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Copper from racks is depositing on stainless parts when pickling

We pickle stainless steel components, mostly 316L, in a solution consisting of sulfuric acid, nitric acid and ammonium bifluoride. We heat the solution to 140 °F, immersion time is 5 to 15 min. depending upon the requirements. Some of the holders are made of copper, thus copper is dissolved in the solution. We cannot easily avoid immersing these fixtures in the solution for a minimum time. The problem is that we get from time to time a copper immersion deposit on the parts. The problem is worse with 316L castings than with sheet stock. Does anybody have any suggestions to alleviate this problem? Thank you.

Thomas Kemp
Chemist - Erie, Pennsylvania, USA
2007

It's probably worse with castings because of lower homogeneity, so more propensity for exposed iron. But you can't stop the immersion deposit once the copper is in solution. Can you not plastisol coat the copper and/or use stainless steel tips on the racks?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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