Occlusion of Gases by Iron

Iron readily absorbs or " occludes " gases, and may contain from ten to twenty times its own volume of gas. Meteoric iron usually contains hydrogen, nitrogen, helium, and oxides of carbon. The gases are evolved on heating the metal in vacuo, the different gases being characterised by a definite temperature of evolution. The gases most usually occluded by terrestrial iron are the oxides of carbon, nitrogen, and hydrogen, and sometimes the last traces of them are held very tenaciously by the metal, a third heating in vacuo to 1100° C. still yielding some gas. Mtiller showed that the gases occluded in iron are, for the most part, mechanically held in the pores of the metal, for they are liberated on boring, only a very little more gas being obtained when the borings are heated to redness in vacuo. Helium is absorbed by iron, but less readily than hydrogen. Iron wire absorbs increasing quantities of hydrogen when heated in an atmosphere of that gas from 400° C. upwards, but on cooling the whole of the gas is evolved. The solubility is independent of the superficial area of the metal, so that the phenomenon is an example of true solution and not of adsorption, which is purely a superficial effect. At constant temperature the solubility of the gas is directly proportional to the square root of the pressure in both the solid and the molten metal. At constant pressure the solubility of hydrogen increases with the temperature. The curve connecting the solubility with the temperature does not indicate any change in the neighbourhood of the A₂ point, but a rapid increase in solubility manifests itself in the neighbourhood of the A₃ point - that is, between 850 ° and 900 ° C. When the molten metal solidifies in an atmosphere of hydrogen, it exhibits the phenomenon of " spitting." The power to occlude gases is closely connected with the amorphous cement layers between the ferrite crystals.