Ni-Cu alloys

Copper alloys include brasses (Cu-Zn alloys), bronzes (Cu-Sn alloys), cupronickels (Cu-Ni alloys) and nickel-silvers (Cu-Sn-Ni-Pb alloys). 

Most cases of crevice corrosion take place in near-neutral solutions in which dissolved oxygen is the cathode reactant, but in the case of copper and copper alloys crevice corrosion can occur owing to differences in the concentration of Cu ions however, in the latter the mechanism appears to be different, since attack takes place at the exposed surface close to the crevice and not within the crevice in fact, the inside of the crevice may actually be cathodic and copper deposition is sometimes observed, particularly in the Cu-Ni alloys. Similar considerations apply in acid solutions in which the hydrogen ion is the cathode reactant, and again attack occurs at the exposed surface close to the crevice. 

To examine the situation with alloys in a little more detail, the Cu-Ni alloys will first be considered. Here the mutual solubility of the two oxides NiO and CU2O can probably be neglected, and these are the only two possible oxidation products. Assume for simplicity that the alloy is thermodynamically ideal, and let and Xn be the mole fractions in the alloy. Consider the reactions... 

When a pure metal A is alloyed with a small amount of element B, the result is ideally a homogeneous random mixture of the two atomic species A and B, which is known as a solid solution of in 4. The solute B atoms may take up either interstitial or substitutional positions with respect to the solvent atoms A, as illustrated in Figs. 20.37a and b, respectively. Interstitial solid solutions are only formed with solute atoms that are much smaller than the solvent atoms, as is obvious from Fig. 20.37a for the purpose of this section only three interstitial solid solutions are of importance, i.e. Fc-C, Fe-N and Fe-H. On the other hand, the solid solutions formed between two metals, as for example in Cu-Ag and Cu-Ni alloys, are always substitutional (Fig. 20.376). Occasionally, substitutional solid solutions are formed in which the... 

According to the data in Table III, the value of the ratio P)Mm is approximately the same for the metals Au, Fe, Co, Ni, and Pd. Binary alloys formed from any pair of these metals can therefore be expected to evaporate without substantial fractionation. On the other hand, films evaporated from Ag-Pd and Cu-Ni alloys can be expected to be enriched in Ag and Cu, respectively. These predictions are largely confirmed by experiment. For example, the composition of Pd-Au films was found to be the same as the wires which were evaporated (46), but in the case of Pd-Ag, evaporation of a 30% Ag-Pd alloy ware yielded a 50% Ag-Pd alloy film (47)- Alexander and Russell evaporated a number of alloys from pellets in the reaction vessel as shown in Fig. 5 (48) The alloy pellet was placed in a small quartz cup with its surface equidistant from the hemispherical top of the reaction vessel. The pellet was evaporated by... 

Spectra for a series of Cu-Ni alloys have been obtained (91) and these are reproduced in Fig. 11. Because of overlapping of peaks from the component metals, separate indications of each element are only obtained from the 925 eV Cu peak and the 718 eV Ni peak. The results have only qualitative significance because the quoted nickel concentrations are bulk values. Nevertheless, they do suggest that for these particular samples of Cu-Ni alloys, the surface composition varies smoothly from pure copper to pure nickel. Auger spectroscopy has subsequently shown that the surface composition of the (110) face of a 55% Cu-Ni crystal was identical with the bulk composition (95a). Ono et al. (95b) have used the technique to study cleaning procedures argon ion bombardment caused nickel enrichment of... 

Early catalytic studies on Cu-Ni alloys were prompted by the suggestion of Dowden and Reynolds (107, 108) that d-band vacancies are... 

Further progress in the study of the Cu-Ni system awaited the preparation and careful characterization of alloy films of known bulk and surface composition. The essential step was taken by Sachtler and his co-workers 28, 88, 114) who prepared Cu-Ni alloy films by successive evaporation of the component metals in UHV. After evaporation the films were homogenized by heating in vacuum at 200°C. The bulk composition of the alloys was derived from X-ray diffraction, and the photoelectric work function of the films was also measured. A thermodynamic analysis, summarized by Fig. 13, indicated that alloy films sintered at 200°C should consist, at equilibrium, of two phases, viz., phase I containing 80% Cu and phase II containing 2% Cu. Evidence was presented that alloys within the... 

Fig. 13. Free energy of mixing at 200°C for Cu-Ni alloys with miscibility gap indicated (33).

Fig. 14. Adsorption ratio a as a function of composition for Cu-Ni alloy films Ni deposited on Cu (O), Cu deposited on Ni (A), both sintered at 200°C Cu deposited on Ni and sintered at 300°C (V) (70).

