Degradation of C-1/2Mo steel pipe by metal dusting in a steam reforming process

A C-1/2Mo steel pipe of a heat recovery economizer in a steam reforming process used in the production of high-purity hydrogen has developed large pits at the inner surface. Light optical microscopy, x-ray diffraction, scanning electron microscopy combined with energy dispersive x-ray spectroscopy and transmission electron microscopy have been used to characterize the microstructure of the pitted pipe. Degradation of the pipe has been correlated with higher than normal temperature enhancing metal dusting as well as a material not intended to function in such an environment. Microstructural characterization shows that the sequence of events leading to the observed pitting follow a model proposed to explain metal dusting of carbon and low-alloy steels.