Kinetic study of the oxidation of the carbides of iron

Abstract

A kinetic study has been made of the rate of evolution of carbon dioxide in the reaction of three different preparations of carbides of iron with gaseous oxygen. All three reactants contained “free” carbon; cementite B contained a considerable amount. Both carbidic and free carbon were oxidized to carbon dioxide and, simultaneously, iron was oxidized initially to Fe₃O₄, though later significant Fe₂O₃ formation occurred. Carbon dioxide evolution was strongly deceleratory and results fitted the Jander equation over the first part (∼50 %) of the reaction. Values of the activation energies were : cementite A(Fe₃C, low surface area) 105±12 kJ mol^–1(405-500° C); cementite B(Fe₃C, high surface area) 102±12 kJ mol^–1(240-340°C); Hägg carbide (Fe₅C₂, low surface area) 100±16 kJ mol^–1(300-400°C).

From quantitative comparisons between these results and the published data for the reaction of iron with oxygen, it is concluded that, in each of the reactants studied here, carbon was oxidized at a metallic phase (carbide)/oxide interface which progressively moved through the reactant particles. On such a model, the rate-controlling process in the oxidation of iron, and of carbon in the iron carbides, is the diffusion of the same species through the adherent barrier layer of product oxide.