Measuring Corrosion by Electrochemical Impedance Spectroscopy

Over the past two decades, electrochemical impedance spectroscopy (EIS) has emerged as the most powerful electrochemical technique for defining reaction mechanisms, for investigating corrosion processes, and for exploring distributed impedance systems (CPE and Warburg). Although the basis of EIS can be traced to the work on operational calculus by Heaviside and to that of Warburg on diffusion processes, more than a century ago, it was the result of work by Epelboin and his group in Paris in the 1960s that propelled EIS into the forefront as a corrosion mechanism analytical tool. Prior to that time, EIS had been dominated by reactive bridge techniques for measuring interfacial impedance, but these techniques, in general, were limited to frequencies above about 100 Hz. The principal reason why EIS did not find extensive use in defining corrosion and electro dissolution reaction mechanisms during this period is that the lowest accessible frequency was too high to detect relaxations involving reaction intermediates, except for the fastest of mechanisms. It took the combined skills of Epelboin’s group and SOLARTRON Instruments, Ltd, to change the EIS world dramatically, with the development of the “frequency response analyzer (FRA)”. As with the introduction of the first electronic potentiostat two decades earlier, the FRA revolutionized the field by allowing the impedance to be measured at frequencies down to 0.1 mHz.

There are numerous books and technical papers that discuss about the electrochemical impedance spectroscopy and their applications. Most of them were written keeping in view of the applications rather than the basics. Not much light is thrown on the basics, which involve system linearity, Laplace transforms, conversion of an electrochemical system to a transfer function, its frequency response analysis and representation of the analysis results in the form of Nyquist and Bode plots. In most of the literatures, we directly find the Nyquist and Bode plots, without even knowing the transfer function of the system.

In my coming posts, I will explain the basics of corrosion resistance measurement using EIS technique and (a) the basic knowledge of the elements of electrochemical systems, (b) their conversion to a transfer function using Laplace transforms and (c) their frequency response analysis and representation of results in the form of Nyquist and Bode plots. I have also included the frequency response analysis of some commonly found electrochemical systems. This report also helps the application scientists to model their impedance results and to cross check the equivalent circuits.