Some polishing processes are based on phosphoric acid, particularly those employed in industry. Solutions containing concentrated phosphoric acid that are heated to 176 degrees F will vigorously attack the surface of aluminum, with the accompanying release of hydrogen. The resulting surface is bright and reflective and doesn’t expose the metal's grain structure. Alternative finishes can be produced by adding small amounts of sulphuric or nitric acid to the initial solution.
Sulfuric acid is used as the electrolyte in the electro-polishing process. The aluminum is immersed in a temperature-controlled bath of sulfuric acid and connected to the positive terminal of a DC power supply. The negative terminal is attached to a cathode. When the current is turned on, the surface of the aluminum is oxidized and becomes dissolved in the sulfuric acid.
Solutions containing small amounts of nitric acid and additional fluorides have been successfully used to polish aluminum. According to the website Aluminium Danmark, the most well-known of these solutions is the Erftwerk process. As well as nitric acid, the ACL-Lichttechnik website notes that the main chemical constituents of the Erftwerk process are hydrofluoric acid and ammonium bifluoride. The aluminum is treated with the solution for between 15 and 30 seconds.
According to Aluminium Danmark, there are a number of aluminum chemical polishing baths based on alkalis, such as sodium hydroxide, potassium hydroxide and calcium hydroxide. Alkali solutions are soapy to the touch and their strength is measured by their concentration of hydrogen ions. The Stone Chemical Co. says that alkali-based baths are non-caustic and do not etch the metal.