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SCC of Stainless Steel Floating Flange Spool Piece

Background: Cracks were detected on floating flange spool piece during routine inspection using snoop liquid test. A metallurgical failure analysis was conducted to identify the failure mechanism.

Figure 1: Visual Inspection and NDT of As-Received Flange Samples Depicting Cracks

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Figure 2: SEM Micrographs Revealing Intergranular Attack Along the Grain Boundaries

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Summary: This flange failure was associated with stress corrosion cracking attack under a polythionic acid environment. The floating flange was used for loading methyl mercaptan into a rail car storage tank and possibly a gasket leak led to exposure of wet HXSOY when reacting with atmospheric moisture. The carbide precipitation at the grain boundaries facilitated intergranular attack but transgranular attack was also observed. The precipitates observed on the microstructure of the spool material, suggests the material was exposed to sensitization temperatures, which could have occurred during manufacturing.

Take Away: Wet exposure to polythionic acid (HXSOY) during operation, coupled with microstructural degradation of stainless steels can make them susceptible to localized corrosion attack.

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