Binary alloys passivity layer composition

Fig. 18. Schematic diagram for a binary alloy with a passivating oxide film in contact to electrolyte with the reactions of (1) oxide formation, (2) electron transfer, and (3) corrosion, including (4) oxidation of lower-valent cations and the indication of ionic and atomic fractions X as variables for the composition of the layer and the metals surface.

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. 

Although less work has been done on the passivation of other binary alloys, many interesting observations have been made and a few selected examples will be considered. In the case of Ni-Fe alloys of varying composition, alloy dissolution results in surface enrichment with nickel (as a consequence of the preferential dissolution of iron) and the formation of a passive film composed of an itmer layer of... 

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. 

Schematic diagram of the chemical composition and structure of passive layers on pure metals and binary alloys as obtained from XPS and ISS investigations. (From Strehblow, H.-H., in Passivity of Metals, R.C. Alkire, D.M. Kolb, eds., Wiley-VCH, Weinheim, Germany, pp. 271 74,2003.)...

If the electrochemistry is understood, one needs the application of surface analytical methods to learn about the chemical properties and chemical structure of passive layers. Then one has to take care that electrochemical specimen preparation has to occur with optimum control in order to get reliable results. This permits to draw clear mechanistic conclusions on the properties of the layers like their growth, reduction, changes of their composition, reactivity, degradation, and stability including realistic environmental conditions. Application of XPS, ISS, and RBS to a wide variety of pure metals and binary alloys has been described in Section 5.6. These techniques provide valuable results especially when applied together with a systematic change of the experimental parameters like the potential and time of passivation, the composition of the electrolyte, and alloy and conditions for layer degradation. 

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