Pitting on trim

Sonata 2.7 auto, 2002 model.
Hi all, no real big deal, but the chromed trim surrounding the rear licence plate panel is pitting. Looks like the sort of corrosion you can get
aluminium. I polished it off, but you can still see the little pitting holes. Seems strange on what I am assuming is a chromed plastic part. Or is it a chromed aluminium part. The car is 3 1/2 years into a 5 year warranty. I'm hoping this will be covered. I purchased the car new and have looked after it. It only has 24000 miles and gets garaged every night.
cheers, nick
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