This section provides overview, applications, and principles of anodized aluminum. Also, please take a look at the list of 13 anodized aluminum manufacturers and their company rankings.
Forming an anodized aluminum film is also called anodizing. Anodizing is a surface treatment in which an oxide film (anodized film) is artificially formed on aluminum's surface. By the substance forming on the aluminum it becomes anodized aluminum. By forming an anodized aluminum film, it is possible to improve corrosion and wear resistance. Unlike the plating process, an anodized aluminum film does not just cover the surface, but is formed in a manner that dissolves and penetrates the product itself to cover the surface, thus providing a more durable surface treatment.
Anodized aluminum is used in a wide range of fields, from household products to industrial products. This is because forming an anodized aluminum film provides high corrosion resistance, high wear resistance, surface properties (water repellency, etc.), and high decorative properties such as coloring.
It's used in household products like lunch boxes, kettles, pots, cell phones, sashes, etc. and in industrial products such as construction materials, automotive parts, optical parts, semiconductor parts, medical equipment, etc.
Aluminum, the base material from which anodized aluminum is formed, easily bonds with oxygen and forms a thin oxide film when in contact with air. For this reason, aluminum has the property of being resistant to rust, in other words, it has good corrosion resistance. However, the thickness of this naturally formed oxide film is very thin.
Depending on the environment, chemical reactions can cause corrosion, which can extend to raw aluminum. Therefore, an artificial oxide (anodized aluminum) film is formed to protect the aluminum.
The main component of anodized aluminum is amorphous alumina (Al2O3). An anodic oxide film made of anodized aluminum has the shape of a porous layer with many pores extending in the direction of film thickness.
The basic formation method of anodized aluminum (anodized aluminum film) is as follows. The anodizing process is similar to this method.
The aluminum product is attached to a jig and placed in an electrolytic solution. Connect an electrode to the jig and apply positive electricity, and at the same time apply electricity to the cathode in the same manner. The electrolysis produces an oxide film (anodized aluminum film) on the surface.
The thickness of the oxide film (anodized aluminum film) produced at this time is proportional to the electrolysis time. Since the anodized aluminum film is formed while dissolving the aluminum surface, its surface shape reflects the shape of the aluminum surface.
In other words, even if the aluminum surface has minuscule irregularities, an anodized aluminum film is formed reflecting the shape of the surface, so the surface cannot be flattened like a coating.
There are five points to be noted when forming anodized aluminum film.
The degree of film growth varies depending on whether the aluminum processing was done by cutting or electrical discharge machining, resulting in differences in the expected dimensions after the anodized aluminum film is formed.
If an anodized aluminum film is formed when materials other than aluminum, such as iron, copper, or stainless steel are joined, there is a possibility that, aside from aluminum, the materials will melt.
If an anodized aluminum film is formed on parts composed of different types of aluminum alloys at the same time, uneven color and thickness will occur.
If anodized aluminum film is formed on a part that has a hole shape designed to fit together, the fit may not function depending on the dimensional accuracy of the anodized aluminum film thickness.
Consider that the depth of the hole in the fit and the shape of the through hole or stop hole, etc., may cause a difference in the dimensional accuracy of the anodized aluminum film thickness.
As mentioned above, the thickness of the anodized aluminum film produced will vary depending on the shape of the part on which the anodized aluminum film is formed, the processing method used, and other factors. Therefore, prior consideration and confirmation are important when forming an anodic oxide film on parts that require precise dimensions.
Anodized aluminum is not flexible and brittle, and parts with anodized aluminum coating may peel or crack when processed or bent. Also, each material has a different rate of thermal expansion (coefficient of thermal expansion). Aluminum alloys before oxidation treatment and anodized aluminum after oxidation treatment have different coefficients of thermal expansion.
Especially in high temperature environments (>100°C), the difference in thermal expansion rates between the two materials will increase, causing the anodized aluminum film to delaminate or crack.
Anodized aluminum can be colored in a variety of colors. There are two main coloring methods: electrolytic coloring and dye coloring.
Electrolytic coloring is a method in which parts of anodized aluminum film are secondarily electrolyzed in an electrolytic solution containing metal salts such as tin and nickel to deposit metal in the pores of the anodized aluminum film, which is a porous layer, for coloring. Anodizing with this method can give anodized aluminum a metallic hue such as black, bronze, or yellow. Electrolytic coloring not only colors the anodized aluminum film but also reinforces it.
Dye coloring is a method of coloring anodized aluminum by placing dye inside the pores of the porous layer of the anodized aluminum film. This is called color anodizing.
After the anodized aluminum film is colored by the dye, a sealing process is applied. To put it simply, this is a process to cover the hole where the dye has been inserted so that the anodized aluminum coloring will not peel off. However, care must be taken because if the anodized aluminum film itself peels off, the coloring will also peel off.
In both the electrolytic and dye-coloring anodizing) processes, the density of the coloring depends on the thickness of the coating, the time and temperature applied to the process, and other conditions. This is because the amount of metal and dye that can enter the hole varies depending on the conditions. The greater the amount, the darker the color.
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