ABSTRACT
M(solid metal) + 0 2(g) = M02(solid compound) (5.2) one has to know the value of Gibbs' free energy change (~G) for the total reaction. If ~G is negative, the reaction will be spontaneous in the forward direction, leading to the formation of metal oxide (M02), whereas if ~G is positive, M02 will not be stable at the temperature and pressure of the oxidant, thereby leading to spontaneous dissociation of the oxide. But thermodynamics deals with the equilibrium attainment for any metal-compound-oxidant system that needs a scrutiny of the standard free energy change (~G0) of the above reaction. For ready reference, to judge the relative stability of various metal-oxidant systems, one has to have a look at the graphical representation of ~G0 vs. T plots [ 1] for various systems (Ellingham-Richardson diagrams). Such types of graphical representation are shown in Fig. 5.l(a)-(d) for metal-oxide-oxygen, metal-sulfide-sulfur vapor, metal-halide-halogen, and metal-carbide-carbon systems, respectively. The preferential choice of alloying elements like Cr, AI, Si, etc., in the development of high-temperature alloys is best judged from such diagrams. The compounds of these elements are comparatively more stable than the base metal compounds, as evidenced by more negative ~G0 values in the above-mentioned figure parts. It would be appropriate to mention that when a metallic component is exposed to an environment containing more than one oxidant, say S02 and 0 2, and the ~G0 values are found to be negative for the formation of both oxide and sulfide; however, preferential growth of either of the compounds will no longer be guided by thermodynamic considerations alone.
