The reflectivity obtained by polishing aluminum metal surface in a flouboric acid electrolyte is almost directly proportional to the purity of aluminum with the reflectivity varying from about 90 per cent at 100 per cent aluminum purity to 60 per cent at 99.2 per cent aluminum purity. In practice, the metal to be polished with fluoboric acid electrolyte should have a minimum purity of not less than 99.8 per cent. The reflectivity of mechanically polished aluminum for visible light amounts to 74 - 75 per cent. Electropolishing can raised it to 80 - 97 per cent.
The fluoboric acid solution is prepared by dissolving boric acid in hydrofluoboric acid. Mixing of the acids should be carried out with constant stirring. The boric acid addition to fluoboric acid is done in small lots with each addition made only after the reaction of the previously added portion has ceased. The concentrated solution of the flouboric acid thus obtained is diluted water. The optimum concentration of fluoboric acid in the polishing electrolyte is 1.25 per cent. An increase in aluminum accompanied by etching of the surface and a reduction in reflectivity.
The operating temperature of the solution is 30 oC. A lower reflectivity is obtained if the temperature falls below 27 oC or rises above 35 oC. The current density range is from 15 to 30 Amp/sq.ft. and the treatment takes 10 - 20 minutes. The highest reflectivity is obtained with an anodic current density of 15 - 20 ampere per sq.ft. and a time of treatment of 10 - 15 minutes. In this process a very porous oxide film is formed on the metal surface which can pick up moisture from the atmosphere and imprint finger marks during handling. The oxide film can be removed by immersing the work in one or the other of the following solution.
- Phosphoric Acid (75 per cent) 45 ml
- Chromic Acid 18 gr
- Water 1 liter
- Chromic Acid 15 gr
- Sodium Carbonat 20 gr
- Water 1 liter
The solution are used at a temperature of 75 oC, the time of immersion being 1 - 3 minutes. After the thin oxide film has been removed, the parts are rinsed with water and anodized.