Originally Posted by ArcticBear
If it is a stainless keg it wont rust at all.
Since you just cut it open, it's like just stale beer and sediment. soak it with some hot PBW for a couple hours and i bet you it will wipe right off
That is not quite true. Stainless can rust under certain circumstances. The resistance to rust is a thin "passivated" layer. Basically a layer of oxidized chromium. Things that will cause pin holes such as bleach, or some juices or scratch and scour away the surface layer will remove the rust protection.
That being said, I agree with everyone else that it is rust. The clean spot in the middle makes me think that sediment dried in spots where there was no fluid and where there was fluid (low spot) it never adhered. Plus I doubt any of those condition were present in this keg.
One of my kegs has rust in the top, fellow I got it from, cut the top with a torch, the splatter dried to the sides and rusted.