EIS has been successfully applied to the study of corrosion systems for thirty years and been proven to be a powerful and accurate method for measuring corrosion rates. But in order to access the charge transfer resistance or polarization resistancethat is proportional to the corrosion rate at the monitored interface, EIS results have to be interpreted with the help of a model of the interface.
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. To make an EIS measurement, a small amplitude signal, usually a voltage between 5 to 50 mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz. The EIS instrument records the real (resistance) and imaginary (capacitance) components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and input by the operator.
The program then fits the best frequency response of the given EIS spectrum, to obtain in fitting parameters. The quality of the fitting is judged by how well the fitting curve overlaps the original spectrum. By fitting the EIS data it is possible obtain a set of parameters which can be correlated with the coating condition and the corrosion of the steel substrate.
Amongst the numerous equivalent circuits that have been proposed to describe electrochemical interfaces only the following apply in the context of a freely corroding interface at or close to kinetic equilibrium (reference):
Simplest RC representation of an electrochemical interface
One relaxation time constant with extended diffusion
Two relaxation time constants
Impedance of pitting processes of Al based materials.