Most people have never heard of a magnesium anode! This is despite the fact they have been a key to water heater longevity for decades…
Magnesium Anode – What is it?
The single most important factor in whether a water heater lives or dies is the condition of its magnesium 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 aluminium) 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 aluminium/zinc sacrificial anodes or powered (impressed-current) anodes can be used to resolve odor problems caused by bacteria in some water. But if you have a holiday home where the water heater sits idle for long periods of time, using them may not be a solution.
When the tank is filled with water, an electrochemical 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.
When two metals are physically connected in water, one will corrode away to protect the other. Sometimes that’s bad, but often it’s good. 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” and are usually made of that metal.
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 separation is necessary on items like copper flex lines when they’re connected to steel nipples.
Magnesium and aluminium are less noble than steel, which is why they’re used for anode rods. For a more in depth explanation check out wikipedia
(why we don’t like aluminium anodes)
Remember, the magnesium anode is screwed into the tank. That means it can be unscrewed and replaced.
A sacrificial anode’s life depends on the quality of the water, the amount of use the tank gets, the water temperature, and the quality of the tank — meaning how well it was constructed.
When salt is added to the water (as in softened water), anodes corrode more quickly. Water softeners help reduce sediment, but anodes can corrode in as little as six months if the water is over-softened. Do not soften to zero. Leave 50-120 ppm of hardness. This may require some plumbing to add unsoftened water to softened water.
People occasionally ask us if Teflon tape applied to the threads of the magnesium anode blocks the electrolytical reaction. Tanks we’ve serviced usually have corroded anodes. We’ve tested with a multimeter and found continuity between the anode and the tank, despite the tape (meaning a connection is still made).
If you’re thinking about adding an anode to a new tank, (that already has 1) make sure both rods are of the same metal. Otherwise, the magnesium rod will be consumed more rapidly in the presence of an aluminium one and you won’t get as long a life.
How do you tell a magnesium anode from an aluminium one?
Simply that the same length magnesium anode is lighter than the equivalent aluminium anode.
Aluminium is usually the factory installed type in most water heaters, but in the past, high-end models have come with magnesium. Today, some of those also offer powered anodes.
If you decide to remove and check your anode, here are some of the states you may find it in 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 fifteen cm 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 questionable.
You might still get several more years out of the next magnesium anode. Or the tank might fail shortly after. It all depends on factors that exist where none of us can see them.
The main kind of magnesium anode configuration is called a hex-head anode and you can see them on the product page. They are found in their own port on top of the tank. With some brands, the hex head is exposed. On many, it may be under a plastic cap about halfway in toward the center from the edge. If there are caps on the edge, they were used to insert the foam insulation. On some older tanks, it may be hidden under the sheet-metal top.