Corrosion is the oxidation of a metal by its surroundings

1. A metal at the surface of an object reacts with substances in the air or water around it.
2. The metal gains oxygen, so corrosion is an oxidation of the metal.
3. The product is a compound, which has none of the strength or shine of the metal.
4. A metal high in the reactivity series corrodes more readily than one low down.
5. Gold does not corrode in air at all, which is why gold objects survive unchanged for centuries.

Rusting needs both water and oxygen

1. The corrosion of iron has its own name, and it happens only when both water and oxygen are present.
2. The reaction that forms rust is: $$iron+oxygen+water\to hydrated\; iron(III)\; oxide$$
3. Rust is hydrated iron(III) oxide, an orange-brown solid.
4. Iron in dry air does not rust, and iron under boiled water with no dissolved air does not rust.
5. Rust flakes off the surface, exposing fresh iron underneath, so the corrosion continues into the metal.

Barriers keep water and oxygen away from the iron

1. Coating the surface stops water and oxygen reaching the iron beneath.
2. Painting is used on large structures such as bridges and ships.
3. Oiling or greasing is used on moving parts, where a paint film would be worn away.
4. A layer of plastic is used on garden furniture and on wire fencing.
5. Every barrier fails once it is scratched, because water and oxygen reach the iron through the gap.

Sacrificial protection uses a more reactive metal

1. Attaching a more reactive metal to iron protects the iron even when the surface is damaged.
2. The more reactive metal loses electrons in preference, so it corrodes instead of the iron.
3. Zinc blocks are bolted to the hulls of ships and are replaced as they are eaten away.
4. Galvanising coats iron or steel with zinc, which acts as a barrier and as a sacrificial metal at the same time.
5. A galvanised bucket therefore keeps working after a scratch, which a painted one would not.

Electroplating improves appearance and resistance to corrosion

1. Electroplating uses electrolysis to lay a thin layer of one metal onto the surface of an object.
2. The object to be plated is made the negative electrode, where metal ions gain electrons and deposit.
3. The positive electrode is made of the plating metal, which dissolves to replace what is deposited.
4. The electrolyte is a solution of a salt of the plating metal, such as copper sulfate for copper plating.
5. A chromium or silver layer makes a cheap object look better and resist corrosion at the same time.