Failure of Hastelloy diaphragm in pressure regulator used for hydrogen fluoride gas

Failure analysis was conducted for a hydrogen fluoride pressure regulator. Hydrogen fluoride gas transformed into highly corrosive hydrofluoric acid owing to the presence of condensed moisture. On the surface of the Hastelloy diaphragm of the pressure regulator, corrosion pits and networklike corrosion structures were uncovered. Crack initiation from a corrosion pit caused the coalescence of cracks of neighboring pits and fatigue crack growth, and the fatigue crack penetrated the diaphragm and triggered gas leakage. The fatigue load was provided by the repeated bending of the diaphragm during pressure regulation. Mo, Ni, and Cr in Hastelloy contributed to the formation of a passivation film, and M6C carbide could have served as a crevice corrosion cell. The stress generated in the diaphragm during pressure regulation was analyzed using finite element analysis. In this paper, apart from a discussion of these observations, an explanation of the detailed results of a metallurgical investigation and energy dispersive X-ray spectroscopy analysis are explained.