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Caustic Stress Corrosion Cracking of Carbon and Stainless Steel

Background: A client contacted KnightHawk Materials Lab when a long section of steam piping fell to the ground releasing steam. Subsequent inspection found widespread cracking of both carbon and stainless steel piping in the steam system.

Figure 1: Macrophotgraphy of the Fractures of Stainless Steel (Top) and Carbon Steel (Bottom)

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Figure 2: Optical Micrographs of Transgranular Cracking in Stainless Steel (Left) and Intergranular Cracking in Carbon Steel (right)

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Summary: Caustic stress corrosion cracking (SCC) was the cause of the cracking in each of the pipes. More specifically, the branching nature of the cracks, the transgranular cracking in stainless steel and intergranular cracking in the carbon steel, are the expected fracture modes of those materials due to caustic SCC. Further, the presence of large amounts of Na and O on the fracture surfaces as evidenced by EDS also supports the conclusion that SCC caused the cracking.

Take Away: There are relatively few types of SCC that affect both carbon and austenitic stainless steels, but through fractographic and process analysis, it is possible to identify the type of SCC that is affecting a system and take the appropriate actions to prevent it in the future.