Degreasing, as the first process of chemical pretreatment of aluminum materials, aims to remove pollutants such as grease and dust from the surface of aluminum, making the subsequent alkaline washing more uniform, and improving the quality of the anodic oxide film. The quality of degreasing can be tested by the effect of surface wettability. After degreasing and washing, the surface of aluminum material is completely evenly watered, and the continuity of the water film is maintained for at least 30 seconds as qualified. The degreasing of aluminum materials can be roughly divided into three types: acidic degreasing, alkaline degreasing, and organic solvent degreasing.

1. Acidic degreasing

Acid degreasing is mostly carried out at room temperature, with easy production control and low cost. Usually, acid degreasing agents are acidic liquids mainly composed of sulfuric acid, and the oil undergoes hydrolysis reactions to generate glycerol and corresponding higher fatty acids. Adding a small amount of wetting agents and emulsifiers to the solution is beneficial for the softening, dissociation, dissolution, and emulsification of the oil, significantly improving the degreasing effect. When the sulfuric acid content in the acidic degreasing solution is insufficient, waste sulfuric acid from the oxidation tank can be added.

2. Alkaline degreasing

Alkaline degreasing is a traditional process for surface pretreatment of aluminum materials. Aluminum is an amphoteric metal that is prone to corrosion by alkaline solutions. Therefore, a solution with lower alkalinity can be used for surface degreasing cleaning treatment. A degreasing solution that is too alkaline may cause uneven corrosion on the surface of aluminum, as it tends to attack the clean surface of aluminum quickly, while the corrosion rate on greasy surfaces is slower, which may lead to cleaning marks on the surface of aluminum. The principle of alkaline degreasing is the saponification reaction between alkali and oil, generating soluble soap. The saponification reaction is used to eliminate the binding between oil and aluminum surface, thus achieving the purpose of degreasing. The reaction principle is as follows:

The biggest problem with alkaline degreasing is that the solution needs to be heated, which not only increases production costs but also brings certain difficulties to production control. The temperature of alkaline degreasing solution is generally controlled between 45~65 ℃, and the temperature often drops during the production process, requiring frequent heating and heating.

3. Organic solvent degreasing

Organic solvent degreasing is a process that utilizes the characteristic of oil being easily soluble in organic solvents for degreasing; Organic solvents can dissolve both saponified and non saponified oils, with strong degreasing ability and fast speed. They are non corrosive to the aluminum matrix, but organic solvents are flammable and toxic. Unless really needed, they should be avoided as much as possible. The main components of organic solvents typically include carbon tetrachloride, trichloroethylene, benzene, gasoline, kerosene, alcohol, and acetone.