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Chemical brightening of aluminium

The surface of aluminium, watched under the microscope, presents an indefinite series of peaks and valleys.
Chemical brightening is a treatment which increase the specularity and the brilliance of aluminum by flushing the microscopic roughness on its surface.
In order to chimically level any chemical imperfection of the surface is essential that the solution in which the aluminium is immersed has an "attack rate" of the peaks faster than in the valleys.
When aluminium is placed in a solution of sulfuric acid, nitric acid and phosphoric acid at high temperature, nitric acid reacts with the aluminum to form a film of aluminium oxide, which is immediately dissolved by the phosphoric acid.
This reaction occurs more quickly in the peaks rather than in the valleys because, given the viscosity of the solution, in the latter there is less agitation.

A limit of the chemical brighting process is the inability to remove all the surface scratches, lines or other imperfections; this limit emphasizes the importance to ensure an appropriate procedure of mechanical cleaning of the surface before the real process of brighting starts.