73. Corrosion & Rusting

1. What is Corrosion?

Corrosion is the process by which metals are slowly broken down by reacting with substances in their environment.

  • Rusting: A specific type of corrosion that occurs with iron.

  • Chemical Reaction: Iron reacts with oxygen and water to form hydrated iron oxide (rust).

  • Redox Reaction: Rusting is a redox reaction because iron is oxidized (loses electrons) and oxygen is reduced (gains electrons).

2. Conditions for Rusting

For iron to rust, both oxygen and water must be present.

  • Water without Oxygen: Boiled water (to remove dissolved oxygen) with a layer of oil on top will not cause a nail to rust.

  • Oxygen without Water: Using a drying agent like calcium chloride to remove water vapor from the air will prevent rusting.

  • Both Present: Iron only rusts when exposed to both air and water.

3. Surface Corrosion Differences

  • Iron: Rust is weak and flaky. As the surface rusts, it flakes off, exposing fresh metal underneath. Over time, the entire piece of iron can be destroyed.

  • Aluminium: When aluminium reacts with oxygen, it forms a thin layer of aluminium oxide. Unlike rust, this layer is very tough and sticks to the metal, forming a protective barrier that stops further corrosion of the metal underneath.

4. Preventing Rusting

Prevention methods are generally split into two categories:

A. Barrier Methods These create a physical barrier to stop oxygen and water from reaching the iron:

  • Painting: Best for large structures like ships.

  • Oiling/Greasing: Used for moving parts like bike chains.

  • Electroplating: Using electrolysis to coat the iron in a thin layer of a different metal.

B. Sacrificial Methods This involves attaching a more reactive metal (like zinc or aluminium) to the iron.

  • The more reactive metal will react with oxygen and water instead of the iron.

  • Even if the surface is scratched, the iron is still protected because the sacrificial metal is "preferred" by the oxygen.

C. Galvanizing This is a hybrid method where iron is coated in a layer of zinc.

  • It acts as a barrier, but if the zinc is scratched, it also acts as sacrificial protection because zinc is more reactive than iron.