Passivation is to make the metal surface show very low reactive activity. Aluminum is a reactive metal. From its reducibility, it is easy to interact with oxygen and water, but it does not interact with either at room temperature, which is because the surface of aluminum forms a layer of oxide film, which passivates the surface of aluminum.
In addition, some metals such as aluminum, chromium, and iron do not interact with cold concentrated sulfuric acid and concentrated nitric acid, but also because these metals will be passivated by cold concentrated sulfuric acid and concentrated nitric acid, so these metals can be used to contain concentrated sulfuric acid and concentrated nitric acid.
In addition to the oxide layer, the nitride layer and the insoluble salt layer will also passivate the metal surface.
In contrast to passivation, some ions will restore the activity of the dull metal surface, which is called activation. Such as chloride ion has a strong activation effect. This is why aluminum products cannot be used for long-term storage of items containing a lot of salt.