I think you mean cathode shadow.
This can occur when something else that is at anode potential (electrically connected to the anode) is placed between the anode (the work) and the cathode. When the distance from the shadowing piece is much closer to the work then is the cathode this can occur. The effect is a thinner anodic coating on the work in the "shadow" of the offending piece. This can be alleviated by repositioning the cathode or by use of multiple cathodes. Cathode shadow is rarely a problem when anodizing one suspended piece at a time, it can become a problem when using racking or anodizing multiple suspended pieces.
If you are using a Constant Current (CC) power supply (highly recommended) anode vs. cathode area is not at all critical, CC adjusts automatically to accommodate the actual ratio. Generally, we recommend that the ratios be more or less equal, there is no harm if the anode : cathode ratio is 3 : 1, or as much as 1 : 100. This is not the case if Constant Voltage (CV) like a battery charger is used, here the ratio will effect the current density if it is not compensated by manually adjusting the voltage.
|