Metal Finishing Guide Book

2013

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chemical environments. Austenitic stainless steels in particular are susceptible to SCC in the presence of chlorides. Sensitizing heat treatments in stainless steels can aggravate problems associated with intergranular corrosion. Sensitization is a process in which a material is heated to a temperature in which chromium carbides, for example, can precipitate out of solution at the grain boundaries and create a chromium depleted region which adversely affects corrosion resistance. Austenitic stainless steels are susceptible to sensitization after heating to ~500°C - 800°C. The addition of carbide stabilizing elements such as niobium or titanium can help to ameliorate the problem; Figure 10. Lab technician conducting the ASTM G47 test, however, processes such a common procedure for detecting pitting corrosion. as welding a susceptible material can still detrimentally affect its microstructural integrity. There are many standardized methods for testing a material's susceptibility to one of the three forms of corrosion addressed. A common test for pitting corrosion is the ASTM G48 test. A sample coupon is polished, dimensioned and weighed prior to testing. It is then placed in a ferric-chloride solution for a specified period of time. The sample is reweighed after testing and visually examined to look for signs of pitting corrosion. This test can also quantify the mass loss rate of certain materials and can very useful in ranking materials' ability to resist this form of corrosion. ASTM A262 Method E is a common test for intergranular corrosion. Two samples are polished for testing, with one as a control. One of the two samples is boiled in a copper-copper sulfate sulfuric acid solution for a specified period of time while the other is desiccated. After testing the two samples are given a U-bend test and the apex of the bend is examined by microscope to look for evidence of intergranular fissures. The presence of these fissures indicates the materials' susceptibility to intergranular corrosion, and can be used to determine if a sensitizing heat treatment has affected a materials susceptibility to intergranular corrosion. The ASTM G36 test is another commonly used test to look for susceptibility to stress corrosion cracking. Samples are polished and bolted into a U-bend configuration. These samples are boiled in a magnesium chloride solution for a specified period of time, and the apex of the bend is microscopically evaluated to look for evidence of crack formation. The presence of cracks indicates suscep517

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