Iron and nickel, model alloys of Ni-Cu and Fe-Cr, and commercial heat resisting alloys were exposed at 650-680oC to flowing CO-H2-H2O gases which were supersaturated with respect to carbon. All ferritic materials, including chromia and alumina formers, developed a coke deposit of carbon nanotubes, the growth of which was catalysed by nanoparticles of Fe3C. Austenitic materials formed graphite filaments and clusters in association with nanoparticles of austenite. Graphite cluster formation was suppressed by alloying copper with nickel. The sensitivity of coking kinetics to alloy copper content was consistent with a mechanism involving graphite nucleation within the subsurface metal. Chromia forming alloys resisted dusting until damage to the scale could no longer be repaired by Cr2O3 regrowth, and carbon gained access to chromium – depleted metal.