Pure aluminium becomes affected by humidity and atmospheric moisture and the surfaces of the sheets and other aluminium manufactures will become dulled in consequence. This condition is not serious unless there has been heavy corrosion such as would arise by actual contact with water and the goods allowed to remain in a damp condition for some time. In the ordinary course of events the dullness can be removed by polishing.
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.
Prolonged exposure to an atmosphere causes slight corrosion resulting in a visible fun which is white to gray in color, 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 alkalis in the packing paper which will assist this corrosion. Even though the paper used by the supplier is 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.
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 wool or a steel wire brush is sometimes used, followed by pickling in a cleaning solution.
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. See also IMDG Code & US CFR.
- Commodity Name: