It's not stained from flux. Stainless does not require flux when welding. Unless you're welding with the SMAW process. Which is highly unlikely.
Where did you get the keg from?
How did you remove the top?
Anyway, you can use an abrasive to remove the staining whatever it is.
Just be sure that the abrasive is either new or has never been used on anything other than stainless steel and you will have no worries.
Don't believe all the hype about passivation and abrasives on stainless.
Passivation is used to remove free iron from the surface of stainless so that when the protective oxide layer reforms it is free of contaminants.
Stainless steel will repair itself if the protective oxide layer is disturbed as long as the surface is exposed to moisture and oxygen.
After using an abrasive to remove the staining, rinse with water and the stainless steel will do the rest.
I'm sure some people on this forum are going to balk at my statements but the only reason I would refrain from using an abrasive is if I had a fancy vessel with a highly polished surface finish, and only because I didn't want to mess up the finish. But given the circumstances I still would because you can bring that surface finish back to it's original condition.
I have been in the welding business for 22 years mostly pipe welding.. Welding stainless steel for close to 15 years. So there's not much anyone on this forum is going to have to say that's going convince me otherwise.
Maybe I just take it for granted that I've been working with stainless for so long that I know how to work with it without compromising it's integrity.
