Pulsed eddy-current measurement of a conducting coating on a magnetic metal plate

A new measurement method has been developed using the pulsed eddy-current (PEC) technique to determine the thickness or conductivity of metallic coatings on a metal substrate for the case when either the coating or substrate is magnetic. We demonstrate this technique for nickel layers (25-200 mum) over copper substrates, copper layers (25-200,mum) over nickel substrates and zinc layers (50-400 mum) over magnetic steel substrates. The theoretical current difference was calculated using the method developed by Tai et al (Tai C C, Rose J H and Moulder J C 1996 Rev. Sci. Instrum. 67 3965-72). All current difference measurements were recorded with a PEC instrument. The determination of coating thickness or conductivity of the metals is based on the comparison of the data taken with air-core coils and theoretical calculation using closed-form solutions. The physical phenomena of the PEC'S interactions with the coated magnetic metals are also discussed.