Small Aluminum Alloy Anodizing Equipment:
Structure of the Anodic Oxide Film: The anodic oxide film consists of two layers:
Outer Layer (Porous Layer): This layer is thicker, loose, porous, and has low resistance.
Inner Layer (Barrier Layer or Active Layer): This layer is thinner, dense, and has high resistance. The porous outer layer grows on top of the dielectric inner layer. Overall, the anodic oxide film forms a hexagonal columnar array, with each column containing a star-shaped small pore filled with the solution, resembling a honeycomb structure. The thickness of the pore walls is approximately twice the diameter of the pores.
Barrier Layer: Composed of anhydrous Al₂O₃, this layer is thin and dense, offering high hardness and blocking electrical current.
Porous Outer Layer: The porous outer layer mainly consists of amorphous Al₂O₃ and a small amount of r-Al₂O₃·H₂O, and contains anions from the electrolyte.
The pore diameter of the oxide film ranges from 100 nm to 200 nm, with a film thickness of around 10 micrometers, porosity of about 20%, and pore spacing of 300-500 nm. The cross-sectional view of the oxide film shows that the pores are essentially tubular structures. The film formation reaction mainly occurs at the bottom of these pores. For typical sulfuric acid direct current anodic oxide films, the pore diameter is about 20 nm. If the oxide film is 12 micrometers thick, it would be equivalent to an extremely deep tube structure. For example, if this were a 1-meter diameter well, the depth would be 600 meters.
The excellent properties of the oxide film, such as corrosion resistance, wear resistance, adsorption, and insulation, are largely determined by the thickness and porosity of the porous outer layer. These factors are closely related to the anodizing conditions, so adjusting these conditions can achieve film layers that meet different usage requirements. The film thickness is a major performance indicator for anodized products, directly affecting corrosion resistance, wear resistance, insulation, and chemical coloring ability. During conventional anodizing processes, the film layer thickens over time. After reaching the maximum thickness, it gradually thins with extended treatment time. Some alloys, such as AI-Mg and AI-Mg-Zn alloys, exhibit this effect particularly clearly. Therefore, the anodizing time is usually controlled within the range to achieve the desired film thickness.
Properties and Applications of Anodic Oxide Film: Anodic oxide film has high hardness and wear resistance, strong adhesion, good adsorption capability, excellent corrosion resistance, electrical insulation, and high thermal insulation. Due to these exceptional properties, it is widely used in various applications.
Major uses include:
- Improving Wear, Corrosion, and Weather Resistance: Enhances the durability of parts.
- Coloring Transparent Films: The oxide film can be dyed to produce various colored films.
- Capacitor Dielectric Film: Used in capacitors.
- Improving Adhesion with Organic Coatings: Serves as a base layer for coatings.
- Base Layer for Electroplating and Enameling: Provides a suitable base for further processes.
- Emerging Uses: Solar energy absorbers, ultra-hard coatings, dry lubricating films, catalytic membranes, magnetic alloy deposits in porous membranes for memory components.
Sealing Process: Aluminum oxide films are porous and need to be sealed before use, whether they are colored or not, to enhance their corrosion and weather resistance. The sealing methods include:
- High-Temperature Water Sealing: Uses hydration reactions between the aluminum oxide film and water to convert the amorphous film into a hydrated crystalline film. This reaction can occur at both room and high temperatures, but high temperatures, especially at boiling point, create very stable and irreversible crystalline films. Thus, the most commonly used sealing method for aluminum oxide films is the boiling water or steam method.
- Inorganic Salt Sealing: Improves the fastness of organic dyes and is commonly used in chemical coloring methods.
- Acetate Method
- Silicate Method
- Organic Sealing Method: Involves oil impregnation, varnish, or coating. This method is less commonly used due to higher costs and additional process steps, with the high-temperature water method being the mainstream approach.