Migration directed metal oxidation

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"-...

In the case of the p-type metal deficit oxides, metal cations produced by the anodic reaction at the metal-oxide interface migrate to the oxide-gas interface by exchange with cation vacancies. Electron charge is effectively transferred to the oxide-gas interface by the movement of electron holes in the opposite direction (toward the metal-oxide interface). The cathodic reaction and oxide growth thus tend to occur at the oxide-gas interface (Fig. 3.9). 

On the basis of these ideas, Macdonald and coworkers [13,14] developed their model of passivity and its breakdown involving the action of vacancies within e passive layer. It is assumed that cation vacancies migrate from the oxide-electrolyte to the metal-oxide interface, which is equivalent to the transport of cations in the opposite direction. If these vacancies penetrate into the metal phase at a slower rate than their transport through the oxide, they accumulate at the metal-oxide interface and finally lead to a local concentration. The related voids lead to stresses within the passive film and its final breakdown. The inward diffusion or migration of cation vacancies is affected by the incorporation of Cl ions at the oxide-electrolyte interface according to the following mechanism The concentration c of metal ion V + and vacancies of are determined by the equilibrium of the Schottky pair formation at the oxide-electrolyte interface (Equation 7.3), which causes an inverse dependence of their concentrations (Equation 7.4) ... 

Independent of the nature of the reacting metallic substrate, i.e. pure metal or disordered alloy, the reactions at internal metal/scale interfaces correspond to the ionization of metal atoms and their transfer, by jumps or interface migration, from the metal lattice to the oxide lattice. To be possible and energetically favourable, these jumps from the metal to oxide lattices must be as short as possible and thus the metal and oxide lattices must be as close as possible. Therefore, the two lattices must be mutually oriented to minimize the distance between metal lattice sites and cation lattice sites of the oxide lattice. Similarly, interface migration implies parallelism of metal and oxide lattice planes if it does not involve atomic jumps. Both requirements usually lead to the parallelism of high-density lattice planes and lattice directions of both lattices, i.e. to specific mutual orientation relationships, or epitaxial relationships, between the metal and oxide lattices that usually correspond to an interface structure of low energy. Therefore, as observed for many metal-oxide interfaces (Ernst, 1995), metal and oxide lattices are expected to share more or less complex mutual epitaxial orientation relationships. 

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