I had the same problem with my Monza wheels on the last GTI....I think this must be a VW problem...
It's been a VW problem since about 1990 when they fitted the Ronals to the Mk2 16v with the polished face - they corroded.
Mk3 Anniversary BBS - the lips corroded.
Mk4s with BBS RXIIs and RCs - they corroded.
Mk5 GTI 18" Monzas - they corroded.
You'd think they's would learn by now...almost every wheel they offer (in a poor selection) have a polished face that will corrode.