Overview of Aluminum Oxide Coatings and Its Uses
Aluminum oxide coatings are 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 may be corroded by chemical reactions or damaged by bending or friction, as mentioned above, and corrosion may significantly progress from the damaged area.
Therefore, aluminum oxide coatings are 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.
There are many products that use anodized aluminum oxide coatings, including kettles, sashes, and smartphones for everyday use, and optical parts, automobiles, aircraft, semiconductors, and medical equipment for industrial use.
Principle of Aluminum Oxide Coatings
In aluminum oxide coatings, an oxide film is formed by electrolysis of aluminum in a sulfuric acid electrolytic solution with aluminum as the anode.
As shown in Figure 1, 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.
As shown in Figs. 2 and 3, this oxide film is an aggregate of hexagonal prismatic cells with pores inside.
Aluminum oxide coatings are performed based on this principle, but the characteristics vary depending on the method of treatment, so anodizing must be performed according to the application.
Types of Aluminum Oxide Coatings
1. General Aluminum Oxide Coatings
This is a commonly used aluminum oxide coatings 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.
2. Hard Aluminum Oxide Coatings
This treatment method is used to achieve even higher hardness than general aluminum oxide coatings, and are performed in an electrolyte at a low temperature over a long period of time. The oxide film is several times thicker than that of general anodizing, and is used for automobile engines and aircraft, where high durability is required.
3. Glossy Aluminum Oxide Coatings
Before applying aluminum oxide coatings, chemical polishing process is performed to make the surface shiny. This process is used for decorative and reflective materials because of its beautiful appearance.
4. Color Aluminum Oxide Coatings
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 light weight and design are required.
Anodized Aluminum Film Thickness and Factors Causing Variation in Film Thickness
1. Aluminum Oxide Coatings Thickness
The thickness of the anodized aluminum oxide film formed by general anodizing, the most common type of anodizing, 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 coatings formed by hard anodizing is in the range of 20 to 70 microns.
Hard anodized aluminum oxide coatings are often applied to parts that require sliding properties, such as automobile engine parts, and a greater film thickness is set than for general anodized aluminum oxide coatings in order to provide wear resistance.
2. Factors Causing Variation in Film Thickness
Despite the anodizing process, variations in the thickness of the anodized aluminum oxide coatings are caused by two factors: one is the distribution of electric currents, and the other is the distribution of temperatures.
Variation due to current distribution
Since anodizing is performed using an electrochemical reaction, uneven electrical distribution causes variations in the thickness of the anodized aluminum oxide coatings.
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 oxide coatings are performed at once, the current distribution differs depending on the position of the anodes and cathodes, which leads to variations in film thickness.
When multiple aluminum oxide coatings are performed at once, dummy aluminum is hung near aluminum products in locations and conditions where film thickness is likely to increase to release the current.
Variations due to temperature distribution
Aluminum oxide coatings are performed in an electrolytic solution, and the temperature distribution of the electrolytic solution can cause variations in the thickness of the anodic oxide film.
During aluminum oxide coatings, the bath is agitated to maintain a uniform temperature in the electrolyte bath. 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.
Disadvantages of Aluminum Oxide Coatings
The anodized aluminum oxide coatings formed on the aluminum surface by anodizing has the disadvantage of being inflexible and brittle, which can lead to cracking and peeling of the anodized oxide coating when the anodized area 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.
Although aluminum oxide coatings improves corrosion resistance and hardness, 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.