Plating Rhodium over most Silver alloys can be a headache; there are
enough variables in the process to make your head spin. Over the last
several years of visiting customers all over Latin America and
Caribbean on behalf of my former company, Technic, and my current
company, Davis-K, (both US manufacturers of Rhodium for Jewelry
applications, I have compiled a list of issues that seem to
represent the majority of these problems. Here are two of them:
- Work pieces are not “active” (you have adhesion or other issues).
Our colleague from Red Sky Plating in Albuquerque did an excellent
commentary on this issue, but it deserves to be emphasized, I see
this issue everywhere, and it is little understood by the operators.
There are two completely separate issues that keep Rhodium (and other
processes) from plating well onto your work when your bath is in
perfect condition and your electrical parameters are correct: they
are: Cleanliness and Activation
- Most operations know how to clean parts, so why aren’t’ your
silver parts always “willing to plate”? It’s because your must deal
with BOTH cleanliness AND proper activation in order for your part to
plate properly.
A clean part that is very passive is still tough to plate well. A
well activated part that is not clean will also give you problems.
Note that many types of silver will become “passive” and therefore
harder to plate just from “sitting around” for 10 minutes or less.
Here is how to make sure your parts are “ready to plate”:
[] If you take a silver plated brass hull cell (a metal coupon with a
2x4 flat surface, bright silver plated) and dip it into a clean water
rinse, what do you see? Does the water stick uniformly to the WHOLE
surface, or does it “bead up” like raindrops on your windshield? This
part could be dirty, or passive, or both. Run the coupon through your
ultrasonic and electrocleaning process. The part should now be CLEAN,
but that doesn’t mean its “active”. Dip the part in water again. More
than likely, the surface may still be rejecting some of the water, it
beads up and there are dry areas with no water at all. If so, the
part is clean, but not active. Now dip the coupon in a 5%
concentration of reagent grade sulfuric acid and clean water, for 15
to 30 seconds, mild agitation. Dip the part in a pure water rinse
again. See the difference? If the water sticks to the whole surface,
that surface will now plate nicely! Same goes for your parts. Teach
your operators to look for that “sheeting action” on your work
pieces, its tougher on small parts, but when they get the hang of it,
they will know before they plate that the parts are ready.
Another problem I see a lot is:
- Out of control casting process temperatures/times can affect the
quality of silver surface, and, similarly: difficult Alloy types.
- You can plate a gold part in your Rhodium bath perfectly, but the
minute you switch to silver, BINGO! You’ve got issues. (this is also
a great test if you are having problems with Silver: Plate a gold
part, if it plates nicely, the problem is not likely in your Rhodium
bath) There are a lot of reasons this can happen, but the two I run
into the most are these:
[] Highly tarnish-resistant or hardened silver alloys (like
“Argentium”, silver with 7% Germanium), tend to be tougher to plate.
Why? Rhodium is angstrom thin, even when plated at “proper” thickness
(30 to 50 u", up to 100u") It does not “level” (like nickel or
copper) and the surface of those alloys are, microscopically, VERY
rough and porous. Rhodium itself is somewhat porous, and does not do
much to help seal up that porous surface.
[] Hot Casting process. Casting at higher temperatures may have some
advantages, but can result in the “stratification” of the alloys
(separation of the homogenous mix), leaving a very porous and less
desirable surface in some areas of the workpiece.
Solution: Try Pre- Plating your work with Palladium or a good “White
Bronze” barrier. There are several other advantages to using
Palladium as a pre-plate before Rhodium, the most important of which
is potential elimination of stresses in Rhodium that lead to
micro-cracking. (the part appears later to tarnish, even though
Rhodium cannot tarnish)
Hope this helps!
Glenn Thompson, Davis K Products Inc.
davis-k.com