Different metals in contact create what amounts to a battery...material moves from one electrode to the other chemically changing the two. This requires intimate contact and a means for the ions to migrate. There is a scale of reactivity. The further apart the two metals are, the higher the reactivity. Steel rusts all on its own. I'd probably pop some brass nipples in there and call it a day. Brass is between copper and iron, so it acts like a moderator and it won't rust all on its own, either.