Definition - What does Passivity mean?
Passivity occurs when an oxide layer forms a continuous film on a metal surface that prevents further oxidation (corrosion). Metals that are normally subject to corrosion will sometimes exhibit passivity.
Corrosionpedia explains Passivity
Passivity is caused by the buildup of a layer of metal oxide on the surface of a metal. In order to provide passivity, this oxide layer must be both stable and tenacious. It is formed by corrosion when the products of corrosion are insoluble in the particular environment to which the metal is exposed. The metal oxide acts as a barrier by separating the metal surface from its environment and prevents corrosion until the reactants diffuse through the oxide film. This diffusion can take a very long time, or may never occur at all.
Metals such as zirconium, chromium, aluminum and stainless steel form oxide films when exposed to the atmosphere or to water. In some cases, the film is so thin that it's invisible to the nake eye. Nevertheless, it is still very effective in giving these metals passivity.
The Corrosion Properties of Aluminum and Its Alloys