Corrosion occurs when a material—usually something metal—reacts with another substance, such as water, hydrogen, oxygen, or certain salts and acids. When this happens, rust is formed, which can compromise the strength and integrity of that material. Since water heaters are made to contain and even facilitate the flow of water, they are expected to have their fair share of corrosion, a process that is slowed down significantly by the presence of an anode rod.