Hard anodizing is similar to ordinary anodizing or anodizing: Here, too, the object is immersed in an electrolyte and connected as an anode. The difference is that in hard anodizing, higher amperages are used, resulting in thicker layers of aluminum oxide than in the simple process. Depending on the purpose, this can be 25 to 50 µm or even more. Hard anodized components are thus particularly well protected against corrosion and mechanical damage and can be used in demanding environments.
High Current, cooled Electrolyte
Hard anodizing is a slightly more complex process than simple anodizing. The electrolyte must be cooled so that the workpieces do not overheat at the high currents. The amperages must be adapted to the respective alloy. The composition of the electrolyte and the current strength are selected in such a way that the pores are as small as possible. The result is worthwhile: The thick protective layer makes the components hard and robust. Bolts, gears or bearings in weight-sensitive industries can also be made from the lightweight aluminum material.
Properties Depending on the Alloy
The exact properties and possible coating thicknesses depend on the respective alloy. This process cannot be used for all compositions. Alloys that are highly conductive are best suited for this. In this regard, alloys with lead or bismuth components should not be used. The anodized layer itself does not become conductive and can, therefore, be used for insulation. As with anodizing, it must be borne in mind that the dimensions change due to the layer structure. The volume increases by one to two thirds of the layer thickness.
Advantages of Hard Anodizing:
The color of a hard anodized component is determined by the alloy and is naturally and usually darker. The possibilities of coloring are limited and are mainly restricted to dark tones.
Hard-anodized parts can then be treated afterwards to close the pores and improve lubricity, for example with a PTFE coating.