Alloys of Magnesium

Alloys of magnesium and zinc resist oxidation more than either of the metals separately. Many other magnesium alloys are very chemically reactive. Le Bon found that magnesium acted much more vigorously on water when it was amalgamated with very small quantities of mercury. In 1864 Phipson described a hard, brittle, lavender-coloured alloy containing 85 parts of tin and 15 of magnesium, which decomposed water at ordinary temperatures and took fire if air were admitted during its preparation. This was a forerunner of a series of very chemically active alloys which contain magnesium.

Alloys containing 5.50 per cent, magnesium and 95.50 per cent. lead rapidly absorb oxygen from moist air. The alloy consisting of the compound Mg₂Pb is the most reactive: heat is required when the magnesium exceeds 35 per cent. The oxidation crumbles the alloy to a black powder of Mg(OH)₂ and Pb₂(OH)₂. In presence of water the Pb₂(OH)₂ oxidises to Pb(OH)₂. Free hydrogen is liberated when the alloys are boiled with water; digestion with water under pressure at about 150° C. completely oxidises the lead to lead oxide and liberates the theoretical quantity of hydrogen. Mg₂Pb is the only known definite compound of magnesium and lead.

The pyrophoric alloys, such as are used in cigarette-lighters, etc., which produce sparks when struck with hardened steel, contain cerium alloyed with other metals - usually iron. These alloys spark easily, because cerium combines energetically with oxygen at a low temperature. A surface film of black cerium suboxide may be an important agent in the process. According to Hirsch, alloys of magnesium with 75.85 per cent, of cerium are highly pyrophoric. The compound CeMg contains approximately 85 per cent, of cerium, and Vogel says that the alloy corresponding with this compound is pyrophoric. He says the same of the alloy corresponding to Ce₄Mg, which contains about 96 per cent, of cerium. Since the combination between magnesium and cerium is endothermic, their alloys are very effective in thermal reduction processes.

The industrially important alloys of magnesium and aluminium, known as magnalium, were originally prepared by making molten aluminium the cathode in a fused salt of magnesium. The earlier magnaliums appear to have contained less than 2 per cent, of magnesium. Magnesium compounds, such as carnallite, are now commonly electrolysed below a red heat and aluminium added during the process.

The tensile strength and elastic limit of the alloys show a maximum at about 8 per cent, of magnesium. They increase in hardness and decrease in density with the magnesium content. The alloys containing 10.30 per cent, of magnesium are malleable, have a density between 2.0 and 2.5, are suitable for castings, easily worked without softening, polish well, and do not fracture easily. They polish better and become more brittle with increase in the magnesium content.1 Magnalium does not easily corrode.

The common magnalium, containing 10 per cent, of magnesium, can be soldered3 and plated with gold or nickel.

Since magnalium retains a silvery lustre it is useful for making certain parts of optical instruments, such as mirrors, and is used in parts of various machines and scientific instruments.

The compound Mg₄Al₃ consists of silver-white, hard, brittle crystals. There is probably a compound MgAl, and there may be Mg₃Al₂. Mg₂Al and MgAl₄ are doubtful.

Lithium and magnesium appear to form solid solutions. Sodium, according to Phipson, forms malleable alloys with magnesium that readily decompose water. According to Mathewson sodium will dissolve about 1.6 per cent, of magnesium at 657° C., but the latter separates out in hexagonal crystals as the temperature falls. Magnesium will dissolve about 2 per cent, of sodium.

Potassium appears not to mix with fused magnesium, though Phipson said it formed malleable alloys that decomposed water.

Copper forms Mg₂Cu and MgCu₂, indicated by the freezing-point curve, that are brittle, crystalline, and coloured like magnesium. Boudouard found an indication of MgCu on the curve, and claimed to have isolated this compound.

Silver. - Alloys of silver and magnesium are harder than their components, brittle, and more easily oxidised or decomposed by water than magnesium itself. Freezing-point and conductivity curves indicate the compounds MgAg and Mg₃Ag.

Gold forms alloys with magnesium that are stable in air at ordinary temperatures. They are yellow when the percentage of magnesium is not over 18 and silver-grey when it is.

Mg₃Au (m.-pt. 83° C.), Mg₂Au (m.-pt. 796° C.), and MgAu (m.-pt. 1160° C.) are indicated on the freezing-point curve. Mg₃Au separates from its alloys with magnesium in large regular crystals. The action between gold and molten magnesium is violent.

Calcium alloys with magnesium in all proportions. The alloys with 10 per cent, and over of calcium are brittle. Ca₃Mg₄, indicated on the freezing-point curve, is silvery, brittle, stable in air, and only slowly acted upon by water.

Zinc. - The compound Mg₄Zn has been described, but MgZn₂ seems to be the only known compound of zinc with magnesium. It has been isolated by distilling off, in vacuo, the excess of zinc from a mixture of its constituents, and can be distilled in vacuo without change.

Cadmium and magnesium form a single compound, MgCd, which is greyish white, slightly harder than cadmium, oxidised in moist air, and acted upon readily by water. Two forms of MgCd, differing in hardness, are indicated by electrical conductivity experiments. It melts at 427° C. and dissolves in all proportions in either metal. Magnesium and cadmium form a continuous series of solid solutions.

Mercury. - Magnesium does not amalgamate easily, but amalgams have been made by introducing magnesium ribbon into nearly boiling mercury, and by acting on crystalline magnesium sulphate with potassium amalgam. Silvery crystals of an amalgam have been prepared by rubbing small quantities of magnesium at a time into mercury contained in a warm mortar. The product dulled in air. The immediate product is a thick fluid which cools to a hard crystalline mass. One part of magnesium and 18 parts of mercury (practically MgHg₂) give the most satisfactory product. These amalgams reduce many organic compounds.

Cambi and Speroni affirm the existence of MgHg₂ and suggest that of MgHg.

Magnesium amalgams decompose water.

Thallium alloys with magnesium blacken in air, especially if the air is moist, through oxidation. The melting-point curve indicates the compounds Mg₈Tl₃, Mg₃Tl, and Mg₃Tl₂.

Tin forms the compound Mg₂Sn when it is melted with magnesium in hydrogen at 700°-800° C. The combination develops heat, and the compound is brittle and easily tarnished in air. It crystallises in regular octahedra.

Antimony forms Mg₃Sb₂ in steel-grey needles that slowly oxidise in air.

Bismuth forms Mg₃Bi₂, a steel-grey brittle compound that slowly oxidises in moist air.

Nickel and magnesium are quite miscible in the fused state. The freezing-point curve indicates the compounds MgNi₂ and Mg₂Ni.