The role of cementite in the metal dusting of Fe–Cr and Fe–Ni–Cr Alloys

Model Fe-25 w/o (weight percent) Cr and Fe-25 Cr-Ni alloys containing 2.5, 5, 10 and 25w/o nickel were exposed to a CO-26H(2)-6H(2)O (vol. pct.) Mixture at 680degreesC under thermal cycling conditions. The supersaturated carbon activity was calculated to be 2.9 (referred to graphite) and M3C was predicted to form on Fe-25Cr and Fe-25 Cr-2.5 Ni, but not on higher nickel content alloys. Metal dusting occurred on all alloys, accompanied by internal carburisation. Transmission electron microscopy of the dusting deposit showed that much of the carbon consisted of hollow graphite nanotubes. Small, metal-rich particles were found at the carbon filament tips. These were identified as single crystal Fe3C in the case of Fe-25 Cr, and M3C, containing low levels of nickel, in the case of Fe-25 Cr-2.5 Ni and Fe-25 Cr-5 Ni. In contrast. the particles found at the filament tips on the higher nickel, two phase, alloys were both M3C and austenitic Fe-Ni. Strong orientation relationships were determined between the graphite and cementite particles, however, no consistent and clear crystallographic relationship was deduced between the graphite and austenite particles. It is concluded that carbon deposition from the gas is catalysed by both Fe3C and austenite. Subsequent carbon nanotube growth reflects the orientation relationship between Fe3C and the graphite.