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 [link by ed. to spec at TechStreet] 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
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
Silver can be chromated. A spec is QQ-S-365 [link by ed. to spec at TechStreet] 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. RET
Pine Beach, New Jersey
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.
chemical process supplier
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