Removal of Passive Layer for Copper Electropolishing

Q. I am currently working for a lab that is conducting experiments in approx. 2 by 2 by .5 cm microwave cavities. The nature of the work requires that the surface resistivity of the copper be as minimal (as close to the resistivity of pure copper) as possible. It was determined that an electropolishing process offered the best chance of achieving this ideal, and I have been searching the literature on a laboratory sized copper electropolishing set-up since the company we consulted was not able to electropolish such a small cavity.

It is my understanding that the bright dip/acid dip recommended before electropolishing, and the electropolishing itself, may leave a residual passive layer on the copper that will have an unfavorable electrical resistivity compared to the copper alone. Are there any processes designed to minimize this layer, or any resources devoted to this subject?

A. Hi Steve. Forgive me for not understanding your technology, but other readers may be in the same boat.

Aren't these cavities electroformed in a copper plating solution. If so, don't the plating conditions affect the conductivity of the metal regardless of whether you remove the surface skin? Is this object kept sealed in a vacuum or inert gas? If not, how do you stop the copper from quickly oxidizing/passifying anyway? Maybe once I'm on board I will be able to make a constructive suggestion :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Ted is available for instant help
or longer-term assistance.
September 27, 2012

Sorry for the ambiguity.

Q. The parts I'm working with were machine milled several months ago. It was originally unknown what effects a clean but milled surface would have on the resistivity of the cavities, and it was determined that the microscopic grooves as a result of the milling process were most likely raising the resistivity significantly.

The cavities since then have been sitting in the open air, and have become heavily oxidized. After the electropolishing process, we plan on storing them in an inert gas or vacuum container.