Physical and Chemical Properties of Magnesium Element. The uses and the alloys, and the compounds of magnesium element.
Physical Properties: The principal physical properties of pure magnesium (99.9 per cent) are given below:
Chemical Properties: Chemically, magnesium is a dyad. It is a very reactive element, as is indicated by the fact that it exists in many combined forms in nature. The metal exhibits good resistance to the atmosphere, although the surface becomes dull grey in color owing to the formation of a protective film of oxide. Magnesium resists attack by pure water and most alkaline solutions, but is not resistant to most acid solutions or to those containing chlorides. It is resistant to many common organic chemicals. Protection of the surface may be desirable in many applications; this protection may consist of chemical surface treatment, application of paint systems, or a combination of these measures.
Alloys: Pure magnesium has only moderate strength; but since its light weight is particularly advantageous from the point of view of many structural applications, it has been alloyed extensively with aluminum, zinc, manganese, or combinations of these, and, to lesser extent, with zirconium, thorium, and some of the rare earths. Aluminum and zinc improve the strength, manganese enhances corrosion resistance, zirconium produces a finer grain structure and improved mechanical properties, rare earths and thorium improve the properties at elevated temperatures.
The magnesium alloys exhibit properties similar to those of pure magnesium, but possess, in varying degrees, greatly increased strength. They are among the lightest of structural metals; being about two thirds the weight of aluminum and one fourth that of steel. They have excellent machining properties; a good surface finish and high dimensional accuracy may be obtained with heavy cuts and at high speeds with low requirements of power.
Magnesium alloys are fabricated in the form of sand, permanent mold and die castings, forgings, sheet and plate, extruded shapes, tubing, and wire. Recent advances in magnesium technology have overcome forming problems associated with its hexagonal crystal structure. Forming of wrought products is accomplished by the commonly used processes. Also magnesium and its alloys may be joined by riveting, bolting, screwing, and by the use of adhesives. Welding is commonly done by modern arc-welding techniques and by resistance methods.
Uses: The uses of magnesium alloys can be divided into two types: nonstructural and structural. Among the important nonstructural applications of magnesium is its use as an important alloying constituent of other metals, the most common application of which is in the production of certain aluminum alloys, where it enhances the strength, corrosion resistance, and welda-bility. It is also used in large quantities as a reducing agent in some metallurgical production processes; for example, titanium, uranium, hafnium, and others. Pure magnesium is also employed as a deoxidizer and desulphurizer in the metallurgical industry. It is used as a scavenger in the production of brass, bronze, and nickel, and in combination with calcium to remove bismuth from lead. In the production of iron it nodularizes the structure and hence increases the ductility of the metal. Miscellaneous applications include the use of pure magnesium in the form of powder as an ingredient of pyrotechnics. Marine and railroad signals, miscellaneous fireworks, and photographic applications are similar, but limited to peacetime uses. Shavings and coarse powder are also used in the well-known Grignard reaction for synthesizing certain organic chemicals.
One of the largest and most important nonstructural uses of magnesium is in sacrificial or cathodic protection of other metals. Magnesium, because of its high position in the electromotive series of metals, acts as an anode when connected to a structure called the cathode, which requires protection from its external or internal environment. It has been used to protect such metal structures as underground oil and gas lines, oil well casings, buried tanks of all types, telephone and telegraph cables, marine structures, such as steel piers and ships hulls, and such equipment as industrial and domestic hot-water heaters. A magnesium anode, when connected to such an installation by a suitable conductor, will dissolve gradually over a period of years. During this time the structure remains intact with this protection.
The structural uses of magnesium in its alloyed forms have grown to rank about equally with the nonstructural uses from the viewpoint of tonnage consumed. Modern developments in production, and particularly in fabricating, techniques have been responsible for extending the structural uses so widely. The best-known applications of magnesium structural alloys have been, and still are, in the aircraft industry, where, in the form of sand, permanent mold and die castings, extrusions and forgings, they form such aircraft parts as landing gear, engine parts, many parts of the fuselage and airborne equipment. In recent years magnesium alloys are also finding very substantial use in highway and railroad transport. Magnesium, with its excellent combination of light weight, high strength, and stiffness, is a logical material for structural members of trucks, trailers, buses, and railroad rolling stock. Similarly, equipment which moves or has to be moved by exertion of energy may be improved by lightweight magnesium alloys. Also, where it is desirable to reduce inertia or reciprocating and moving parts of machines, magnesium alloys are a logical choice as the material of construction. Somewhat similarly, the portable tool industry uses magnesium alloys for a wide variety of manually handled equipment.
Compounds: The most important compounds of magnesium are those from which the metal is derived. Some of these same materials are used for the production of caustic-calcined and refractory magnesia, the important applications for which are magnesia bricks and refractories for high-temperature uses.
Other important magnesium compounds are magnesium chloride, used as a dressing and filler for cotton and woolen fabrics and as a filler for papers and building products, in cement, refrigerating brines, and ceramics; magnesium carbonate, magnesium bicarbonate and magnesium hydroxide of various grades, used as filler for insulating materials and for chemical and medicinal purposes; and magnesium sulphate (Epsom salts), used chiefly for medicinal purposes.