System Linearity

About EIS Technique

EIS is a powerful technique for characterization of electrochemical systems. The fundamental approach of the EIS is to apply a small amplitude sinusoidal voltage excitation to the system under steady state and measure its linear current response at a wide range of frequencies. Analysis of the system response contains information about the interface, its structure and reactions taking place at the interface.

The value of EIS derives from the effectiveness of the technique in isolating the individual reaction and migration steps in a multi-step process. This is possible because each reaction or migration step has ideally a unique time constant associated with it. These steps can be easily separated in the frequency domain.

Generally, there are two types of EIS techniques:

1. Measurement of the impedance as a function of the frequency of a small-amplitude sinusoidal potential perturbation. This technique is called as “Electrochemical Impedance Spectroscopy” or “Impedance Voltammetry”.

2. Superimposition of a single-frequency sinusoidal potential on a scanned or stepped direct potential (open circuit potential) and measuring the sinusoidal current response as a function of the direct potential (OCP). This technique is named as "Alternating Current Polarography" or “Alternating Current Voltammetry”.

EIS cannot give all the answers. It is a complementary technique and the other methods must be used to elucidate the interfacial processes.

Electrochemical Work Station

A commercial Electrochemical work station used to quantitatively measure the corrosion resistance and the corrosion rate.

Back Ground of EIS

The rapid and accurate measurement of electrochemical phenomena is of considerable importance for a wide range of studies like corrosion, effectiveness and life of surface coatings, battery testing, electrode kinetics, double-layer studies etc. Of these, corrosion and surface treatments are discussed in detail in this work, although the basic mechanisms are broadly similar for all the applications.

It is possible to measure the corrosion rate by direct analytical methods like weight loss measurements etc; but, because the corrosion process is very slow, these methods are time consuming and inefficient. Further, they are restricted to systems in which the products formed by corrosion do not form adherent layers. As the processes under consideration (corrosion and surface treatments) are electrochemical in nature, it is possible to analyze them using electrical methods, based on faraday’s laws, which relate the change in mass per unit area to the current flow.

The advantages of this approach are a relatively short measuring time, high accuracy and the possibility of monitoring the process continuously. The major disadvantage of this approach is that the system under investigation has to be perturbed from its normal state by an external signal, which inevitably changes the properties of the system. The perturbation itself can be due to an AC or DC signal and it is primarily with AC methods.

The DC technique is widely used for corrosion rate measurement. But this method generally requires large perturbation signal and can infact fail when the corrosion process is taking place in a low conductivity medium. AC methods are finding increasing applications in electrochemical research because only small perturbation signals, which do not disturb the electrode properties, are used and can be investigated in low conductivity media.

A corroded nut and bolt. It is estimated that each year, there is approximately Rs.1 lakh crore loss due to corrosion in India and in US, it is about 4% of its GDP!!!!!

Studying Electrochemical Interfaces

At any electrochemical interface, charge transfer occurs only at the end of a succession of the following coupled elementary phenomena:

1. Transport of reactive species in the bulk of the solution (mass transport).

2. Adsorption of the reactive species on to the electrode.

3. Electrochemical and chemical interfacial reactions.

Both adsorption and reactions occur on the electrode surface; but, mass transport is a homogeneous phase phenomenon. The electrochemist has to isolate each of the elementary phenomenons from others to study them. Hence, he has to use a technique which is capable of separating the elementary phenomenons and extract the data for each of them separately.

Some techniques like AUGER that are able to characterize the adsorbed species on the interface require the use of vacuum techniques; therefore they can’t be for “in-situ” analysis of the electrochemical interface. Other techniques using electromagnetic waves like SEM are being used to study the electrochemical interface. But these cannot be used when dissolution of the surface or deposition on the surface occurs. Hence electrical methods are often the only possible methods for “in-situ” investigations of electrochemical interface.

Use of electrical methods allows studying the reaction kinetics, which permits the separation of the coupling between the elementary phenomena by control of the reaction rates. This enables the individual electronic steps in the reaction mechanism to be distinguished and the often unstable reaction intermediates involved in these reactions to be counted. Even if these techniques do not allow a real identification of the reaction intermediates and bondings from a chemical point of view, they at least give information on the kinetics of the reaction mechanism governing the behavior of the electrochemical interface and some characterization of these intermediates.

Steady state techniques are used to study simple processes; where as non-steady state techniques are necessary to study more complex electrochemical systems. Disturbing the reaction from the steady state by applying a small perturbation to the electrochemical system allows the system to attain a new steady state. As the various elementary processes change at different rates, their response can be analyzed to separate the overall electrochemical process. The choice of the technique to be used depends on the application. Transient (time analysis) techniques are generally used for testing models or determining kinetic parameters of a known mechanism, as they are well suited for extracting kinetic parameters when the mass transport is negligible. However, when complex heterogeneous reactions interact with mass transport, transient analysis will lead to very poor results and a frequency analysis is more efficient. Hence, the use of impedance measurements over a wide frequency range is a better technique for studying the electrochemical interface.

Devising the measurement procedure and elaborating the models which have to be compared with the experimental data require an accurate description of the kinetic and electric laws governing the interface.

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.