Oxide/gas interface

The logaritlrmic law is also observed when the oxide him is an electrical insulator such as AI2O3. The transport of elecuons tlrrough the oxide is mainly due to a space charge which develops between tire metal-oxide interface and the oxide-gas interface. The incorporation of oxygen in the surface of tire oxide requhes the addition of electrons, and if this occurs by a charging process... 

Oxide movements are determined by the positioning of inert markers on the surface of the oxideAt various intervals of time their position can be observed relative to, say, the centreline of the metal as seen in metal-lographic cross-section. In the case of cation diffusion the metal-interface-marker distance remains constant and the marker moves towards the centreline when the anion diffuses, the marker moves away from both the metal-oxide interface and the centreline of the metal. In the more usual observation the position of the marker is determined relative to the oxide/ gas interface. It can be appreciated from Fig. 1.81 that when anions diffuse the marker remains on the surface, but when cations move the marker translates at a rate equivalent to the total amount of new oxide formed. Bruckman recently has re-emphasised the care that is necessary in the interpretation of marker movements in the oxidation of lower to higher oxides. 

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

Chromia is thought to grow via outward diffusion of cahons from the metal to the oxide/ gas interface. When the scale formation proceeds by diffusion along short-circuit paths such as grain boundaries in the oxide layer, Eq. 1 describes the usual diffusion controlled oxidation kinetics ... 

Conversely, at the oxide/gas interface, the cation and electron will combine with oxygen according to... 

If it is assumed that regeneration does not disturb the oxide already present on the surface, then further oxidation should be governed by the transference of cobalt cations from the metal-oxide interface to the oxide-gas interface where reaction takes place. The rate would be dependent on the total film thickness if the theory of Mott and Cabrera were valid. The limiting thickness then would be the sum of the defined limiting thickness Xl for a particular oxidation plus the thickness of the oxide layers previously formed. These total limiting thicknesses for successive oxidations, desig-... 

Figure 3. Schematic location of inert markers before oxidation (on the surface of the pure metal) and after oxidation (at the metab-metal oxide interface). From this limiting case one may infer that the mobile species diffuses from the metal-metal oxide interface outward through the scale or tarnish layer. If the marker were found at the oxide-gas interface, the inference would be that the mobile species diffused from the oxide-gas interface to the metal—oxide interface.

F is Faraday s constant, Nq is Avogadros number, q is the electronic charge, t denotes a transference number and the subscripts 1, 2 and 3 denote the metal ion, the oxygen ion and electron, respectively. The total electrical conductivity is a. Local equilibrium is assiuned to occur at the metal-oxide interface and also at the oxide-gas interface. Therefore.the chemical potential of oxygen is fixed at each interface. The oxygen pressure at the metal-oxide interface is fixed as the dissociation pressure of the oxide and denoted as pq. The oxygen partial pressure in the gas phase, pft, is at equilibrium at the oxide-gas interface. (See Figure 7) This equation may be written as... 

Figure 7. Schematic of transport processes through an oxide layer growing on a metal. Two limiting cases may be distinguished. First, metal ions and electrons may migrate from the metal toward the oxide gas interface and second, oxygen ions may migrate toward the metal-oxide interface with electrons migrating in the opposite direction. In any volume element of the oxide, electrical neutrality is required. The chemical potential of oxygen is fixed at both the metalr-oxide and-the oxide-gas interface. The former is fixed by the dissociation pressure of the oxide, po/, and the latter by the ambient oxygen partial presure, po"-...

The limits of integration are the activities of O2 at the metal/oxide interface and the oxide gas interface pO. In most oxides of interest, the ionic conductivity is very much less than the electronic conductivity... 

An n-type cation interstitial oxide grows through interstitial cations diffusion to the oxide-gas interface (Fig. 11.8). The electrons travel to the same interface where the overall reaction, Eq. (11.3), occurs. The metal oxidation and oxide formation, Eq. (11.1), and oxygen reduction, Eq. (11.2), are shown in Fig. 11.8. 

Figure 4.6 Schematic representation of the 656, X. Carrier et al, influence of aging on molecular-scale phenomena occurring at the M0O3 formation in the preparation of oxide/water interface (impregnation) and alumina-supported Mo catalysts, 231-238, oxide/gas interface (calcination) during Copyright 2003, with permission from alumina-supported Mo catalysts Elsevier.)...

Figure 8.4 summarizes the reactions that contribute to the growth of a compact oxide film. At the oxide-gas interface, the oxygen is reduced to by accepting two electrons. At the metal-oxide interface, the metal atoms oxidize into cations and thereby liberate n electrons. These electrons have to move across the oxide in order to react with the oxygen. Furthermore, the cations move towards the outside surface of the film, while the anions migrate towards the interior. The slowest of these processes determines the growth rate. 

Wagner s theory of oxidation provides a quantitative description of the growth rate of compact oxide layers as a function of the difference in electrochemical potential between the metal-oxide and the oxide-gas interfaces. The following analysis uses concepts developed in Section 4.3 for aqueous electrolytes. This simplifies the theoretical developments proposed by Wagner [4], while yielding the same results. 

At the oxide-gas interface, the concentration of interstitial cations is normally very low Ci,g ci,m. By setting, VoxdPoxd = 3 oxd. where Moxd represents the molecular mass of the oxide, and by substituting the subscript 1 by M, the expression (9.50) becomes ... 

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