Gases other than hydrogen in iron and steel

The paradox contained in the title of this article raises the preliminary question of the reference to the gaseous state, since species (elements or defined compounds) that are gaseous under conditions considered normal by thermodynamics occur, in combination in metals, either as solutes or as inclusions. The gas-metal reactions involved in corrosion processes are not considered in this article. Gases react in metals according to their electronic structure, which suggests that elements should be considered according to their position in the periodic table. Hydrogen, because of its peculiar behavior due to its small size, is excluded from the scope of this paper. It is certainly the element that raises the most questions because of its role in brittleness, plasticity, corrosion, stress corrosion, etc., and also the possibility of storing it in certain alloys.

Nitrogen, following hydrogen in order of increasing electronegativity, is found in interstitial form in most metal lattices, particularly in steels. When it comes to oxygen, electronegativity is such that, in most metals, it is found in the form of O ^2– ions, significantly larger than the metal atom, leading to the formation of oxides. A small amount of unreduced oxygen may nevertheless exist in interstitial form. The size of halogens, and a fortiori that of their anions, prevents the formation of interstitials, but makes them active in corrosion phenomena. Noble gases, used as a protective atmosphere due to their chemical inertness, may have a low solubility in liquid metals. In solids, they play no role, unless they result from nuclear reactions.

The reactions of polyatomic gases (H ₂ O, H ₂ S, CO, CH ₄ , NH ₃ ) on metals, apart from corrosion reactions, are also considered. They can be destructive or constructive (case-hardening, refining), in which case the end result is an improvement in the mechanical properties of the steel. The examples given focus on nitrogen and nitriding gases, which offer the widest range of applications in steels.