Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 492 of 707

DROP TESTS The drop test for measuring plating thickness is based on the rate of attack of spe- cially prepared corrosive solutions on a metal coating. For consistent results, the drop size, drop rate, temperature, time, and solution composition must be con- trolled. The test is performed by allowing the solution to drop on a particular spot at a rate of 100 drops per minute. The operator then observes the time at which the coating is penetrated. For the most common thicknesses, the elapsed time is less than one minute. Reproducibility of the test is dependent on the skill of the operator. The operator must detect the point at which the base metal is exposed and record the time. An experienced operator can reproduce his readings within ±5%. For best accuracy, the operator should standardize his technique with a standard of known thickness. The accuracy of the system is generally considered to be ±15%, because the oper- ator cannot control some of the factors that affect the test. These factors include drainage of the solution, alloying at the coating/substrate interface, and com- position of the coating. Low cost and the ability to measure thickness quickly on irregular shapes are the chief advantages of the drop test. The greatest disadvantages are destruction of the coating and objections to the use of corrosive solutions in some areas. The largest application is in the fastener industry. Standard guidelines for the use of this test are contained in ASTM B 555. 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 .6H2 O), 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 30O C (70 to 90O F). 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 491

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