D. Macdonald 2 Point Defect Model

While these six generalizations are not all encompassing, in that exceptions may exist, they are sufQcient to differentiate between various theories that have been proposed for the growth of barrier oxide layers on metals and alloys. A number of models that have been developed to describe the growth of anodic oxide films on metals are listed in Table 4.4.2, together with some of their important features and predictions. Of the models listed, which were chosen because they make analytical predictions that can be tested and because they introduced new concepts into the theory of passivity, only the point defect model (PDM) in its latest form (D. Macdonald [1999], Pensado-Rodriguez et al. [1999a,b]) accounts for all of the observations summarized above. 

The electric field strength within the barrier layer is buffered by internal (Esaki) band-to-band tunneling, such that the field strength is independent, or at least is only weakly dependent, upon the appUed voltage (Chao et al. [1981], D. Macdonald [1999]). For mathematical convenience, it is assumed that the potential varies 

Model Applicability Field Strength Rate Control Film Dissolution Interfacial Pot. Diff. Electronic Structure 

Verwey [1935] Cation condnctors Unconstrained Cation motion in oxide No No Not addressed 

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