ABSTRACT

This research evaluates laboratory data collected in an environmental chamber using the new ASTM D8370 Field Measurement of Electrochemical Impedance on Coatings and Linings test method. The study simulates field testing conditions by placing the field potentiostat and sample within controlled environments, such as an environmental chamber. The experiments controlled for relative humidity (RH) and temperature independently across 14% RH to 100% RH and 35 F (2 C) to 115 F (46 C), respectively, to evaluate the effect of environment on impedance data. The coating systems evaluated were solvent-borne epoxy, 100% solids epoxy, polysulfide-modified epoxy, and solution vinyl. The results showed the strongest effect for temperature, with each coating type exhibiting impedance values changing inversely with temperature.

INTRODUCTION

The recent development of ASTM D8370-22 provides a field-applicable technique for measuring impedance on protective coatings.1 The test method expands the use of electrochemical impedance spectroscopy (EIS) beyond the laboratory and standardizes the approach for various applications to polymeric coatings on conductive substrates, e.g., barrier coatings on steel structures. Example applications include condition assessments and quality control testing. The interpretation of test method results requires an experienced engineer or technical specialist to properly account for the test conditions and confirm the data is valid.

Comparison of Traditional EIS to Field EIS Testing

The field test method has two primary differences from traditional laboratory EIS testing. The first is a configurational change in the EIS setup. Traditional three-electrode EIS uses a single test cell containing a reference and counter electrodes (RE and CE). The conductive substrate is the working electrode (WE), which is the location of the electrochemical evaluation in EIS testing. Therefore, the data provides the electrochemical (corrosion) response of the small voltage perturbations applied to the working electrode. The voltage perturbations are kept within the linear region on potential-current plot to minimize polarization of the WE and changes to the reaction kinetics.

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