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Rhodium plating on sterling turns oxidized


#1

Hello.

Most of silver jewels I make are rhodium plated. Recently, some of my
own silver jewels presented an oxidized surface, smelling like
vinegar. Icleaned them up and observed that the plating was taken
off by the oxidation. Silver surface remained without oxidation, such
as all other silver jewels (non plated) I had inside the jewel box.I
could not find articles explaining the substances that may cause
oxidation on rhodium. The only thought I have by now is that the
foam inside the jewel box may contain something that is harmful for
rhodium. Does someone know the caution we shall have on dealing with
this situation? Should we warn our customers regarding the way they
must keep their jewels (including white gold ones)?

Thank you.
Best regards, Evelise.
Evelise Gonsales Pimenta


#2
I could not find articles explaining the substances that may cause
oxidation on rhodium. 

Evelise,

Simply put, rhodium does not tarnish. If what started out as a good
looking rhodium electroplate now looks tarnished, it is probably not
the rhodium itself that is at fault. With sterling silver, to
properly rhodium plate the metal, you must first plate it with
nickel. Usually, one puts a copper plate on the silver before the
nickel. With both these platings, and the subsequent rhodium
plating, the metal must be clean, and the platings applied at the
proper temperature and current density or voltage. And with nickel,
it must be rhodium plated immediatly after the nickel is put on, or
if not, the nickel has to be dipped in an acid activator in order to
remove a passive nickel oxide layer that will form if it’s allowed to
dry. If any of these steps are not done correctly, you could have a
rhodium electroplate that is not durable, or might be porous, or not
well adhered to the silver. These situations could allow chemical
reactions to occur with the metals underneath the rhodium.

So the real answer to your question is that you do not have to take
unusual measures to protect the rhodium plating from oxidation, but
you DO have to take care that the rhodium plating is done correctly,
or it will not perform as it should. Rhodium plating onto silver is
considerably more complex than rhodium plating on gold alloys.

By the way, the initial underplating layers of copper and nickel on
the silver are not just to give a good plated layer. It’s also to
protect the rhodium plating solution itself. While silver is not
highly soluable in sulphuric acid, it does dissolve a very little
bit, and if silver is put directly into the rhodium electroplating
bath (which is based on sulphuric acid), it will contaminate the bath
with a little bit of silver, and if that happens, your rhodium bath
will not work well. Again, the solution to your problem is not
special care of the rhodium electroplated jewelry. Rather it’s the
care of the process being done right, and the rhodium plating
solution so as not to contaminate it.

Peter Rowe


#3
Most of silver jewels I make are rhodium plated. Recently, some of
my own silver jewels presented an oxidized surface, smelling like
vinegar. Icleaned them up and observed that the plating was taken
off by the oxidation. Silver surface remained without oxidation,
such as all other silver jewels (non plated) I had inside the jewel
box

I wish I knew what did this, because a simple way to remove rhodium
plating would be very handy! I have some chain I bought without
realizing it had rhodium (it should have to be marked, but that’s
another thread…) and it is useless to me.

Noel


#4

Noel,

If you want to remove rhodium which was properly applied (Cu flash
and Ni under plate) reverse plating in ~30% sulphuric with a lead
anode. Around 6V and with the wires reversed I supose the lead
technicaly is a cathode. Activity stops when down to just the silver,
silver not damaged but it does come out white. A quick steel tumble
and all is fine.

All of the usual safety procedures, after all that much electricity
will drop a gerbal in an eye blink, lead is toxic, and the acid
solution is three times as strong as traditional pickle.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#5
Icleaned them up and observed that the plating was taken off by the
oxidation. 

I’d have to agree with Peter, the rhodium process had a problem. The
above statement suggests that the rhod flaked off? Maybe it was never
really there? Rhod does not oxidize(at least not in the work a day
world) so the discoloration had to be something else. The nickel
plate tarnishing maybe? Contaminated bath? Electrocleaner too hot?
Wrong voltage? Reversed polarity? Rinse water IS distilled, right?..
and fresh? Anode still good?

What color was the initial ‘oxidization’? Yellowish whitish suggests
nickel tarnish which might mean the rhod never stuck. Greyish whitish
dullish splotchy suggests contaminated rhod bath or too hot
electrocleaner.

