Measurement of the noise resistance for corrosion applications

Measurement of the noise resistance (R-n) or frequency-dependent noise impedance (Z(n)), 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 was proposed originally 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 as a result of unequal corrosion of the electrodes. Also, a number of researchers have used asymmetrical cells to limit the corrosion phenomena to a single electrode, but the exact meaning of the measured values of R-n and Z(n) is still questionable. To clarify the interpretation of electrochemical noise measurements, the experiments carried out in the laboratory on symmetric or asymmetric configurations were reviewed here. Results of the measurements of R-n and Z(n) are discussed and explained on the basis of a theoretical model previously published.