Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 496 of 707

1. Weigh coating directly after dissolving the base material without attack- ing 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 cor- rosive solutions and to measure the mass and area with sufficient accuracy. After the area and mass have been determined, the thickness may by deter- mined 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. MICROMETRIC A micrometer may be used to check the thickness of coatings over 0.001 in. If the micrometer is equipped with a dial indicator, thicknesses of 0.0005 in. may be measured on uniformly coated cylindrical parts. It is necessary to measure the same spot before and after plating. Measurements may be obtained by masking the deposit and dissolving the unmasked coating, then measuring the step produced by this procedure. MICROSCOPIC The microscope can be used as a length measuring instrument when it is equipped with a filar eyepiece. The specimen must be carefully polished to pre- pare a smooth reflective surface and then etched to reveal the various metals exposed. This generally requires the services of an experienced metallographer. The measurements are generally made on a transverse section of the deposit so that the various layers of plating are exposed for viewing. Measurements are also made on tapered sections to increase the length to be measured. The thick- ness is then calculated by correcting the observed length for the taper angle. This method is destructive and time consuming. The thickness results have a precision of ±2%; however, the accuracy has a constant uncertainty of about 0.8 µm (30 microinches). Therefore, despite the precision of the method, it should not be considered as a reference to resolve questions about thicknesses less than 8 µm (300 microinches). The filar eyepiece is calibrated by comparing the divisions on the filar microm- eter to the known distance between divisions on a stage micrometer. This method is a true measure of length and does not require a plated standard for cal- ibration. Due to the high cost of this technique, its use has been reserved for those occa- 495

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