One thing you might try is Naval Jelly-- phosphoric acid. It won't attack any silver that might be present, but will eat up the iron oxide. I don't know what it would do to brass or bronze, though, so research is warranted before committing to this kind of treatment.
I also knew a guy some time ago that found a rusted concretion while detecting. He said he got a good signal but didn't know what to do with the object. He left it sitting on a shelf in his garage. 6 months later he bumped it when he was doing something else-- the thing fell off and hit the floor. Inside was a gold ring! He had it tested, it was an odd karat, primitively made, and the jeweler said it had to be very old!