Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 494 of 707

cadmium deposits with the chromic acid/sulfuric acid reagent. Lacquer or other films are removed from the area to be tested, which is then cleaned with a suspension of magnesium oxide in water. The specimen is held at an angle of 45O below the dropping tip. To ensure that the reagent impinges on the same spot throughout the test, it is preferable to clamp the specimen in place rather than to hold it by hand. The apparatus may consist of a separatory or dropping funnel, which is fitted with a short length of tubing terminating in a drawn-out tip. Special funnels are available with two stopcocks. One of these is fully opened and the other used to set the desired drop rate. An automatic drop-test apparatus is available in which the drop rate is automatically maintained at 100 drops per minute by means of a synchronous motor-driven mechanism. EDDY CURRENT Eddy current thickness gauges are electromagnetic instruments designed to measure the apparent change in impedance of the coil that induces the eddy cur- rent into the base metal. They are calibrated by comparing the apparent change in impedance to known thickness of coatings on selected base materials. It is the difference between the conductivity of the base material and the coating that influences the change in impedance; therefore, the instrument has its greatest accuracy when testing nonconductive coatings on conductive base 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 avail- able 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 approx- imate 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 alu- minum, nonmetallic coatings on nonmagnetic base metals, and to a lesser extent, to metallic coating/substrate combinations that have different electrical conduc- tivities. 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. 493

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