Are 2015 Sierra 20" rims coated?

[Garykk]

Noticed some pretty good scuff marks on my rims today after picking it up from the dealer who had rotated and balanced the tires. Of course the denied doing it. In any event I managed to rub most out with a light compound. Noticed that the rag turned black like bare aluminum would produce and I was very careful not to rub too hard. If these things are coated, it must be microscopic.