About the EIS Technique
Typical standard coating immersion tests can take hundreds or thousands of hours. However, the electrochemical impedance spectroscopy (EIS) technique can provide reliable quantitative data in rather short time.
The electrical impedance of a coating is the most direct measure of a coating’s barrier properties and ability to protect against corrosion. Electrochemical Impedance coupled with exposure to the intended environmental stressors is the most direct method of establishing the quality of corrosion preventative coatings.
Electrochemical Impedance measurements
EIS technique is used to evaluate materials and coatings in corrosive environments. EIS measures impedance of the coating at different frequencies. This is an indication of barrier effect of the coating. Barrier coating will exhibit high impedance in corrosive exposures.
Changes in electrical properties determined by EIS experiments have been found to closely resemble long-term performance of coatings. This way EIS method can detect deterioration at metal/coating interfaces well before defects become visible, and with greater accuracy than artificial testing methods such as salt spray.
Percentage of Ideal Protection
The new terminology of primary EIS measure used for comparisons between coatings as EIS measurements are made during the course of an environmental exposure is the “percent of ideal protection.” This is the area under the log (impedance) – log (frequency) plot expressed as a fraction of the initial area, which is measured before initiating the environmental exposure.
The percentage of protection measure is used to summarize the changing frequency- dependent impedance of a coating, which reflects both the water, corrosive ions penetration and the low frequency impedance which approximates the pore resistance. In traditional EIS measurement, an estimate of the low frequency impedance is given. Therefore, the percentage of ideal protection forms a reliable method of high performance durable coating evaluation and rating.
It has been successful for us to rate and rank the high performance coatings in most cases using this method during the course of an environmental exposure. In general, 80% or above EIS protection indicates a sufficient protection according to our experience. The following examples exhibit how the EIS can be e protective organic coatings.
In summary, EIS is an excellent cost effective technique that can provide information on barrier properties, permeability, in addition to information that can be used for determining remaining life based on electrical properties of organic coatings. It is capable to determine the effectiveness of coatings in corrosive application in rather short times.
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