The measurement of the noise resistance Rn or of the
frequency-dependent noise impedance Zn, both derived from the fluctuations of current and voltage of electrodes corroding at the open circuit potential, is more and more widespread for corrosion applications. These quantities have been shown to be related, under certain conditions, to the corrosion rate of the material under investigation. This
type of measurement has been originally proposed for a symmetric cell with identical electrodes, and much of its analysis has been based on the validity of this assumption. However, it is common that initially symmetrical cells become asymmetrical, due to unequal corrosion of the electrodes. Also, a number of workers have employed asymmetrical cell to limit the corrosion phenomena to a single electrode, but the exact meaning of the measured values of Rn and Zn is still questionable. To clarify the interpretation of electrochemical noise measurements, the experiments carried out in our laboratory on symmetric or asymmetric configurations are reviewed here. The results of the measurements of Rn and Zn are discussed and explained on the basis of a theoretical model previously published.
Keywords: electrochemical noise, voltage or current transient, noise resistance, noise impedance, symmetric cell, asymmetric cell, corrosion monitoring.