What happened to the edges of this coin

This is a N#18070 Mainz 5 sol siege coin, it's authentic and all but what happened to the edges? Is it mint damage or just water corrosion

It broke off. Why? Most likely nobody will ever now for sure. Could have been a flaw in the planchet and then dropped or shattered on impact in the mint or corrosion weakened the material.

Siege coinage was notoriously poor quality, I would say that it is post mint damage due to the poor quality of the planchet