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Electrochemical Impedance Spectroscopy (EIS) can be a useful tool for differentiating today’s high-performance coating systems.  These advanced coatings with their improved barrier properties perform longer in-service environments that are notoriously challenging. The use of accelerated weathering/corrosion testing in the laboratory to differentiate coating performance is still viable, but can take a year or longer, versus 3-4 months that were used just a decade or so ago, which creates longer lead times to market and increased testing costs. 

In order to discriminate between the performance of these coatings, new testing methods such as EIS have been developed to detect signs of deterioration before visible deterioration is evident. EIS measurements are relatively quick and non-destructive to the test surface, which is an advantage since the coated samples can be evaluated at designated intervals and the testing continued. The ability to repeat the testing on the same panels provides consistency in the data and is an improvement over other commonly used methods of evaluation that often require replicate panels so that destructive evaluations can be performed at each designated test interval.

The mathematical and theoretical basis of EIS is complex and beyond the scope of this article. Interested readers are referred to publications by MacDonald[1] and Orzem[2] for in depth discussion of these topics.  An EIS analysis consists of measuring the alternating current that results from application of an oscillating voltage to an electrode (i.e. the sample). Ohm’s law (Voltage = Current / Resistance) explains the relationship between voltage, direct current and electrical resistance. When examining electrochemical reactions such as corrosion, the alternating current equivalent to resistance, impedance is used because the chemical and physical processes of interest do not completely follow Ohm’s Law.  The impedance can be measured over a range of frequencies to assess different electrochemical parameters such as the charge transfer resistance and the coating capacitance. The impedance magnitude (|Z|) is frequently used as the metric for assessing coating performance.

Application of EIS to Coating Performance

The reason that EIS is effective in evaluating coating performance is that the degradation mechanism causes the coating material to become less resistant to moisture permeation (i.e., loses barrier properties).  The EIS procedure involves placing an electrolyte (usually dilute sodium chloride) solution on the surface of the coating.  Most methods require a contact time of at least 2 hours, with some requiring a minimum contact time of 24 hours.  If there is degradation (loss of barrier properties) of the coating, a change in the permeation of the salt solution is signified by a decrease in the impedance measurement.  While an individual data point does not provide enough information to make a judgement on the condition of the coating material, by obtaining a baseline reading and intermittent impedance values of the weathered or stressed coating, useful information on the barrier properties of the coating can be revealed.  The more similar the simulated test environment is to the actual service environment, the more useful the information will be for projecting the service life of the coating. Most of the EIS methods reference ISO 16773 “Paints and Varnishes – Electrochemical Impedance Spectroscopy (EIS) on High-Impedance Coated Specimens” for the testing of coatings.

A tightly cross-linked or thick industrial coating with good barrier properties is expected to have low permeability at the start of the testing, which translates to a value of 10⁹ ohms at 0.1 Hz or higher with most equipment limited to a reading of 10⁹ ohms (giga-ohms); although some equipment can obtain data to 10¹² ohms.  A change (decrease) in the exponent number is the indication that the coating is degrading.  As the exposure continues, some specifications list a maximum reduction of two units in the exponent value (e.g., a coating’s barrier properties starting value of 10⁹ would be suspect at a value of 10⁷).

EIS has also been used to examine other systems. In this article, the application of this method in applications related to oil and gas, microbial corrosion and stress corrosion is discussed. Method limitations in these specific applications are also described. In the oil and gas industry, EIS is used to study various corrosion phenomena, which include the following: 9) corrosion of carbon steel in a solution of saline water by carbon dioxide, and 2) corrosion of pipes buried in the soil. In the study of carbon dioxide corrosion, EIS is used to continuously monitor the formation and growth of corrosion products (mainly carbonate) on the steel surface. Different equivalent circuits are used to fit EIS information due to the multi-stage process. The multistage nature of the process is that the formation layer is non-uniform at the beginning of the corrosion and then completes over time. [93.] Therefore, equivalent circuits are similar to corrosion in solutions. For example, Choi et al. [96–94] used the equivalent circuit of Figure 3-b to fit the EIS spectrum in the early stages of corrosion when the rust layer on the metal has not yet formed. The rust layer had two time constants in the spectrum and therefore the model in Figure 3 should have been used. Microbial corrosion has also been investigated by the EIS. Biofilms are created on the metal surface. Because this process is a bio-sieving process. It is trochemical, electrochemical measurements such as EIS can be used to momentarily monitor this type of corrosion. However, recent studies [97] have shown that the application of this method can lead to delays in biofilm formation and lead to erroneous observations. Therefore, the use of complementary methods such as electrochemical noise is recommended. Finally, the EIS method has been used to study stress corrosion; However, the information obtained from the method will contain errors due to the existence of several phenomena in these phenomena. For example, Petit et al. [97] observed a shift or transmission at specific frequencies of the EIS spectrum, which they attributed to crack formation. Bosch [98] proposed a model for the presence of cracks in impedance information, a stress corrosion process, and suggested that phase change at specific frequencies is related to the length of the cracks. In addition, it is noted that this phase shift can be much smaller than systems detection.
S Among the many electrochemical methods used in corrosion monitoring, S is one of the least degraded due to the very low voltage range applied. The AC properties of this method also make it ideal for studying high resistance systems. These advantages have led to the widespread and successful application of the EIS method in corrosion monitoring. In this paper, EIS method is used to study and monitor atmospheric corrosion, corrosion in concrete, performance of coatings and thin films, performance of inhibitors, corrosion corrosion, corrosion in oil and gas industry, microbial corrosion and stress corrosion and configuration of different electrode systems. Explained in different contexts. Finally, it should be noted that although this method is relatively easy to use, analyzing and fitting EIS information is usually a complex task and therefore great care must be taken in selecting the appropriate equivalent circuit models for data analysis and mathematical models based on the actual physical properties of systems. , Be offered.