Aluminum and aluminum alloy chemical polishing method

- Jan 25, 2019 -

The chemical polishing of aluminum and aluminum alloys is a 70% (mass percent) phosphoric acid based solution. Commonly used is a mixture of phosphoric acid, nitric acid and sulfuric acid. When the composition of the alloy changes, the sulfuric acid content (mass percentage) is 9%. The change between l5% and the nitric acid content (% by mass) vary between 3% and 9%. Since the working temperature is around 105 °C, a large amount of nitrogen oxides are generated to pollute the environment. Therefore, based on the phosphoric acid and sulfuric acid mixture, the S-type copper brightener is used as an additive to produce a yellow-free chemical polishing process. However, this yellow-free chemical polishing is only suitable for pure aluminum or aluminum-magnesium alloys, and it is inferior to other aluminum alloys. Later, there was a formula with only 2% nitric acid, and similar additives were used, which did not expand the scope of application. However, to date, the phosphoric acid, nitric acid, and sulfuric acid mixed solutions are still in the mainstream formulation because of their strong adaptability.

For aluminum alloys with high copper and zinc, it is helpful to add 2~5g/L chromic anhydride during chemical polishing. The manual contains a high-silicon-containing aluminum alloy. For chemical polishing, a mixture of 60-65 mL/L nitric acid, 15-20 mL/L hydrofluoric acid, and glycerin 1-2 mL/L is immersed for 2 s at room temperature. However, this method is more like etching.