Heavy Metal: Should You Choose Aluminium, Zinc or Magnesium Anodes For Your Hot Water System?

Sacrificial anodes are a vital part of any modern hot water system, protecting the internal components of your water heater from corrosion and rust. However, the performance and longevity of your system's anodes can be altered by a number of factors, including what the anode itself is made from. Aluminium, magnesium and zinc are all commonly used as the sacrificial metal in anodes, and while they all essentially perform the same function, they also have unique properties that can affect water heater operation. If you're in the market for a new heater, or seeking to replace old or damaged anodes, make sure to familiarise yourself with the different metals on offer before you drop your hard-earned dollars.

Aluminium anodes

Aluminium anodes are commonly found fitted as standard in older and/or cheaper water heaters. They are generally the least expensive options on the market, and very easy to find. Aluminium anodes are also relatively light and are generally very easy to install without having to call in expensive professional help. Crucially, aluminium anodes may even be the only effective option in areas with very hard mains water.

Unfortunately, aluminium anodes can serve as an unlikely food source for anaerobic bacteria that may be present in your water supply. While the presence of these bacteria is completely normal and is not harmful to humans or animals, the byproducts they create when feeding on aluminium can create a persistent and deeply unpleasant 'rotten egg' smell. These bacteria can also cause aluminium anodes to degrade more quickly.

Zinc anodes

Zinc anodes are not actually made from pure zinc, as they would be impractical and very short-lived (not to mention expensive). Instead, they are made from zinc-aluminium alloys and are used and installed in the same way as pure aluminium anodes. The addition of zinc discourages the previously mentioned bacteria from feeding on the anode and can effectively reduce problems with unpleasant smells when fitted to ageing heaters. However, they tend to be significantly more expensive than pure aluminium anodes and lose some of their effectiveness in hard water conditions. Zinc anodes also lose some of their effectiveness in very hot temperatures, although this is generally only a problem for high-capacity immersion heaters and other very powerful water heaters.

Magnesium anodes

Magnesium anodes are the most reactive anodes commonly available, and as such they confer the highest level of corrosion protection to your water heater, even at very high temperatures. This high reactivity also means that they degrade in a uniform fashion, extending their lifespan and making it much easier to tell when an anode needs replacing. They are a very popular choice and are very easy to find -- modern water heaters generally have magnesium anodes fitted as standard. 

However, magnesium anodes do not perform well in hard water conditions and degrade much more quickly due to the electrical resistance hard water possesses. They are also not immune to the 'rotten egg problem', although they are less likely to suffer from it than aluminium anodes.