plating, anodizing, & finishing Q&As since 1989
What is actually anode polarization? I know polarization can form black film on copper anode (soluble anode) in copper cyanide bath, but what is the effect on insoluble anode eg. titanium anode.
The anodes of some metals (Cad, Copper, Tin, etc.) can have have a tendency to become passive i.e. act as inert (the way to introduce only current into solution) anodes. Unless anodes are deliberately inert, it is desirable if the anodes replenish the metal deposited. In some plating baths anode activators (specific ions) are added to the bath chemistry to keep the anodes active. Another way to keep anodes from going passive is to keep the anode to cathode area ratios at about 2 to 1 (or higher) to keep the rate of reaction low in order to prevent the evolution of oxygen. Oxygen will react with the metal of the anode and form an insulating film. This causes the anode to go passive. Titanium anodes are passive to start with and so will pass current without break down to the anode. Keep the voltage below 8.5 volts and watch out for the fluoride ion, both can break down the normal oxide film of Titanium and dissolve the Titanium anodes.
Regards,Fred Mueller, CEF
- Royersford, Pennsylvania
Fred has explained to you why the anode is polarized. I would like to make addition to explain why the anodes cannot be polarized in some cases while insoluble anodes are used in other cases.
If anodes act as the source of a plating substance, such as in copper cyanide baths, the anodes must be kept in an active dissolution that also balances the metal concentration in the plating solution. Metal ions are reduced to metal depositing on parts acting as cathodes, in the meanwhile the metal dissolves to become metal ions getting into the solution at anodes theoretically at the same speed as the metal deposition at the cathodes (in the quantity of weight/unit time).
In the cases that insoluble anodes are used in plating, the metal salts are added to the plating solutions in accordance with the metal ion consumption. The anodes are only designed to complete the electrolytic circuit. Anodic reaction is mainly oxygen evolution in most cases. Anodes must be kept in passive state otherwise dissolved products from anodes may contaminate the plating baths. The insoluble anodes often have catalytic effect on the anodic reaction.
A plating system determines what kind of anodes should be used. The anodes must then work properly and consistently under the condition required by the plating system.
- Grand Rapids, Michigan