Dissimilar metal corrosion, often referred to as “electrolysis”, is a condition that occurs when metals of dissimilar composition are joined and come into contact with an electrolyte. Water is a weak electrolyte. Hence, when two dissimilar metals are joined and come into contact with water (or other electrolytes), corrosion occurs at an accelerated rate. The “less noble” metal, or metal with higher reactivity, will corrode before the other.
Corrosion occurs when the metal with the lesser galvanic reactivity (more noble) attracts ions from the less noble metal. This chemical reaction produces electrical current in the same manner as an automotive battery. However, the current created in the water heater is negligible.