Alkaline Electro-Brightening and Anodising of Aluminium

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Abstract
The paper reviews the present state of development of a well established process for electro-brightening aluminium. The brightening operation is carried out in a solution containing 15% wt./vol. anhydrous sodium carbonate and 5% wt./vol. anhydrous trisodium phosphate at 80–82°C. After an initial etch for 10–30 seconds, 12V d.c. is applied between stainless steel cathodes and the aluminium anode, the anode current density falling rapidly from 35A/ft2 at the start to about 20 A/ft2. Treatment is for 5–15 minutes, after which the article is rapidly removed and rinsed in water. The thin oxide film produced on the metal surface by this treatment is removed by immersion in a hot solution of phosphoric and chromic acids, since it forms an easily-marked outer layer on subsequent anodising, and the article is finally subjected to the normal protective anodic treatment in sulphuric acid, after which it is sealed and waxed. For certain decorative applications the anodic film may also be dyed. The process works best with super purity aluminium (99·99% Al) and certain alloys made from this metal, particularly those containing small amounts of magnesium, but good results can also be obtained with 99·8% Al. The treated surface has a high reflectivity for both light and radiant heat, and the process is extensively used in the production of aluminium reflectors, as well as for a wide range of decorative purposes.

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