But add water to the join and suddenly corrosion blossoms. That's because water is a conductor and becomes the facilitator of the current flow. This is why mating dissimilar metals is much less of a problem inside your house than it is on your boat. All forms of galvanism involve metals, but all metals don't look like metals. Carbon is a metal that is used in making rubber, and so carbon rubber when mated to stainless steel can produce quite a reaction.
Galvanism is a very complex issue.
Boats, of course, have a lot of different metals in them, including those below the water line. This is complicated by the fact that all bronzes, brasses and all stainless steels are not the same. This accounts for why there is such a wide range of performance of these metals, and sometimes why they corrode when they shouldn't. If the right alloys aren't used, we have a problem. We attach pieces of zinc to the underwater metals of boats to protect those metals.