3: Passivation

Passivation of a metal surface through the formation of an oxide layer is found in many common metals and alloys. Aluminium naturally forms a protective oxide layer (or scale) which slows down further oxidation and corrosion. Stainless steel has chromium added to it, which forms a very protective oxide layer that prevents further corrosion.

 

Not all oxide layers that form on metals are protective. If the oxide does not form a continuous layer on the surface of the metal, it will not be able to reduce the amount of oxygen reaching the metal surface.

 

 

If the metal forms an oxide whose area is less than the area of the metal then the surface will show gaps in the protective layer.

If the metal forms an oxide whose area is greater than the area of the metal then the surface will show blistering and flaking of the protective layer.

Only if the area of the oxide that forms is the same as the area of the metal will the oxide layer protect the metal from further corrosion.