It was claimed that this model helps to explain earlier catalytic results using Cu-Ni alloys, but comparisons with alloys in granular, or other massive form, are difficult. The available catalytic results on Cu-Ni alloys show that the method of preparation of the catalyst can have a profound influence upon the observed activity pattern. The promoting effect on the catalytic activity, caused by cooling in hydrogen rather than in vacuum... 

Fig. 15. Benzene hydrogenation at 150°C over Cu-Ni alloy films symbols are as defined in Fig. 14 (70).

Fig. 16. Butene-1 hydrogenation over Cu-Ni alloy films, annealed in hydrogen at 530°C (117).

We have seen that the early hope that catalytic studies on Cu-Ni alloys would provide clear confirmation of the simple d-band theory has... 

Fig. 19. Rate of H2-D2 exchange (at -40°C) divided by nickel content as a function of composition for Cu-Ni alloy films deposited at 300°C and sintered at 400°C 84).

The Pt-Au films were not used in catalysis, but the chemisorption of CO was studied. The work function of Pt was only raised by 0.03 eV and there was no change with the alloys after short exposures to CO. It was therefore not possible to titrate the Pt content of the surface with CO in the same way as hydrogen was used with Cu-Ni alloy films (2). Long-term exposure of the films to 10 5-10-4 Torr CO at 20°C for periods up to four days caused the work function of the alloys to increase slowly Fig. 31. After 16 hr this increase was more evident in the Pt-rich region, but the effect was observed on the Au-rich regions after longer exposures. The effect was accelerated if the films were maintained at 100°C. These results were cited as direct evidence for the enrichment of the surface with platinum... 

Technogenic phases, such as various Fe-alloys ((Cr,Ni)-ferroalloy, (Cu,Ni)-alloy, and (Fe, Si)-alloy), Fe-oxides ((Cr,V,Fe)-oxide, (Mo,W,V,Cr,Fe)-oxide, (Monoxide, (W,Cr,Fe)-oxide), and spherical particles ((Cr,Ni)-oxide or chrome-nickel-spinel (Fig. 3), (Cr.Fe)-oxide or chrome-spinel), were recognized in the area of the Ravne ironworks. 

However, surface segregation (cs / Cb) has a radical effect on AE, as can clearly be seen in Fig. 6.3(b). Cu/Ni alloys are known (Kelley and Ponec 1981, Ouannasser et al 1997) to have an enriched Cu concentration in the surface layer for all bulk concentrations. As a result, the alloy shows a more Cu-like behaviour than it would if it were non-segregated. In particular, AE has a value significantly closer to that for pure Cu than in the case where cs = Cb, and this occurs at all bulk concentrations c. The smallest change in AE occurs in Cu-rich alloys, which is understandable, because these alloys have mostly Cu in the surface layer anyway, so the effect of surface segregation is relatively small. Thus, surface segregation has a lesser effect in these alloys than in Ni-rich ones, which have mostly Ni in the bulk, but may have a Cu majority in the surface layer. Clearly, then, the concentration cs of the surface layer is the primary parameter in determining the chemisorption properties of the DBA. 

Vibrating Electrode Atomization (VEP) 300-500 Mild steel, Cr-Ni steel, Cu-Ni alloy, W — -0.2 — Spherical, high-purity particles, Simple Low volume productivity... 

In the VEP, currents used are between 600 and 1200 A at potentials between 30 and 60 V. The vibration frequency of the wire electrode is up to 500 Hz. The materials atomized via VEP include mild steel, Cr-Ni steel, Cu-Ni alloy and tungsten. The VEP is carried out in an inert atmosphere (typically argon) for most alloys, but the arc is struck under water for tungsten wire. Wire diameter is 1-4 mm, and its feed rate is 1.7-4.3 m/min. The feed rate and current density must be determined properly according to the relationship between these two variables. At lower current densities, the wire electrode tends to stick to the rotating electrode. At higher current densities, the wire electrode becomes overheated, causing it to bend or even rupture. 

The origin of the attractive interaction and its difference on Ni(l 10) and Ni(lOO) is somewhat uncertain, but it is reasonable to speculate that it arises from dipole-dipole attractions in the adlayer. Quantitative TPRS (99) and results on (110) oriented Cu/Ni alloy surfaces (100) showed that the anhydride required four nickel atoms for stabilization on the (110)... 

Fig. 21. Plot showing the falloff in CO produced from formic anhydride on a Cu/Ni alloy with changing surface composition (100). Reprinted with permission from Journal of Inorganic Chemistry 17, 1978. Copyright 1978, American Chemical Society.

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