Thin oxide film formation, metal mechanism

F.P. FeUner, M.J. Graham, Thin oxide film formation on metals, in P. Marcus, J. Oudar (Eds.), Corrosion Mechanism in Theory and Practice, Dekker, New York, 1995, pp. 123—155. 

Fehlner, F. R Graham, M. J. Thin Oxide Film Formation on Metals in Corrosion Mechanisms in Theory and Practice, New York, Hong Kong, Marcel Dekker, 1995,123. 

The theory of Mott and Cabrera for the growth of very thin oxide films did not satisfactorily explain the results. The governing kinetic factor was found to be the increase in oxide thickness rather than the total oxide-film thickness. A mechanism based on the formation of metal lattice vacancies and their elimination by heating is proposed. 

The reaction between a metal and oxygen is one of the most common phenomena encountered. Yet the mechanism of the formation and growth of the oxide film is still not understood, particularly in the region of very thin oxide films. There remains, too, a lack of sufficient, reliable data for the initial stages of oxidation. 

It is not appropriate here to consider the mechanism of passivation (see Section 1.5), but it is apparent from Fig. 1.33 that the transition from the active to the passive state must be associated with a fundamental change in the nature of the metal surface, and it is now the generally accepted view that passivity is due to the formation of a very thin solid film of metal corrosion product, usually of oxide, on the metal surface. In this connection it should be noted that metal oxides are thermodynamically unstable in acid solutions so that the oxide formed during passivation must be regarded as a metastable form of the oxide that is stable at a higher pH (Fig. 1.18). It follows that the... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

The initial reaction results in the formation of a continuous film of oxide that is firmly attached to the metal surface. The rate of growth of the film is controlled by the slow diffusion of the Cu ions. However, no corrosion could occur without the transport of electrons, as the mechanism depends on electron transport. The electronic conductivity of the film is therefore of major importance. The reason why both aluminium and chromium appear to be corrosion-resistant lies in the fact that, although oxide films form very rapidly in air, the films are insulators and prevent reaction from continuing. As the thin films are also transparent, the metals do not lose their shiny appearance. 

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