The hidden ingredient

The single most important factor in whether a water heater lives or dies is the condition of its sacrificial anode. For more than 60 years, it has been used as a key part of the rust protection of a tank, although few people know it's there.

This is a rod made of magnesium or aluminum that's formed around a steel core wire and is screwed into the top of the tank. A six-year-warranty residential tank will have one, while a 12-year-warranty tank will have two, or an extra-large primary anode. Commercial tanks have from one to five. Special aluminum/zinc sacrificial anodes or powered anodes may be used to resolve odor problems caused by bacteria in some water.

When the tank is filled with water, an electrolytic process begins whereby sacrificial anodes are consumed to protect a small amount of exposed steel. Powered anodes replace that process with electricity and are not consumed.

Electrolysis simply means that when two metals are physically connected in water, one will corrode away to protect the other. Although few people have heard of this, the principle is used all over the place -- anywhere that someone wants to protect metal exposed to water. In marine applications, anodes are known as "zincs."

All metals fall somewhere on the galvanic scale of reactivity. When two are placed together in water, the "nobler" -- or less reactive -- one will remain intact while the more reactive one corrodes. When steel and copper are together, steel will be the one that corrodes. Indeed, steel is more likely to rust in the presence of copper than it would have been by itself. That's why dielectric unions are necessary on items like copper flex lines when they're connected to steel nipples.

Magnesium and aluminum are less noble than steel, which is why they're used for anode rods.

And there is no way to tell metal type if a tank has a combo rod, but you're probably going to be replacing it anyway.

As to other brands, as far as we know, American/Whirlpool always uses aluminum anodes. Bradford White always uses magnesium, but many of their tanks have one combo rod in the hot port. State/Reliance/Kenmore offers both combo rod tanks and tanks with a hex anode, which can be either aluminum or magnesium. As of February 2008, only the Premier line has magnesium. All the others use aluminum anodes. A.O. Smith makes both hex and combo rod tanks and uses both aluminum and magnesium.

If you decide to remove and check your anode, we'll tell you some of the possibilities and what they mean.

If there is rough, seemingly chewed-up metal all up and down the rod, that's normal. It's doing what it's supposed to do. If you can see six inches of the steel core wire, replace the rod. If all you have IS the steel core wire -- or less -- then extending the life of the tank by replacing the anode becomes more iffy. You might still get several more years out of the next anode. Or the tank might fail shortly after. It all depends on factors that exist where none of us can see them.

If the rod looks perfectly intact, with no sign of corrosion, then it has passivated. That means it has sort of gone to sleep. It might not be doing its job. It's a wild-card situation. We've seen tanks with passivated rods last a long time. We've seen them break. We're never sure what will happen next, and apparently none of the experts in this field whom you think might know are any more certain than we are. About all you can do is replace the rod and hope for the best. Or leave it and hope for the best. Wild-card situation.