Metal Finishing Guide Book

2013

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is simply to run a small amount of water over the surface of the test piece. If it is free of oils and contaminants the water will run over it in a sheet and off in one fall with no breaks. Finally, ASTM D609 discusses the handling of cleaned test pieces and storage in paper impregnated with volatile corrosion inhibitor (VCI) if the test pieces are not to be run immediately after cleaning. EVALUATION OF RESULTS Some primary ASTM Standards that have been developed to assist in the evaluation of corrosion test results are: ASTM D1654—Standard Test Method for Evaluation of Painted or Coated Specimen Subjected to Corrosive Environments; ASTM D610—Standard Test Method for Evaluating Degree of Rusting on Painted Steel Surfaces; ASTM D714—Standard Test Method for Evaluating Degree of Blistering of Paints; and ASTM B537—Standard Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure. Irrespective of the title wording, the text of ASTM B537 states that it is also used to evaluate panels subjected to laboratory accelerated corrosion tests. Other standards are also available for evaluation of specific materials and finishes. ASTM D1654 is used for the evaluation of substrates, pretreatments, coating systems, or a combination thereof. This standard is also used when the coating has been scribed prior to exposure to the corrosive environment. ASTM D1654 recommends a straight-shank tungsten carbide tip or equivalent scribing tool to produce a cut through the coating to the substrate of the test piece. The scribe serves as a deliberate failure of the coating system prior to corrosion testing. Once the test piece has been through the selected accelerated corrosion test it is then cleaned by rinsing with a gentle stream of water at 45°C (110°F). Loose coating is then removed using compressed air to blow it off or a scraper to scrape it off. The "creepback" (perpendicular distance of failure from the scribe line) of the coating is measured at uniform intervals and the rating system described in the standard applied. ASTM D610 provides a series of photographic reference standards that demonstrate degrees of rust on the surface of a test piece. The test pieces are first gently cleaned then the amount of rust observed is reported from comparison to the photographic standards and the additional use of established standardized terminology from the test standard. It is important to compare only rust and not surfaces stained by rust. ASTM D714 is used to evaluate coating adhesion by observing the amount of blistering that has occurred. The test pieces are first gently cleaned then compared to the series of included photographic references and the additional use of the appropriate "standardized" terminology indicated in the test standard. They are graded first by the size of the blisters that appear on the surface and then the number of blisters that appear per unit area. CONCLUSION The corrosion tests discussed in this article are some of the most utilized tests of this type. They also demonstrate the diversity of the tests and some of the advancements in accelerated corrosion testing and complexity of the subject. They also provide a solid basis for evaluating the performance of a wide variety of products and materials. These standards have been developed, evolved and refined through cooperative efforts of experts and interested parties from: ASTM International, industry, government, and other trade organizations worldwide. 546

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