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