Characterization of Coatings on Magnetic Metals Using Swept-Frequency Eddy Current and Transient Eddy Current Methods
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Begun in 1973, the Review of Progress in Quantitative Nondestructive Evaluation (QNDE) is the premier international NDE meeting designed to provide an interface between research and early engineering through the presentation of current ideas and results focused on facilitating a rapid transfer to engineering development.
This site provides free, public access to papers presented at the annual QNDE conference between 1983 and 1999, and abstracts for papers presented at the conference since 2001.
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Abstract
Eddy currents can be used to characterize the conductivity and thickness of coatings on metals. However, when the same techniques were applied to magnetic metals, some uncertainties were found. We have discovered that the broadband behavior of eddy current coils in proximity to ferromagnetic surfaces depends dramatically upon very thin surface layers. For nickel, we found a 10∼100 micrometers thick dead layer at the surface that reduces the apparent relative magnetic permeability substantially [1]. Conversely, this extreme sensitivity to surface conditions means that measurement methods can be devised that will be sensitive to very thin surface coatings, on the order of a few micrometers thick or less.