It's completely normal. If you want to prevent this you can have the hubs stripped and painted with Sperex VHT ('Very High Temperature') paint. It's the same on everything. I find that 'Copperease' grease prevents the alloy wheels from fusing to the cast iron hubs of my Fiat. Aeronautical engineers tell me that introducing a third metal (copper) is not the right thing to do, but I think it becomes a sacrificial anode. Whatever, it works.