A novel approach for metallic coating detection through analogizing between coil impedance and plane wave impedance

For conductive plates with tens of micrometres of metallic coating, a novel approach is proposed to simultaneously measure the coating thickness, coating conductivity and substrate conductivity by analogizing eddy current testing to the parameter measurement process using the apparent conductivity in the field of magnetotelluric sounding. A normalisation method is presented for both coil impedance and plane wave impedance and the approximation relationship between them is theoretically deduced under a certain condition. Comparing the experimental curve of the equivalent conductivity of coil impedance measured by swept-frequency eddy current testing and theoretical curves of normalised apparent conductivity of plane wave impedance, all parameters of the coated plate can be measured. Two measurement experiments were performed on a 304 stainless steel plate with titanium coating, and the results show that all relative measurement errors are less than 3% for the plate with 32 μm-titanium coating and 5% for the plate with 23 μm-titanium coating. The effect of the difference between the coating and substrate conductivity values on the measurement error, and the selection of measurement frequency range were discussed.