It's what I call DEFDAM - Definitely Damaged! Your Cent is a Copper Plated Zinc coin. The plating was damaged/compromised and caused the Zinc inside to start corroding away.
We discontinued the half-cent as "not worth the trouble" when its buying power was about the same as today's dime. As long as we insist on using a coin that won't buy anything by itself, and is worth less than the time it takes to count it out and put it away, I'm afraid we're getting what we deserve.