Metal Finishing Guide Book

2013

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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 prepare 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 thickness 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 micrometer 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 calibration. Due to the high cost of this technique, its use has been reserved for those occasions that require more information than may be obtained from other thickness-gauging methods. Information concerning porosity, surface roughness, grain structure, and adhesion may be gathered from the specimen prepared for a thickness test. ASTM B 487 is a standard method outlining the conditions for accurate results. The wavelength of light limits the resolution of the light microscope to about 10 microinches. The scanning electron microscope utilizes the shorter wavelength of electron waves to measure metallurgical specimens with a 4 microinch resolution. ASTM B 748 is the standard test method for this procedure. Attachments are available for the metallurgical microscope that allow it to be used as an interferometer. The method is mentioned, because its accuracy with thin coatings can be within ±5% of the true thickness. Conditions necessary for this procedure are contained in ASTM Standard Method B 588. THE SPOT TEST This test was developed as a rapid and inexpensive thickness test for chromium coatings on nickel and stainless steel. The test has an accuracy of ±20% for coatings up to 1.2 μm thick. A wax ring is outlined on the part to be tested. A drop of hydrochloric acid is placed in the ring, and the time between the onset and cessation of gassing is recorded. ASTM B 556 provides a standard guide for the performance of this test. 504

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