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Rust spots on copper plated steel balls

I have been trying to copper the steel balls from ball bearings. The steel the balls are made from contains about 1.5% chrome. Using a solution of muriatic acid ⇦ this on eBay or Amazon [affil links] (HCl) and sal ammoniac ⇦ this on eBay or Amazon [affil links] (NH3Cl) I am able to plate a hard copper plate onto the bearing. The plating will polish with #0000 steel wool ⇦ this on eBay or Amazon [affil links] or automotive polish and looks good. Problem is when I get it wet it develops rust (iron oxide) spots. The spots can be buffed off but I want to be able to expose the bearings to the elements, so I can't tolerate the rust. I am going to try pre-cleaning with a bath of caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] followed by a rinse in water and finally a weak bath of muriatic acid. I'm also going to try a lower current, right now I am using about 500 mA per square inch.

Any other ideas?

Tom Marek
- Carpentersville, Illinois
2003

Hi, Tom. To reply to you in broad brush strokes, chloride is not really a good ion for plating because it is corrosive and it creates highly stressed deposits. Sulfuric acid would be a better base for your plating solution, but even that isn't quite right for depositing copper onto steel.

The balls should probably be either nickel plated first or copper plated in a cyanide-based bath, but cyanide is completely inappropriate if this is not an industrial environment. Please consult the Metal Finishing Guidebook or Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] for detailed suggestions about the plating solution. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 12, 2009