Electrochemical Noise - the Third Octave

Eden, David A. (InterCorr International Inc)

OnePetro 

ABSTRACT
Electrochemical noise has been used as a corrosion monitoring tool for almost twenty-four years. During this time, the technique has received a high level of interest in both laboratory and field applications. Various analytical approaches in both the time and frequency domains have been developed for data evaluation. In addition, different probe arrangements/geometries and electronic instrumentation have been applied to help capture essential mechanistic information. This paper describes how the technique and some of its variants have evolved in the last few years. The paper also discusses some of the fundamental aspects of how the technology has been used to identify localized corrosion effects, with emphasis on interpretation of the observed electrochemical response as it evolves.

INTRODUCTION
The development of the techniques for studying and understanding electrochemical noise phenomena has been an ongoing task for the last twenty four (or thereabouts) years. During the last few years, the technique has gained a fairly wide degree of acceptance, with some broadening of the scope of the instrumentation, methodologies and analysis techniques. In the opinion of the author there has probably been too much emphasis on the comparison of the electrochemical noise technique with other standard techniques such as polarization resistance, electrochemical impedance, etc.. In general terms, such comparison is difficult from a theoretical standpoint, since there is a reasonably solid foundation for the techniques using potential or current perturbation, whereas, on the other hand, electrochemical noise is essentially the study of stochastic behavior of corroding systems, and as such the noise behavior is difficult to model in a classical theoretical sense. On the other hand, it is possible to model observed electrochemical noise response in terms of our understanding of the electrochemistry of the corroding interface, in particular the resistor / capacitor networks, developed for interpreting the deterministic techniques. However, the classical approach to the electrochemistry of corrosion involves the adoption of a steady state model, i.e. we must make the assumption that the corrosion rate is stable during the period of the measurement. Whereas this premise is not unreasonable, there are periods when the corrosion rate is unstable and the deterministic model is not applicable, and instead we must evaluate the corrosion processes somewhat differently. From the noise perspective, we are trying to evaluate what processes are responsible for the instability, and to judge whether these processes are likely to result in some catastrophic failure mode (for example pitting corrosion) of the material being studied.

The initial part of this paper will focus on the theoretical aspects of the electrochemistry of the corroding interface, and the relationship between electrochemical noise response (non-steady state) and the classical deterministic (steady state) response. The classical approach typically utilizes some form of potential perturbation (potential step, sine wave & etc), and the response is analyzed in terms of the modified Butler- Volmer1 approach. With electrochemical noise on the other hand, the self-perturbation may be considered to arise from the stochastic kinetic control of the system, i.e. random current fluctuations, with consequential generation of potential noise. It will be shown that this type of behavior can be readily simulated to provide responses characteristic of real systems.