The main reason for anodizing aluminum after it has been electropolished with an alkaline or fluoboric acid type electrolyte is that the oxide film formed on the metal surface does not offer satisfactory protection against corrosion. When aluminum is electropolished in solution containing phosphoric, sulfuric or chromic acid its surface has higher resistance to corrosion and does not necessarily require an anodizing treatment subsequently. In solutions containing phosphoric, sulfuric and chromic acids, the acid concentration may vary within wider limits – 37 percent for phosphoric acid, 15 – 46 per cent for sulfuric acid and 4 – 12 per cent for chromic acid. The optimum composition of the electrolyte recommended for electro polishing of Duralumin contains 43 per cent phosphoric acid, 43 per cent sulfuric acid, 3 per cent chromic acid and balance water. In the case of pure aluminum an electrolyte which does not contain any sulfuric acid may be used but it may leave some risk of pitting.
The polishing of aluminum in these solutions is accompanied by a reduction in current strength due to the formation of surface oxide film with a considerable electrical resistance. With use the maximum current density obtainable at a fixed voltage in a solution decreases. Higher operating temperatures produce bright finish. The time of treatment can be reduced by rising the current density. Although solutions of this presence of other metals in aluminum has the effect of lowering the reflectivity and the quality of finish. Thus the treatment of 99.6 per cent of aluminum in these electrolytes give a reflectivity of 75 – 90 per cent and 99.16 percent aluminum 68 – 80 percent reflectivity only.
The presence of sulfuric acid in the solution helps to stabilize the electrical conditions of process by maintaining the current density at a relativity high level. High sulfuric acid concentration of the electrolyte and also result in infrequent adjustments in concentration of electrolyte and also reduces the tendency of the solution to etch the surface of the anode. The presence of chromic acid is of lesser importance.
If the process it may be noted that the hexavalent chromium starts reducing into trivalent chromium resulting in formation of compounds which raise the viscosity of the solution which has adverse influence in the quality of the finish. The aluminum which accumulate in the solution is possible until upto about 30 – 35 gram aluminum per liter of electrolyte are present. Correction of the solution involve the oxidation of the trivalent chromium and the periodically oxidation of water and acids. Water is added with a view to maintaining the specific gravity of the solution at 167 – 170. the amounts of acids required to be added are determined by chemical analyses.