Dusting of iron results from partial disintegration of a cementite scale which grows on the metal surface during reaction with carbon-supersaturated gas. Scaling kinetics are shown to be consistent with diffusion of carbon through the cementite. Further diffusion of carbon into the iron supersaturates it to a very high degree. Dusting of nickel and austenitic alloys leads to no carbide formation. Instead graphite grows into the metal, supported by diffusion of dissolved carbon to growth sites. Variations in rate with alloy iron content reflect the known effects of iron on carbon solubility and diffusivity. Alloying with copper also changes coking and dusting rates, although copper does not affect carbon permeability. The effect is shown to be due to interaction of copper with graphite nucleation sites.