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What are anodizing1

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When aluminum is exposed to air, aluminum oxide is formed on the surface of bare aluminum and its corrosion resistance can be excellent. However, since the surface layer of aluminum oxide is thin, it can be corrosive with chemical reactions, or other circumstances such as higher pressure, vibration, and more friction areas. Thus, surface treatment on aluminum, or an electrolytic passivation process, is commonly used to prevent corrosions.
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As sulfuric acid or oxalic acid is thinned with water in a bath (an electrolysis vessel) and an aluminum part (or parts) is connected to positive electrode, electricity passes in electrolysis solution. The oxygen made by electrolysis of water will adhere on the surface of aluminum rapidly, and it will make the film of an aluminum oxide. In this process, the surface of the aluminum is penetrated, and an oxide cell or a layer grows unevenly.
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Aluminum oxide (aprx. 20A) is formed when the metal is exposed to air
Surface Point of Bare Aluminum Part,
Natural Oxidation Film, aprx. 20A*
* A --- Angstrom
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Aluminum oxide (AL2O3) grows in
an electrolysis solution

Penetrated and Grown Oxide Surface
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Sulfuric acid ion enters in pores and a pores
grow deep with higher electrolysis causing
sulfuric acid aluminum to be eluted and
pores to be bored deeper
After 100A~200A of film, pores start being generated
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At the bottom of pores, oxidation reaction and
elution reaction of oxidation film process
simultaneously, and pores grow deeper
Penetration and growth of oxidation film simultaneously in the portion of pores’depth growing
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Thickness of oxidation film, or anodic film, is proportional to the charge of electrolysis = Current * Time, and distance among pores is proportional to electrolytic voltage
Diameter of pores: aprx. 100A
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main The relationship between porosity and electrolytic voltage is as follows;
Electrolytic
Voltage
Number of Pores per 1cm2
15V 77 X 109
20V 52 X 109
30V 28 X 109
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