OLD brass.

[Mr. 16 gauge]

I got a bunch of .38 special brass from a guy several years ago that his dad had.....some of it from the 50's as well. Some of it was corroded, and I did the same as you (discard the heavily corroded stuff). I reloaded it all, and all I can tell you is some of the stuff is so brittle that the cases split when fired....no stains or corrosion, ect. Some with the stains have been fired a couple of times without incident.
Take from my experience what you will..............