Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 295 of 331

exposure in real-world environments. The development of variations on basic accelerated corrosion tests have allowed engineers, technicians, and quality professionals to better evaluate durability of products to be used in a wider variety of environments, under multiple corrosive influences, and under constantly changing industrial conditions. The issue of consistency in test results has been addressed with the develop- ment of ASTM methods for evaluating consistency in a large or small number of test chambers to ensure repeatability and reproducibility. These standards are ASTM E 691 Practice for Conducting an Inter-laboratory Study to deter- mine the precision of a Test Method and the Mass Loss Corrosion Coupons such as described in ASTM B 117 Appendix X3. A variation of this Mass Loss Cor- rosion Coupon procedure is described in Paragraph 8.7 of ASTM B 368, using nickel coupons. ACCEPTABLE CRITERIA FOR CYCLIC TESTS The American Iron and Steel Institute in cooperation with the Automotive Cor- rosion and Prevention Committee of the SAE conducted a study of existing cyclic corrosion tests in order to develop tests that could be used to rank per- formance of materials used in au- tomotive applications. Approximately 20 existing tests were selected for study. These in- cluded static exposure condition tests, such as ASTM B117, and cyclic exposure condition tests such as ASTM G 85 Annex 5 and GM 9540P, which subject test pieces to a variety of different exposure at- mospheres and conditions over the duration of the test. Ten different test materials were exposed in each test. The results of the tests were compared to real-world exposure results from 10 sites around the globe utilizing duplicates of the same materials. It was concluded from results that an acceptable cor- rosion test must: (1) simulate the rank order of performance of coatings in ac- tual service; (2) produce the same type of failure as found in real-world experience (example:blistering); (3) be reproducible; (4) be of relatively short duration; and (5) be feasible and cost effective. The SAE J-2334, GM-9540P, and ASTM G 85-Annex 5 meet these criteria and are some of the more commonly used cyclic corrosion tests in the U.S. The SAE J-2334 and the GM-9540P were originally developed for testing cosmetic com- ponents on automobiles. Fortuitously, they have been shown also to be applic- able to a much wider variety of materials,finishes, and applications. For example, ASTM G 85, Annex 5, is particularly useful for testing both architectural coat- ings and industrial maintenance paints as well as other applications. Fig. 1. Schematic drawing of a typical salt fog test chamber. 294

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