Captainwiffo has the most accurate answer. Environmental damage caused this. It likely was exposed to something acidic, but it was likely not purposefully done. Coins sit in the ground or in car floorboards and get eaten away by moisture and whatever else happens to be in the area. Coke and other soft drinks are also very corrosive to copper based coins.
While it is possible that a purposeful acid bath could have caused this, there are far more possibilities that leave a conclusion of a purposeful bath as premature.