Letter 31084

Silver contacts are tarnishing [Texas]  

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Does SO2 tarnish on silver affect conductivity or produce intermittant electrical component? Is there a safe way to remove the SO2 tarnishing from the silver without damaging any of the plating process?

Jaime Enrique Garcia
Switch Manufacturing - El Paso, Texas, USA

Your question is a bit vague and abstract, Jaime. All metals except precious metals tarnish, and it affects the conductivity and can cause intermittant contact. Despite this, non-precious metals are fine for contacts in some cases, but totally unsuitable for contacts in other cases. The A.C. adaptor for charging your cell phone may use electroless nickel plated contacts whereas the signal contacts require gold plating. I didn't think silver was used for contacts, despite it's conductivity.

Tarnish can be removed with silver polish, but this may be a ludicrous suggestion depending on the real-world situation.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Silver will tarnish in SO2, especially if it is moist. If the tarnish layer is thick enough, it will result in an electrically insulating layer that could result in an intermittant electrical fault. Removal can be achieved by polishing with a suitable silver polish, but it is all horses for courses.

Trevor Crichton R&D practical scientist - UK

Yes, the Silver tarnish does cause problems in switches when used for low voltages; that's why Gold is used in low voltage electronic switches. A higher voltage (24 to 50 Volts if I remember correctly) is able to 'punch through' the tarnish and the switch operates normally. And at higher currents, the small amount of arcing tends to clean a Silver contact but corrodes a Gold contact rather rapidly.

Tom Gallant
- Long Beach, CA USA

I've seen some telegraph key makers enclose acid free polishing paper strips to periodically pull through closed silver contacts. Most of the contacts I deal with are solid coin silver rather than plated and I just take my dremel cloth buffing wheel with a dab of polish paste on it and hit them a couple whacks. Usually, I only have to do that to a key I just got from someone that never ever cleaned the contacts on and I never have had to reclean one I buffed to a mirror finish. Silver is a better conductor than gold by the way, but gold doesn't oxidize like silver does. Some makers of keys wised up and made what wss described as a "wiping" contact that cleaned itself with each closure. I am more concerned about dust giving me intermittant contacts than oxidation on silver.
Now, carbon from arcing on contacts can really mess you up.
Dealing with high voltage and with RF goes a bit deeper than I am prepared to go with this response.

George D. Ballentine
- Orlando, Florida, USA

I want to answer or follow-up on this subject publicly (in non-commercial fashion).

 

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