Eddy current evaluation of metallic coating thicknesses over nonmagnetic metals

The use of titanium and nickel alloys in metal coating technologies have been gaining rapid popularity due to their high strength characteristics and excellent corrosion resistance behaviour operating at different temperature ranges. Phase-sensitive eddy current technique set by the International Organization for Standardization to assess metal coating thicknesses over nonmagnetic metals delivers ±30% uncertainty in ±25.4 μm lift-off variation from that used over metal-coated calibration blocks. This study takes the first step towards adopting AECC spectroscopy as an alternative forward measurement technique, which offers a reduced sensitivity to lift-off and sample conductivity deviations from those used over the calibration blocks in a broad range of inspection frequencies. To map the depth-dependent conductivity profile from the frequency-dependent AECC measurement, a new AECC-based inversion algorithm is developed to estimate metallic coating thicknesses over nonmagnetic metals. This work is validated following the plane-wave approximation and COMSOL simulations over different coating thicknesses relevant to the industry. Results indicate that AECC spectroscopy can potentially deliver ±3% uncertainty in metallic coating estimation in ±25.4 μm lift-off range. That is a significant improvement over existing eddy current measurement methods.