An anode rod is a sacrificial metal rod inside a water heater tank that corrodes in place of the tank's steel lining, protecting it from rust. Made of magnesium, aluminum, or zinc, it slowly dissolves through electrochemical corrosion and needs periodic inspection and replacement to keep protecting the tank.
Every tank-style water heater has a weak point. The steel tank will rust from the inside out unless something else corrodes first. That something is the anode rod. It is a rod of reactive metal that runs down through the top of the tank into the water. It is built to wear away so the tank does not have to.
The rod works through galvanic corrosion. This is the same process that lets two different metals in water act like a battery. The rod's metal is usually magnesium, aluminum, or zinc. Each one is more reactive than the tank's steel lining. Corrosive minerals in the water attack the rod first and leave the tank alone. Magnesium rods give the strongest protection and are the default in most tanks. Aluminum rods hold up longer in hard or well water. Zinc rods work best when sulfur bacteria in the water cause a rotten-egg smell, because zinc slows that reaction better than the other two metals.
An anode rod is a wear part, not a permanent fixture. Both major water heater makers treat it that way. Rheem's owner's manual for its e-control electric residential water heaters calls for pulling the rod out once a year to check it. Rheem recommends replacement once more than 6 inches of the rod's steel core wire is exposed at either end. A.O. Smith goes further and leaves anode rods out of its warranty entirely. It lists them as a consumable maintenance part, not a covered component, the same way a car warranty skips brake pads.
Once a rod is fully used up, the tank loses its sacrificial protection and the steel itself starts to corrode. Arizona's hard water speeds up this process, which is one reason Phoenix-area water heaters often need anode checks sooner than a manufacturer's baseline schedule assumes. Picture a rod pulled at the one-year mark that comes out thin, chalky with calcium buildup, and missing chunks of its outer coating: that rod has stopped protecting the tank and should be swapped right then, not left for the next inspection cycle.