At room temperature, iron, aluminum and other metals can be passivated in concentrated nitric acid. There are usually two theories to explain this problem:
One is the adsorption theory. It is believed that the passivation of metals is due to the formation of oxygen adsorption layer composed of oxygen molecules, OH- and O2- ions on the surface of metals in nitric acid. There are four possibilities for oxygen adsorption layer to lead to metal passivation:
(1) The oxygen adsorption layer increases the overpotential of metal oxidation, which increases the difficulty of metal oxidation;
(2) It is believed that the unsaturated bonds of the atoms on the metal surface reach saturation after adsorbing oxygen, so the metal surface loses its activity;
(3) It is believed that the oxygen adsorption layer suppresses other chemical media that the metal surface was originally in contact with;
(4) It is believed that the oxygen atom adsorbed on the metal surface is induced by the electrons on the metal surface to form an oxygen dipole, with the positive end on the metal surface and the negative end toward the solution, thus forming a double electric layer, which blocks the oxidation of the metal.
The other is the thin film theory. It is considered that when the concentration of metal ions in concentrated HNO3 exceeds the solubility product of metal hydroxide, a film precipitation of hydroxide is formed on the metal surface, which stops the further reaction between metal and concentrated nitric acid. The results of polarization analysis show that the passivation of iron in concentrated nitric acid is due to the formation of a layer of 2.5-3.0 nm γ-Fe2O3 thin film on the surface of iron, which stops the further reaction of iron with concentrated HNO3.
Aluminum passivation also occurs in concentrated HNO3, and the passivation film of aluminum is analyzed to be a layer of non-porous γ-Al2O3, which is also covered with a layer of porous Al2O3·3H2O, so the stability of aluminum in concentrated HNO3 is better than that of iron (in the storage and transportation of concentrated HNO3, people often use aluminum containers instead of iron containers).
It is worth saying more that: passivated iron, aluminum and other metals have high stability, for example, iron passivated in concentrated nitric acid, changed to dilute HNO3 will no longer be oxidized by nitric acid; Adding passivated iron to CuSO4 solution does not replace copper.
Iron will be passivated! A phenomenon in which the chemical stability of metals or alloys is significantly enhanced by some factors is called passivation. The phenomenon of metal passivation caused by certain passivating agents (chemicals) is called chemical passivation. Such as concentrated HNO3, concentrated H2SO4, HClO3, K2Cr2O7, KMnO4 and other oxidants can passivate metals! Iron dissolves quickly in dilute HNO3 or dilute H2SO4, but in concentrated HNO3 or concentrated H2SO4 the dissolution almost completely stops. Carbon steel is usually easy to rust, if the appropriate amount of Ni and Cr is added to the steel, it becomes stainless steel.