Aluminum and its alloys are commonly chemically polished using a solution based on 70% phosphoric acid. A typical formulation includes a mixture of phosphoric acid, nitric acid, and sulfuric acid. The sulfuric acid concentration typically ranges from 9% to 15% by mass, while the nitric acid content varies between 3% and 9%. This process is usually carried out at around 105°C, which leads to the release of significant amounts of nitrogen oxides, causing environmental pollution. To address this issue, a modified method was developed by adding S-type copper brighteners to a mixture of phosphoric and sulfuric acids, resulting in a yellow-nosed chemical polishing process. However, this technique is only effective for pure aluminum or aluminum-magnesium alloys and performs poorly on other types of aluminum alloys. Later attempts introduced formulas with just 2% nitric acid, but no significant improvement in application range was achieved. Despite these limitations, the traditional phosphoric, nitric, and sulfuric acid mixture remains widely used due to its versatility.
For aluminum alloys with higher levels of copper and zinc, chemical polishing can be done using 2–5 g/L of chromic anhydride. Additionally, for alloys with high silicon content, a solution containing 60–65 mL/L nitric acid, 15–20 mL/L hydrofluoric acid, and 1–2 mL/L glycerin is often used. This process is typically performed at room temperature for about 2 seconds, though it is more similar to etching than true polishing.
Another formulation for pure aluminum and 2A12 (LY12) alloys contains acetic acid. This method offers an alternative approach depending on the specific alloy and desired finish. Overall, the choice of chemical polishing solution depends on the composition of the aluminum alloy and the required surface quality. Different formulations have been developed over time to suit various applications, balancing effectiveness, cost, and environmental impact.
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