Corrosion is the process by which something deteriorates because of oxidation, a chemical action that creates oxides that flake away from the base.
When you see a rusty, shoddy looking car, corrosion is the culprit. Although the word is most often associated with the physical breakdown of a metal through rusting, the erosion of rock by wind and water is a form of corrosion. The word can also be applied to other situations, like the corrosion of a once-strong friendship. There could be corrosion of relations between two countries. Whenever something's being worn down or eaten away, you can call it corrosion.