Ruined pennies

I found this in Mother's hoard. A good illustration of what happens when coins are stored in non-archival materials. I pulled out the undamaged coins.

True, but still worth three cents. Dump them back into circulation please.

Interesting that the pure copper surface reacted but the bronze did not.

I expect it was the underlying zinc reacting through the copper. I also expect the bronze would also react, it would just take longer. 

I plan to keep them in the case as a stark reminder. I wonder what they will look like in 50 years.

The zinc could only react if the copper was breached.  To me it looks greenish which would be copper corrosion.

rsirian1, the evidence might suggest otherwise. A layer of copper measured in microns doesn't offer much of an impenetrable barrier over long contact. 

However, there is an anomaly. There should be four corroded coins, not three.

All true (~13 microns) but zinc doesn't have green corrosion byproducts and copper does.  Also I have 40 plus year old pennies that the copper protected the zinc just fine.  Regardless, the observation is still very interesting.

I find generally anything packaged before the early 1990s will rot.

Generally paper from the 1970s or even the late 80s, was acidopherous and always tones or corrodes, same with the plastic always rigid PVC stuff that cracks, warps and makes coins turn green and gungy. Better quality acid free paper started to show up in the late 80s and PVC free plastic in the 1990s, but uptake was slower and budget products like this always used cheaper and lower grade materials.

I have noticed with my own sets, the 1980s ones have toned paper and rotten plastic, but 1990s cardboard sleeves have survived well, still flexible, paper is still white and nice and thats likely when people got their sh*t together with packaging.