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Metals
Aluminium
Most aluminium objects found in museum collections are alloys containing copper as a minor component. The addition of only 3% by weight of copper trebles the mechanical strength of the parent metal.
As aluminium corrodes, an oxide layer forms on the surface and protects it against further corrosion. Therefore, under normal environmental conditions the metal does not corrode to any great extent.
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| Aluminium cylinders showing the effects of attack by chloride ions.
Photograph courtesy of the Western Australian Museum
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Deterioration of aluminium
If in contact with metals such as copper and iron, or in the presence of chloride ions from sea water, aluminium and its alloys will corrode appreciably.
Aluminium must never come into contact with mercury. Since mercury prevents the formation of the protective oxide patina, subsequent corrosion of aluminium is rapid.
| For more information |
| For more information on the effects of chloride ions on aluminium and its alloys, please see More About Metals, later in this chapter. |
Cleaning aluminium
Aluminium should be cleaned only with industrial methylated spirits, to remove dirt. Heavy deposits of oil, grease and petroleum products, commonly encountered on vintage car parts, can be removed by using kerosene or similar products.
If the metal is heavily stained or corroded, a 1% solution of phosphoric acid can be used. This will produce a mild, uniform etch on the metal surface which, after thorough washing and drying, should be left for a day to enable the protective corrosion film to reform through contact with the air. Consult a conservator for more information about this treatment.
Aluminium that has been contaminated with chloride ions can be treated by washing the corroded metal with a solution that contains ammonia and ammonium sulphate. This is a long term treatment which is best left to conservators. A corroded sea plane float, for instance, was only stabilised after soaking for a period of 12 months.
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| Sea plane float after treatment.
Photograph courtesy of the Western Australian Museum
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| CAUTION: |
| Never use caustic soda to remove grease or paint from aluminium products, because it reacts vigorously with them. Don't use abrasives on aluminium, because these can remove the protective oxide layer. |
Coating aluminium
After cleaning, the aluminium surface may be coated with a protective, clear, acrylic lacquer. This protection is generally not needed unless the aluminium is likely to be affected by salt—for example, because of a sea-side location.
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