@MrPink
I know this is a late response, but that coin was part of an ignored collection. It contained coins in 2x2s using PVC plastic here and there and was stored in various hazardous environments including a rented storage building, attic and metal shed while being around other chemicals in Texas summer heat. So, I'm surprised that some of the coins turned out unscathed. An acetone bath was all that was needed for some to be right as rain.

I think the specific alloy of this coin made it especially susceptible to corrosion from PVC residue.
In Memory of Crazyb0 12-26-1951 to 7-27-2020
In Memory of Tootallious 3-31-1964 to 4-15-2020
In Memory of T-BOP 10-12-1949 to 1-19-2024