Oxidization is the chemical reaction of oxygen in the air with another substance. It's the same force that causes exposed metals to rust. In pennies, oxygen reacts with the copper to form a new compound, copper oxide. This causes the dull, rust-colored or greenish-hued corrosion that are sometimes observed on very old pennies.
Zinc corrodes relatively slowly compared to other metals. Wind, moisture and exposure to corrosive fumes can slowly break down zinc. If submerged in water for a time, the zinc in pennies can corrode if the water contains impurities. Because modern-day pennies wreath zinc in a copper shell, the copper must be degraded first in order to expose the zinc to corrosive forces.
Corroded pennies can be partially restored to their original brilliance with a simple procedure. Immersing the pennies in vinegar cleans some of the oxidized build-up off the metal. This is because the acid in vinegar reacts with the copper oxide.
A University of Southern California experiment compared the corrosion of zinc-based pennies (minted after 1982) and copper-based pennies (minted before 1982). Specifically, the experiment explored the corrosive impact of gastric acid on each metal. The zinc-based pennies broke down much faster than their copper-based counterparts.