What's alumite?- Why does aluminum need surface treatment?

1. The necessity of the aluminum surface treatment

It is usually said that the corrosion resistance of aluminum is good, because the surface of aluminum is protected by the natural oxidation film.
However, this film is very thin, so there is some environment where the film can not withstand.
Aluminum is activated in general,so even under the protection of the natural oxidation film, aluminum can be corroded by the chemical reaction.
Especially under the moisture and acid or alkali environment , it occurs remarkably.
Therefore, when making the surface treatment, the anodic oxide film or the painted film should be necessary to protect the surface from corrosion depend on the usage.
Frankly speaking, corrosion-resistance and hardness of the natural oxidation film is not enough under the normal condition.
To improve the surface condition, it is necessary to make the surface treatment to form the artificial film or the painted film.

As a way of surface treatment, the anodic oxide film process, the composite coating process and painting process are used in general.
Recently, all these ways of surface treatment are combined to adapt the demand of diversified, specialized and high valued product.
The property of aluminum is varied by adding other metals to make an aluminum alloy. Please see the table 1 below as a general property of pure aluminum.

[Table-1] Physical and chemical property
of pure aluminum

item property
Chemical symbol Al
Atomic weight 26.97
Atomic number 13
Specific gravity 2.70 (20℃)
Dissolving point 658℃
Boiling point 2057℃
Dissolving latent heat (Cal/g) 94.55
Vaporization latent heat (Cal/g) about 2,000
Specific heat 0.21-0.22(0-100℃)
Heat expansion coefficient 2.97x10-5(0-100℃)
Heat conductivity (c.g.s.) 0.48-0.50 (0-100℃)
Electrical resistibility (Ωcm) 2.48x10-6 (at room temperature)
Its temperature coefficient 4.34z10-3 (0-100℃)
Lattice Face-centered cubic lattice
Reflection rate 94-98 (electropolishing)

2. Anodic oxidation

Usually the anodic oxidation treatment of aluminum is called "anodizing process.". This is a process to make an oxidation film artificially by the electrochemical way of using aluminum as an anode.
When aluminum is electrolyzed as an anode in sulfuric acid electrolytic solution;

As the anode reaction

Al Al3+ + 3e-
OH- O2- + H+
HSO4- SO42- + H+

As the cathode reaction

H+ + e- H2

These reaction happens.

However, the following reaction is also occurring simultaneously inside the micro-hole of the anode.

2Al3+ + 3O2- Al2 O3
Al3+ + So42- Al2(So4)3

That is, the formation of oxidation film which contains a sulfuric acid root is proceeding.
There is a figure-1 which shows what change is occurring on the surface of aluminum while the above mentioned reaction is proceeding.

[Figure-1] How to form the porous film.
(It is quoted from "The Story of Anodizing"
(published by "Tokyo Alumite Industry Cooperation")

3. Difference between the anodic oxidation(anodizing) and the plating

Sometimes the anodic oxidation and the plating is recognized as the same process, because both of them is meant to cover the metal.
However, these process are quite different,basically.
The difference is shown in figure-2.
As for the plating, the metal film of different kind is piled up on the main material, so it becomes thicker than that of original metal.
However, in case of anodic oxidation treatment, there is a difference that the film erodes inside. Because it dissolves aluminum and the dissolved aluminum changes into the oxide compound called "Al2O3"and then it becomes the oxidation film.

[Figure-2] Difference between the anodic oxidation and the plating

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