What could be causing this problem with adhesion?
Be certain that your plating company actually knows how to put
rhodium onto sterling silver. While rhodium can go directly over gold
and some other metals, on silver, it requires an under plate to
adhere properly, as well as to avoid issues with the silver
contaminating the rhodium bath. Usually, a copper flash and then a
nickel under plate are used, though there are other combinations
that also work.
For any good adhesion with plating, proper cleaning is essential.
The metal must be oxide free, as well as free of any dirt or oils.
I’d assume most electroplating firms use a proper electro cleaning
stage for this, but some jewelers do not, using simply an ultrasonic
and steam clean. These are not always adequate. In the case of
silver, the need for it to be oxide free is also significant. If your
work has fire stain on it (not black fire scale, but rather the
somewhat subtle creamy colored or reddish tinted cast on otherwise
silver looking metal, caused by penetrating copper oxide), then this
can interfere with the adhesion of the plated layer.
Also, if a nickel under plate was used, there is the possibility of
the nickel layer being exposed to air for too long before the rhodium
is applied. Nickel also oxidizes, and if the nickel under plate is
allowed to dry and sit before the rhodium goes on, then it can form a
transparent thin oxide film. The fix to this is a mild acid bath dip
to remove this oxide layer. Called an activating acid dip, phosphoric
acid is a common agent used, but even ordinary pickle will do it in a
pinch. If the nickel doesn’t get this activating dip, and has been
allowed to sit too long between plating steps, then the rhodium will
not adhere well, and can do just what you’ve described.
Finally, it’s also possible to mess up a rhodium electroplate, or
the under plate layers, by simply trying to put too much metal on too
fast. Too high a current density can give a thick but poor deposit
that won’t adhere well.