Binary alloys passivity corrosion

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.

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. 

Cobalt-base alloys. The corrosion behavior of pure cobalt has not been documented as extensively as that of nickel. The behavior of cobalt is similar to that of nickel, although cobalt possesses lower overall corrosion resistance. For example, the passive behavior of cobalt in 0.5 M sulfuric acid has been shown to be similar to that of nickel, but the critical current density necessary to achieve passivity is 14 times higher for the former. Several investigations have been carried out on binary cobalt-chromium alloys. In cobalt-base alloys, it has been found that as little as 10% chromium is sufficient to reduce the anodic current density necessary for passivation from 500 to 1 mA cm". For nickel, about 14% chromium is needed to reduce the passivating anodic current density to the same level. 

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]. 

The critical compositions for passivity in the Cr-Ni and Cr-Co alloys, equal to 14% Cr and 8% Cr, respectively, can also be related to the contribution of electrons from nickel or cobalt to the unfilled rf-band of chromium [49]. In the ternary Cr-Ni-Fe solid solution system, electrons are donated to chromium mostly by nickel above 50% Ni, but by iron at lower nickel compositions [50]. Similarly, molybdenum alloys retain in large part the useful corrosion resistance of molybdenum (e.g., to chlorides) so long as the d-band of energy levels for molybdenum remains unfilled. In Type 316 stainless steel (18% Cr, 10% Ni, 2-3% Mo), for example, the weight ratio of Mo/Ni is best maintained at or above 15/85, corresponding to the observed critical ratio for passivity in the binary molybdenum-nickel alloys equal to 15 wt.% Mo [51]. At this ratio or above, passive properties imparted by molybdenum appear to be optimum. 

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