The sacrificial anode protects the tank using a process called cathodic protection through the principle of galvanic corrosion. It is the same technique that is used to protect the hulls of ships and their mechanical systems that are in contact with sea water, underground pipelines and tanks, sea based drilling platforms and even outboard motors on everyday pleasure craft.
Galvanic corrosion occurs when two dissimilar metals are connected electrically and placed into a corrosive liquid. An electrical current is generated and the metal which is more easily corroded (less noble) becomes the anode while the metal that is less subject to corrosion (more noble) becomes the cathode. With a complete electrical path, the anode (magnesium rod) supplies electrons to the cathode (tank wall). The process results in the release of positive hydrogen ions and positive metal ions into the water. This corrodes the anode resulting in its depletion while the tank walls are protected and remain intact with no corrosion taking place.
Eventually, the sacrificial anode is fully depleted or in some cases, may become inactive. Ultimately, no protection is afforded to the water heater any longer. The exposed steel tank wall will now begin to corrode on its own or at an accelerated rate in the presence of any available copper or brass as found in the attached plumbing or drain. These may now become the cathode, and the tank’s steel the anode - The unprotected steel tank will corrode until it eventually fails and leaks (usually in a few more years.)
This table shows a listing of many of the more common metals and their position on the galvanic scale which identifies them with regards to their relative reactivity.
Metals that have a lower number in this chart are more reactive (less noble) while metals with a higher number are less reactive (more noble). The metals with the lower numbers will corrode when paired with metals that have a higher number. For instance, our hot water tank made of steel will be the cathode when paired to a magnesium rod that will act as the anode. The magnesium will be sacrificed to protect the steel as it is less noble than the steel is.
However, upon depletion of the magnesium, the steel tank becomes anodic in the presence of any copper or brass which is now cathodic. The steel will now be sacrificed to protect the copper and brass or be left to corrode on it’s own.
What Exactly Does the Sacrificial Anode Rod Do?
As the sacrificial anode rod is a consumable item, it therefore requires replacement when it is worn out or no longer effective in protecting any exposed steel. It is an inexpensive form of protection to ensure that the water heater tank walls are not compromised due to the devastating effects of corrosion.