A Simple Formula for Calculating the Coating Thickness of Galvanized Steel Wires

This paper presents the development of a simple but accurate formula to obtain the coating thickness of galvanized steel wires from the impedance of a solenoid containing a sample of the wire. The basic assumption behind the formula is that the skin depth of eddy currents in the coating is much larger than the coating thickness, which can be obtained by a proper frequency selection. The results of the formula compare very well with results of the exact solution, leading to errors <;0.08% in the normalized solenoid impedance, for coating thicknesses up to 80 μm. A simplified version of this formula is also presented, which led to errors <;1.3% in the normalized impedance, for coating thicknesses up to 80 μm. This simplified formula provides the coating thickness directly from the input data by performing some arithmetic operations. The formulas are used in the analysis of experimental data available in this paper, and the results are consistent with the uncertainties of the input data. A better representation of the experimental data was obtained by postulating a thin-intermediate layer between steel and zinc. The developed formulas are useful in the implementation of nondestructive testing methods based on eddy current inspection.