What Is an Anode Rod and How Does It Work?
What is an anode rod? The dictionary answer is "a sacrificial rod used mainly in water heaters. It helps protect the lining of the water heater and generally lengthens its life." But what does that really mean?
Plumbing involves metals and water. When these combine, you get this thing called galvanic corrosion.
Galvanic corrosion is defined as "an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte." So your piping, which is one kind of metal, and your tank - which is another kind of metal - and the water together set the stage for some fantastic galvanic corrosion.
This is not good. To prevent the tank from rusting or your heater element from corroding, the brilliant plumbers and chemists of long ago created a sacrificial rod for the water heater tank. The idea was that the anode rod would corrode first, leaving the metal of the tank (and element if you have an electric water heater) alone - saving you from dealing with a rusty behemoth that randomly springs leaks. This works because the anode rod possesses a lower, more negative, electrochemical potential than that of the water heater's steel composition.
The negatively charged electrons create a higher voltage to flow from the anode rod to the steel tank causing the anode rod to corrode instead of the steel water heater tank, or other exposed metals such as electric elements. The anode rod is "self-sacrificing" and will continue to corrode until eventually it must be replaced. When there's no sacrificial metal left on the anode, the tank can rust out - which is why it is so important to have your rod checked or even changed as part of routine maintenance.
When To Change The Anode Rod?
Most anode rods that come pre-installed in water heaters are formed aluminum or magnesium around a stainless steel cable. When you check your anode rod, you'll probably see some pitting, or tiny holes; that's exactly what should be happening. However, to keep protecting your tank the anode rod must be replaced when a good chunk of the cable becomes visible. Waiting a long time is not a good idea: having a depleted anode rod will shorten the life of your water heater.
Another problem with waiting too long? The possibility of the old water heater anode rod breaking off and falling to the bottom of the water heater. Doesn't sound so bad, does it? Unfortunately, the problem then becomes the issue of the loose anode rod bouncing around inside the water heater. This is bad: it will cause cracks in the heater's glass lining, allowing the underlying metal to rust and drastically shortening the life of the unit.
Anode rods have a life expectancy of about five years, but as always it really depends on the quality of your water and how much of it travels through the heater. When sodium is added to the water (like when a water softener is used), anode rods can corrode more quickly: in as little as six months if the water is over-softened! Take care not to over-soften water, and make sure to check your anode rod more often if you have a water softener (at least every six months).