Estimation of electrochemical noise impedance and corrosion rates from electrochemical noise measurements.

Abstract

Electrochemical noise refers to the spontaneous fluctuations in potential and current that can be observed on a corroding metal. The use of electrochemical noise for obtaining information on the corrosion process generates much interest in research fields. One important application is the measurement of corrosion rate. This can be achieved using the electrochemical noise of a pair of electrically coupled corroding metals to obtain an estimate of electrochemical impedance - an abstract quantity that reflects various aspects of the corrosion process.There are a number of problems associated with estimation of impedance information from the electrochemical noise data, particularly regarding data pre-treatment, accuracy and precision. In addition, the present methods are incomplete: current literature does not offer information regarding the phase of the impedance; and assumptions regarding symmetry of an electrode pair cannot be tested without additional measurements.The thesis addresses the above mentioned problems. Specifically,analysis of the impedance estimation process is given to determine how precision can be affected by various factors;a novel signal processing technique is described that is shown to yield a local optimum precision;the application of the proposed signal processing to time varying systems is demonstrated by use of a time varying, frequency dependent impedance estimate;a technique for recovering phase information, given certain conditions, is suggested so that Nyquist impedance diagrams can be constructed; anda technique for testing the symmetry of a coupled pair of corroding metals is described.An integral part of electrochemical noise analysis is the software used for numerical computation. The Matlab package from MathWorks inc. provides an extensible platform for electrochemical noise analysis. Matlab code is provided in Appendix A to implement much of the theory discussed in the thesis.Impedance analysis and many other electrochemical corrosion monitoring techniques are primarily used for uniform corrosion, where the corrosion patterns occur uniformly over the exposed surface. In order to map localised corrosion, where the corrosion is typically concentrated within a small area, a wire beam electrode can be used. A wire beam electrode is a surface that is divided into a matrix of mini-electrodes so that the corrosion rate at different points can be monitored. However, manual connection of each mini-electrode to the measurement device can prove cumbersome. The final chapter of this thesis describes the design and testing of specialised multiplexing hardware to automate the process.In general, the thesis shows that by careful conditioning of the electrochemical noise prior to analysis, many of the problems with the technique of impedance estimation from the electrochemical noise data can be overcome. It is shown that the electrochemical noise impedance estimation can be extended to encompass a time varying, frequency dependent quantity for studying dynamic systems; that phase information can be recovered from electrochemical noise for the purpose of constructing Nyquist impedance diagrams; and that asymmetric electrodes can be detected without requiring additional measurements.