Preventing corrosion

Removing substances that cause rusting

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

  • oxygen can be excluded by storing the metal in an atmosphere of unreactive nitrogen or argon

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.

The oil lubricates the chain, helping it to move smoothly. Paint just flakes off when the bike is ridden, exposing the steel chain to air and water again.

Electroplating

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

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.

Sacrificial protection

Iron can be protected from rusting if it is in contact with a more reactive metal, such as zinc. The more reactive metal oxidises more readily than iron, so it ‘sacrifices’ itself while the iron does not rust. Once the sacrificial metal 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.A nail wrapped in magnesium does not rust. A nail alone rusts but a nail wrapped in copper rusts more.
  • 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 galvanisation. 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.

Oxygen and water can reach the steel when the layer is broken. Iron is more reactive than tin, so it oxidises more readily than tin. The damaged can rusts faster than if the tin was not there at all.