General approach of mechanism

The oxidation of metals by water vapor is carried out in experiments fixing the partial pressures of water and hydrogen (produced gas). The simultaneous presence of these two gases involves, by the action of the chemical equilibrium in gas phase, a partial pressure of oxygen. 

Some authors considered that the oxidation of metal by water vapor is, in fact, an oxidation by oxygen essentially present. But many experiments have shown that it is not tme and that the kinetic curves and the influences of pressures obtained with water were not as those obtained with pure oxygen under tte same partial pressure. Water, without a doubt, has a direct reaction with the metal. 

In all exactness, we must consider that the system will be the seat of two parallel oxidation reactions, one by oxygen and the other by water. If hydrogen and water pressures are sufficient, the reaction of oxygen is very low and if its pressure has an influence on the reactivity, we can neglect its oxidizing action. We will thus consider the specific oxidation of metal by water. 

The general mechanism will be roughly similar to that developed in section 15.2 for the oxidation of metals by oxygen with adsorption of water on the surface of oxide, two interface reactions that create and consume the defect and the diffusion of the defect. With these steps, it will be necessary to add desorption of the formed hydrogen at the external interface. 

We will solve the mechanisms by adopting the approach of the prevalent Wagner defect and considering only pure modes with only a single rate determining step. The experimental conditions are in general far from equilibrium and we will not have to take into accounts the reverse reaction of the rate determining step. 

---