Metal Finishing Guide Book

2013

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Commercial instruments are available with analog and/or digital thickness display, memory, hard copy printout, and computer prompting of calibration procedures. The effective thickness range is dependent on the choice of probe system (magnet or reluctance) and the coating/substrate combination. The ranges for the magnet type are: 1. Nonmagnetic coating on magnetic base, 5 to 25 μm. 2. Nickel coating on magnetic base, 5 to 50 μm. 3. Nickel coating on nonmagnetic base, 5 to 25 μm. The ranges for the reluctance type are from 5 μm to 1 μm for all three coating/substrate combinations. Both types of instruments are sensitive to marked differences in the surface contour of the test specimen. Particular attention should be given to the distance from an edge, surface roughness, and curvature at the testing point. Erroneous results may be avoided by calibrating with standards that approximate the surface condition and geometry of the specimen to be tested. When properly calibrated, the magnetic system can determine the actual thickness within 10%. The instrument manufacturer's instructions should be carefully followed for the most accurate results. Two standard methods have been developed for additional guidance to more reliable readings. They are ASTM B 499 and ASTM B 530. GRAVIMETRIC This method requires the measurement of the area to be tested and the determination of the mass of the coating in that area. The area may be determined by standard measuring techniques. The coating mass may be determined by one of the following procedures: 1. Weigh coating directly after dissolving the base material without attacking the coating. 2. Determine coating mass by analyzing the solution used to dissolve the coating and all or a portion of the base material. 3. Determine coating mass as the difference between the weight before and after dissolving the coating without attacking the base material. 4. Determine coating mass as the difference between the weight before and after coating. Usually this method is assigned to a laboratory equipped to handle the corrosive solutions and to measure the mass and area with sufficient accuracy. After the area and mass have been determined, the thickness may by determined by using Equation (1). Procedures 1, 2, and 3 are destructive; procedure 4 is nondestructive. This method has the capacity to yield extremely accurate results and is frequently used to determine the exact mass of metal used for cost purposes, particularly with the more precious metals. This method (procedure 4) is used as a technique for making thickness standards. ASTM has developed procedures for several specific coatings. They are Method A 90 for zinc, Method A 309 for terneplate, Method A 630 for tin plate, and Method B 137 for anodized aluminum. ASTM B 767 serves as a guide for use of this procedure for a variety of plating and substrate combinations. 503

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