An inert gas is generally non-reactive with other substances, used to avoid unwanted chemical reactions degrading a sample. The term ‘inert’ means non-reactive. We refer to gases as being chemically inert if their atoms don’t combine with other atoms in chemical reactions under a set of given conditions. The noble gases often do not react with many substances and were historically referred to as the inert gases. These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in the air.
Unlike noble gases, an inert gas is not necessarily elemental and is often a compound gas. Like the noble gases, the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases. This is a tendency, not a rule, as noble gases and other “inert” gases can react to form compounds. Inert gases, in particular helium, are used as coolants in gas-cooled reactors. In laboratory experiments and accident simulation tests, inert gases, mostly argon, are applied as a cover gas or a filling gas for the cladding tubes to detect cladding damage.
The presence of inert gases has the following consequences. The inert gases decrease the partial pressure of the reactive gases like oxygen, steam or hydrogen, and under special conditions, they can isolate the reactive gases from the reaction site. The decrease in the partial pressure of hydrogen by inert gases was discussed earlier. The vapor pressure of oxygen is so low that oxygen impurities of the order of ppm in noble gases result in oxidation reactions. The second effect, the formation of a boundary layer between the reactive gas and reaction site, plays a more important role. In the case of lamellar gas flows, the reactive gases flowing at the metal/gas interface are consumed and the inert gas remains.
New reactive molecules have to diffuse through the inert gas layer at the interface. However, diffusion in gases is fast and gas diffusion does not influence or only slightly influences the reaction rate. The situation changes when the concentration of the reactive gases is so low that starvation conditions are reached, at least locally. Inert gases play a determining role under conditions of limited diffusion. Such conditions occur, for instance, in breakaway oxide layers during oxidation. The gases penetrate into the cracks and are transported to the oxide/metal interface. The oxygen or steam is consumed there and the noble gases remain in the cracks. New oxygen or steam then has to diffuse through the cracks filled with the inert gases. The result is an, at least partly, suppressed breakaway effect.