4.1 This test method is suitable for in-service condition assessment and quality control (QC) testing.

4.2 This technique is used to investigate a polymer barrier coating over a conductive substrate and is limited to exposed and accessible coating surfaces.

4.3 This test method is applicable to polymer barrier coatings of all thicknesses provided the impedance is within equipment capabilities. Special considerations are required for evaluation of coatings exceeding 2 mm thickness or containing conductive media, such as metal pigments and conducting polymers.

4.4 This test method provides the experimental method needed to ensure proper application of field EIS testing and reporting of its results. This test method uses two test cells per measurement with no electrical connection to the substrate (1-4) (a deviation from the traditional three-electrode measurement) to prevent the need for electrical connection to the underlying structure.

Note 1: The two-test-cell method measures the impedances beneath the two cells plus the impedance of the path between them. This arrangement has additional risks of false negatives/positives that are not encountered using the traditional three-electrode measurement in which an electrical connection to the substrate is made. For this test method, a false positive is defined as a higher impedance value than is typical for the coating, and a false negative is defined as a lower impedance value than is typical for the coating. A traditional three-electrode measurement in the field is possible, but a reliable electrical connection to the substrate can be challenging and may require damage to an otherwise good coating.

4.5 This test method may be used at any time during the life of a coating system. If used for QC, allow for any manufacturer's recommended cure or drying time unless otherwise agreed upon between the participating parties.

Note 2: The results obtained by using this test method could be used for informed coating maintenance decisions, for example, whether to replace a coating system, and may also be applicable as QC measurement for coating contracts.

4.6 The results obtained by using this test method shall not be considered as a means for estimating the structural properties of the underlying structure.

4.7 The results obtained by using this test method do not measure the corrosion susceptibility of the underlying structure because it uses two test cells with no electrical connection to the substrate. The open circuit potential and voltage perturbation are applied to the test cell electrode, per 5.3, and not the underlying structure.

4.8 The electrochemical impedance measurements shall be interpreted by engineers or technical specialists experienced in the fields of protective coatings and corrosion testing. It is often necessary to use other data such as visual inspection, dry film thickness, and adhesion testing, in addition to electrochemical impedance, to formulate conclusions concerning corrosion activity of the underlying structure or the remaining service life of the coating system. See Test Methods D660, D661, D714, or Practice D610 for more information on coating visual inspection.