If the brass finish is coming off and you see bare metal then the cases you have are not brass, but brass plated. Toss those in the trash... Brass cases that you keep for reloading are made entirely of brass, it won't wear or flake off.

Not all ammunition is made from brass. There are some that are brass plated steel such as the cheaper bulk range ammo by Tula, Monarch and Winchester steel and some of Freedom munitions brass plated steel range ammo.

Stick to Winchester white box, Green & white boxes of Remington, UMC, Blazer Brass, Federal, Sellier & bellot, Magtech, to name a few. these bulk packages of range ammo are brass and are good for reloading.