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 and Orzem 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).
In other test protocols, the EIS values are monitored over the duration of the exposure and compared to replicate panels with intentional scribes (through the coating to the substrate) that develop corrosion over the duration of exposure. The critical value (hours of exposure to measurable degradation) of that material is then determined. In yet other protocols, the values are reported throughout the exposure cycle and used for reference as more data on long term exposure of the coating in actual service environments are collected. That is, the known performance of the coating to both an actual service environment and simulated environments, such as salt fog and cyclic salt spray/weathering, can be useful for comparisons with new formulations of coating materials.
EIS testing can be conducted with a 2-, 3- or 4- electrode configuration. The standard arrangement in Figure 1 shows the coated sample (working electrode), a reference electrode and a counter electrode used to conduct the measurement. It should be noted, that as with most testing methods, the testing parameters are important to the production of the data. The testing parameters should be closely monitored, provided as qualifiers for any reported data, and repeatable. The sample surface area, electrolyte contact time, electrolyte composition, and electrolyte temperature are all important variables that can change the data generated.
Also, laboratory data are required to be obtained in a Faraday Cage (Figure 2) to decrease interference from external electromagnetic fields. The measurements are typically obtained over the range of 100kHz to 0.01Hz and the |Z| value reported at 0.1Hz. The qualifier of laboratory data is included here since there are efforts to use this technique in field applications that would limit the availability of a Faraday Cage.
Testing performed by T. Gichuhi, et al rates the resistive (barrier) properties of coatings using EIS data as follows:
Excellent – >108 ohms·cm2
Good – 107 – 108 ohms·cm2
Fair – 106 – 107 ohms·cm2
Poor – <107 ohms·cm2
Today’s coating materials/systems can easily surpass the 5,000 hours of salt fog (ASTM B117) exposure that was once used to differentiate the high-performance coating materials in only a few thousand hours. EIS data can be used to determine if the coating materials are starting to degrade in exposure, even if the coating remains in visually good condition. The EIS method can also be used to monitor the coating materials throughout the exposure process since the testing is not destructive. The resulting EIS values can be compared to baseline values to determine when the coating is beginning to show signs of chemical degradation through the loss of barrier properties.
 Gichuhi, T., Balgeman, A., Prince, S., Wagner, C., O’Brien, S., and Adams, A., Employing Electrochemical Impedance in Predicting Corrosion Events,” JCT CoatingsTech, 8, 32–38 (June 2011)