Passivity and Polarization of Metal:

Metals which generally fall victim to corrosion will sometimes exhibit passivity to corrosion. Passivity is the features of a metal exhibited while that metal does not turn into active within the corrosion reaction. Passivity is caused through the buildup of a stable and tenacious layer of metal oxide on the surface of the metal.  This oxide layer is formed through corrosion on a clean metal surface; whereas the corrosion products are insoluble within the particular environment to that the metal is exposed. At the layer, or film, is created, it acts as a barrier dividing the metal surface from the environment.  For additional corrosion to occur, reactants must diffuse by the oxide film.  Such diffusion is extremely slow or nonexistent, therefore corrosion either decreases stops or markedly.

Metals such as zirconium, aluminum, chromium, and the stainless steels form thin, tenacious oxide films while exposed to the atmosphere or to pure water at room temperature. In a few cases, the film is very thin and might be invisible to the unaided eye, other than it is still extremely effective in giving these metals a marked passivity.