Handling and Storage of Aluminium
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.
Traffic marks on aluminium
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 damage by flexing or bending usually are 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.
Water stains on aluminium
Appearance: non-metallic in appearance and, while usually whitish, may appear iridescent depending upon the alloy or degree of oxidation. They are caused by the entrapment of moisture between the adjacent surfaces of closely-packed material. The 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 having such stain are not affected.
Prevention and Treatment: If a shipment of aluminium arrives in wet condition, it should be thoroughly dried before storing. This may be done by evaporation in air or by means of dry air currents. When the moisture is removed in this manner within a short period 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 safely dry, the metal should not be stored near such obvious water sources as steam and water pipes, and it should be kept reasonably away from open doors and windows.
Condensation is perhaps the most troublesome cause of water stains. Under severe conditions, condensation may also cause surface deterioration which may only become apparent if the material is subsequently etched and anodised. It may be prevented by avoiding conditions where the moisture of the air increases enough to carry the dew point above the metal temperature. It is thus important to ensure that a sudden fall in temperature or increase in humidity does not occur in the places of storage. Aluminium packed in original boxes should never be left in the open, because the greater variations in temperature and humidity outdoors increase the possibility of condensation. Even if the package is wrapped with 'waterproof' 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. Placing new shipments in temporary storage where its 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, the stain is merely 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 per cent by volume of sulphuric acid and 3 per cent by weight of chromic acid at about 10°C may be employed.
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