Thickness evaluation of thermally sprayed coatings after exposure to boiler tube environments by eddy current testing

This paper discusses the feasibility of determining the thickness of Ni-based alloy sprayed coatings that have been heated in plant environments using swept frequency eddy current testing based on the measurement of the impedance spectrum of the measurement coil. Through a comparison of the experimental and computed signals, an electromagnetic model simulating the test geometry including the coating and the substrate is presented, with particular focus on the effects of heat treatment. Using these models, the coating thickness is estimated using the measured impedance spectra, and the estimation error was found to be around 30 mm.