It is anodized, an oxidation process that hardens the surface to resist scratching (aluminum is very soft, aluminum oxides very hard) while coloring. The finish might not be stripped off in that area, rather stained over it.
It is going to take a pretty good abrasive to cut through the anodized surface, and once through, that will start rubbing away aluminum very quickly.
If not anodized, or protected with a coating, aluminum can corrode very quickly to either a duller gray (which may be stable), a black, or a white, depending which chemicals are attacking it.
