Failure Analysis of a C-276 Alloy Pipe in a Controlled Decomposition Reactor

Failure analysis was carried out on a ruptured C-276 pipe heated externally at 1050 °C, which had been used for a few months in a controlled decomposition reactor (CDR) system. To catch the decomposed perfluorinated compounds (PFCs, e.g., CF₄, SF₆, NF₃, C₃F₈ and C₄F₈) present in the exhaust gas, the C-276 reactor was periodically purged with water mist, which caused a temperature gradient from the external to the inner surface of the pipe. The precipitation of large amounts of intermetallic compounds along the grain boundaries were found to be corroded preferentially. The internal surface of the used pipe was covered with many fine cracks. The corrosion and cracking of grain boundary precipitates accounted for the short service life of the C-276 pipe. Compositional measurements by electron probe micro-analyzer (EPMA) and phase identification by electron backscatter diffraction (EBSD) confirmed the presence of δ and μ phases in the ruptured pipe. The coarse intergranular precipitates were the δ phase (Mo₇Ni₇), which were enriched in Mo and Cr. Moreover, the fine precipitates dispersed intergranularly and intragranularly were the μ phase (Mo₆Ni₇), which were abundant in Mo and W. The numerous precipitates present in the matrix and along the grain boundaries were responsible for an obvious loss in the strength and ductility of the used C-276 pipe.