Characterisation of the coke formed during the metal dusting of low alloy steels

Abstract

The deposition of carbon from carbonaceous gas mixtures onto the surface of critical components is of particular concern to the petrochemical and chemical industry. In highly carbonaceous atmospheres, a type of corrosion occurs by which the metal (Fe, Ni or Co) disintegrates into a dust of fine metal-rich particles and graphite. This is called metal dusting.In this project, low alloy 1Cr 0.5Mo ferritic steels were exposed to a 50% CO and 50% H2 (vol.%) gas mixture at temperatures between 500°C and 650°C for times up to 100 hours. Metal dusting occurred on all the test samples. Analysis of cross-sections through the samples in a scanning electron microscope (SEM) showed the formation of a cementite layer together with a layer of coke/graphite deposited on the surface after short times. The amount deposited on the alloy’s surface increased with both time and temperature over the temperature range studied.High levels of sample coking were observed after 100 hour tests. Deposited carbon from these tests was characterised by SEM, X-ray diffraction, and (scanning) transmission electron microscopy, S(TEM). TEM of the carbon deposit/coke on selected samples showed a complex convoluted network of carbon filaments with small iron-rich particles at their tips. High resolution TEM was used to study these carbon filaments in greater detail and showed that there were two completely different orientation relationships between the catalyst particles and the graphite.