Contact them ASAP.
This is a known problem with the dark silver wheels.
There was a special coating that was not properly done.
The new wheels do not exhibit this problem. Only '15 and '16 ones.
Mine was replaced free of charge (all four, new ones). New ones last very well so far.
Talk to your dealer ASAP. Since yours is out of warranty, if I were you, I would be nice and plead the case.
Dealers are aware of this problem.
If dealer is not helpful, call MazdaUSA with case number and dealership name.