C Class wheel corrosion

[steve-V8s]

Both rear wheels on my 27K miles C350 are corroding from the centre towards, they are otherwise pristine. Fronts have similar corrosion starting but are no where as near as bad. Car is still well within warranty but Merc have refused the claim. Had to go and ask the dealer what Merc said as the initial enquiry was about a month ago but they had “forgotten to let me know”

Is this corrosion pattern unique to my car or have others suffered the same ?