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Prevention of crack in anodic oxidation film of aluminum

Release time:

2020-11-27 11:10

Aluminum alloy anodic oxide film cracks for several reasons, clear the main reasons for the cracks in order to find a solution.

1. Conditions for cracks in aluminum anodic oxide film

(1) The difference of the linear expansion coefficient of the aluminum-aluminum anodic oxide film.

The linear expansion coefficient of the aluminum-aluminum oxide film is approximately 5 times different. In addition, these two materials are close to each other and linked together. When the temperature rises, the aluminum expands greatly, and the anodic oxide film cannot reach the corresponding level, resulting in cracks.

(2) Internal stress of anodic oxide film accompanied by anodic oxidation reaction

Due to the volume expansion of aluminum in anodic oxidation, the compressive stress of the product on the anodic oxide film tends to prevent cracking. However, the anodic oxide film becomes thicker, the current density increases, and the stress moves in the direction of tensile stress. Therefore, a crack occurs only by the force of bending.

(3) The growth of the anodic oxide film at the protruding part (angle).

In the protruding portion, since the anodized film expanded from the original surface to the outside along with the growth of the anodized film cannot sufficiently cover the surface of the aluminum material, the anodized film is cracked from the surface to the aluminum substrate.

2. Feasible countermeasures

(1) Improve the linear expansion coefficient of aluminum anodic oxide film

The method of increasing the coefficient of linear expansion is as follows:

1) giving surface roughness

2) Formation of anodic oxide film with high porosity: AC oxidation and anodic oxidation solution with high solubility are used.

3) Anodic oxide film using radioactive microporous structure: The growth of chromic acid anodic oxide film micropores is radioactive. This anodic oxide film structure gives aluminum anodic oxide film flexibility and improves the linear expansion coefficient.

(2) Control the internal stress of anodic oxide film

The residual compressive stress in the anodic oxide film is more difficult to crack. For this reason, a low current density, a high temperature treatment, and a thin anodized film are effective, and are formation conditions for forming a fragile oxide film.

(3) Growth of anodic oxide film at protruding parts

An anodized film having a large curvature grows on the protruding portion, and propagation of cracks can be suppressed.

Oxidation, anodic, crack, growth, linear expansion coefficient, generation, anodic oxide film on aluminum, aluminum alloy, oxide film