Passivity binary alloys

Broadly speaking the binary alloying additions fall into two categories (1) those that improve passivity of Ni, viz. Cr, Si, Sn, Ti, Al and (2) those... 

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... 

A W One of the partners plays, relative to the other, the role of a passive host lattice. However, this case is not trivial, because of the difficult problem that arises in the topology of the crystal the existence, at a given concentration, of a threshold of a topological percolation (for which an infinite coulombic aggregate appears in one of the two species) suffices to show the complexity and the variety of the random binary alloy. 

Mechanistic studies are often performed on binary alloys. Hf-Zr-binary alloys are excellent examples [118]. Because of the lan-thanoid contraction, both elements show an extraordinary chemical similarity. The atom radii are identical and hence both can substitute for each other easily. Both elements form stable passive films and so do all their alloys. The physical parameters of the oxides such as density, permittivity, and crystallinity show a continuous changeover with variation of the atomic ratio. 

P. King, H. Uhhg, Passivity in the iron-chrominm binary alloys, J. Phys. Chem. 63 (1959) 2026—2032. S. Fujimoto, R.C. Newman, S.P. Kaye, H. Kheyrandish, J.S. Colligon, Passivation thresholds in iron-chromium alloys prepared by ion-beam spnttering, Corros. Sci. 35 (1993) 51-55. 

D.E. Williams, R.C. Newman, Q. Song, R.G. Kelly, Passivity breakdown and pitting corrosion of binary alloys. Nature 350 (1991) 216-219. 

Because the binary nickel-molybdenum alloys have poor physical properties (low ductility, poor workability), other elements, for example, iron, are added to form ternary or multicomponent alloys. These are also difficult to work, but they mark an improvement over the binary alloys. Resistance of such alloys to hydrochloric and sulfuric acids is better than that of nickel, but it is not improved with respect to oxidizing media (e.g., HNO3). Since the Ni-Mo-Fe alloys have active corrosion potentials and do not, therefore, establish passive-active cells, they do not pit in the strong acid media to which they are usually exposed in practice. 

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. 

I. Aimeigren, M. Keddam, H. Takenouti, and D. Thierry, Modelling of the passivation mechanism of Fe-Cr binary alloys from ac impedance and frequency resolved RRDE. 2. Behaviour of Fe-Cr alloys in 0.5 M H2SO4 with an addition of chloride, Electrochim. Acta 42 1595 (1997). 

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. 

Model of passive layer for a binary alloy and related variables. (From Strehblow, H.-H., in Passivity of Metals, R.C. AUdre, D.M. Kolb, eds., Wiley-VCH, Weinheini, Germany, pp. 271-374,2003.)... 

Almost all pure metals and binary alloys form passive layers with a multilayered structure. Even for pure metals with only one oxidation state of the cahons, a bilayer structure is observed with an inner oxide topped by an outer hydroxide. The situation becomes more complicated with several oxidation states and for alloys. 

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. 

Many different alloying elements, when added to FeTbCo in the amount of about 5%, have been found to improve the corrosion resistance [135,165]. Both improved passivity and reduced pitting susceptibility have been achieved in this fashion. The primary protection of MO disks, however, derives from protective overlayers. The MO layer is typically covered by a dielectric layer for optical and thermal reasons. Some designs employ a metallic layer such as A1 as a reflector, which covers the dielectric layer. Sputter-deposited A1 binary alloys, which display for superior pitting resistance compared with pure Al, have also been used. The metallic reflective layer in MO disk is the first application of these remarkably corrosion-resistant alloys that have generated significant attention [166-171]. 

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