Noise Resistance Applied to Corrosion Measurements: VII. Deriving the Full Impedance from Noise Measurements?

Electrochemical impedance modulus can be obtained by electrochemical noise measurements (ENM) using two identical electrodes connected through a zero-resistance ammeter, via the “noise impedance,” defined as the square root of the ratio of the power spectral density (PSD) of the potential fluctuations to the PSD of the current fluctuations at each frequency. This paper examines the possibilities of further extending the method to derive the phase angle of the impedance. An attempt was made to integrate the noise impedance using the Kramers-Kronig formula method; however, the poor precision of the noise impedance and its limited frequency range significantly restrict the usefulness of the approach. The calculation of the electrode impedance from ENM using the cross-power spectrum method, as employed in spectrum analyzers, was generally ineffective because the computed impedance depends on the noise levels on the two electrodes, which cannot be measured individually. Nevertheless, good results over a wide frequency range were obtained when corrosion induced isolated current and potential transients, such as during metastable pitting, or when one electrode exhibited significantly higher noise signals than the other, in which case the full impedance of the latter electrode could be determined.