Approximation in Ellingham diagram and calculation approach using heat capacity of oxides and metals constituent in stainless steels

Abstract

The Ellingham diagram is constructed to express the stability of phases e.g. a metal and its oxide or compound of which the isothermal Gibbs free energy change of the reaction is exhibited as a function of temperature. The approximation is done by assuming that the enthalpies and entropies of formation of the oxide phases insignificantly vary with temperature, and then their values at the standard state are applied. This paper assesses the deviation of the reaction Gibbs free energy when the calculation is made without and with this assumption, focussing on the formation of oxides of metals which are constituent of stainless steel, e.g. Fe, Cr, Si, Al, Ti, Nb, Mo, Zr, Ce and Y. Using thermodynamic data of heat capacities compiled by Kubaschewski et al. (1993), the deviation of the reaction enthalpies and entropies are found to be in the range of 4.13% and 12.97%, respectively. As a result, the values of the reaction Gibbs free energies calculated using general approximation are up to 5.90% different from the ones calculated using the data of heat capacities.