Metal Finishing Guide Book

2012-2013

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100% Relative Humidity (ASTM D 2247) This test has wide applicability for protective coatings that are exposed indoors, in sheltered areas, or in areas where water (condensation) can accumulate on the surface of the specimen. Many variations of this test are employed to more accurately reflect service conditions. These include varying humidity levels from 60 to 95% relative humidity; cycling humidity with periodic dips into corrosive liquids (ASTM G 60); cycling humidity with drying cycles for coatings on wood (ASTM D 3459); and combining humidity with severe temperature fluctuation (ASTM D 2246). The corrosion mechanism employed is oxygen concentration cell, accelerated by high humidity level, elevated temperature, and inclination of test specimen. ASTM B 287 (Acetic Acid-Salt Spray) This test is intended for coating systems that provide excellent corrosion-protection results in long-term salt spray (B 117) exposure. This test is approximately twice as severe as the salt fog (B 117) test, although this may vary significantly with each application. The corrosion mechanism employed is oxygen concentration cell accelerated by operation at lowered pH (3.1-3.3 vs. 6.5-7.2 for the salt fog test); use of an electrolyte with a chloride content of 5% weight (more has been shown to be unnecessary); elevated temperature; inclination of test specimen; and utilization of a finefog mist. Copper Accelerated Salt Spray (CASS) (ASTM B 368) This test was developed for use on copper-nickel-chromium coatings over ferrous and nonferrous substrates. The oxygen concentration cell corrosion mechanism is accelerated by operation at lowered pH (3.1-3.3 vs. 6.5-7.2 for the salt fog test); use of an electrolyte with a chloride content of 5% weight (more has been shown to be unnecessary); elevated temperature; inclination of test specimen; utilization of a fine-fog mist; and the addition of cupric chloride to provide galvanic effects. Corrodkote (ASTM B 380) This test was also developed for specific use on copper-nickel-chromium coatings on ferrous and nonferrous substrates. The corrosion mechanisms employed are oxygen concentration cell and galvanic effects produced by cupric and ferric ions, plus complicated chemical effects produced by the nitrate, chloride, and ammonium ions. The test utilizes a kaolin paste that holds the corrosive ions to the surface of the test in a ���poultice��� fashion, similar to accumulated dirt and scale on exterior automotive parts. FACT (ASTM B 538) This test is limited to anodized aluminum coatings and is basically a porosity test used to obtain rapid corrosion protection results based on the porosity level found. Substantial service background is needed to correlate test data with service life for any specific application. The test consists of an electrolytic cell with salt spray (ASTM B 117) or CASS (ASTM B 368) solution as an electrolyte. The cell is placed onto the test specimen with a gasket to prevent leakage. A potential is applied between a platinum anode and the test specimen (cathode). As hydrogen is discharged from the cathodic sites (pores), alkalinity is developed, 562

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