Absorption of Nascent Hydrogen by Iron

Iron readily absorbs nascent hydrogen. When saturated with this gas, iron is characterised by unusual brittleness, but, on prolonged exposure to air, or more rapidly on heating, the hydrogen is evolved and the metal regains its usual physical properties.

A ready method of studying these changes consists in immersing iron wires in very dilute sulphuric acid. Under these conditions the metal will absorb some twenty times its volume of hydrogen. The brittleness appears to be due to the adsorption of the gas by the thin intercrystalline cement joining the ferrite crystals together, the cement thus increasing in volume to such a degree as to force the crystals apart somewhat, thus reducing the intercrystalline cohesion. The property of absorbing nascent hydrogen is shared by steel and cast iron, but the purer forms of the metal are more readily affected. Cast iron was found by Ledebur to require a very prolonged exposure to a relatively concentrated acid to produce a well-defined effect, possibly on account of its high silicon content. Iron and steel wires were used, ranging in diameter from 0.065 to 0.140 inch. It will be observed that the hydrogenised wires possessed substantially the same tenacity, but their elongation decreased, whilst their brittleness showed a substantial increase. Exposure to air at the ordinary temperature gradually restored the wires to their normal state, - a change that was rapidly brought about by ignition to cherry-red heat in an inert atmosphere.