Alloys of Palladium

Palladium yields a few alloys, the most important of which are those containing gold.

With copper the freezing-point curve falls steadily from the freezing- point of palladium to that of copper, the portion representing alloys rich in copper being nearly horizontal. There is no evidence of the formation of compounds, the two metals yielding complete series of mixed crystals. The alloys are harder than the individual metals, exhibiting a maximum hardness with equal parts of each by weight.

With gold, palladium likewise yields no compounds, the freezing- point curve falling continuously, and lying concave to the axis of concentration. The hardness of the alloys increases up to 70 per cent, of palladium and then decreases. Alloys containing more than 10 per cent, of palladium are white.

A couple consisting of gold wire and a wire containing 60 per cent, of gold, the remainder being palladium, has approximately six times the thermo-electromotive force of a platinum-rhodium couple. Its utility, however, is curtailed by the liability to undergo disintegration which characterises palladium alloys.

The addition of gold to palladium first increases and then reduces the power of the latter metal to occlude hydrogen, the reduction being proportional to the amount of gold in the alloy. The occlusion is reduced to nil in the presence of 75 per cent, of gold.

Alloys of palladium and gold, containing from 60 to 90 per cent, of the latter metal, are known as rhotanium, and, on account of their high melting-point, strength, and incorrodibility, have been recommended as substitutes for platinum.

An alloy of palladium and gold has been placed on the market under the name of Palan as a platinum substitute. It is claimed to be even superior to platinum in certain respects. It melts at 1370° C., and has a density of 17.22. When heated strongly it loses only very slightly in weight, being superior in this respect to platinum containing 2.4 per cent, of iridium, although inferior to that containing only 0.6 per cent, of iridium. Its comparative freedom from iron is an advantage, and in its resistance to hydrochloric, nitric, hydrofluoric, and sulphuric acids, and to sodium hydroxide solution, and fused sodium carbonate compares favourably with that of platinum. It is not so good, however, for potassium pyrosulphate fusions.

The freezing-point curve of mixtures of palladium and silver is similar to that of palladium and gold, and no compounds appear to be formed. The hardness of the alloys lies between that of the components.

An alloy containing 5 parts by weight of palladium and 4 of silver was found by Graham to be still capable of absorbing hydrogen.

Palladium silver alloys admit of receiving a high polish, and retain their bright surface. An alloy containing 38 per cent, of palladium, the remainder being silver, was formerly used for dental purposes.

With lead the freezing-point curve exhibits two maxima, namely, at 454° C. and 1219° C., corresponding to the compositions PdPb₂ and Pd₃Pb, respectively. Three breaks occur at 495°, 596°, and 830° C. respectively. The first and last, namely, at 495° and at 830° C. indicate the existence of the compounds PdPb and Pd₂Pb, and which decompose below their melting-points. It is not yet certain what the composition of the alloy represented by the break at 596° C. may be.

Interesting support to the belief that the compound Pd₂Pb can exist is afforded by the results of experiments to determine the difference of potential between various alloys and pure lead in a normal solution of lead nitrate. The alloys were prepared by melting the palladium and lead under a mixture of lithium chloride and either potassium or barium chloride. Alloys containing less than 33 per cent, of palladium have a potential practically identical with that of pure lead, whilst those containing more than this amount of palladium exhibit a higher potential, which at first rapidly increases with the palladium. Between 20 and 90 per cent, of palladium the alloys are harder than the individual components, a maximum occurring with 65 per cent, of palladium.

Palladium and nickel yield a continuous series of solid solutions. The equilibrium diagram exhibits a flat minimum near 1208° C.,that is, between 40 and 60 per cent, of palladium. The alloys are all more readily susceptible to attack by concentrated nitric acid than are the individual metals themselves. Alloys up to 87 per cent, of palladium are magnetic, but the magnetic power rapidly declines from 40 per cent, of palladium upwards. The alloys can be forged to a certain extent, those containing a high percentage of nickel being somewhat more easily worked.

Palladium Amalgam is readily prepared by triturating finely divided palladium with mercury. Union takes place with evolution of heat and sometimes accompanied by explosion.

The finely divided metal is obtained for this purpose by reduction of palladous chloride with strips of metallic zinc, the palladium deposit being carefully washed and dried. After prolonged exposure to air it appears to lose its affinity for mercury, possibly on account of slow oxidation.

Palladium amalgam is sometimes used as a filling for teeth.