Description / Application / Shipment/storage / Risk factors
The most important source of this metal is through the processing of bauxite ore, of which there is a great abundance in the earth’s surface. The characteristics of the metal are its lightness in weight when compared with other metals, resistance to corrosion, high electrical and heat conductivity, and its chemical resistance. With regard to steel making it is used as a deoxidising agent, hence the term all-killed or aluminium killed steel. Aluminium coatings are applied to the surfaces of steel through a patented process of alloying the surfaces of carbon or alloy steel with aluminium by diffusion. This process is referred to as ‘aluministing’ which produces a coated steel offering good resistance to heat and corrosion.
Aluminium is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is non-magnetic and does not easily ignite. A fresh film of aluminium film serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.
Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 Kelvin and a critical magnetic field of about 100 gauss (10 milliteslas).
Corrosion resistance can be excellent due to a thin surface layer of Aluminium Oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is also often greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semi-conductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.
Ingots are manufactured by the freezing of a molten liquid (known as the melt) in a mold. The manufacture of ingots has several aims. Firstly, the mold is designed to completely solidify and form an appropriate grain structure required for later processing, as the structure formed by the freezing melt controls the physical properties of the material. Secondly, the shape and size of the mold is designed to allow for ease of ingot handling and downstream processing. Finally the mold is designed to minimize melt wastage and aid ejection of the ingot, as losing either melt or ingot increases manufacturing costs of finished products.
A variety of designs exist for the mold, which may be selected to suit the physical properties of the liquid melt and the solidification process. Molds may exist in top, horizontal or bottom-up pouring and may be fluted or flat walled. The fluted design increases heat transfer owing to a larger contact area. Molds may be either solid “massive” design, sand cast (e.g. for Pig Iron) or water-cooled shells, depending upon heat transfer requirements. Ingot molds are tapered to prevent the formation of cracks due to uneven cooling. Crack or void formation occurs as the liquid to solid transition has an associated volume change for a constant mass of material. Formation of these ingot defects may render the cast ingot useless, and may need to be re-melted, recycled or discarded.
Wire, Rod, & Bar
Wire is a long, thin string of aluminum that can carry electrical current. Made from rod or bar, wire is, by definition, less than three-eighths of an inch in diameter. Rod and bar are larger. Rod is round, while bar can have any number of flat sides.
Aluminum rod, bar, and wire products can be produced by several different processes.
One method of making these products is similar to rolling sheet. A long, square ingot is heated, progressively reduced in cross-section by passing it through a series of rolls, and then coiled. The coils are heated for softening and, if they are slated to become wire, the rod is pulled through smaller and smaller dies on a wire-drawing machine. Electrical conductor is made by stranding several wires into a single length.
Bar, rod, and tube can also be made by the drawing method directly from stock produced by hot extrusion. This process is also completed by the progressive thinning of the aluminum material through the use of dies.
Electrical conductor rod can also be drawn and stranded directly from molten aluminum.
Electrical transmission lines are the largest users of aluminum rod/bar/wire products. In fact, this is the one market in which aluminum has virtually no competition from other metals. Aluminum is simply the most economical way to deliver electrical power.
Aluminum wire and cable are also used almost everywhere there is an electrical impulse to conduct: in commercial buildings, in machinery and equipment, and in transportation and consumer durables.
Rod and bar become the rivets, nails, screws, bolts, and parts of all kinds of machinery and equipment.
Drawn tube carries liquids in heat exchangers, food processing equipment, water treatment plants, and other industrial applications.
Aluminium is used in building and construction, corrosion-resistant chemical equipment (desalination plants), die-cast auto parts, electrical industry (power transmission lines), photoengraving plates, permanent magnets, cryogenic technology, machinery and accessory equipment, miscible food processing equipment, tubes for ointments, toothpaste, shaving cream, etc. Also as a powder in paints and protective coatings, as rocket fuel, ingredient of incendiary mixtures (thermite) and pyrotechnic devices, as a catalyst, foamed concrete vacuum metalizing and coating. Other uses are as foil in packaging, cooking, decorative stamping, and as flakes for insulation of liquid fuels.
When dispatched from works sheets and other semis products are usually interleaved with tissue paper or treated with oil or lanolin, according to conditions of transport and usage to which they are to be subjected. Coils are treated similarly and are wrapped in heavier paper or fabric.
The surface of aluminium and aluminium alloy products are relatively soft. Sheets, long tubes and extrusions may be scored by being dragged over one another. Black spotting may be caused on sheets by rubbing together of surfaces through loose packing. The high resistance of aluminium to corrosion is due to a natural film of Aluminium Oxide which is integral with the parent metal. This film begins to form immediately when cut metal is exposed to the air and slowly increases in thickness until after some days no further oxidation takes place.
Aluminium can be affected by humidity and atmospheric moisture and the surface of sheets and other aluminium products may become dulled in consequence. This dullness can be removed by polishing.
Prolonged exposure to a moist atmosphere causes slight corrosion resulting in a visible film which is white to grey in colour, but under normal atmospheric conditions this visible film is easily rubbed off, leaving a permanent protective film behind. When sheets are stacked together inside cases or in stores, moisture, which may be condensed in considerable quantities, can penetrate by capillary action between the sheets and cause considerable corrosion. In industrial locations such moisture is usually contaminated by acid products in the atmosphere or by the alkalis in the packing paper which will assist this corrosion. Even though the paper used by the supplier is neutral and free from sulphates or chlorides, the packing may still absorb moisture from the air, and this paper should therefore be removed as soon as possible. When stacked, sheets should be kept in a warm store free of wide variations in temperature. Failure to observe these precautions may result in staining or corrosive attack which may render the sheets unsuitable for their purpose if a high quality surface is necessary.
Corrosion by moisture is accelerated by various alkaline compounds which may be present in cement, mortar, plaster, etc., and therefore aluminium materials should not rest on unprotected concrete floors or be stowed in close proximity to such goods. Superficial corrosion arising out of condensation is rarely harmful to the metal itself, and can usually be removed by rubbing with a hand cloth and whiting. In more severe cases steel wire or a steel wire brush is sometimes used following by pickling in a cleaning solution.
Oil and grease can be removed by hand cleaning with petroleum solvents or kerosene or with emulsions which can readily be washed away by water after application. Cutting oils and other lubricants may result in corrosion if in contact for any length of time. If aluminium articles of a small nature have been in contact with moisture, they should be forced air dried.
Aluminium Discs – Subject to damage by humidity causing corrosion and stains. Provided the corrosion is not too deep, the metal is perfectly workable without any special treatment. If the finished article is of the frosted type, no sign of corrosion or staining will be visible in the final product. The conventional process of frosting is carried out by dipping the finished article in hot Caustic Soda followed by a weak nitric acid swill.
- Moisture damage