Nickel plating can be removed by reverse plating using a lead
cathode and around a 50% sulphuric acid bath at room temperature and
6 volts. ( from "Electroplater's Handbook" by C.W. Ammen ) None of
the numbers are very critical. Any rhodium or gold plating over the
nickel is also removed. It will attack silver but only very slowly,
keep an eye on it. As I remember the current drops significantly when
the nickel is gone. The silver will have a frosted white surface
which responds well to tumbling with steel/stainless shot.