Binary alloys passivity film structure

The analysis of several pure metals and binary alloys yields generally at least a duplex and in some cases a multilayer structure of the passive film, as depicted schematically in Fig. 19. These systems have been examined with surface analytical methods, mainly XPS, but also ISS in some cases. The systematic variation of the electrochemical preparation parameters gives insight to the related changes of layer composition and layer development, and support a reliable interpretation of the results. Usually the lower valent species are found in the inner part and the higher valent species in the outer part of the passive layer. It is a consequence of the applied potential which of the species is dominating. Higher valent species are formed at sufficiently positive potentials only and may suppress the contribution of the lower... 

XPS has been applied to study the composition and formation of passivating layers on various pure metals and binary alloys. Usually, a multilayer structure is found with the lower valent species in the inner and the higher valent species in the outer part of the film. Generally, hydroxides are located at the surface and oxides at the inner part of these layers. The distribution and accumulation of cations within these films are characteristic for the alloy components and are closely related to their contribution to the improvement of passivity of these metals (Strehblow, 1997). XPS is a valuable tool to detect the role of alloy components and to understand their influence on the corrosion properties of metals. 

In this chapter, these thermodynamic and kinetics aspects of passivity are presented after a brief historical survey The following section discusses the electrode kinetics in the passive state. Next the chemical composition and chemical structure of passive films form on pure mefals are reviewed wifh an emphasis on iron. This is followed by a compilation of data for binary alloys. The elecfronic properties of passive layers are fhen discussed, and the last section covers the structural aspects of passivify. 

The role of alloying elements in the passivation process has been briefly discussed. Alloying additions such as chromium and molybdenum can substantially influence the structure and composition of the passive oxide film and thereby the process of passivation. The alloys discussed have been of the fairly simple binary type, where it is easier to analyze the surface oxide films by surface analytical techniques and to understand the results. This treatise provides a basis for the following discussion of stainless steels, where the number of alloy additions is increased as is the complexity of the passivation process. 

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