Metal Finishing Guide Book


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protect surfaces providing the highest quality product with the least amount of overhead in order to yield maximum profit. Accelerated Corrosion Testing Accelerated corrosion testing allows base material verification prior to final application of surface treatments such as plating Figure 8. Conforming and non-conforming G48C corrosion coupons. Pitting indications correspond to dark spots on left-hand samples. or polishing. Unwanted corrosion in stainless steels and other related alloys is of significant concern in their engineering. Rough surface finishes, elevated temperatures and corrosive environments encountered in service can accelerate corrosion and lead to material early failure. Different forms of corrosion are commonly encountered in service, and many can be strongly influenced by heat-treating. Fortunately there are many types of standardized test methods, which seek to qualify materials and quantify the rates corrosion that the alloy may experience. Pitting corrosion is a localized form of corrosion found in stainless steels and other related alloys, which do not typically experience uniform corrosion. Pitting corrosion is typically caused by the breakdown of a passivating surface layer and results in small areas of corroded "pits" to form. These pits can act as sites of crack initiation and are detrimental to a material's performance. Intergranular corrosion in stainless steels is usually associated with a precipitated inter-metallic phase found at the grain boundaries of a material. For example, chromium carbides precipitation at the grain boundaries can deplete the surrounding material of corrosion-inhibiting chromium. In the presence of a corrosive environment, these grain boundaries are preferentially attacked and are subsequently weakened. Stress corrosion cracking is another form of corrosion that can cause normally ductile materials to fail in a brittle manner. When a material is both stressed (especially at elevated temperatures) and also exposed to a corrosive environment the grain boundaries can be attacked and the materials mechanical properties can be compromised. Stress corrosion cracking is very alloy specific in that certain alloys may Figure 9. Intergranular corrosion of metallographic cross-section of only experience SCC in 304 stainless steel. the presence of specific 516

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