These were made out of very cheap zinc as Germany was saving copper for the war effort. 1942 was not a good year for the thousand year reich as metal/resource shortages were becoming increasingly common. Nearly every single zinc pfennig has some sort of corrosion/environmental damage to it. I am yet to see one in mint state. I'm afraid there isn't really anything you can do to help that coin. It has been too far gone with the corrosion, luckily it isn't worth a whole lot. As @furryfrog02 said, it's toast.