a sacrificial anode

A sacrificial anode is a cheap block of an easily-corroded metal that you deliberately attach to something valuable so that it agrees to rust first. It is the bodyguard that steps in front of the bullet: because it is more active than the metal it protects, it becomes the anode in the corrosion cell and dissolves away in place of the ship's hull, the pipeline, or the tank, which is left as the protected cathode. You do not stop corrosion — you redirect it onto a lump that is meant to be sacrificed and later replaced.

It works purely by the galvanic series. Choose a metal that sits well above the structure on the active end — zinc, magnesium, or aluminum for protecting steel — and connect it electrically. Now the pair is a galvanic couple in which the anode does all the corroding and its released electrons flow into the steel, holding the steel at a potential where it cannot dissolve. Magnesium, being the most active, gives the strongest push and suits high-resistance soils; zinc and aluminum give a gentler drive and are favored in seawater. The anode is sized to last a planned number of years and is designed to be inspected and swapped out — it is a consumable, like a printer cartridge for corrosion.

You have already met sacrificial anodes as galvanic corrosion turned into a weapon: the same effect that destroys a small steel bolt next to a big copper plate is harnessed deliberately by making the sacrificial metal the small-loss part you can afford to lose. The dull grey zinc slabs bolted around a ship's steel rudder and propeller housing, the anode rod in a water heater, and the magnesium pucks wired to buried pipelines are all the same idea. Two honest points: it only works where a continuous electrolyte connects anode and structure, and it is a form of cathodic protection, so the anode genuinely wears out and must be replaced on schedule or the protection quietly ends.