Stress corrosion cracking of ring type joint of reactor pipeline of a hydrocracker unit

Failure of a ring type joint (RTJ) of reactor pipeline of a hydrocracker unit has been reported. Cracks were noticed on the flange as well as the ring gasket of the RTJ during inspection following a periodic shutdown of the unit. These components of the RTJ were manufactured from stabilized grades of austenitic stainless steel, namely, type 321 and 347, respectively. Study showed that the failure of the RTJ components occurred by transgranular stress corrosion cracking. It was further established that formation of polythionic acid due to presence of H2S in the process gas and H2O in the system was responsible for initiation of SCC in RTJ components. Detailed metallurgical investigation was carried out to identify the probable causes that could have been responsible for formation of polythionic acid in the process gas