Pretreatment, which includes five steps:
- Degreasing treatment: the purpose is to remove lubricating oil and other dirt on the surface of the product, so as to ensure uniform corrosion on the surface of the product in the alkali washing process and improve the quality of oxidized products. Generally, sulfuric acid with concentration of 5-25% is used for degreasing treatment, and the degreasing temperature is 60-80c.
- Etching treatment: the purpose is to further remove the dirt on the surface of the product, remove the natural oxide film with a thickness of about 25-1000a on the surface of the product, and expose the surface of the base metal, so as to facilitate the smooth oxidation and coloring. Generally, 40-80 ° NaOH solution is used.
- Pickling treatment: the purpose is to remove the black corrosion products left on the product surface after corrosion, so as to obtain a bright metal surface.
- Extinction treatment: the purpose is to make the surface of aluminum materials or aluminum products dull and form a non gloss surface after anodizing treatment to form spot corrosion. Generally, 20-40c ammonium fluoride solution is used.
- Polishing treatment: in order to eliminate mechanical damage and corrosion spots on the surface of aluminum products and improve surface smoothness and gloss depth, polishing treatment is generally required.
Basic principle: aluminum cathode oxidation is essentially the electrolysis of water. When water is electrolyzed, O2 - ions react with anodic aluminum to form alumina (Al2O3). Generally, sulfuric acid, chromic acid, phosphoric acid and oxalic acid are selected as the electrolyte to form a hard porous oxide film in sulfuric acid at about 0C. Different materials have great influence on the oxide film. For 6000 series Al Mg Si (6063.6061.6065), it can form not only protective oxide film, but also dyeing oxide film and bright oxide film. However, for cast aluminum alloy, it can form protective oxide film, but dyeing oxide film can only form thick color.
For coloring treatment of cathode oxidation, there are the following methods:
A: Electrolytic coloring:
The method is as follows: the anodized aluminum alloy parts are electrolyzed again in the electrolyte of metal salt, so that the cations of metal salt are deposited on the bottom layer of pinhole of oxide film and colored. In practical application, bronze color system and black system can be obtained. Usually, the salt solution of Ni, CO and Sn is used as electrolyte, usually sulfate and AC electrolysis.
B: Dyeing method
The method is as follows: put the anodized aluminum alloy parts into the solution containing dyes, and the pinholes of the oxide film will absorb dyes and color. The sulfuric acid film is the best for the dyeing method. The representative dyes are ammonium ferrous oxalate (dyed into golden yellow) and cobalt acetate (bronze). The holes need to be sealed after dyeing.
There are mainly two methods:
- A: Nickel salt sealing: good weather resistance.
- B: Boiling water sealing: the defect is that the dye is easy to overflow during sealing, resulting in uneven tone.
Organic coating treatment
After cathodic oxidation and coloring treatment, aluminum can be treated with organic coating in order to improve its corrosion resistance and loading corrosion effect.
A: Electrostatic painting
Atomize the coating with fine particle size and then blow it to the surface of the coated object to make it become an attached material. Generally, the coating particles are charged on the cathode first, and 100kV DC voltage is applied to the coated object as the cathode. The coating is generally acrylic resin.
B: Electrophoretic painting
In acrylic resin aqueous solution, the oxide film is used as the cathode and electrolyzed with 200V DC voltage to coat an organic film on the oxide film