α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.