Metal Finishing Guide Book

2013

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materials and vice versa. The test can be applied to poor electrical conductors over good electrical conductors with some loss in accuracy. The thickness test is performed with the aid of a specially designed probe. Measurements are made by holding the probe perpendicular to the surface and with the probe point in contact with the area to be measured. The measurements are rapid and nondestructive; therefore, some problems with accuracy may be resolved by statistical evaluation of many readings. Thickness gauges are available with digital display, memory, hard-copy printout, and computer prompting of the calibration procedure. In the range of 5 to 50 μm, the thickness can be determined to within 10% or 1 μm, whichever is greater, of the true thickness. This test is sensitive to marked differences in the surface contour of the test specimen. Particular attention should be paid to the distance from an edge, surface roughness, and curvature at a test point. Erroneous results may be avoided by calibrating with standards that approximate the surface condition and curvature of the specimen to be tested. The type of electroplating solution used can influence the electrical conductivity of the deposit and, therefore, the thickness measured. Zinc plated in cyanide, chloride, or alkaline baths is the most prominent example of this problem. Calibrating the instrument with standards from the same or similar solutions can greatly reduce error. Eddy current thickness testing is widely applied to anodic coatings on aluminum, nonmetallic coatings on nonmagnetic base metals, and to a lesser extent, to metallic coating/substrate combinations that have different electrical conductivities. The instrument manufacturers' instructions should be followed precisely for best results. A standard method for the application and performance of this test is available in ASTM B 244. MAGNETIC METHOD The magnetic method uses the magnetic influence of the coating and substrate on a probe as the basis of a measuring system. Two types of probe systems are in common use. The first to be developed makes use of a mechanical system to measure the influence of the coating thickness on the attractive force between a magnet and the base material. An electromagnetic probe was later developed that measures the influence of the coating thickness on the reluctance of a magnetic flux path through the coating and base material. Three types of coatings can be measured with this system: 1. Nonmagnetic coatings on ferromagnetic base metals. 2. Nickel coatings on ferromagnetic base metals. 3. Nickel coatings on nonmagnetic base materials. The test is performed with the aid of specially designed probes. With the permanent magnet type, measurements are made by placing the probe perpendicular to the surface to be measured and observing the force necessary to remove the probe. The electromagnetic-type probe requires placing the probe perpendicular to the surface to be measured and observing the reluctance measurement. The measurements are rapid and nondestructive; therefore, some problems with accuracy may be resolved by the statistical evaluation of many readings. 502

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