Crazing defects (cracks) in the anodic layer.

Crazing defects in the anodic layer formed by sulfuric acid anodizing is a well known phenomena. It is often readily seen by the naked eye as a kind of a spider web.

The anodic oxide layer will always have some cracks, so it is how to minimize these which is important. The crazing defects are usually always found on anodized aluminum which has been exposed to high temperatures or any considerable sudden changes in temperature.

The reason for this is the difference in thermal expansion for the aluminum material and the coating.

Anodic coating: 5 x 10-6/°C
Aluminium: 23 x 10-6/°C

The temperature to which cracking becomes visible is dependent on the thickness . Thick films are more prone than thin films to crack. A 5 µm the oxide layer will crack at 320°C whereas a 25 µm thickness will crack at 120°C.

Some times the cracks disappear after a couple of days in room temperature with high humidity. In a dry room cracking occurs at a lower temperature compared to humid rooms.

During the anodizing process higher current density increases the possibility of the crazing defect and higher temperature in the anodizing tank will decrease the probability for crazing.

Colored parts especially the electrolytic colored generally tends to crack less than clear anodizing.

Hot sealing will increase the temperature where cracks happen. To decrease the possibility for crazing while cold sealing a final short hot rinse will decreases the crazing.

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