Currently, aluminum profile hardening treatment plants widely use conventional oxidation methods such as sulfuric acid oxidation, oxalic acid oxidation, and chromic acid oxidation. This article provides a brief introduction to some new processes currently under development both domestically and internationally.
Ceramic Oxidation:
Ceramic oxidation primarily uses chromic acid, boric acid, and potassium titanium oxalate as electrolytes, employing high voltage and high temperature electrolysis. The resulting film resembles the glaze on porcelain, exhibiting high corrosion resistance and good wear resistance. The film can be dyed with organic or inorganic dyes, giving it a unique luster and color. It is currently widely used in aluminum cookware, lighters, and fountain pens, and is very popular.
Oxalic Acid-Formic Acid Mixed Solution AC Rapid Oxidation:
An oxalic acid-formic acid mixed solution is used because formic acid is a strong oxidizing agent. In this solution, formic acid accelerates the dissolution of the inner layers (barrier and wall layers) of the oxide film, thus creating a porous layer (i.e., the outer layer of the oxide film). The conductivity of this bath solution can be improved (i.e., current density can be increased), allowing for rapid oxide film formation. Compared to the pure oxalic acid oxidation method, this solution increases productivity by 37.5% and reduces power consumption (oxalic acid oxidation consumes 3.32 kWh/m², while this method consumes 2 kWh/m²), saving 40% of electricity.
The process formula is:
4-5% oxalic acid, 0.55% formic acid, three-phase AC 44 ± 2 volts, current density 2-2.5 A/dm², temperature 30 ± 2℃.
Defense Color Oxidation: Defense color oxidation is mainly used for the decoration of military aluminum products, thus requiring special protective properties. The oxide film is military green, matte, wear-resistant, durable, and has good protective performance. The process is: first, oxalic acid oxidation is performed to generate a golden yellow film layer, followed by anodizing treatment with a solution of 20 g/L potassium permanganate and 1 g/L H₂SO₄.
Mixed Acid Oxidation:
This method was officially incorporated into the Japanese national standard in 1976 and adopted by Hokusei Nichikatsu Household Products Co., Ltd. Its characteristics include rapid film formation, and the film's hardness, wear resistance, and corrosion resistance are higher than those of ordinary sulfuric acid oxidation. The film layer is silvery-white and suitable for printed and colored products. After a study tour to Japan, the Chinese aluminum products industry began recommending its use in 1979. The recommended process formula is: H₂SO₄ 10-20%, COOHCOOH·2H₂O 1-2%, voltage 10-20V, current density 1-3A/d㎡, temperature 15-30℃, and time 30 minutes.