In the solid state, iron can have several allotropic forms depending primarily on its temperature. These are:
III a-Iron: magnetic and stable to 768°C, crystallizes in a body centered cubic lattice, i.e., iron atoms are arranged at the centre and the apexes of unit cubes. It dissolves very little carbon (0.025% at 721°C). Alpha iron with the carbon traces dissolved in it is calledftrrite
Beta-Iron: it is a form stable between 768 and 910°C. It is alpha iron that has lost its magnetism. It does not dissolve carbon.
III Gamma-Iron: this form is stable between 910 and 1390 °C. The crystallographic appearance is a face centered cubic lattice, i.e., iron atoms are arranged at the apexes and centers of the sides of unit cubes. It is nonmagnetic and dissolves 2% carbon at 1102 0c. Gamma iron with carbon in solution is called austenite.
The physical properties of iron are changed by the presence of foreign elements (metalloids or metals) in very small amounts. Iron has a great affinity for o~’Ygen. It rusts in moist air, i.e., the surface gradually becomes converted into iron oxide hydrate. Compact iron reacts with dry air only above 150°C. When heated in air it forms the intermediate oxide, Fe30 4, which is also formed
during the forging ofred hot wrought iron. In a very finely divided state – such as is obtained, for example, when iron oxalate is heated in hydrogen – iron is pyrophoric.
Iron objects are protected from rusting by covering them with coatings of other metals (e.g., zinc, tin, chromium, nickel) or with paint (red lead). A particularly effective protection from rust can be achieved by converting the iron superficially to iron(II) phosphate (“phosphatizing”). This is done by treatment with a solution of acid manganese or zinc phosphate, Mn(H2P04h or Zn(H2P04h. Iron dissolves in dilute acids with the evolution of hydrogen and the fonnation of iron(II) salt<;. The nonnalpotential of iron in contact with iron(II) salt solutions is +D.440 V at 25°C, relative to the normal hydrogen electrode. If iron is dipped into a copper sulfate solution, it becomes covered with metallic copper:
Fe + cu2> -7 Fel> + Cu
At ordinary temperature, iron is hardly attacked by air-free water. If air has access, however: the porous iron(III) oxide hydrate is formed, and corrosion goes on continuously as a result. Concentrated sodium hydroxide attacks iron, fairly strongly, even in the absence of air, especially at high temperatures, since the Fe(OHh goes into solution through the formation ofhydroxo salts. Iron combines energetically with chlorine when heated, and also with sulfur and phosphorus,
but not directly with nitrogen. It has a strong tendency, however, to unite or alloy
with carbon and silicon. These alloys are most important for the properties of technical iron. Oll.)’gen is dissolved by molten iron, as FeO; some of this is retained in solid solution on cooling (o-iron can dissolve up to 0.12%, airon only up to 0.04% of°in solid solution).
The ox)’gen content of iron reduces its workability when hot. Nitrogen is absorbed from the air by molten iron only in minimalamounts. However, if iron is heated in ammonia gas, an iron-nitrogen compound, Fe2N, is fonned, which displays a considerable solubility in solid iron, and confers great hardness.
This property is utilized in the surface-hardening of iron articles, by heating them in ammonia (nitriding process). In addition to the foregoing, there is a second compound, Fe4N, with a rather narrow range of homogeneity. Above 660°C, this is transfonned without change of composition into crystals of Fe,N, in which only one half of the available lattice positions are occupied by nitrogen atoms.
Hydrogen is also absorbed by iron at a red heat. The amount absorbed is small, and is proportional to the square root of the pressure. Electrolytic iron, however, may contain larger amounts of hydrogen, which make it hard and brittle. The hydrogen is driven off on heating, and the iron then becomes ductile.