An anode rod is a sacrificial metal rod, usually magnesium, aluminum, or a zinc-aluminum alloy, threaded into the top of a tank water heater. Through galvanic corrosion it corrodes in place of the glass-lined steel tank, so the tank stays protected only while rod material remains. Once the rod is consumed, the tank itself starts to rust.
Why it matters to you
The anode rod is the main thing standing between a tank's steel walls and rust, and a spent rod is one of the most common reasons a tank water heater starts leaking years earlier than expected.
Practical guidance
Have the anode rod inspected every two to three years, sooner with softened or very hard water, and replaced once the steel core wire shows through. A rotten-egg smell in hot water often points to a reaction with a magnesium rod, and an aluminum-zinc rod is a common fix.