Failure analysis of secondary reformer burner nozzles made from Incoloy 825 in an ammonia production plant

Abstract

The aims of this study were to perform failure analysis and to assess the microstructural changes of secondary reformer burner nozzles made from Incoloy 825 in an ammonia production plant. The nozzles were in service for about 3 years under the atmosphere of synthesis gas (syngas) and wet air. Various techniques including on-site visual inspection, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), grazing incidence X-ray diffraction (GIXRD), and micro-hardness testing were employed to assess the root causes of failure. Results of visual inspection showed green rot, pitting, cracking, spallation, and coking in the secondary reformer burner nozzles. Microscopic data and Phase analysis of the external surface of the nozzle indicated the formation of a substantial amount of oxides and spinel from elements such as Fe, Cr, Mn, and coke in the form of carbon nano-filaments. Moreover, some voids, cracks, and intergranular and intragranular internal oxidation were observed in the cross-section of the outer wall of the nozzle. The formation of oxides and spinel, as the product of in-situ oxidation of carbides, is suggestive of the occurrence of type III metal dusting. Under these conditions, the alloy underwent considerable volume expansion that led to the nucleation and growth of voids and cracks, and ultimately, the failure of the nozzle. Some of the microstructural changes observed include the nucleation and growth of massive secondary carbides of Cr21Mo2C6 type and platelet carbides of (Ti,Mo)C type, and degeneration of primary precipitates of Ti(C,N) type formed in the nozzles.