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Copper plate w/ Trivalent Chromate Finish

2005

I am plating a low carbon steel part which has a copper plating of approx. .000080"; a clear Trivalent Chromate is then applied. The finished part has an appearance of being stained or what you might term as dark spots.
The issue is my customer says parts should not exhibit the described stain. My plater says it is the process and anybody who knows plating would agree. Cosmetics would seem to be an issue here.
Can you offer me some opinion either way? Thank you.

William F. Cade, Jr.
Stewart EFI, LLC - Yonkers, New York

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Such a question is difficult to answer as a generality without seeing the parts and without reading the rest of the process parameters. But in general, again in general, there is merit to what the plater is saying. Chromating is considered a functional finish rather than a decorative finish, and staining is common. If a brighter, cleaner finish is required you might consider an application of benotriazole in lieu of chromating.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

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2005

I feel that the problem is that the chromate finish is not acting as a passivate/tarnish resist as would a hexavalent passivate dip for copper. I believe your plater is probably using the same type of chromate they use for zinc plating since I am unaware of any trivalent passivates for copper.
The staining may be due to selective oxidation due to the lack of a truly protective film, poor rinsing either of the chromate or possibly of the plating (a bleed out of alkali from the plating is a possibility(.
You and your plater needs to first determine where the spotting is coming from. Ask the plater to work with his supplier to try to determine the answers.

Gene Packman
process supplier - Great Neck, New York