Metal Finishing Guide Book


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X-RAY FLUORESCENCE This method is similar to beta backscatter in that the area to be tested is the target of radiation, and the energy emitted from that surface is measured. In this method, the radiation used is X-rays produced by an X-ray tube. The radiation measured is secondary emissions from the interaction of the X-rays with the coating and substrate. Unlike beta backscatter, the emissions measured are specific for each metal. Among the unique characteristics of this method are the following: 1. No physical contact with the measured surface is required, thereby protecting that surface. 2. Extremely small areas may be measured. 3. Since the emissions are specific for each metal, alloy compositions may be determined. 4. With proper calibration, intermediate coatings may be measured in a multilayer system. Thickness may be measured in the range 0.25 to 10 ��m, depending on the metal being measured. With proper calibration, the thickness may be determined to within 10% of its true value. ASTM B 568 outlines a standard method for this measurement system. Due to the noncontacting characteristic and the rapid test time, this procedure is used to continuously monitor thickness on continuous-coil plating machines and automated plating machines. STANDARDS Thickness standards are required for calibrating thickness gauges. In most cases, the standards should be plated from a similar plating solution and on the same base metal as the product to be tested. After receipt, it is important to have a system to ensure the standard���s reliability after use. One system makes use of primary, secondary, and working standards. The working standards are used for calibrating the thickness gauge daily. Once a week, the working standards are calibrated against the secondary standards. The secondary standards are then calibrated against the primary standard once a month. The time between calibrations can be varied based on experience with the expected life of the standard. When a new standard is purchased, it becomes the primary standard, and the remaining standards are lowered in rank. SUMMARY To measure the thickness of a coating, many properties have been utilized. Measurements have been described that use the atomic configuration, electrochemical equivalent, corrosion rate, electrical conductivity, magnetic properties, density, and actual linear measure of the coating. In addition to these methods, tests have been successfully performed by using the electrical resistance and transparency of the coating. Each of these methods has its proper application. It is obvious that one system is not capable of satisfying the needs of every plater. Certainly, a plater coating 10,000 fasteners per barrel load at a rate of 50 barrel loads per shift has a problem that is substantially different from a plater that hard chromium plates printing rolls. Experience and judgment are required, and the responsibility for choosing the most economical thicknessmeasuring system adequate for a particular problem is not a simple task. 557

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