Kinetics of the carburization of iron alloys in methane-hydrogen mixtures and of the decarburization in hydrogen

The surface reaction kinetics have been investigated for the reaction CH₄ (gas) = C (dissolved) + 2H₂ (gas) on Fe and Fe-M foils (M = Mn, Cr, Co, Ni, Sn) at temperatures between 800 and 1000°C. The resistance relaxation method was applied in a flow apparatus. The following rate equation was observed for the rate of carbon uptake where is the carbon concentration and k and k′ are the rate constants for carburization and decarburization respectively. For Fe, Fe-10 Ni and Fe-10 Co the value of v = 3, for the alloys with > 20% Ni as well as for all Fe-Mn and Fe-Cr alloys the value v = 4 results from the measurements, which indicates differences in the rate determining step. With increasing Cr, Mn, Ni, Co content the rate constants k and k′ increase. The decrease of the equilibrium carbon concentration with increasing Ni content in Fe-Ni alloys is correlated to a fast increase of k′ and a slow increase of k with increasing Ni content. Beyond the minimum of at about 70 wt.% Ni both rate constants show a strong increase. By holding Ni at high carbon activities (a_C > 0.7 at 1000°C) segregation of carbon induces a surface reconstruction which is characterized by formation of (111) planes and which reduces the reaction rates. The increase of the equilibrium carbon concentration in the systems Fe–Mn and Fe–Cr with increasing content of alloying element is correlated to a slow increase of k′ and a fast increase of k with increasing Mn or Cr content. Sn segregates to the surface and retards the reaction rate by blocking the reaction sites for the surface reaction, similar to the effects of P and S.