The Electrochemical Impedance Spectroscopy (EIS) is a technique for characterizing electrochemical systems.
The principle of this technique consists of applying a low amplitude sinusoidal signal to the electrochemical system and measuring its response at different frequencies.
In the case of an electrochemical impedance measurement applied to the aerosol generators, the test method consists of bringing the coating or varnish (container) into contact with the product which will serve as electrolyte (content).
The test is based on the application of a low alternative potential at the input, by means of a reference electrode (RE) and a counter electrode (CE) connected to a potentiostat* and the measurement of the ouput current, between the reference electrode and the working electrode (WE), which will allow the impedance (alternating current electrode) to be calculated. In this case, it is the metal aerosol can (or a container, metal bottle) which constitutes the working electrode.
In order to be able to measure an output current, it is necessary that the prodcut – the electrolyte – has a minimum conductivity. This measurement is carried out at different frequencies during the test, typically between 100 mHz and 100 kHz.
The physical model used to quantify the response to the product, the electrolyte, is a RLC model: resistance, inductance, capacity (see figure below). To quantify the quality of the coating used, we are interested by the resistance and capacity values and and their changes over time. In this goal, several measurements are carried out at regular intervals, over a period of one month, to characterize the stability of the coating to extended contact with the product.
The Electrochemical Impedance Spectroscopy (EIS) is a technique that allows a rapid and reliable initial assessment of the compatiblity of a coating towards a given product. This method is complementary to storage tests for the evaluation of container-content compatibility, as it is not possible to evaluate compatibility in the gas phase with this technique.
This is an effective method to compare different varnishes between them and to anticipate faster incompatibilities. Thanks to the new generation of potentiostat*, with which the LEREM is equipped, it is possible to measure very low output signals (currents), which makes it possible to obtain a response with resistance and capacity values even with very poorly conductive formulations.
* Potentiostat: accurate device that generates and measures low electrical signals with electrodes for electrochemical studies.
Electrochemical studies include other techniques as tin rate measurements on tinplate sheets as well as corrosion current measurements by amperometry. These studies will be the subjects of a future article.
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