2. Hot gas corrosion

The chemical reactions that degrade ceramics require that the reactants (gas and ceramic) have "chemical affinities". This question is resolved by the study of thermodynamics, which determines whether a reaction is possible or not: a thermodynamically impossible reaction never occurs. On the other hand, if thermodynamics indicates that a reaction is possible, it is not certain that it will actually take place quantitatively, because the reactants may be separated by a layer of reaction products, sometimes only very thin, forming a screen between the ceramic and the reacting gases (cf. § 1.3.1