Concentrated sulfuric acid and concentrated nitric acid at room temperature can make aluminum, iron, chromium passivation, the principle is to generate a dense oxide film, blocking further reaction (this oxide film is insoluble in concentrated sulfuric acid, concentrated nitric acid, because concentrated sulfuric acid, concentrated nitric acid ionized hydrogen ions are relatively small), when diluted (with dilute strong acid to remove) or heating, this layer of oxide film will be destroyed.
Iron and aluminum are elements with moderate activity, which are easy to passivate and stop the reaction when they encounter concentrated sulfuric acid. Whether it can be passivated depends on the activity of the metal, and on the concentration and temperature of the acid.
Not only iron and aluminum, but also many other metals can be passivated. For example, Cr,Ni,Co,Mo,Ta,Nb and W, among which the most easily passivated metals are Cr,Mo,Al,Ni; Not only concentrated nitric acid and sulfuric acid.
Other strong oxidants such as chloric acid, iodic acid, potassium dichromate, potassium permanganate, etc., can cause the passivation of some metals.
Even oxygen in the air can blunt some metals (such as aluminum, chromium, titanium, etc.).