Victim of Bipolarity

Question:

While installing new anode bags, the enclosed nickel deposit was discovered attached to a cathode-facing corner of a titanium anode basket (one of six) containing nickel rounds. What could cause this in a nickel sulfamate electroforming bath and what deposition problems might result?

Answer:

Si Gary, CEF, of Scientific Control Labs., Chicago, took a close look at the specimen you sent and had the following reply. (Note: The nickel electrodeposit, about 0.03 in. thick, conformed to the shape of the inquirers titanium anode basket.)

Dont feel you have been picked on or are a victim of foul play! You have experienced a classic case of bipolarity. To explain bipolarity, you should first understand that:

ï Electricity will always flow through the path of least electrical resistance.

ï Metals and metal particles are better electrical conductors (they offer less electrical resistance) than plating solutions.

ï Anode bags may be neutral as to electrical conductivity if they are open and the electrical current may flow through them as though they didnt exist. That is, the resistance will be the same as that of the plating solution. If the anode bags are plugged with non-conducting materials, they will be very poor conductors; conversely, if they are plugged with metal fines, they will be quite conductive.

Lets look at a simple plating cell with a metal bar floatingóelectrically speakingóin solution (Fig. 1). There are no electrical connections to the bar (consider it as being suspended by a fine nylon string).

The path of least electrical resistance from the anode to the cathode is from the anode to Point C through the plating solution, from Point C to A through the bar, and then from Point A through the solution to the cathode (or vice versa if we consider the current as flowing from the cathode to the anode). Point A will be anode relative to the cathode and Point C will be cathodic relative to the anode, even though the bar in total will be electrically neutral. Point A will dissolve or be an insoluble anode depending on whether the plating solution will attack the material that the bar is made of. Point C will receive a deposit of metal of the type being plated.

This phenomenon is sometimes put to good use by constructing racks with bipolar electrodes to facilitate plating in recesses. Bipolarity is also a very common source of trouble at entry and exit stations of automatic plating machines, especially when plating nickel and chromium.

And now for the specific problem you have posed. In this case, the titanium anode basket actually became bipolar. The area of the basket that plated became similar to Point C in Fig. 1, while some other area of the basket became equivalent to Point A.

Things that could cause this to happen include:

1. Poor electrical contact to the chips, so the basket is fully neutral or somewhat neutral (electrically speaking) compared to the other five baskets.

2. Insufficient chips in the anode basket.

3. Poor conductivity through the anode bag because the bag is too tight.

4. Clogging of the bag by crystallized plating salts.

5. Too many conductive anode fines in the anode bag.

6. A great disparity in size and/or location of the anode basket and the parts being electroformed.

As to deposition problems, there is a corresponding area of the basket that is acting as an anode and probably as an insoluble anode. This could cause problems with your stress level.