Failure analysis of FeCrAl heating coils exposed to a high-temperature, high-pressure ammonia environment

Abstract

This work presents an investigation into the premature failure of FeCrAl heating coils after 48 h of continuous exposure to a high-pressure (12 bar) and high-temperature (700 °C) ammonia (NH3) environment while conducting a material nitridation test. Herein, we delve into the underlying causes and mechanisms associated with such a failure and the degradation of the coils. The study involves a detailed analysis of the formation of cracks and their initiation and propagation, with examination using optical microscopy, scanning electron microscopy (SEM), and electron dispersive spectroscopy (EDS). Examination of the topography of the fracture surface revealed the formation of brittle nitrides and cracks propagated through a quasi-cleavage fracture mechanism. Subsequent nitriding attack was attributed to the result of imperfections in the formed protective Al2O3 layer. This led to the primary conclusion that the premature failure of the heating elements was predominantly triggered by the nitriding process associated with the dissociation of NH3 and the diffusion of nascent nitrogen into the surface of the FeCrAl coils.