Failure analysis was conducted on two Alloy 625 nozzle lateral pipe sections removed from Suncor’s 52C-101 diluent tower after approximately seven years of service. The investigation determined that failure originated from fatigue cracking at the weld toes of spray nozzle branch connections, likely driven by flow-induced vibration under cyclic operating conditions. Metallography, SEM/EDX, XRD, and mechanical testing confirmed transgranular, non-branching fatigue cracks initiating from the external surface. Corrosion scale on the fracture surfaces formed post-failure and contained nickel, chromium, sulphur, and oxygen. The pipe material met ASTM B444 Alloy 625 Grade 1 requirements.
