The authoritative public forum
  for Metal Finishing since 1989
-----

Reducing chloride level in electroforming by high current treatment

We are running a fairly standard sulfamate nickel electroforming operation but have concerns about residual stress and pitting. A recent bath analysis showed 42 g/l chloride which I realize is quite high. We have heard that running this bath at very high current (no parts, just dummy electrodes) for several hours can bring the chloride level down. Is there any truth to this?

Ken Holko
Engineer - San Diego, California, USA
2007

---

The discharge of chlorine at the anode is not the preferred anodic reaction, so it is unlikely that it will significantly reduce the chloride concentration. There is a chance you may slightly reduce the concnetration if you use inert anodes (or passivate the nickel), but the dominant reaction will be the release of oxygen. Your chloride level is much too high for stress free or low stress nickel deposits. In theory you don't need any chloride for sulphamate nickel as long as you use S-nickel rounds as the anode. In practise, most electroformers use 10g/l or less nickel chloride. I would recommend you aim at very low levels, say about 5g/l. However, I must ask, how did the chloride level get so high - I would strongly recommend you solve that problem and ensure it doesn't happen again. Removal of chloride is not easy; it can be done by selective preciptation, but you run the risk of contaminating the bath with another cation, so I would recommend you dump the bath or at least remake it and slowly use this bath as a chloride top-up system for the new bath. Alternatively, you can add stress reducers to the bath, but these tend to be quite expensive and can cause other problems - talk to your chemical supplier about what options they may have. Good luck.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2007