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.