Preventing corrosion

Removing substances that cause rusting

can be prevented by keeping oxygen or water away from the iron or steel:

oxygen can be excluded by storing the in an atmosphere of nitrogen or argon
water can be excluded by storing the metal with a such as calcium chloride

A desiccant is a substance that absorbs water vapour, so it keeps the metal dry.

Physical barriers to oxygen and water

Many methods of rust prevention rely on creating a physical barrier to oxygen and water. These include:

painting
oiling and greasing
coating with plastic

Different methods are used depending on the situation.

Question

Explain why a bike chain is protected from rusting by oiling it, rather than by painting it.

Electroplating

involves using to put a thin layer of a metal on the object:

the (negatively charged ) is the iron or steel object
the (positively charged electrode) is the plating metal
the contains of the plating metal

For example, steel cutlery can be electroplated with silver using a silver anode and silver nitrate solution. Electroplating improves the corrosion resistance of metal objects. It also improves their appearance and may be used to produce gold-plated jewellery.

Sacrificial protection

Iron can be protected from rusting if it is in contact with a more metal, such as zinc. The more reactive metal more readily than iron, so it 'sacrifices' itself while the iron does not rust. Once the has corroded away, it can simply be replaced.

Worked example

Three nails are left in contact with air and water for a few days. A nail wrapped in magnesium does not rust. A nail alone rusts but a nail wrapped in copper rusts more. Explain these observations.

Three nails, one with copper wire wrapped around it, one by itself and one with magnesium wire wrapped around it.

Magnesium is more reactive than iron. It oxidises more readily than iron so the nail does not rust.
Iron is more reactive than copper. This means it oxidises more readily than copper, so it rusts faster than the nail alone.

Galvanising

When iron is coated in zinc, the process is called . The zinc layer stops oxygen and water reaching the iron. Zinc is more reactive than iron, so it also acts as a sacrificial metal. This protection works, even if the zinc layer is scratched.

Question

The inside of a steel food can is electroplated with tin, a less reactive metal than iron. It provides a physical barrier to oxygen and water, stopping the can rusting. Explain why the inside of the can rusts very quickly if the layer of tin is broken.

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Transition metals, alloys and corrosion - EdexcelPreventing corrosion