Today I noticed the same corrosion on the front wheel caps. I have only ever used soap and water on the wheels, no acid based products. The wheels are waxed once a month so the brake dust comes off easily. I am an hour from nearest dealer who wants me to post the caps for inspection! then after analysis will order new ones if covered by warranty.
BUT
If this is not a known problem then why is it that the center caps are on back order and will take 15 working days to arrive.

While inquiring about the center caps I asked about the Startech Sports Exhaust, this can be with the dealer in 4 DAYS, the cost including fitting. ANYONE GUESS

£ 826.38
I'll ask next for price excluding fitting.