APPENDIX B
Eddy Current
Theory
The eddy current technique is utilized for measuring nonconductive coatings on
nonmagnetic metal substrates, metallic coatings on ferrous substrates and
conductive coatings on nonconductive substrates. The eddy current measuring
probe contains a coil that produces an oscillating electromagnetic field. When
the probe is applied to the surface of the coating being tested, a flow of eddy
currents is induced in both the coating and the substrate of the sample. Eddy
currents flow in closed circular paths and the depth of penetration is inversely
proportional to the square root of the probe frequency and also to the square root
of the electrical conductivity of the material in which the eddy currents flow. The
eddy currents create a counter-magnetic field that reacts upon the probe by
reactance and alters its output voltage. The change in output voltage created is
used to calculate a thickness. In the case of nonconductive films over
nonmagnetic metals (lift-off), increasing the thickness of the coating increases
the separation from the conductive substrate. This increased separation causes
a decrease in the magnitude of the eddy currents induced in the base, and
therefore a decrease in output voltage of the probe. In the measurement of
metallic coatings on ferrous substrates, the eddy currents flow in both the coating
and the base. The intensity of the eddy current reaction depends on the
conductivity of both metals involved. As the coating thickness increases, more of
the eddy currents flow in the coating than in the base metal, and the reaction on
the probe corresponds more closely to that of the pure coating metal.
Base Variations
Calibrating an eddy current probe for a nonconductive over conductive
application, commonly referred to as lift-off, consists of placing nonconductive
shims of known thicknesses directly over the bare conductive substrate. Since
all of the eddy currents are flowing through the base, this application is very
susceptible to changes in the base. Because of this, calibrations must be made
on a bare sample of the part being measured.
Base Metal Thickness
If the base metal thickness is less than infinite, any variations in the thickness of
the base will affect the coating measurements. The magnitude of this effect
decreases with increasing coating thickness. Limit the effect of the base
variations as follows. Take numerous readings on the bare sample during the
calibration to average out the base variations. When taking measurement on the
coated samples, several measurements must be also taken over the entire
surface of the sample to determine the average coating thickness.
Inherent Base Variation
Base variation can also be due to inherent variations in electrical conductivity or
magnetic permeability due to the manufacturing processes. To determine the
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