1998/03/01 by A. Ptchelintsev, B. de Halleux
Engineering · Materials Science · #Carbon steel #Coating #Composite material #Conductivity #Corrosion #Eddy current #Electrical conductor #Electrical engineering #Electrical resistivity and conductivity #Electromagnetic coil #Magnetic Properties and Applications #Materials science #Non-Destructive Testing Techniques #Skin effect #Substrate (aquarium) #Welding Techniques and Residual Stresses
paper · doi:10.1063/1.1148784
openalex publication_date 1998/03/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
An eddy current technique for determining parameters of a thin nonmagnetic conductive coating on a ferromagnetic conductive sheet metal is reported. Under several conditions the electrical impedance of a coil is shown to be a function of two independent quantities: the substrate permeability-to-conductivity ratio and the coating thickness-conductivity product. Thus, thickness or conductivity of the coating can be determined independently of the variations of magnetic and electrical properties of the substrate. Simple analytic formulae for the electrical impedance of circular and rectangular surface coils are obtained and applied to predict the behavior of split wound surface coils. The performance of the method was tested in the frequency range 20–100 kHz on 1.5 mm thick low carbon steel sheets coated with 15–45 μm thick aluminum layers and on two series of 0.75–2.0 mm thick hot dip galvanized low carbon steel sheets of 9–20 μm coating thickness. The experiment was carried out using printed surface coils of rectangular and circular shape. Agreement between theory and experiment is excellent. The mathematical inversion carried out using the obtained formulae and the Newton–Raphson method takes about 450 ms on a Pentium 133 MHz PC. Discrepancies between the eddy current thicknesses and those obtained by other techniques were typically below 1 μm. An uncertainty in the thickness or conductivity determination better than 2% is obtained. The method has an extremely low sensitivity to variations in the substrate permeability. A large change in the substrate permeability, implying large changes in the coil electrical impedance, does not significantly influence the determined coating thickness. When the permeability change is about a factor of 10, additional errors in the thickness determination typically do not exceed 0.5 μm.