There are a number of answers to this question. The first is that your tank water heater can rust if it is old enough or if it doesn’t receive regular maintenance service from professionals. If you notice rust developing on any part of the tank of your water heater, call technicians right away. Sometimes they can replace the corroded sections to save the water heater; at other times it may be better simply to replace the water heater.
However, in general, water heaters do not suffer from rust. This might seem strange because the tank is made of steel and it holds a large amount of water. Why doesn’t the water cause corrosion in the steel?
Glass-Lined Tanks and Expansion Tanks
There are a number of defenses that a storage-tank water heater has against corrosion. The inside of the tank is glass-lined to keep the water from the steel. To prevent air from entering the tank (oxygen is necessary for corrosion to develop), the air cushion used to keep water pressure under control is located in a separate tank called the expansion tank.
The Sacrificial Anode Rod
The major defense against corrosion, however, is a component called the anode rod, or sacrificial anode rod. This is a rod made of either magnesium or aluminum encasing steel. The rod is attached to the top of the tank and extends down into the mater. The presence of the two different types of metals causes a chemical reaction that draws corrosion to the magnesium/aluminum, rather than the steel so that it corrodes instead. This protection also extends to the steel in the tank. Basically, the outside of the rod “sacrifices” itself to corrosion so the rest of the tank won’t corrode.
You must keep up with regular maintenance for your water heater so that the anode rod can be replaced regularly. Once it corrodes all the way, it won’t be able to stop corrosion starting elsewhere.