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Substituting silver plating for cadmium plating

We manufacture a wide variety of threaded locknuts. Our typical finish is cadmium chromate underplate primer with a black dry film lubricant on top per MIL-L-46010/MIL-PRF-46010 [from DLA] type I.

Our customers are pushing us to eliminate the cadmium. We have silver plated parts effectively - the silver provides good lubricity to the threads/locking feature. However, without a supplemental coating for the silver, it quickly tarnishes.

Does anyone have a good suggestion for a supplemental coating to silver plate that will extend the shelf-life prior to tarnishing, but still allow the silver plating to provide good lubricity for the locking thread in our nuts?

Thanks for anyone's help!

Pat Kelly, Senior Applications Engr
- Danboro, Pennsylvania
1998

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There is a Mil Spec call out for a "plated" dichromate treatment for silver. It is a little touchy, but does help some.

Consider looking at zinc-tin alloy plate. It comes very close to cadmium for lubricity and will take a chromate of your choice. Boeing has done quite a bit of work on this and may share some knowledge with you if asked nicely.

James Watts
- Navarre, Florida
1997

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Silver can be chromated. A spec is QQ-S-365 [from DLA] Grade A.

But when all is said and done, cadmium offers cathodic protection to the locknuts whereas the silver plating if breached will accelerate the corrosion of the locknuts. I think you need to look at tin-zinc plating rather than silver.

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

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There is an electric chromate conversion coating to silver that gives good protection and does not change the apperance of the plated silver. About lubricity, I have no info on this asspect but the passivation is very good.

Sara Michaeli
Tel-Aviv-Yafo, Israel
1997