Single-frequency apparent eddy current conductivity assessment of metallic coating thicknesses over nonmagnetic metals

摘要

Advantages offered through eddy current technology provide a promising solution to assess metallic coating thicknesses over nonmagnetic metals. Existing single-and multi-frequency impedance-based eddy current techniques are sensitive to both coating/substrate combinations and lift-off deviations from those used over the coated calibration blocks. The adoption of apparent eddy current conductivity (AECC) spectroscopy demonstrated the potential benefits to overcome these limitations and deliver one-order of magnitude improvement in coating thickness estimation within a +/- 25.4 mu m lift-off range. To meet the plane-wave approximation, this technique requires capturing the AECC spectrum over a broad frequency range using a relatively large coil design, which makes the technique less practical. This study takes the AECC-based technology a step further to operate at a single frequency in assessing metallic coating thicknesses of such nonmagnetic-layered structures. The criteria for reducing the coil diameter and selecting the inspection frequency are demonstrated in agreement with the plane-wave approximation and COMSOL simulations. Moreover, a new AECC-based inversion model is developed and validated for single-frequency AECC measurements over different coating thicknesses relevant to the industry.