There is an anode rod in most hot water heaters made of aluminum or magnesium that is used to protect against corrosion of the tank. It is sometimes called a sacrificial anode since it dissolves slowly and sacrifices itself to protect the metal tank. As the anode reacts with substances in the water, it undergoes an electrochemical reaction and draws corrosion to itself rather than the glass lined steel tank. The rod should last for five years but could fail earlier. Please contact a plumber if you are experiencing this problem. If you are experiencing this problem please contact a plumber or your water heater provider.