The same thing happens with any two dissimilar metals, like an aluminum intake on iron heads. With time it is possible to corrode things to the point where they are ruined. One way to fight it is placing a piece of reactive metal such as zinc or magnesium (I can't remember for sure) in the coolant. I seem to remember a company a few years ago that had a special radiator cap that had a piece of one of these metals attached to it. The idea is that the metal acts as a sacrificial anode and it corrodes instead of your engine parts. It might be worth looking into.

Aaron