Characterisation of the coke formed during metal dusting of iron in CO–H 2–H 2O gas mixtures

Abstract

Carbon deposits formed on the surface of iron samples during carburisation at 700 °C in a gas mixture of 75%CO–24.81%H 2–0.19%H 2O were characterised by using scanning electron microscopy (SEM), X-ray diffraction (XRD), Mössbauer spectroscopy and transmission electron microscopy (TEM). Cross-section observation of the iron sample by light optical microscopy revealed the formation of cementite after only 10 min reaction, together with a thin layer of graphite. After 4 h reaction, a thick coke layer was formed on top of the cementite surface. SEM surface observation indicated the formation of filamentous carbon in the coke layer. Further analysis of the coke by XRD and Mössbauer showed the presence of mainly Fe 3C and small amount of Fe 2C but no metallic iron in the carbon deposit. TEM analysis of the coke detected very convoluted filaments with iron-containing particles at the tip or along their length. These particles were identified to be cementite by selected area diffraction. Carbon deposits produced at the same temperature but with other gas compositions were also analysed by using XRD. It was found that with a low content of CO, e.g. 5%, both α-Fe and Fe 3C were detected in the coke. Increasing CO content to more than 30%, iron carbide was the only iron-containing phase.