What is a Sacrificial Anode Rod and Why is it in My Water Heater?

What is a Sacrificial Anode Rod and Why is it in My Water Heater?

Here’s something to think about: your water heater has one primary function, and that is to heat water. It does this pretty well most of the time, by conducting heat through metal, and warming up gallons of water that is then pumped through your home when you turn on the tap. Yet this goes against everything you’ve ever learned – water rusts metal, right? Or were your mother’s warnings that leaving your bike in the rain would rust it just a lie?

No - it’s not a lie. Water rusts metal, and your water heater would be a crumbling mess without a small rod inserted into the top of the tank. This rod is called an anode rod (or sometimes a sacrificial anode rod) and it is the only reason your water heater hasn’t left you bathing in rusty water.


How does an Anode Rod Work?

Rust, or corrosion of metal, happens with three things: iron (or steel), oxygen and water. These are all plentiful in a water heater tank.  Although in modern water heaters the tank is encased in a thin layer of glass, water can still get into cracks and rust the water heater’s tank. So, water heater manufacturers place an anode rod into the tank.  An anode rod is made up of magnesium or aluminum. Both magnesium and aluminum are less-noble metals, meaning they corrode (rust) quickly in water. Let’s put on our chemistry hats and explore this a bit further.

The chemical reaction for rusting begins by oxidation, which is when iron loses two of its electrons to the oxygen found in the tank. When you place a magnesium or aluminum rod into water, this also occurs, just much faster. The bonds between the molecules of magnesium and aluminum give up their electrons faster than the bonds in steel or iron. Thus, when you place an aluminum or magnesium anode rod into a iron or steel tank of water, the oxygen in the equation takes the two electrons of the anode rod instead of the tank, as they gave them up quicker. This will rust the anode rod, but not the tank itself.

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