Unified equivalent circuit of dielectric permittivity and porous coating formalisms for EIS probing of thick industrial grade coatings

Abstract

The equivalent circuit Re(Q1[R1(Q2R2)]) was fitted to the impedance spectra of the two water based and the two solvent based industrial coating systems measured during long term exposure to 3.5 % NaCl. The initial states of the samples were the intact state and the salt cabinet aged state. The elements of Re(Q1[R1(Q2R2)]) were ascribed physical meaning within the dielectric permittivity and EIS porous coating formalisms. A real capacitor behavior of a coating was simulated by a lossless capacitor in series with a resistance and the distributed time-constant behavior was simulated by a set of parallel circuits representing either space related or frequency related heterogeneity of the system. The equivalent circuit parameters and the impedance modulus at 0.1 Hz reflected various degrees of coatings sensitivity to environmental conditions that correlated with the harshness and length of exposure. The additional parameters suggested for rating of the coating barrier properties were tan⁡δ, equal to the resistive to capacitive current ratio, and its response to various cathodic offset potentials indicating the retention or loss of wet adhesion. The compromised barrier property of one of the water based coatings was deduced from a continuously decreasing low-frequency impedance and the appearance of the low frequency tan⁡δ peak that increased upon cathodic polarization. These observations were linked to blistering, adhesion strength decrease, corrosion of the substrate revealed after the adhesion test, cratering defects at the coating surface and pronounced rust leaching at the sample edges and welds that were lacking the stripe coats.