Corrosion

Iron rusts in the presence of oxygen and water.

Barriers such as paint or grease can be used to prevent corrosion.

Zinc blocks can be attached to iron objects. As zinc is more reactive than iron it will corrode preferentially thus preventing the iron from corroding. This is called sacrificial protection.

Aluminium is a very reactive metal but it can be used without protection against corrosion. This is because it has a thin layer of oxide which sticks very firmly to the aluminium and protects it against further corrosion.