Efficient Eddy Current Models for Evaluation of Thin Conductive Coatings on Ferromagnetic Substrates

Abstract

<p>Eddy current testing is widely used to determine physical characteristics of materials and to detect flaws by measurements of the electrical impedance of an eddy current probe. In this paper two analytical models allowing to determine properties of non-magnetic conductive coatings on ferromagnetic conductive substrates, are reported. Operating at a single frequency, two following quantities can be determined: permeability-to-conductivity ratio of the substrate and thickness-conductivity product of the coating [1, 2]. The method was validated using both long solenoids and air core surface coils, and was applied to the evaluation of zinc coatings on steel wires and sheets. The theoretical solutions given for high arguments are compact, and allow fast inversion, respectively around 400 and 10 ms for a pancake surface coil and for a long encircling solenoid. Two series of samples: Ø2.2 mm low carbon steel electro galvanized wires and 0.75–20 mm thick hot dip galvanized sheets, were inspected. Steel sheet samples with artificial coatings, as aluminum foils glued from both sides, were also examined. Experimental data of the coil electrical impedance were compared to those predicted. Agreement between theory and experiment is excellent. The technique developed has an extremely low sensitivity to the substrate conductivity and permeability variations [2]. A DC magnetic field, significantly diminishing the permeability of the substrate, almost does not influence results of the coating thickness determination. The agreement between measured thickness and that obtained by other methods is excellent. The accuracy of the thickness determination typically about 1 μ is obtained.</p>