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Handling and Storage
Aluminium is one of the easiest materials to keep in good condition. It has a high natural resistance to corrosive conditions normally encountered during shipment and storage, and a little care will maintain its original appearance for a long time. Generally, guard against conditions that might cause surface abrasions or water stains. Suppliers make every effort to pack aluminium so that traffic marks or rub marks do not occur during shipment and so that it remains dry. All incoming shipments should be inspected promptly, however, since suppliers generally have a time limit in which damage claims will be honoured.
Appearance: Scratches, surface abrasions or a condition resembling cinders embedded in the metal. They result from mechanical abrasion and subsequent oxidation of the abraded areas which is unsightly and has a detrimental effect on finishing operations.
Prevention and Treatment: Suppliers usually pack the metal so that it is not subjected to undue flexing or twisting and so that the units within a package do not rub against each other. Products subject to such damage are usually packed on skids or in timber boxes. Paper or cardboard may be used for cushioning thin or soft metal. Steel strapping is used to reinforce skids and boxes and to bind wrapped bundles. In storage, do not place aluminium with other metals. Use wood faced shelving, racks and bins. Store away from caustics, nitrates, phosphates and acids. Use older stock first.
Appearance: Non-metallic usually whitish, perhaps iridescent depending upon the alloy or degree of oxidation. Caused by the entrapment of moisture between the adjacent surfaces of closely-packed material. Purer aluminium alloys are more resistant to water stain, while the condition seems most pronounced on those alloys having high magnesium content. Water stain is a superficial condition and the mechanical properties of the metal are not affected.
Prevention and Treatment: If a shipment of aluminium arrives in wet condition, dry thoroughly before storing - evaporate moisture in air or dry air currents. When the moisture is removed shortly after the metal becomes wet, no stain will result. If stain has occurred, and the moist condition causing it is removed, the stain will not develop further. Once dry, store the metal away from such obvious water sources as steam and water pipes, and reasonably away from open doors and windows.
Condensation is perhaps the most troublesome cause of water stains. in severe cases, it may also cause surface deterioration which may only become apparent if the material is subsequently etched and anodised. Prevent it by avoiding conditions where the airborne moisture increases enough to carry the dew point above the metal temperature. Ensure that a sudden fall in temperature or increase in humidity does not occur in storage areas. Aluminium packed in original boxes should never be left in the open, because greater variations in temperature and humidity outdoors increase the possibility of condensation. Even if the package is wrapped with water-resistant paper, the impossibility of obtaining a perfect seal makes outdoor storage highly undesirable. So-called waterproof packages are designed solely for the protection of the metal during shipment and are not meant to withstand extended exposure to the weather.
Try to store cold metal in a dry storage place until its temperature has increased substantially before bringing it into heated areas with higher humidity. Place new shipments in temporary storage where their temperature is raised slowly to that of the permanent storage room.
Where water stains have occurred, the degree of staining may be judged fairly accurately by the relative roughness of the stained area. If the surface is reasonably smooth overall, the stain is superficial, and its appearance can be improved by mechanical or chemical treatments. Scratch-brushing or the use of steel wool and oil is effective in removing water stain. If a chemical dip without undue etching is preferred, an aqueous solution containing 10% by volume of sulphuric acid and 3% by weight of chromic acid at about 10°C may be used.