There are however other types of corrosion that go on, but they take a very long time to get to the point to where they are a problem. After a while these will make the water conductive enough to cause the battery effect. If you change the water now and then with freash didtilled water and whatever chemical to keep growth down there shouldn't be anything to worry about. But I never had a water cooling system go over 6 month strait without tearing it down and replacing everything.
In other words I can be totally wrong, but I never had any major problems.