Electrochemical noise measurements (ENM) for SS 316L and Ti-6Al-4V exposed to Ringer?s solution have been analyzed in the time and frequency domains using power spectrum density (PSD) and spectral noise (R~n) plots. Excellent agreement between spectral noise plots and impedance spectra was observed. The noise resistance Rn was found to be dependent on the bandwidth Af of the ENM and was much lower than the polarization resistance Rp.
As discussed in an extensive review by Eden electrochemical noise measurements (ENM) have traditionally been analyzed in the time domain. Recently, analyses have also been performed more frequently in the frequency domain using power spectrum density (PSD) plots. Xiao and Mansfeld have extended the analysis in the piquancy domain by introducing spectral noise plots in which the ratio of the Fast Fourier Transform (FFT) of potential and current noise is plotted as a function of frequency f in the Bode plot format2-5. This type of electrochemical noise analysis (ENA) requires simultaneous collection of potential and current noise fluctuations2-5. Late#8 it was shown that spectral noise plots (R~n(f)) can also be obtained from the ratio of PSD plots according to:
RJf) = lV~f)/I~f)l = lVPs~(f)/IP#)lln (l),
where Vm (f) and Im (f) are the FITs of the potential and current fluctuations, respectively and VPsD (f) and IPsD (f) are the corresponding PSD data. Bertocci et al have given a theoretical analysis? and provided experimental noise data which have been compared to impedance spectra8. Mansfeld et al have provided experimental noise data in the form of spectral noise plots for polymer coated steel exposed to natural and artificial seawater and have demonstrated that spectral noise plots and 9-lZ Due to instrumental limitations caused by the high impedance impedance spectra (log IZIvs. log 5 gave similar results .
of the polymer coatings the bandwidth Af = fmm - ftin of the noise spectra was quite limited with a maximum fmax = 1/2 f,, where f is the sampling frequency, and fh = I/T, where T is the measurement time. Usually f~ was 2 Hz, while T was 1024 secb9-12. The relationship between ENM and impedance data has been further evaluated for a passive system ? SS 316L and Ti-6Al-4V exposed to Ringer?s solution ? by comparing spectral noise plots with impedance spectra13. In addition it was of interest to compare the noise resistance Rn obtained from ENM by statistical analysis with the polarization resistance RP obtained by electrochemical impedance spectroscopy (EIS) for this system.
