The chemical polishing of aluminum and aluminum alloys is a solution based on 70% (mass percentage) phosphoric acid. Commonly used is a mixture of phosphoric acid, nitric acid, and sulfuric acid. When the alloy composition changes, the sulfuric acid content (mass percentage) is 9%. The nitric acid content (mass percentage) varies between 3% and 9%. Since the working temperature is around 105°C, a large amount of nitrogen oxides are produced and pollute the environment.
Therefore, on the basis of the mixed solution of phosphoric acid and sulfuric acid, the S acid copper brightener is used as an additive to produce a yellow smoke-free chemical polishing process. Yine de, this non-yellow-smoke chemical polishing is only suitable for pure aluminum or aluminum-magnesium alloys, and has poor effect on other aluminum alloys. Later, a formula with only 2% nitric acid appeared, and similar additives were used, but the scope of application was not expanded. Yine de, the current blend of phosphoric acid, nitric acid, and sulfuric acid is still the mainstream formulation because of its strong adaptability.
For aluminum alloys with high copper and zinc content, it is helpful to add 2-5g/L chromic anhydride during chemical polishing. There is a description in the manual that aluminum alloys with high silicon content are chemically polished with a mixture of 60～65mL/L nitric acid, 15～20mL/L hydrofluoric acid, and 1～2mL/L glycerin, and soaked at room temperature for 2s. But this method is more like etching.
There is also a formula containing acetic acid, suitable for pure aluminum and 2A12 (LYl2):
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