Permeability to Gases by Iron

Closely connected with the power to occlude gases is the property of transfusion, by virtue of which gases are able to pass through iron. In 1863 Cailletet showed that nascent hydrogen could pass through an iron vessel immersed in sulphuric acid at the ordinary temperature, and this has been confirmed by numerous later investigators.

This diffusion of nascent hydrogen is not inhibited even by a pressure of 14 atmospheres. Molecular hydrogen only passes through iron at a measurable velocity when the temperature is raised. Below 325° C. the velocity is inappreciable, at 350° C. it becomes perceptible, and at 850° C. the velocity is some forty times as great.

Iron is softened by prolonged exposure at 500° to 1000° C. to the transfusion of hydrogen, for the gas reduces and removes the small quantities of non-metallic impurities usually existing in the metal. Thus, for example, sulphur, phosphorus, and carbon are reduced and escape as volatile hydrides, the metal being purified to a corresponding extent.

The optimum temperature-range for this purpose lies between 950° and 1000° C., hydrogen containing small quantities of water vapour proving most efficient. The time of decarburisation is necessarily somewhat lengthy, and averages not less than some 4 or 5 days. The following results may be regarded as typical: -

Time of Decarburisation. Days.	Carbon %		Sulphur %
	Before.	After.	Before.	After.
4	0.094	0.076	0.025	0.017
5	0.182	0.015	0.030	0.017
4	0.326	0.027	0.026	0.018
5	0.683	0.026	0.016	0.006
5	1.05	0.035	0.011	0.003
12	1.67	0.220	0.042	0.008

No doubt if the decarburisation process were very greatly prolonged, the carbon and sulphur would be reduced to infinitesimal quantities. Analogous reactions take place between hydrogen and molten iron.