I would replate something expendable using the same steps as the bad
batch, and really look at everything that happens, hoping to catch
the offending mistake. Step back because sometimes a thing can be
right in your face and you just don’t see it. Forest and trees.


#6

It’s the silver that’s oxidizing, not the rhodium. To prevent silver
from oxidizing you need to apply a barrier coat of something that
will not allow the gases in the air to penetrate to the silver.
Unfortunately rhodium is not a good barrier coat. Typically nickel
plating is used after a flash of copper plating on silver (the
nickel plates better on the copper). The nickel prevents anything
from getting to the silver and causing oxidation. The same thing
would have happened if you had gold plated your silver pieces, the
oxidation starts on the surface of the silver and then bleeds
throughthe plating. It’s a bummer because nickel is much trickier to
plate than rhodium, it’s more difficult to do it beautifully.

Mark


#7
If you want to remove rhodium which was properly applied (Cu flash
and Ni under plate) reverse plating in ~30% sulphuric with a lead
anode. Around 6V and with the wires reversed I supose the lead
technicaly is a cathode 

Cool! I can do that! As long as it doesn’t require more than 1 amp,
anyway. I think I already have everything, even! I know that’s a lot
of exclamation points, but I’m excited to try this (not today, maybe
not tomorrow, but soon, and for the rest of our lives…)

Noel
P.s. In case you’re too young, that’s a quote from Casablanca


#8
It's also to protect the rhodium plating solution itself. While
silver is not highly soluable in sulphuric acid, it does dissolve a
very little-- 

That was excellent, Peter.

One question,

If the degradation of the plating solution did not matter, would
directly plating rhodium onto silver last for a long time? As in if
you made a piece out of silver and there were places where polishing
is not possible when the piece is finished.

Cheers, Hans


#9

Hans,

If the degradation of the plating solution did not matter, would
directly plating rhodium onto silver last for a long time? As in
if you made a piece out of silver and there were places where
polishing is not possible when the piece is finished. 

Check out the price of rhodium solution. Bad idea to poison it with
silver Copper flash is cheap and easy, Ni plating also cheap and
easy. I rarely use rhodium but do know the rules and procedures.

Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#10

Noel,

I can do that! As long as it doesn't require more than 1 amp,
anyway. I think I already have everything, even! I know that's a
lot of exclamation points, but I'm excited to try this (not today,
maybe not tomorrow, but soon, and for the rest of our lives...) 

Try it. 1 amp is low (slow) but should still work. I think I even
did it using really strong sparex once. My suggested recipe came out
of a plating book but I don’t think there is any really advanced
rocket science involved.

All I can add is that good exhaust is nice to have, fumes and
spatterrs are not nice. I’ve only needed to use the technique a
couple of times but it was magic.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#11
One question, If the degradation of the plating solution did not
matter, would directly plating rhodium onto silver last for a long
time? 

Frankly, Hans, I don’t really know. I already do as little rhodium
plating on silver as I can manage, since I much prefer the color of
silver over that of rhodium, and when I do rhodium plate on silver, I
always use the copper and nickel underplates. So I really have no
experience with directly plating rhodium on silver. The main thought
is that, as other posts mentioned, rhodium itself doesn’t form an
impervious seal to the surface. It’s somewhat abrasion resistant
since rhodium itself is quite hard, and it’s used for the color. But
just as tarnish will blead through a gold plate applied directly to
silver (unless it’s very thick), I belive tarnish will also bleed
through rhodium over time. But frankly, I’ve never tried it. so this
is just a guess.

And for that matter, given the cost of rhodium solution, I would
tend to think ruining even a small amount of the plating solution
WOULD matter. Note that the contamination problem doesn’t show up
only slowly over time with increasing contamination. I recall once a
coworker tried to directly plate a single silver ring without knowing
it needed the underplate. It was just the one instance, and a fairly
new bottle of rhodium solution. But it was ruined with just that
single use. After that, the rhodium would plate unevenly and with a
blotchy, spotty surface. Even a run through activated charcoal in a
filter paper (which will clean up contamination from various organic
contaminants that can also mess up rhodium solution over time) didn’t
fix the problem.

cheers
Peter


#12

Hello.

I would like to thank all those who spent time on reading and
answering my question! Information was useful for understanding
better the “secrets” of applying rhodium on silver.

All my best
Evelise Pimenta.