Partial oxidation of chemical compounds

If the initial solid substance is a chemical compound (an intermetallic, a silicide, etc), then its oxidation can proceed via two different mechanisms, depending on the experimental conditions. Two oxides are formed in the severe oxidation (combustion) usually resulting in the disintegration of the compact solid phase. In the partial (soft) oxidation the chemical compound undergoes a partial decomposition giving another chemical compound of the same class and an oxide. 

Therefore, two solid layers occur at the interface between initial substances. One of these is an oxide layer, while the other is the layer of a chemical compound of the same A B multiphase binary system, enriched in the non-oxidising element. In the course of partial oxidation both layers remain more or less compact and adherent to the surface of the initial solid phase. 

Consider the oxidation of Zr2Al3 as a typical example. M. Paljevic established372 that the ZrAl2 and Zr02 layers occur during its oxidation. The overall chemical reaction of their formation is 

However, it provides no information about a real mechanism of layer growth. Indeed, reaction (5.38) cannot proceed any more immediately after the occurrence of the ZrAl2 and Zr02 layers because these separate the reactants Zr2Al3 and oxygen from each other. Instead, partial chemical reactions are to take place at appropriate interfaces. 

Note that reaction (5.39) reflects the phase transformation of Zr2Al3 into ZrAl2 under the influence of reaction diffusion of the zirconium atoms from interface 1 to interface 2. It can proceed at any temperature whenever this reaction diffusion takes place.138-140 

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