Two Metals Present

The electrical resistance, measured on a dry assembly, should be as high as possible, typically several hundred thousand ohms. In order to achieve this, the bolts, whatever their material (steel, stainless steel or aluminium alloy), must be carefully insulated from the two metals in presence, by using insulating sleeves in the bores and insulating washers under the bolt heads and nuts. 

Such a luxury insulation is necessary only if the heterogeneous assembly is immersed (or located in an area that will retain water), and provided that safety rules do not obUge the use of a common grounding. In this case, the common grounding will transform the immersed aluminium structures into sacrificial anodes for the cathodic protection of steel constructions. 

The insulation of flanges in aluminium and steel in a circuit can be achieved by using an elastomer joint maintaining a gap between the metals that is sufficient to avoid galvanic corrosion. 

It should be recalled that mbber charged with carbon or graphite may lead to severe galvanic corrosion by its simple presence as a charge. 

---

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal on to one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and mqst Vapid procedures are those in which a fixed applied potential or a cqqp nt cell current is employed, but in both cases selectivity is poor and they are generally used when there are only one or two metals present. Selective deposition of metals from multi-component mixtures can be achieved by controlling the cathode potential automatically with a potentiostat. This device automatically monitors and maintains the cathode potential at a pre-determined value by means of a reference electrode and servo-driven potential-divider. The value chosen for the cathode potential is such that only the metal of interest is deposited and there are no gaseous products formed. 

To obtain a metallurgical bond between two metals, the atoms of each metal must be brought sufficiently close so that their normal forces of interatomic attraction produce a bond. The surfaces of metals and alloys must not be covered with films of oxides, nitrides, or adsorbed gases. When such films are present, metal surfaces do not bond satisfactorily (see Metal surface treatments). 

In the foregoing it has been assumed that the complex species does not contain more than one metal ion, but under appropriate conditions a binuclear complex, i.e. one containing two metal ions, or even a polynuclear complex, containing more than two metal ions may be formed. Thus interaction between Zn2+ and Cl ions may result in the formation of binuclear complexes, e.g. [Zn2Cl6]2-, in addition to simple species such as ZnCl3 and ZnCl -. The formation of bi- and poly-nuclear complexes will clearly be favoured by a high concentration of the metal ion if the latter is present as a trace constituent of a solution, polynuclear complexes are unlikely to be formed. 

There is an appreciable difference between the stability constants of the CDTA complexes of barium (log K = 7.99) and calcium (log K = 12.50), with the result that calcium may be titrated with CDTA in the presence of barium the stability constants of the EDTA complexes of these two metals are too close together to permit independent titration of calcium in the present of barium. 

For analytical purposes silicates may be conveniendy divided into the following two classes (a) those ( soluble silicates) which are decomposed by acids, such as hydrochloric acid, to form silicic acid and the salts (e.g. chlorides) of the metals present and (b) those ( insoluble silicates) which are not decomposed by any acid, except hydrofluoric acid. There are also many silicates which are partially decomposed by acids for analytical purposes these will be included in class (b). 

The raw materials needed to supply about ten million new automobiles a year do not impose a difficult problem except in the case of the noble metals. Present technology indicates that each car may need up to ten pounds of pellets, two pounds of monoliths, or two pounds of metal alloys. The refractory oxide support materials are usually a mixture of silica, alumina, magnesia, lithium oxide, and zirconium oxide. Fifty thousand tons of such materials a year do not raise serious problems (47). The base metal oxides requirement per car may be 0.1 to 1 lb per car, or up to five thousand tons a year. The current U.S. annual consumption of copper, manganese, and chromium is above a million tons per year, and the consumption of nickel and tungsten above a hundred thousand tons per year. The only important metals used at the low rate of five thousand tons per year are cobalt, vanadium, and the rare earths. 

The same result as in the case po2 = Po2 is obtained if the electrolyte is not present but the two metals are brought in direct contact so that jlw = Pr, from which Eq. (5.36) is directly derived. 

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

Scheme 5.1. Alloy formation and segregation in bimetallic systems with one of the metals present as a minority. The scheme qualitatively predicts whether two elements form a surface alloy or a solid solution. The results are valid in vacuum. As soon as an adsorbing gas is...

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

Further evidence for different reaction pathways is obtained in isotope labelling experiments, illustrated in fig.6. Here we present a comparison of the effects of preadsorbing various amounts of deuterium with methyl groups cm the two metal surfaces. 

With ex = e2 = 1, capacitances were very small compared to experimental ones, so that Cs(dip) would have to be much smaller than generally accepted values to get agreement with experiment. With a dielectric present but no 6-function barrier, more reasonable results were obtained. The addition of the barrier changed 6 M, but had little effect on capacitances. Of course, these calculations are of interest only in comparing the contributions of two metals, or in investigating the importance of modifications of the model, since there is no solvent. 

---