With internal headsets you only have to worry about a few things:
First, water infiltration normally eats at the fork "race" and bearing seat. Both are steel and corrode. This generally only means you have to replace both. You can help prevent by placing a good layer of HQ grease at this interface point.
Second: The same applies to the top bearing, but to a lesser extent and I normally only see corrsion on Tri bikes because those folks just sweat buckets in their aerobars and it drips right on the stem/headset.
The shoulders that Internal Headsets sit on in the headtube are generally aluminum or carbon (depending on frame) and are pretty stout and I have never seen issues with them.