What are Bipolar Anodes, and How are they used in electroplating?

A. A bipolar anode should not outgas. It is only a conduit for the transfer of current or voltage from one area to another. Several of the basic plating books address this adequately since it is very simple in theory. You put it in between an area of higher CD and an area of lower CD. I have used them on sulfamate nickel. They work, but are a pain if they cannot be permanently attached to the rack. SS worked fine in Sulfamate.

CORRECTION:

Tom Pullizzi pointed out a reference in Lowenheim's "Electroplating" ⇦[this on eBay , Amazon, AbeBooks affil links] that says it will outgas. Also Graham's "Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] " p. 103 shows bipolar building on one side.

I will concede to the experts.

I had used it in sulfamate nickel that was operated at under 4 volts with very little buildup on the negative side and it increased throw into the sharp corner of the part. No attack on the anode. Unable to tell if it outgassed as it was over a foot down in a well agitated solution. No damage noted to the solution, but it is a robust bath.

If you are going to the trouble of making it out of soluble material, why not make it a powered aux anode and make it work even better. A second power supply may be required to reduce voltage applied to the anode at the closer space. A voltage reduction circuit could be used also, but this will mess up the amp hours as the circuit wastes the extra electricity as heat.

Thanks, James.

Specialty Testing & Equipment offers a device that may address that ampere-hour meter problem. It's a small 'current controller' that you can power the aux anode from, and it's powered from the main bus bars rather than a separate AC supply. If you put the power draw leads beyond the shunt, and if the device is efficient, it would seem that the ampere-hour meter should remain accurate.

I am not sure if I have seen this device or not. If it has been at Sur-Fin, I have. If it has huge cooling fins of extruded aluminum, it is there to dissipate heat that is generated by the voltage reduction. This heat shows up on the amp hour meter the same as if it were plating. I am not an electronics person, just have not seen any Voltage adjuster that did not generate some heat. It is possible that it is irrelevant or is constant enough that it could be compensated for with a slight increase in amp hours.

If it is the one that I have seen, it is really a nice accurate piece of equipment.

The auxiliary anode can be wired to the main buss with a resistive voltage reduction device. The 'resistor' should not affect the total amp hours as the amp hour measurement is not a measurement of power. It is a measurement of amperage. All components of a series circuit will have the same amperage flowing through them. This means the auxiliary anode will have the same amperage as the resistor. They do not add. The main buss will have the amperage from the bath and the amperage through the auxiliary leg. The heat on the auxiliary resistor can be calculated: watt losses are voltage times amperage. Since amp hour measurements do not use voltage you are safe to use the main shunt as an accurate source of measurement of the total amperage flowing in the tank via whatever path you give it.

TUTORIAL FOR NEWBIES:

In electroplating, the workpiece which we want to plate the metal onto is connected to the negative pole (cathode) and the metal which we wish to dissolve into solution and plate out onto the workpiece is connected to the positive pole (anode). A battery or rectifier pumps electrons from the anode to the cathode via the external copper wiring (or bussing).

This "theft" of electrons from the anode converts the anode metal to positively charged, soluble, ions which dissolve into the solution (the anode metal is "reduced"). The positively charged ions migrate through the solution, attracted to the negatively charged workpiece. When the metal ions reach the cathode and rejoin their electrons, the ions are "oxidized" back to metal onto the workpiece.

The "Bipolar Effect" . . .

Electrical charge travels much faster and easier through metal (as electrons) than through a plating solution (as ions). So if we offer the electricity a choice by placing some metal into the plating bath in between the anode and the cathode, electrons will flow through that piece of metal. Lowenheim illustrates this:

Lowenheim's piece of metal is item DE, and as reflected in his sketch, because the electrons run through the metal, a shortage of electrons develops at the end nearest the anode (the right hand end of item DE), and a surfeit of electrons builds up at the cathode end (the left hand end of item DE). This bipolar effect can be used constructively, as Lowenheim illustrates, to channel current into the inside cup shape of item C, to produce thick plating there whereas this recess would otherwise be a low current density area.

The bipolar effect can also be inadvertent and destructive, as when parts fall off of a plating rack and lie at the bottom of the tank, offering a bipolar "short circuit".