The black Fe3O4 is a mixed-valence oxide of iron. It has a melting point of 1597℃ and a density of 5.17g/cm3. It is insoluble in water and soluble in acid solutions. It appears in the form of magnetite in nature. Strong submagnetism and high conductivity. (There is also literature that the melting point of Fe3O4 is 1538°C, which is insoluble in acid).
Ferromagnetic and ferrimagnetic materials undergo a second phase transition above the Curie temperature to transform into paramagnetic materials. The Curie temperature of Fe3O4 is 585°C.
Combustion of iron wire in oxygen will generate ferroferric oxide. Comparing the free energy of Gibbs generated by the standard mole of iron oxide, it is concluded that Fe3O4 has the greatest thermodynamic stability, so the product is Fe3O4.
Iron and air will form oxides on its surface. At this time, the chemical composition of the oxide film itself is not uniform. For example, a piece of low-carbon steel can be covered by three kinds of oxide films: FeO is in contact with the metal, Fe2O3 on the side in contact with air, and Fe3O4 in the middle. More precisely, perhaps a mixture of saturated solid solutions of three oxides constitutes the oxide film on the surface of the steel.
At the same time, the thickness of the oxide film also varies depending on different environmental conditions during oxidation. At room temperature, the thickness of the oxide on the relatively pure iron in dry air does not exceed 20 angstroms (1 angstrom = 0.1 nanometers), but the thickness of the oxide film in moist air increases significantly, and rust spots on the surface can be seen. At this time, the oxide deposition is layered, the side close to the metal is a dense amorphous anhydrous layer, and the side close to the air is a thick porous hydration layer.
Fe3O4 has anti-corrosion effects. For example, the bluing of steel parts (also known as blue-burning and baking blue) is to use the oxidation effect of alkaline oxidizing solution to form a blue-black or dark blue Fe3O4 film on the surface of steel parts. Used to increase corrosion resistance, gloss, and aesthetics.