Measurement of Plating Thickness with High Liftoff Using Eddy Current Testing

This study deals with measurement of the thickness of a zinc layer deposited on steel in the process of zinc plating. A short distance between the blower that blows liquid zinc and the sensor that measures the thickness of the layer, and a long distance between the sensor and test samples are required in the process. We attempted to apply eddy current testing to measure the thickness of the zinc layer with high liftoff. This paper demonstrates the eddy current distribution calculated using the finite element method and the measured liftoff and thickness dependencies at each excitation frequency of an eddy current sensor. The frequency that results in a phase difference of approximately 90° between dependencies was determined. This indicates that the thickness and liftoff length signals can be separated from the measured signal when an appropriate excitation frequency is selected. These results demonstrate the possibility of making thickness measurements using an eddy current with high liftoff.