cathodic protection
/ kath-ODD-ick /
In a corrosion cell the metal that dissolves is always the anode; the metal that is spared is the cathode. Cathodic protection turns that fact into a defense with disarming simplicity: force the whole structure to become the cathode, and it stops corroding. If you can guarantee that a buried pipeline or a ship's hull is always on the receiving end of the electrons rather than giving up its own, its metal has no reason to dissolve. You are not sealing the metal away from the world — you are electrically bribing it to keep its atoms.
There are two ways to do it. The first is a sacrificial (galvanic) anode: bolt on a lump of a more active metal — zinc, magnesium, or aluminum — which sits higher on the corrosion series, so it becomes the anode and corrodes in place of the structure, feeding it protective electrons until the lump is used up and must be replaced. The second is impressed-current cathodic protection: connect the structure to the negative terminal of a DC power supply and use an inert or long-lasting anode elsewhere, pushing protective current in from the outside. Sacrificial anodes are simple, need no power, and suit smaller or well-coated jobs; impressed current is stronger and adjustable, used for big pipelines and tank farms, but needs a power source and careful control.
It is one of the most important protection methods in engineering — steel pipelines, offshore platforms, ship hulls, and water heaters all rely on it, usually paired with a coating so the CP only has to protect the few spots where the coating is scratched. Two honest cautions: the metal must be immersed in or buried in a continuous electrolyte (soil, seawater) for the current to flow, so it does not protect metal in dry air; and you can overdo it — driving the potential too far negative generates hydrogen at the surface, which can cause hydrogen embrittlement of high-strength steels, so the protection level must be controlled, not simply maximized.
A domestic hot-water tank quietly uses cathodic protection: a magnesium 'anode rod' is screwed down inside it and corrodes away over a few years in place of the steel tank. Replace the spent rod and the tank lasts far longer; ignore it until it is gone, and the steel becomes the anode and the tank rusts through.
The cheap replaceable rod is meant to be sacrificed; the real cost of ignoring it is a rusted-through tank.
Cathodic protection needs a continuous electrolyte to carry the current, so it guards buried and immersed metal but does nothing for metal exposed to dry air; and over-protection can charge high-strength steel with hydrogen and embrittle it, so more current is not always safer.