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 figures show the differences in structure of type II Anodizing compare to type III Anodizing. The anodized layer is seen from the top and down into the porous hexagonal structure.
The structure of the porous aluminium oxide layer is highly ordered as explained in an earlier post with the great slide from a hard coat presentation by Mr. Leonid Lerner from Sanford Process Corp. at International Hard Anodizing Association symposium in Las Vegas.
The slide show the two different directions of external stresses in the anodized oxide layer depending on if we test it or use it in normal applications.
According to MIL-A-8625F shall type III coatings be a result of treating aluminium and aluminium alloy electrolytically in a sulfuric acid based electrolyte to produce a uniform, hard anodic coating, often called Hard Coat in US and Hard Anodized.
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-8625F the hard-anodized coatings are characterized by their layer thickness and the coating 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.
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