Hard Anodizing of aluminum and aluminum alloys
Hard anodizing of aluminum at low temperatures produces a surface with excellent wear resistance. It also provides good corrosion protection with electrically insulating properties.
Suitability of alloys
Almost all aluminum alloys can be hard anodized, and the purer the alloy, the thicker the layer can become. Thicker layers provide harder and better corrosion protection. The normal layer thickness ranges from 25-80 µm.
An alloy containing more than 2% copper is not suitable for hard anodizing. Aluminum alloyed with silicon, zinc, or copper achieves lower hardness. The achievable hardness in hard anodizing is generally around 500-600 HV (SIS 4212 and equivalent alloys).
It is not possible to dye the anodized layer in the usual way. The choice of alloy and layer thickness instead determines the color nuance achieved: from natural to gray or from brown-gray to black-gray.
No allergic reactions have been observed in humans when using hard anodized items.
Guidelines for designing parts
The hard anodizing layer, which is formed from the base material, causes a dimensional increase of the surface-treated part by about 40-50% of the layer’s thickness. Therefore, this should be considered when designing parts with high dimensional accuracy. Additionally, sharp edges and small radii should be avoided.
A hard anodizing layer can be ground, repaired, and honed, and the machining can be performed similarly to machining steel. Anodizing does not cause deformation but has a negative effect on fracture strength. Therefore, the method should be used with caution on thin-walled structures that will be subjected to significant mechanical stresses. The same applies to parts with sharp corners and edges.