Because of a neurological condition, I had to leave 500 rounds of brass in various stages of preparatation sit idle in ziploc bags for 20 years. When I recovered and returned to it, much of it was not only tarnished, but corroded.

Even corrosion that is confined to the surface layer of the case may be remediated. My approach was:

• Soak the brass in a weak acid solution; either Vinegar at the rate of 1/4 cup per quart of hot water or Lemishine at the rate of one teaspoon per quart of hot water, for about a half-hour agitating the mix every ten minutes.
• Rinse and dry the brass.
• Tumble in dry media overnight (i.e. at least 8 hours).
• Inspect the cases and return to service any cases that shown no cosmetic defect.
• For the remaining cases, cut paper towels or napkins into strips large enough to cover the whole length of the case.
• Saturate the strip in Vinegar.
• Wrap them tightly around the case and let sit overnight.
• The next morning, strip off the saturated paper and the cases will be pink and the corroded areas tan.
• Tumble in dry media overnight (i.e. at least 8 hours).
• Inspect the cases and return to service any cases that shown no cosmetic defect.
• For the remaining cases, polish thoroughly with a metal polish like Brasso. Ignore the people who tell you it will damage the brass. The concentration of ammonia in Brasso is much less than the concentration of ammonia that ammunition was exposed to when it was stored over a manure pit. Read about "season cracking" on Wikipedia and compare those conditions to a few minutes exposure to 1% ammonia in Brasso.
• I know there are people who have reported cracking or splits in Brasso polished brass, but without comprehensive records it is not possible to distinguish that from cracking that occurs as a result of work hardening.
• After polishing, discard any cases that still have any cosmetic defects as it must be considered that these are no longer confined to the surface of the brass.