Metal Finishing Guide Book


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troubleshooting, testing, & analysis COATING THICKNESS MEASUREMENT DAVID BEAMISH DEFELSKO CORP., OGDENSBURG, N.Y. A variety of recognized methods can be used to determine the thickness of organic coatings. The method employed in a specific situation is most often determined by the type of coating and substrate, the thickness range of the coating, the size and shape of the part, and economics. Commonly used measuring techniques are (1) nondestructive dry-film methods such as magnetic, eddy current,ultrasonic, or micrometer measurement; (2) destructive dry-film methods such as cross sectioning or gravimetric (mass) measurement; and (3) wet-film measurement. MAGNETIC The magnetic technique is used to nondestructively measure the thickness of nonmagnetic coatings on ferrous substrates. Most coatings on steel and iron are measured this way. Gauges use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction. Magnetic Pull-Off These gauges consist of a permanent magnet, a calibrated spring, and a graduated scale. The attractive force between the magnet and the magnetized steel pulls the two together. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. Coating thickness is determined by measuring this pull-off force. The weaker the force, the thicker the coating. Testing is sensitive to surface roughness, curvature, substrate thickness, and alloy content. Magnetic pull-off gauges are rugged, simple, inexpensive, portable, and usually do not require any calibration adjustment. They are the instruments of choice when a low number of readings per day is required. Pull-off gauges are typically classified as either pencil-type or roll-back dial models. Pencil-type models mount a magnet to a helical spring (see Fig. 1). The spring acts perpendicularly to the surface to pull the magnet off. Most have large magnets and are designed to work in only one or two positions, which partially compensate for gravity. A more accurate version is available, which has a tiny, precise magnet to measure on small, hot, Fig.1 Pencil-type magnetic pull-off thickness gauge. 547

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