The anodizing process needs these cathodes to run. So anodizing consists of two processes an oxidation process (anodic reaction) and a reduction process (cathodic reaction), and both of them are necessary to run the anodizing process but most of the time we actually neglect this second reation, the cathodic reation.
When anodizing in sulfuric acid the major cathodic reation is the hydrogen evolution.
The position of the cathodes (the blue parts in the drawing) in the anodizing tank is very important.
The Anodizing solution has a good throwing power compared to most plating solution. The reason for this is the high electrical resistance of the aluminum oxide film. This high electrical resistance will produce an anodic film on both sides of a sheet of aluminum close to a single cathode.
So in theory there shouldn´t be any problems placing the cathodes where they fit best in the tank. The film formation starting on the back side as soon as the resistance between the cathode and the near side is equal to the resistance between the cathode and the back side of the plate.
Even so you will often find thickness variation on complicated shapes and over large complex loads. The reason for this is often an insufficient agitation in the tank, or cathode placed in areas where there never are any parts to be anodized.
For example cathodes sticking deeper into the tank than the work load. This will increase the growth in the lower area of the parts being anodized.
The most common cathode material is aluminum, where the prefered aluminum alloy is AA6063 T5 or T6, or the aluminum alloy 6101 also in T5 or T6.
The placement of the cathodes is along the tank sides, so as much as possible of the cathode is facing towards the anode. The cathode area/anode area ratio should be as close to 1:3 as possibly.
The depth of the cathodes should not exceed the maximum length of a normal work load.
Joints between the cathode and aluminum bar should be as easy to maintain as possible because aluminum and the sulfuric acid reacts to form aluminum sulfacte which is very voluminous. This corrosion product can force the cathodes away from the aluminum bar with the loss of contact between the two.
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