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Silver Plated Contacts Galling

2005

We manufacture and assemble silver plated Copper and Brass parts (we do our own plating) that are contacted by silver plated Beryllium Copper contact fingers. The contact "fingers" have to slide on the surface of the parts. Parts that are plated just a few days ago will gall when contacted. We developed a method to avoid the galling but don't quite understand how and why it works: We wipe the parts with a damp paper towel. Alcohol or acetone ⇦ this on eBay or Amazon [affil links] Warning! highly Flammable! gives the same effect. Immediately after wiping the surface becomes like hardened and the contact fingers slide instead of biting into the surface.
Plated parts that are sitting for 2 weeks or longer have no problems with galling even without this "procedure". Can you tell me what the chemistry behind our "hardening"-procedure is?

Holger Schult
Electronics mfgr. company - Williamsport, Pennsylvania, USA

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Fascinating! I've never heard of this before, so can only surmise as to what is going on. Firstly, it could be that when you rub the silver with something, you are smoothing it down on a microscopic scale. Do you get the same effect if you just dip the connector into acetone? This will stop the two mating surfaces from galling, as it reduces the mutual keying. Secondly, it could be that you are putting another layer on the silver, in this case it would be either silver oxide or silver sulfide. This again will level out the microscopic surface and allow better traversing between mating surfaces. Thirdly, galling is usually due to a strong mutual attraction between the metal surfaces and the same clean metal intimately mating is always prone to galling. So, by leaving the part to change its surface chemistry over a few days, or by dipping or rubbing it, you are changing and/or contaminating the surface and reducing the attraction level. I suspect to get a complete understanding, you will need to get down to the micro or nano structure level. Can you patent it!?

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005