The interaction of carbon and sulfur with iron-base alloys in predominantly carburizing environments

Carburization is a major mode of corrosive degradation of alloys in many high temperature processes. Under conditions where the environmental chemistry and alloy chemistry are such that a Cr₂O₃ scale can form on the alloy surface, such a scale can resist the entry of carbon into the material. However, when such a scale does not form, carbon is transferred from the gas phase onto the alloy surface and diffuses to the alloy interior with concurrent precipitation of chromium-rich carbide phases. The kinetics of this overall process have been determined by thermogravimetry in the temperature range 1173-1373 K using Fe-20Cr (where the composition is given in approximate weight per cent) as a model alloy and H₂-Ch₄ gas mixtures to provide the carburizing medium. Mathematical expressions for the carburization kinetics have been developed in terms of surface reaction and diffusion steps. Both processes contribute to carburization, the influence of the surface reaction step increasing with an increase in temperature. Additions of sulfur as H₂S to the gas phase slow down the surface reaction step sufficiently that the whole carburization process can become surface reaction controlled.