Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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control, analysis, and testing Accurate Thickness Testing Via Phase-Sensitive Eddy Current BY MIKE JUSTICE, PRESIDENT, UPA TECHNOLOGY, WEST CHESTER, OHIO With the advancements in science and electronics in recent years, metal finish- ers currently have many choices for the measurement of plating and coating thick- ness. This article focuses on using the phase sensitive eddy current method. Phase-sensitive eddy current (PSEC) is an instantaneous, non-destructive and contact method used worldwide for the thickness measurement of many dif- ferent coatings and platings applications. PSEC measures coating thickness based on the conductivity difference between the coating and the base material. (A conductivity difference of 20% or more is required in order to measure using this technique.) Eddy current is designated by ASTM B659-90, which states "that the magnitude of eddy currents is a function of the relative electrical conductivities of the coating and substrate materials and the coating thickness." Specially designed eddy current probes can be chosen according to the appli- cation thickness range of the coating, and the size of the parts being measured. Measurements are made by holding the probe perpendicular to the part with the probe tip in contact to the surface. There are actually two types of eddy current techniques that are used for measuring coating thickness. The most common type is "amplitude" eddy cur- rent, which is capable only of measuring non-conductive coatings over conduc- tive (non-ferrous) substrates. The most common applications for amplitude eddy current are for measuring anodizing and organic finishes on aluminum sub- strates. While the phase-sensitive eddy current technique also measures anodizing, the real advantage comes in the possibility of measuring many different applications, even with the same probe. Since PSEC measurements are based on having at least a 20% difference in conductivity between the coating and substrate, the variation of applications is almost unlimited. We could not list all of the various applications that PSEC is capable of mea- suring, but some of the many possibilities include: Ni/Fe, Cu/Fe, Ag/SS, Cu/EPG, RFI & EMI coatings on plastic, Silicide/SS, Sn/Fe and Cu/Fe, Anodizing/Al, Al/Glass, Al/SS or Cr/Al. The most common use of PSEC is for measuring zinc, copper or cadmium thickness on steel fasteners. INSTRUMENT CALIBRATION AND THICKNESS STANDARDS Typical applications for measuring plating thickness such as zinc/steel require calibration of the instrument using plated thickness standards. The specific thickness of the calibration standards depends on the frequency of the probe. ASTM specifies using calibration standards identical to the parts being measured. In the case of PSEC, this means making calibration standards having the same char- acteristics in both the coating and substrate as the parts being plated. Most eddy current instruments incorporate a base correction feature that adjusts for different properties of substrates. Differences in plating deposit conductivity can also be corrected by incorporating an algebraic factor. Though 438

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