. In general, it has to b e concluded that for protective coatings for which impedance spectra a re capacitive between 1 Hz and 1 mHz, the frequency region in which me aningful noise data can be collected, R-n is a function of the measure ment bandwidth. Therefore interpretation of R-n in terms of a single c oating property is not possible.