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Metal dusting of Fe-Ni alloys was investigated in a CO-H2-H 2O-Ar gas corresponding to a C = 19.6 at 650 °C. Thermogravimetric analysis showed that increasing the nickel content in the alloy decreased the initial rate of carbon uptake. A uniform Fe3C scale formed on pure iron, a layer with mixed structures of Fe3C, γ and α-Fe developed on ferritic Fe-5Ni, and small amounts of Fe3C developed at the surface of an austenite layer grown on two-phase (α + γ) Fe-10Ni. At nickel levels above 10%, no carbide appeared. These observations are shown to be broadly consistent with local equilibrium according to the Fe-Ni-C phase diagram. However, the failure of higher nickel austenitic alloys to form the (Fe,Ni)3C expected at high carbon activities indicates a barrier to nucleation and growth of this phase. Graphite deposition was catalysed by (Fe,Ni)3C on ferritics and by the metal itself on austenitics. The rates of carbon deposition on Fe-60Ni corresponded to the existence of three parallel and independent paths: the synthesis gas, the Boudouard and the carbon methanation reactions.