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The formation of rust on iron, tarnish on silver, and the blue-green patina that develops on copper are all examples of corrosion.
In the United States, the total cost of corrosion is more than half a trillion dollars a year.
Every American recognizes the Statue of Liberty.
The Statue of Liberty's height, stance, and blue-green color make it easy to identify it.
The change in appearance was caused by something.
The statue's primary component slowly oxidizes from the air.
The protective layer on the surface was created by the formation of the patina.
The protective layer is a form of passivation that is discussed in a later chapter.
The formation of rust on iron is one of the most common examples of corrosion.
Oxygen and water can cause iron to rust.
Once exposed to the atmosphere, iron rapidly oxidizes.
Oxygen is reduced in the air by the electrons.
We call it rust because it is hydrated iron(III) oxide.
The formation of rust does not create a protective layer and so the iron is exposed to fresh iron in the atmosphere.
Rust begins to form when the paint is scratched on an iron surface.
In the presence of electrolytes, such as the salt used on roads to melt ice and snow, the speed of the reaction is increased.
Iron can be painted to keep it from corroding.
The water and oxygen needed for rust formation are prevented by the layer of paint.
The iron is protected from damage if the paint is intact.
The iron can be alloyed with other metals.
The steel is mostly iron with a small amount of chromium.
There is an oxide layer that protects the iron when the chromium collects near the surface.
Zinc has a lower reduction potential than iron.
zinc is a more active metal since it has a lower reduction potential.
The zinc will oxidize before the iron even if the zinc coating is scratched.
This suggests that this approach can work with other active metals.
A galvanic cell is an important way to protect metal.
It is possible to prevent or greatly reduce the rusting of underground iron storage tanks and pipes by connecting them to a more active metal such as zinc or magnesium.
The metal being protected does not oxidize.
The useful lifetime of the iron storage tank can be greatly extended if the anodes are monitored and periodically replaced.
cathodic protection is a way to protect an underground iron storage tank.
The storage tank is prevented from corroding by using an active metal like zinc or magnesium.
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