Aluminum

Called aluminium in England, aluminum is a white metal with a bluish tinge (symbol Al, atomic weight 26.97), obtained chiefly from bauxite. It is the most widely distributed of the elements next to O2 and silicon, occurring in all common clays. Aluminum metal is produced by first extracting alumina (aluminum oxide) from the bauxite by a chemical process. The alumina is then dissolved in a molten electrolyte, and an electric current is passed through it, causing the metallic aluminum to be deposited on the cathode. The metal was discovered in 1727, but was obtained only in small amounts until it was reduced electrolytically in 1885.

Pure (99.99%) aluminum has a specific gravity of 2.70 or a density of 2685 kg/m3, a melting point of 660°C, electrical and thermal conductivities about two thirds that of copper, and a tensile modulus of elasticity of 62,000 MPa. The metal is nonmagnetic, highly reflective, and has a face-centered cubic (fcc) crystal structure. Soft and ductile in the annealed condition, it is readily cold-worked to moderate strength. It resists corrosion in many environments as the result of the presence of a thin aluminum oxide film.

Iron, silicon, and copper are the principal impurities in commercially pure aluminum, wrought products containing at least 99% aluminum and a foil product containing at least 99.99%. Such unalloyed aluminum is available in a wide variety of mill forms and constitutes the aluminum 1XXX series in the designation system for wrought aluminum and aluminum alloys. Annealed sheet is quite ductile — 35 to 45% tensile elongation — but weak, having a tensile yield strength of 27 to 35 MPa. Cold reduction of 75% increases yield strength to 124 to 166 MPa. Thus, the unalloyed metal is used far more for its electrical, thermal, corrosion-resistant, and cosmetic characteristics than for its mechanical properties. Applications include electrical and thermal conductors, capacitor electrodes, heat exchangers, chemical equipment, packaging foil, heat and light reflectors, and decorative trim.


Aluminum flake enhances the reflectance and durability of paints. Aluminum powder is used for powder-metal parts. Aluminum powder and aluminum paste are used in catalysts, soaps, explosives, fuels, and thermite welding. Aluminum shot is used to deoxidize steel, and aluminum foam, made by foaming the metal with zirconium hydride or other hydrides, is an effective core material for lightweight structures.

Anodized aluminum is aluminum with a hard aluminum oxide surface imparted electro-lytically using the metal as the anode. The coating, which is much thicker than the naturally formed aluminum oxide film, provides additional corrosion and weathering resistance in certain environments, but is generally no more protective in acidic or alkaline solutions outside the 4 to 8.5 pH range. The coating is noncon-ductive, wear resistant, and can be produced in various colors, thus enhancing decorative appeal. Whereas the naturally formed oxide film is less than a millionth of an inch thick, anodized coatings range from 0.005 to 0.008 mm for bright auto trim to as much as 0.030 mm for architectural applications.

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