The significance of electrochemical noise measurements on asymmetric electrodes

The technique of electrochemical noise measurement has now reached a relatively mature state, in that the methods required to make reliable measurements are well-understood, and some aspects of the analysis of such measurements have achieved general acceptance. Thus, the analysis of the resultant data in terms of the noise resistance, Rn, is widely accepted, and methods for the extraction of information about corrosion type are starting to become available.One requirement of most of the analysis methods is the assumption that the two electrodes used to determine the current noise are similar. This assumption is always something of a concern, and a number of methods have been proposed to enable the simultaneous or sequential determination of both potential and current noise associated with a single electrode, so that the assumption of similarity is not necessary. This paper is concerned with methods that use deliberately asymmetric electrodes.The normal justification for using deliberately asymmetric electrodes is to permit the study of what is happening on one of the two electrodes. The situation that applies when two asymmetric working electrodes are coupled together and the current and potential noise determined has been analysed previously. Unfortunately, however, the significance of this work has not generally been appreciated by those groups using asymmetric electrodes. In part this may be because the prior analysis did not explicitly consider the theoretical and experimental justification claimed for this type of experiment, and this paper aims to review this analysis, and to rationalise the claims and observations of previous workers.