Aluminium

Description

A metal produced from the ore bauxite. 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 handcloth 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.