Metal Finishing Guide Book

2013

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Figure 2. Failed painted power transfer assembly after exposure to NSS. Figure 3. Base metal and surface corrosion in galvanized steel construction after 24 hours of exposure to NSS. Figure 4. Failed galvanized plating on a spring after 48 hours of exposure to NSS. Figure 5. Failed powder coating on a ductile iron casting after 72 hours exposure to NSS. 514 manufacturer or end user determine the efficacy of their coating/finishing process. By examining test specimens throughout the duration of the test, obvious surface effects are noticeable sometimes within a single day or two, preventing expensive and time-consuming rework and recalls. This comparison to an unfinished, or control specimen will help to determine the best surface finish for the intended application (Figures 1-5). The following images illustrate failures of coating application and/or performance. There are many specifications and standards written to help perform this testing and, equally as important, evaluate the results. The most common specifications for performing an NSS test are ASTM B117 and ISO 9227, which outline the test conditions required for the neutral salt spray test. These specifications unify the test parameters across the globe in the effort to evaluate the efficacy of paints, coatings and finishes. These specifications unify the test parameters worldwide in the effort to evaluate the efficacy of paints, coatings and finishes. In this test, an atmosphere of a 5% solution of sodium chloride is atomized and introduced at a constant rate throughout a sealed chamber. (Figure 1a) Test specimens are inclined and placed in the cabinet so that the test surface is parallel to the primary direction of flow of the sodium chloride mist. The mist wets the surface and the excess solution runs off into the cabinet. Humidity testing is another measurement of the effectiveness of the corrosion resistance properties of a metal part. This test can be performed is several ways. The first takes place in an unused NSS cabinet by simply turning off the salt solution flow and plugging the drain. The second test takes place in an environmental simulation cabinet specifically designed for this type of test. The requirements for this test can be

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