Introduction to hard anodizing

The main purpose of hard anodizing is to form a thick and dense oxide layer with a high wear resistance with thicknesses above 25 µm (1 mil). A dense oxide layer is an oxide layer with narrow pores and very thick cell walls.

The figure above shows the difference in the structure of hard anodizing a) and conventional anodizing b) seen from the top of the anodic layer.

This can be done by a low electrolyte temperature and a low concentration of the electrolyte in order to slow down chemical dissolution of the oxide layer. Production of very thick coatings will usually involve very high voltages and/or high current densities, which lead to high local temperatures therefore agitation of the electrolyte is most important.

According to MIL-A-8625E the hard-anodized coatings are characterized by their layer thickness and the weight of the formed layer. These types of coatings are named Type III coatings. Usually these coatings are used in the engineering industry for components such as pistons, cylinders and hydraulic gear, where a severe abrasive wear is found.

Apart from the wear resistance of the oxide layer, the hard-anodized oxide layer has other properties. Properties such as low friction and non-stick are very important. These hard coatings are usually unsealed to maintain a high wear resistance, but can be impregnated with different materials such as waxes and silicone.

If sealed in hot water the wear resistance will decrease with 20 - 50 % depending of the sealing process used.

If the corrosion resistance is the most important property for the surface, a sealing will enhance this property. The sealing will normally be in hot water or dichromate, which increases the corrosion resistance remarkably.