All aluminum will start to oxidize if there is no coating on it.Wheels ususlly have a thick protective finish on the face but not very much on the inner sides.Depending on how much has oxidized and how pitted it is,I would suggest cleaning it up with some scotch-brite pads and then spraying on a clear plastic coating available at any hardware or parts store.The tire installer should be able to apply a bead sealant to the rims and your problem is pretty much solved.
I agree with boghog, clean them with scotch-brite pad, to remove the corrosion. However, what I use is a heavy grease on the bead. This has the advantage of protecting from corrosion, adding a lubricant for the removal and installation of your tires. Plus it also aids in sealing the bead.