Corrosion On Proof Nickels

Any possible attempt to remove corrosion on any metalic object almost always depends on the type of corrosion. How much corrosion is there on the coin also is a determining factor. Where some may call something corrosion, others may say it's just toning.
Could you post a photo of that? Also, how do you know it's a proof coin if you found it in circulation? What is the date and Mint mark?
You stated the corrosion started after you placed them in a 2x2. May be from your hands. How did you handle the coins?
I would suggest you start with a simple dip in Acetone. If that doesn't work, may well be real corrosion and really difficult to remove without damaging the coin futher.