The concentration of free alkali in the solution is one of the major factors which determines the program of the polishing process when treating aluminum in an alkaline solution. Although alkali is not one of the constituents used in making up the solution. It is formed in the bath as a result of hydrolysis. As the solution is used, the concentration of free alkali rises as a result of reaction of aluminum with sodium hydrogen phosphate. The optimum of alkali in the solution is 15 – 18 gram per liter. Any variation on either side in this concentration result in a considerable reduction in the reflectivity obtained. The operating temperature of the solution is 90 oC. At a lower temperature anodic oxidation occurs and at higher temperature there is a risk of etching of the metal surface.
The electropolished parts are washed in water, anodized and sealed. The sealing of anodic film result in a slight reduction in the reflectivity, the extent to which this occurs depends on the anodizing process. The smallest reduction in reflectivity is obtained by anodizing in a 15 – 30 percent solution of sodium bisulfate at an anodic current density of 5 A/sq. ft. a voltage of 8 – 12 volts and temperature of 30 – 40 oC for 10 – 20 minutes. A drawback of the alkaline solution is its rapid exhaustion and the consequent need to undertake frequent corrections and also the occurrence of etch figure on the polished surface. The reflectivity obtained in this case is somewhat lower than that obtained in the fluoboric acid electrolyte.
The next article is phosphoric/sulphuric/chromic acid solutions.