Iron nitride, Fe₂N

Iron nitride, Diferro-nitride, Fe₂N or Fe₄N₂. - That iron combines with nitrogen was first observed by Depretz, who gave to the compound the formula Fe₄N₂. A natural nitride of iron has been found in the lava of Etna as the mineral siderazote. When heated in hydrogen ammonia is evolved, metallic iron constituting the residue. The importance of carefully studying the manner in which iron combines with nitrogen has been increasingly emphasised within the last decade, because the presence of nitrogen in steel has been shown to affect very materially the mechanical properties of the metal. For example, the absorption of 0.3 per cent, of nitrogen entirely suppresses the critical changes occurring in pure iron, and prolonged heating in vacuo is necessary to effect the complete expulsion of the gas. Nitrogenised steels are brittle, showing an increase in hardness and tenacity, accompanied by a decrease in elongation and ductility - these properties resulting from the tendency of the nitrogen to retain the iron in the gamma form, and the carbide in solution. Tubes of malleable iron, after exposure for several days to ammonia at 800° C., become so brittle that they can be broken like porcelain with a blow from a hammer. A rod of soft charcoal iron, after like treatment, becomes so hard that it can be used as a drill.

Nitrogen is slightly soluble in iron, the solubility being proportional to the square root of the pressure.

Iron and steel absorb small quantities of nitrogen when heated to above 1200° C., and when melted under a high pressure of the gas. Under ordinary conditions, however, merely heating the metal in nitrogen yields no perceptible quantity of nitride.

Continuous heating of the reduced metal is stated to result in a small amount of nitrogen absorption, as also exposure of the finely divided reduced metal at the ordinary temperature to nitrogen.

To obtain iron nitride indirect methods must be adopted. These may be enumerated as follows: -

1. Heating ferrous chloride in a current of dry ammonia, the optimum temperature for the reaction being 480° C. Ferrous bromide may be used instead of the chloride.
2. Heating reduced iron in a current of dry ammonia at about 414° C., when hydrogen is rapidly evolved. The heating is continued until the reaction is complete and no more hydrogen is liberated.
3. Heating electrolytic iron foil in dry ammonia at temperatures ranging from 600° to 1000° C. Hanemann by this means succeeded in preparing iron nitride containing 11.1 per cent, of nitrogen, the product thus corresponding to the formula Fe₂N or Fe₄N₂. The most favourable temperature appears to be 650° to 700° C., in a rapid current of ammonia, the nitride peeling off in thin flakes if the metal is thick. Any carbon combines with the nitrogen and is removed.
4. Heating spongy iron in dry ammonia, the optimum temperature being 450° to 475° C.
5. Iron amalgam when heated in ammonia vapour loses its mercury content and is gradually converted into nitride.

As prepared by any of the above methods, iron nitride is a grey powder of density about 6.25, and soluble in dilute hydrochloric and sulphuric acids, yielding ferrous and ammonium salts. Nitric acid, even when concentrated, only acts slowly. Gaseous hydrogen chloride attacks it at 220° C., the reaction becoming vigorous at 350° C., ammonium and ferrous chlorides resulting. When heated in oxygen, ferric oxide is produced, nitrogen (not its oxides) being set free; the reaction is detectable at 200° C. Hydrogen reduces it, the reaction beginning at 350° C. and being very rapid at 600° C.

Heated in vacuo or in nitrogen it begins to decompose at 500° C., and at 600° C. nitrogen is rapidly evolved. This reduction takes place even under a pressure of 18 atmospheres of nitrogen. For this reason there is no nitride formation when iron is heated at these temperatures in nitrogen.

It has been suggested that the constitution of iron nitride is represented by the formula: -



The heat of formation of iron nitride is as follows: - [Fe] + (N) = [Fe₂N] + 3040 calories.

Two other nitrides of iron have been prepared, namely, ferrous nitride, Fe₃N₂, and ferric nitride, FeN. The former is described as a black, oxidisable powder, obtained by heating lithium nitride with ferrous potassium chloride. The ferric nitride, FeN, resulted when lithium nitride was heated with ferric potassium chloride. It is a black substance, which, on being heated in air, oxidises to ferric oxide. These substances are, presumably, ferrous and ferric substituted ammonias, the constitutional formulae being: -

and Fe≡N

respectively.