Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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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 mea- sure the influence of the coating thickness on the attractive force between a mag- net 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 coat- ing 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 per- manent magnet type, measurements are made by placing the probe perpendicu- lar 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. Commercial instruments are available with analog and/or digital thickness dis- play, memory, hard copy printout, and computer prompting of calibration pro- cedures. 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 coat- ing/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 avoid- ed 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 manu- facturer's instructions should be carefully followed for the most accurate results. Two standard methods have been developed for additional guidance to more reli- able readings. They are ASTM B 499 and ASTM B 530. GRAVIMETRIC This method requires the measurement of the area to be tested and the deter- mination 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: 494

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