Thin oxide film formation, metal protective layer

In general, anodic passivity of metals, regardless of type of corrosion, is associated with the formation of a thin oxide film, which isolates the metal surface from the corrosive environment. Films with semiconducting properties, such as Fe, Ni, Cu oxides, provide inferior protection compared to metals as Al, which has an insulating oxide layer. ... 

From a thermodynamic point of view, aluminium would have to react with water to form hydrogen. However, A1 and A1 alloys have proven to be very corrosion-resistant in a wide range of practical applications. This corrosion resistance is attributable to the reaction of A1 with oxygen or water vapor and the formation of a thin but compact natural oxide film when it is exposed to air, i. e. A1 is passivated. In contrast to the oxide layers formed on many other materials, this oxide layer is strongly adherent and thus protects the underlying metal against further oxidation. This property explains the good re-... 

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