Kinetics of monticular carbon growth on polycrystalline iron

The kinetics of carbon deposition on polycrystalline iron surfaces from methane at 750 Torr pressure and at temperatures between 1013 and 1133 K have been studied using the change in power dissipation of an electrically heated filament caused by the increased emissivity as the surface is blackened by deposit. The carbon formed has a monticular-type structure. The growth kinetics follow a direct logarithmic rate law which is explained in terms of the availability of active sites at the surface. The total carbon uptake at maximum coverage of the monticular layer, i.e. at maximum emissivity, depends on the filament temperature and can be correlated with the amount of carbon taken up into solution during the prior induction period in accordance with the phase diagram. The activation energy has a value of 66 ± 8 kJ mol–1. The rate-determining step for monticular growth cannot be established, but certain possibilities, such as carbon dissolution, bulk diffusion of carbon in the metal, and surface and bulk self-diffusion of iron, can be eliminated. Various aspects of the mechanism of formation of the monticular growths are discussed.