Some ways in which a metal (mostly steel) may be protected:
Passivating the surface
Adding Chromium (passivating metal) to steel forms a passivating layer on the surface of the steel which is impermeable to oxygen and water (also known as stainless steel, which resists corrosion).
Tin plating
Covering the surface of steel with a layer of tin provides a physical barrier so that water and oxygen cannot come in contact with the steel. The tin oxidises to form a stable oxide layer. e.g tin cans, food packaging cans.
Surface alloys
A stainless steel-like coating can be produced by bombarding the steel surface with chromium atoms. The process is carried out in a high temperature discharge and are directed at this steel surface where they become embedded as atoms to form the surface alloy. The steel under this surface does not change while the surface forms an impermeable passivating layer that resists corrosion.'
New paints
Stops oxygen and water from coming into contact with the iron so rust cannot form. Re-paints are needed to cover any scratches, which would otherwise encourage corrosion.
Galvanising
Covering the steel with zinc is galvanising. Zinc is more reactive than iron and will oxidise in preference to iron when the metals are in contact:
Fe -> Fe2+ + 2e- Eo=0.44v
Zn -> Zn2+ + 2e- Eo=0.76v (higher oxidising potential)
