Was watching a repair shop the other day, and saw some chemically applied silver plating.
Would this be suitable for plating some copper electrical contacts (low voltage) that get dirty/oxidised easily, and make a poor connection?
Just for info, it's on the old Rover. The switch gear has got contacts rated for high current - but many actually switch just relays. Those that switch high current remain clean, or clean up with a little use. The ones switching relays stop doing so until physically cleaned. Not a problem on a car used every day, but is on one not much used. All the contacts are plain copper. The switches themselves, easy to dismantle.
I doubt if it would stand up to the duty. Only one way to find out though.
My instinct is that it is only a cosmetic decorative silver plating and will not stand up to the rigors of being a switch contact.
I'm not sure what the formulation is for the modern physical plating solution that is now sold. It cannot be old style cyanide complex nor the classic two part ammoniacal mirror silvering formula (which can go to fulminate if you mess up). My guess is thiourea and something else.
Silver contacts are all right, but they require a current to be passed through them occasionally to keep them clean. I don't know about silver plating.
The window switches, for example, are also used on the Triumph Stag. Maybe others too. On the Stag, they switch motor current directly, and seem to stay clean (my brother has one)
On the Rover, they switch relays, and need cleaning if not used regularly.
I meant that if copper is going to0 oxidise in whatever environment it is, then silver is likely to as well. It's normally used in protected envirinments, e.g. in enclosures (switches, etc).
Find some specs for switches and see if they use Palladium.
And this is metallurgy after all, and layer after layer of dissimilar metals are normally involved. Maybe copper followed by nickel followed by Pd. I have not managed to find a lot of honest answers about exactly how many layers are involved in this. But nickel seems to be one of them, perhaps put there to passivate the thing underneath.
While everyone knows silver nitrate is the stuff to possess, for dumping silver somewhere, there are some materials that "stabilize" the plating process. These are similar to "photographic solutions". Ours in school was quite toxic, and had sodium thiosulphate as well as HCN (cyanide). And that's not all it had. That was not an electroplate, and the solution (which includes silver nitrate), would slowly but steadily deposit silver metal on copper. The protection afforded, the quality of the plating, was not particularly good. It's a function of surface prep, and how quickly you can plop it into the solution after clean and wash.
I only mention that, to show that while one material might be "a good source of raw metal", the uncontrolled dropping of that metal, results in flakes of precipitate, black in colour, as metal fines are dropped, and no significant plating to speak of. It takes some kind of redox chemical setup to "encourage" the site of the plating, to be other than the entire solution turning into sludge. Electroplating, presumably, being one way to encourage a redox reaction.
When I made chloroplatinic acid in high school, it performed poorly at plating out platinum where I wanted it. No fuel cell for Paul... :-) Just solution with black (very thin/fine) flakes.
Your best bet, would be the nearest engineering school, and see if someone there can help you with your plating project. The amount of Pd on high current contacts is thick enough, you can take an emery board and file it a bit, in a pinch. It isn't a 50u coating.
You can start by examining relay specs or high current switch specs, and see what they specify as coatings.
Bare copper might be OK for switching elements that wipe under significant contact pressure (the switch in my lawn mower), but for face to face contact closures, I would think some other choice would be needed. There wouldn't be enough wiping to keep the copper clean.
Well that depends. Nickel usually, but silver on the more robust ones, even gold in some cases. The main issue with switches is that they oxidise to some extent no matter what you have them plated with, or simply the plating wears through. Common on slide and rotary switches like those in portable radios. Thing is, without knowing how this chemical plating was done its hard today if its any good. Normally the contacts need to be degreased and de oxidised before being normally electroplated to allow it to actually make a good contact. Brian
It is OK on sliding contacts, but not really those that only close with pressure. You need gold for that. Also of course if they are dis similar metals this makes things a whole lot worse. I guess the answer is try it and see. I recall the paint on silver stuff for repairing ribbon cables in the days of the zx spectrum. this worked well for the membranes, but was not much help on the tails where they had to push into a socket. Brian
A thought occurred to me. If its a high current it needs, perhaps some kind of large capacitor that is switched with the load could provide a burst of high current at switch on. You would need a high value resistor across the capacitor to make sure it was discharged for the next switch operation though. Yes it would waste some power, but it would be minimal. Brian
When you say "chemically applied" you mean by reduction, rather than electroplating? IIRC that's normally only done for very thin films. I don't think it is too difficult to electroplate silver, though.
I'd be more inclined to go for gold electroplating although that's a bit more work.
Silver is commonly used on 240/110 V ac relay contacts and is very effective. Gold is preferable for 24/12/5 V dc, as silver will not remain clean enough for them.
There is also the question of whether this type of plating would thick enough and well enough bonded to work for any period of time.
Experience says that the spark you get when switching a current of a few amps is enough to keep them fairly clean. But not when switching about 0.1 amp.
Palladium oxide decomposes at high temperatures, so a bit of arcing keeps it clean. Palladium melts at 1555degC so it is quite robust. I once used it for brazing platinum with a gas torch and found that the final surface was bright and shiny.
The reason for having a nickel layer under gold is that gold diffuses into copper and eventually disappears. Such diffusion does not happen (at a significant rate) with nickel, so it is used as a barrier for gold plated onto copper.
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