I had the same problem on 3 of the alloys on my 07 159. Corrosion at the edge on the back of the spokes. This is the sharpist edge on the wheel and is always going to be the weakest point for corrosion. I complained to the dealer, who first refused to do anything. Warranty from the wheel manufacturer is only 12 months. I then took photos and sent to ARUK. They offered 50% of the cost to repair the alloys. I argued and finally they offered to refurbish FOC.
I am in the aluminium coating industry, so was able to technically argue with them. Both ARUK and Perry's of Amersham assured me that they would strip and recoat the alloys. Car was left with Perry's for one day (Mito 155 courtsey car was fun for the day). On collection, the rework was a disgrace. They had just overcoated the wheels with silver paint, with the corrosion marks still visible under the finish. The back of the wheels were still covered in brake dust, so defintely not re-worked as my discussions with ARUK/Perry's. The corrosion will come through, it is only a matter of time.
I completely lost it with ARUK and demanded a new set of alloys. They refused and going up the chain to the customer services supervisor did not help. All they would offer was to have another go at the re-work. I decided not to take up their offer and will live with the wheels over the winter and will then refurbish them myself in the spring. Luckily I work for a powder coaters and will be able to strip and recoat to a quality HIGHER than anything that Alfa Romeo can achieve, not to mention a choice of finishes. (before anyone asks, I can't offer this service to others).
In over 20 years of motoring, I have never had an alloy wheel paint failure. The cause of the corrosion is either down to the metal, the chemical pretreatment or the powder coating. It has nothing to do with the brake pads and alloy wheel cleaners if applied correctly are not a problem (I've used then for years).
Good luck with your warranty claim. I hope you have more success than I did. I am very ****ed off with ARUK's attitude on this and because I can sort the alloys myself, it will not spoil my ownership of the car. But it is just the sort if thing that could put off someone from buying a second car of the same brand. ARUK/FIAT will never get this. Perry's will not be getting any future business from me - they had their heads too far up their own a****, to know what customer service is. RANT OVER.