Corrosion: the gradual destruction of metal due to a chemical or electro-chemical reaction to the environment. Or, to look at a case you’re probably more familiar with, metal that rusts because of the presence of water. There are different types of corrosion, but when we’re talking about a residential water heater, it’s the corrosion that occurs because of the mix of oxygen and water in contact with metal.
A water heater is designed to resist corrosion through a number of methods. The interior of its hot water tank is lined with glass. A pressure relief valve on the top of the tank allows for an air cushion without allowing air into the tank itself. A component called the sacrificial anode rod that runs through the tank draws the forces of corrosion to it so that the rod corrodes rather than the rest of the tank.