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"*What interface(s) control alloy oxidation and how? "*What parameters provide the driving potential for oxidation? "*How does this potential respond to temperature (T), pressure (P) and compositional changes?

will be discussed. Progress on these questions have allowed models to be proposed to predict alloy oxidation behavior under any set of conditions. These models are already providing some predictive power in how the oxide overlayer and passive film structure develop. By using surface studies of the oxidation behavior of the following alloys and their thin films: Cu-Mn, Ag-Mn, Ni-Ti, Ni-Zr, Ti-Cu and Ti-Al, we have been able to delineate the factors which are most important to the oxide formation process and provide insight into the prediction of oxide layer structures. These will be illustrated with actual experimental results on selected alloys. INTRODUCTION Metal alloys, which allow the design of optimal structure-property relationships for specific applications, have become one of modem society's most important materials. One of the most frequently sought-after property is oxidation resistance; however, all alloys are thermodynamic unstable with regards to oxidation and must resist the tendency to form their oxides. Although considerable progress in both the physics [1-4] and chemistry [5-13] of oxidation and corrosion has been made, we do not have an adequate fundamental physical-chemical model of how alloys oxidize and corrode and the mechanisms by which they resist these processes. The reasons for this have been the inability to identify and measure the potentials that develop at important interfaces. The prediction of these potentials and how the oxide structure develops to maintain the maximum driving force (i.e. overall potential) for oxidation provides a model to predict how the oxide overlayer and passive film structure develops. ALLOY OXIDATION Cocke and co-workers have performed extensive surface oriented low temperature oxidation studies on a number of alloy systems (bulk and thin films prepared in vacuum): Cu-Mn [14], Ag-Mn [15], Ni-Zr [16-18], Ni-Ti [19], Ni-Hf [20], Ti-Cu [21] and Ti-Al [21-23]. The results have allowed delineation of the factors which are most important to the oxide formation process and provide insight into the prediction of passive layer structures. They have shown that the modified Cabrera-Mott (C-M) model [2] provides a qualitative guide to understanding alloy oxidation. The model describes the driving force for the oxidation process as a potential, A0, across the growing oxide formed by the charges at the metal-oxide (+) and the gas-oxide (-) interfaces. This potential is described as the sum of two terms: 421 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society

A(D = -AGO

2e

+ka 1

2e

4e2Nsa"x1

L kTE 0o1

(1)

The first term involves -AG'f, which is the free energy of formation of oxygen anions at the surface (1/2 02 + 2e + surface __> 02- surface) and can be approximated by the free energy of formation of the oxide per 02- species. The second ter