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Iron Contaminated Cobalt Hardened Gold Plating Bath

Q. I am trying to remove Iron from a potassium cyanide gold citrate based plating solution. The bath has a gold content of 4 g/l with a pH of 4.2 - 4.6 and a cobalt content of 300 - 500 mg/L. The bath has a Iron content of 200 mg/L and it is causing a problem as to meeting a purity of deposit spec. The Iron is being plating out in a matter of 0.298% in deposit. I have tried a variety of resins but to no success.

The Iron has to be possibly lowered to 25 - 35 mg/L to pass our purity of deposit spec.

Is there a chemical treatment process which will not remove all of the Gold and most of the iron. Dummy plating is not an option.

Doug Duda
- Southington, Connecticut

A. I know with the case of Rhodium plating, you can utilize potassium ferrocyanide [affil link] to precipitate out Iron impurities. We have utilized this method very efficiently, however, the affect on Gold and/or Cobalt concentration is unknown to me.

I would take a small sample size ~ 100 mL of solution and try to add potassium ferrocyanide [affil link] to this. I would try a 1:1 stoichiometry ratio. The solution may need to sit for a couple of days and try to be very careful with siphoning out the solution, as these complexes are very fluffy and will go back into solution easily.

Try to run the analysis on the solution, following the precipitation method and see how much of the iron has been depleted.

James Danford
- Temecula, California
March 8, 2011

Q. Is 200 PPM of Iron contamination in a cobalt hardened gold tank enough to cause galling and why? The gold concentration is at .3 tr oz/gal.

Bert Barrett
- Riverside, California
April 2, 2013

A. Hi Bert. Are we on the same wavelength regarding the meaning of "galling"? To me it means something similar to "cold welding", where material is pulled from one the surface when rubbed against another surface. Is that what you are referring to, cold welding of contacts or something like that? Thanks.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
April 3, 2013

A. Hello Bert,
I can tell you that the iron concentration in your bath is too high and can cause the problems you are experiencing. This is especially true operating the bath at 0.30 tr. oz/gal. When acid gold baths were in the formulation stage, Fe was considered as a hardening agent. The problem that was discovered is that the Au deposit was too brittle using Fe at the same level as lets say nickel and cobalt. In moving contacts there is a transfer of the deposit from one part to the other. This is also true when other alloys are used as a hardening agent and the concentration is too high.

Mark Baker
Process Engineer - Mesa, Arizona, USA
April 9, 2013

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