Metal Finishing Guide Book

2013

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laid out in a manufacturer's specification or in an ASTM publication such as ASTM D2247. This type of testing helps to evaluate primarily paints and their tendencies to bubble and blister. These two types of cabinet corrosions testing can also be very effective as free iron detection tests. Free iron testing can be an excellent predictor of corFigure 6. Free iron on threaded portion of powdered metal ring. rosion resistance properties and surface finish success. Locations, on a casting or finished part, which are high in free iron, can become epicenters for corrosion while in service. In most cases, a free iron test can be done via salt spray testing in about 2 hours or in a humidity chamber in about 24 hours (See Figure 6). These different combinations of coatings and hours of exposure have led to a number of different conclusions for these companies. The most important being cost effectiveness of coating and surface finishes vs. corrosion resistance. Additional Free Iron Detection Tests In addition to these two cabinet style tests, another test that can effectively detect free iron is a potassium ferricyanide-nitric acid (Ferroxyl) test and in accordance with MIL-STD-753 and ASTM A380. The ferroxyl test will illuminate even the lowest levels of iron contamination. In this test, reactions with the chemicals in the test solution and yields a chemical reaction in the form of a bluish coloration demonstrating the presence of free iron (See Figure 7). Once the testing has begun, the challenge becomes the evaluation of the constantly changing surfaces of interest. Sometimes it is helpful to add a point of certain failure to the test specimen, or scribe, to see just how bad a failure becomes if it should occur via damage or deficiency of the coating. There are many evaluation procedures such as ASTM D610, D714 and D1654 that oversee the evaluation of test results. These specifications use numerical values to evaluate the severity of the corrosion, blistering and/or creep. These laboratory tests have one goal in common: to assist the surface preparation industries evaluate the efficacy of their coating material and/or process. Corrosion testing can help everyone from an epoxy powder coating companies to chemical coating companies to zinc/cadmium/ chrome plating companies and everything in between. The ultimate goal in applyFigure 7. Free iron indications after ferroxyl test on high alloy stainless steel casting. The blue coloration indicates the ing a coating or perform presence of surface-free iron. surface treatments is to 515

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