Electromagnetic material characteristics of steel grades used in continuous hot-dip galvanization and their impact on the coating thickness estimation bias in eddy current testing

Precise determination of coating thickness of metallic-coated steel sheets is essential for various industrial applications. This study explores the parameter estimation of hot-dip galvanized steel by an eddy current testing method, employing a multi-frequency model-based estimation approach. While the method performs well for linear substrate materials, systematic errors arise when the substrate material exhibits magnetic nonlinearity. To quantify these deviations, finite element analyses are conducted, followed by thickness estimation using the well-known eddy current model, which assumes linear material. The focus is on steel grades commonly used in continuous hot-dip galvanizing processes, including titanium-stabilized interstitial-free, dual-phase, and micro-alloyed steel, with the aim of quantifying the worst-case estimation bias. The electrical properties of the materials are characterized using the van der Pauw method, their magnetic properties through measurement of ring-shaped specimens. The findings provide insights into the impact of material nonlinearity on coating thickness estimation and contribute to improving the accuracy of eddy current testing in industrial applications.