About the software for fitting EIS

One of the most frequent problems with the interpretation of results of Electrochemical Impedance Spectroscopy (EIS) is fitting experimental results to an equivalent scheme. I will try to compare here some programs, both free and commercial, with which I have some experience. All these programs will be tested on a EIS spectrum measured in a corrosion experiment, in which a specimen of a magnesium alloy was immersed into salt water until a stable corrosion potential had been achived. The experimental EIS spectrum is given below:

Bode plot for a corrosion process

Nyquist plot of a corrosion process

The plots were built in IviumSoft available for free download on the website of Ivium.

There are two capacitive loops and one inductive loop in the Nyquist plot. Presumably, there is a faradaic process in the interphase Metal-Oxide (where "Oxide" stands for the layer of corrosion products, for the sake of brevity), a faradaic process in the interphase Metal-Electrolyte (occuring, for examples, inside corrosion pits) and a sorption process, which apparently corresponds to the sorption of ions from the electrolyte on the "oxide" surface. The following three equivalent circuits were found in the literature as reasonable models for the system in question:

1. R(RC)(RQ[RL]), presented, for example in J. Chen et al. / Electrochimica Acta 52 (2007) 3299–3309:

R(RC)(RQ[RL])

2. R(RC)(RCR[(Q[RL])]), ibidem:

R(RC)(RCR[(Q[RL])])

3. R(RC)(RCL), F. Cao et al. / Corrosion Science 76 (2013) 60–97:

R(RC)(RCL)

I am not going to discuss here, which of these three equivalent models (or of a dozen others also found in the literature) is more justified or describes better the experimental impedance spectrum. My objective is rather to compare the fitting procedures and results obtained with different software. As the experimental plots were built with IviumSoft, it seems to be fair and logical that we should start with this program.

(to be continued)