Analysis of electrochemical noise measurement on an organically coated metal

Electrochemical noise measurement (ENM) has found a credible place among the electrochemical methods applied to organic coatings, with a large number of reports in the literature of using the technique as a reliable method for the evaluation of the corrosion protection afforded by an organic coating on a metal surface. This has commonly been performed by calculating the noise resistance, Rn, or spectral noise resistance, Rsn, from the two main elements of electrochemical noise signal, the electrochemical current noise and potential noise. Several studies have shown that in practice Rn is a good measure of corrosion protection provided by an organic coating and affords good agreement with other measures of corrosion resistance from more established methods such as electrochemical impedance spectroscopy and DC resistance measurement. However, the theory of the electrochemical noise signal has not been fully analysed to elucidate the influence of the coating on the noise acquired. In this study a mathematical model is advanced in accordance with the equivalent electrical model in an electrochemical system consisting of a corroding metal substrate which has on it an organic coating. Experiments are also performed to evaluate the presented model in practice. Results of both theoretical and physical modelling show that potential noise is not influenced by the effect of coating while the current noise is attenuated due to the large impedance of coating.