Metal Finishing Guide Book


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Some typical procedures are provided below. Reagents 1. Cadmium deposits: Ammonium nitrate, 110 g/L Hydrochloric acid, 10 ml/L 2. Zinc deposits: Ammonium nitrate, 100 g/L Nitric acid, 55 ml/L 3. Zinc and cadmium deposits: Chromic acid, 200 g/L Sulfuric acid, 50 g/L 4. Tin deposits: Trichloroacetic acid, 100 g/L 5. Copper deposits: Ferric chloride (FeCl3.6H2O), 450 g/L Antimony trioxide, 20 g/L Hydrochloric acid, 200 ml/L Acetic acid (CP, glacial), 250 ml/L Operating Conditions Drop rate: 90 to 110 drops per minute (100 preferred). Temperature: 20 to 30OC (70 to 90OF). Penetration rate: For zinc and cadmium (using separate reagents listed above), each second = 0.00001 in. For copper deposits, two seconds = 0.00001 in. Figure 1 shows the penetration factor as a function of temperature for testing zinc and cadmium deposits with the chromic acid/sulfuric acid reagent. Lacquer or other films are removed from the area to be tested, which is then cleaned with a suspension of magnesium oxide in water. The specimen is held at an angle of 45O below the dropping tip. To ensure that the reagent impinges on the same spot throughout the test, it is preferable to clamp the specimen in place rather than to hold it by hand. The apparatus may consist of a separatory or dropping funnel, which is fitted with a short length of tubing terminating in a drawn-out tip. Special funnels are available with two stopcocks. One of these is fully opened and the other used to set the desired drop rate. An automatic drop-test apparatus is available in which the drop rate is automatically maintained at 100 drops per minute by means of a synchronous motor-driven mechanism. EDDY CURRENT Eddy current thickness gauges are electromagnetic instruments designed to measure the apparent change in impedance of the coil that induces the eddy current into the base metal. They are calibrated by comparing the apparent change in impedance to known thickness of coatings on selected base materials. It is the difference between the conductivity of the base material and the coating that influences the change in impedance; therefore, the instrument has its greatest accuracy when testing nonconductive coatings on conductive base 501

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