How Do You Stop Steel Rusting Underwater? Give the Corrosion Something Else
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Bolting a lump of zinc to a steel hull makes the zinc corrode instead of the steel. The metal is deliberately destroyed and replaced, which is cheaper than replacing the structure.
Why metals corrode at all
Corrosion in water is an electrical process rather than a simple chemical attack. Metal atoms at the surface give up electrons and dissolve away as ions, and those electrons must go somewhere, which they do by travelling through the metal to another spot where they are consumed by a reaction with dissolved oxygen and water. That makes any wet metal surface a collection of tiny cells, with some regions dissolving and others receiving the electrons. Anything that supplies electrons to the steel from elsewhere therefore prevents the steel's own atoms from having to give them up, which is the whole basis of the protection.
How the trade works
Connecting a second metal sets up a cell deliberately:
- •Metals differ in how readily they give up electrons
- •Zinc, aluminium and magnesium do so more readily than steel
- •Connecting one to steel in water makes it the dissolving side
- •Electrons flow from it into the steel continuously
- •The steel is therefore supplied and does not dissolve
- •The attached metal is consumed and must be replaced
Where they are used
The method protects an enormous amount of infrastructure and the lumps are visible once known. Ships carry them on the hull near the propeller and rudder, where flow and dissimilar metals make corrosion worst. Domestic hot water cylinders contain a rod suspended down the middle, which is why a tank lasts decades and why replacing that rod every few years extends its life considerably. Buried pipelines, harbour piling, oil platforms, bridge foundations and the steel reinforcement inside concrete are all protected this way. Boat owners inspect and replace them on a schedule, and a hull with badly wasted anodes is a recognised warning sign at survey.
Choosing which metal
The three metals used are not interchangeable and the choice follows from the water. Zinc is the standard for seawater, being well matched to steel in salt water and cheap, and it performs poorly in fresh water where it can form a coating that stops it working. Aluminium alloys deliver more protection per kilogram and last longer, and have largely replaced zinc in commercial marine use. Magnesium gives the strongest drive and is used in fresh water and in soil, including in water heaters and buried pipelines, and it is consumed quickly and can overprotect a structure in seawater. Fitting the wrong one for the water is a common and expensive mistake.
The powered alternative
Where the structure is very large or the water is poorly conducting, the same protection is applied with a power supply rather than a consumable metal. A rectifier drives current from a durable anode through the water into the structure, supplying the electrons directly and letting the operator set the level precisely rather than accepting whatever the metals happen to produce. That approach protects long pipelines and large vessels efficiently and needs monitoring, since too little current leaves the structure unprotected and too much generates hydrogen at the surface, which can strip coatings and, in high strength steel, make the metal brittle.
The takeaway
Corrosion happens when metal atoms give up electrons, so connecting a metal that gives them up more readily supplies the steel from elsewhere and stops its own atoms dissolving. Zinc, aluminium or magnesium is consumed instead and is replaced on a schedule, which protects hulls, pipelines, harbour piling and hot water cylinders. Large structures use a powered version, which must be set carefully.