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Removal of Iron from Gold Plating Solution

We are using Nickel hardened Acid Gold bath, operating at the pH 4.0-4.5. Recently we found the iron content reached to a level of 300 ppm. It would be great if someone help me in solving following doubts / issues.

1. Will the coating deposit get affected by 300 ppm of iron, when it is alloyed with Nickel?

2. Presently the deposit fails for NAV (Nitric Acid Vapor) test, when subjected to 120 minutes. Is that reason could be of high level of Fe content?

3. If it felt at high, how to remove the Fe impurity from Gold bath?

Thanks in advance

Dhanasekar D
Employee - Bangalore, Karnataka & India
June 29, 2009

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With a Fe concentration of 300 ppm in a nickel hardened bath, the hardness and contact resistance of the Au deposit will be greatly affected. Also the color of the Au will be brownish. EDTA has been effectively used in the past to chelate or "tie up" contaminants, but it has carcinogenic hazards.
Let the Au concentration in the tank drop to the lowest level on your spec. Take a liter of Au solution into the lab, lower the ph to 3.8 with citric acid ⇦this on eBay or Amazon [affil links] , and lower the temp to 85 degrees F. Plate at 3 ASF for one hour. Check the solution for Fe again to see if it has been reduced. At a lower ph, temp, and Au concentration you will preferentially plate out more of the alloy in the bath, or in your case Fe and Ni. You will still plate out some Au which involves a cost, but it will be alot cheaper than changing out the bath. If this experiment proves satisfactory to you, you can perform the process on the Au bath. Your gold supplier should be able to help you as well.

Mark Baker
Fellow Plater - Syracuse, NY, USA
June 30, 2009

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When I worked for a gold solution supplier this was a common problem. Although you can reduce the problem, there is no permanent cure that I can recommend.
Speak to your supplier. We would always supply a new solution and give the customer full value for the gold in the old one.

EDTA is not a carcinogen. EDTA is authorised as a food additive (E385), and used in laundry products and medicine.

Geoff Smith
Hampshire, England
July 3, 2009

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Geoff,
You are right about EDTA being used in food products as a preservative, and medical products. What I should have said is that when EDTA comes in contact with certain chemicals it can form Benzene, which is a known carcinogen.

Mark Baker
Fellow Plater - Syracuse, NY USA
July 16, 2009