Any time you have two dissimilar metals in direct contact you will have electrolytic corrosion to some extent. The question is how bad is it? My guess is the "deterioration" you see on the mounting flange is actually coming from the wheel and most of the time this is just surface discoloration and is nothing to worry about.

Examine the wheel and flange for any signs of pitting or cracking. If you don't see any then you have nothing to worry about. If the discoloration bothers you, you can remove it with a stiff-bristle non-metallic brush.