What Is an Aluminum Oxide Coating?
1. Overview of Aluminum Oxide Coating Treatment and Uses
Aluminum oxide coating is a type of surface treatment that artificially forms an oxide film on the aluminum surface.
Aluminum is easily oxidized by combining with oxygen in the air, and a very thin oxide film is formed on its surface when exposed to air.
Because it is protected by this naturally formed film, it is said to have relatively high corrosion resistance.
In addition, aluminum is lightweight and highly workable and is used in all kinds of products around us, including daily necessities.
However, due to its high workability, the surface is easily scratched by bending and friction.
The oxide film that naturally forms on the aluminum surface is very thin, and depending on the operating environment, it can be corroded by chemical reactions or damaged by bending or friction, as mentioned above, and corrosion can significantly progress from the damaged area.
Therefore, aluminum oxide coating is performed to artificially form an oxide film that protects the aluminum surface by passing an electric current through the aluminum in an electrolytic solution to promote oxidation.
The formation of this oxide film is expected to improve corrosion resistance, wear resistance, insulation, and strength.
Many products are treated with aluminum oxide coating, including kettles, sashes, and smart phones for everyday use, and optical parts, automobiles, aircraft, semiconductors, and medical equipment for industrial use.
2. Principles and Types of Aluminum Oxide Coatings
2-1. Principle of Aluminum Oxide Coatings
In aluminum oxide coating, an oxide film is formed by electrolyzing aluminum in a sulfuric acid electrolytic solution on an anode.
An anode and cathode are placed in the electrolytic solution, and when the aluminum product is placed on the anode side and energized from the electrode, an oxide film is formed on the surface of the aluminum product. This oxide film is an aggregate of hexagonal prismatic cells with pores inside.
The aluminum oxide coating is performed based on this principle, but the characteristics vary depending on the treatment method, so the aluminum oxide coating process must be tailored to the application.
2-2. Types of Aluminum Oxide Coatings
General Aluminum Oxide Coating
This is a commonly used aluminum oxide coating treatment that can be applied to small parts with complex structures as well as large products. This method is used to improve corrosion resistance and hardness.
Hard Aluminum Oxide Coating
This treatment method is used to achieve even higher hardness than general aluminum oxide coating and is performed over time in an electrolyte at a low temperature. The thickness of the oxide film is several times greater than that of general aluminum oxide coating and is used for car engines and aircraft, where high durability is required.
Luster Aluminum Oxide Coating
Before aluminum oxide coating, the surface is chemically polished to a luster. This process is used for decorative and reflective materials because of its beautiful appearance.
Color Aluminum Oxide Coating
Immediately after forming an oxide film, the surface is immersed in a dye solution to color it. Coloration can be controlled by the dye concentration, immersion time, and thickness of the oxide film. It is used for water bottles, etc., where lightweight and design are required.
3. Anodized Aluminum Film Thickness and Factors Causing Variation in Film Thickness
3-1. Anodizing Film Thickness
The thickness of an anodic oxide film formed by general aluminum oxide coating, the most common type of aluminum oxide coating, is usually in the range of 5 to 25 microns and is set in consideration of usage conditions.
The thickness of the anodized aluminum oxide film formed by hard aluminum oxide coating is set in the range of 20~70 microns.
Hard aluminum oxide coating is often applied to parts that require sliding properties, such as automobile engine parts, and a greater film thickness is set compared to general aluminum oxide coating to demonstrate wear resistance.
3-2. Factors Causing Variation in Film Thickness
Despite the aluminum oxide coating process described above, variations in the thickness of the anodized oxide film occur, one factor being the distribution of electric current and the other factor being the distribution of temperature.
3-2-1. Variation Due to Current Distribution
Since aluminum oxide coating is performed using an electrochemical reaction, uneven electrical distribution causes variations in the thickness of the anodic oxide film.
Depending on the distance between the anode and cathode where the aluminum product is held, variations in film thickness can occur between multiple aluminum products. In addition, when multiple aluminum products are anodized at once, the current distribution differs depending on the position of the products, which leads to variations in film thickness.
When multiple aluminum products are anodized at once, dummy aluminum is hung near the aluminum products in positions and conditions where the film thickness tends to increase to release the current.
3-2-2. Variations Due to Temperature Distribution
The aluminum oxide coating is performed in an electrolytic solution, and the temperature distribution of the electrolytic solution causes variations in the thickness of the anodized aluminum oxide film.
During aluminum oxide coating, the temperature in the electrolyte bath is kept uniform because the bath is agitated. When the temperature is kept uniform, the electrolyte can flow freely and the temperature distribution of the electrolyte becomes uniform.
However, in the area of the diffusion layer near the aluminum product, the electrolyte has a relatively difficult time moving and the temperature distribution becomes non-uniform. This causes variations in the thickness of the anodized aluminum oxide film. To counter this problem, methods to promote the flow of the electrolyte, such as the use of injection nozzles, are used.
4. Demerits of Aluminum Oxide Coatings
The anodized aluminum oxide film formed on the aluminum surface by Anodizing has the disadvantage of being inflexible and brittle, which causes the anodized aluminum oxide film to crack or peel off when the anodized part is bent or processed.
In addition, there is a weakness in heat resistance, and there is a concern that the normal anodized aluminum oxide film may crack or peel due to thermal expansion under high-temperature environments exceeding 100 ºC.
Aluminum oxide coating improves corrosion resistance and hardness, but it is weak in solutions of strong acids and bases, and there is also the problem of dissolution in such solvents.
In addition, wet contact with metals increases the risk of corrosion. Therefore, it is necessary to devise a treatment method according to the intended use.