In the context of water heaters, an “anode” is a means of protecting the water heater tank from corrosion.
Most tanks that store water or another liquid that is conducive to galvanic corrosion have a galvanic anode. Nearly all water heaters are shipped to the customer with galvanic anodes that eventually break down. These galvanic “sacrificial” anodes protect the tank/structure from corrosion because they are comprised of metals (such as magnesium or aluminum) that lose their electrons more easily than the metal of the tank.
This same (or even better!) tank protection can also be gained through the use of actively impressing an electrical current into the tank/structure through the use of a powered anode rod, and this is called cathodic protection. The powered anode transmits a positive current, while the tank/structure is made the negative cathode and attracts the positive charge and is thus prevented from losing its own electrons: therefore the powered anode preserves the tank. This use of electricity for the protection of metal structures has been around for over a century and a half, and for the last 100 years it has been widely used to protect underground oil pipes, ship hulls, and the metal reinforcement (rebar) in expensive commercial projects.
Cutting to the chase:
All modern water heater tanks have anodes
Factory-installed anodes typically last 3-7 years
Your water heater needs a working anode to prevent corrosion and subsequent leaking & tank replacement
By ensuring a continuously working anode, your tank has the potential to last over a human lifetime
Many homeowners can replace the anode themselves (please see video below, or our written instructions [link]). Some home-owners opt to have a plumber install the anode for a typical labor cost of $250-$400.
(Unaffiliated with STOLTCO)