Metal Dusting Corrosion of Alloy Cr5Mo in H₂-CO Gas Mixtures

Metal dusting is a catastrophic phenomena of high temperature corrosion, which occurs in severe carburizing environments (carbon activity a_C>1.0) at temperatures 400–900 °C. It causes not only phase changes but also removal of materials (pitting or thinning) and serious material deterioration. The present study focuses on the fundamental understanding of the corrosion of low alloy steels Cr5Mo in carbon-supersaturated environments (50CO-50H₂) at 600 °C over different holding times. Scanning-electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), nano-indentation, and nano-scratch tester are used to investigate the microstructure and its mechanical properties. An interesting continuous thick layer composed of Ha¨gg carbide (Fe₅C₂) and less cementite (Fe₃C) was present on top of the samples, which was ever observed in pure iron but not in alloy steels. This layer grew thicker with increasing holding times and showed very different mechanical properties with the carburized layer which was below the Ha¨gg carbide layer. And the carburized layer could not form a continuous and homogeneous layer of Fe₃C even in longer holding times. The cementite only formed at grain boundaries. The results show that also as for low alloy steels at very high carbon activities a second iron carbide, Ha¨gg carbide (Fe₅C₂), forms on the surface instead of the decomposing process of the metastable carbide, cementite (Fe₃C).