Thermodynamic Aspects of Iron in Metal Dusting

Abstract

A theoretical analysis has been conducted into the thermodynamic role of iron and the dependence of possible metal-dusting occurrence on temperature, volumetric ratios of carburizing species CH₄ and CO, and total pressure. The analysis indicates that the equilibrium carbon activity with cementite (a _{C(Fe 3 C/Fe} )) decreases with increasing temperature, and becomes lower than unity in excess of approximately 1000°C. This critical temperature could be the upper-temperature limit for decomposition of cementite in metal dusting. The temperature regime for possible metal dusting in either CH₄–H₂ or CO–CO₂ gas mixtures increases with increasing volumetric ratio of CH₄ or CO. Increasing total pressure leads to a reduction of this temperature regime in CH₄–H₂ but an expansion in CO–CO₂. Comparatively, this temperature regime in CO–CO₂ is shifted more towards lower temperatures, and iron is likely more susceptible to metal-dusting attack in CO–CO₂ due to the higher reverse carbon-activity gradient established.