%0 Journal Article
%T What If the Protection against Oxidation of Chromia-Forming Alloys Was Not Always Due to the Chromia Layer?
%A Boris Contri
%A St&#233
%A phane Valette
%A Marina Soustre
%A Pierre Lefort
%J American Journal of Analytical Chemistry
%P 286-302
%@ 2156-8278
%D 2024
%I Scientific Research Publishing
%R 10.4236/ajac.2024.159019
%X Chromia-forming alloys have good resistance to oxidizing agents such as O<sub>2</sub>, CO<sub>2</sub>, &#8230; It is accepted that the protection of these alloys is always due to the chromia layer formed at the surface of the alloys, which acts as a barrier between the oxidizing gases and the alloy substrates, forming a diffusion zone that limits the overall reaction rate and leads to parabolic kinetics. But this was not verified in the study devoted to Inconel<sup>&#174;</sup>625 the oxidation in CO<sub>2</sub> that was followed by TGA, with characterizations by XRD, EDS and FIB microscopy. Contrary to what was expected and accepted in similar studies on other chromia-forming alloys, it was shown that the diffusion step that governs the overall reaction rate is not located inside the chromia layer but inside the alloy, precisely inside a zone just beneath the interface alloy/chromia, this zone being depleted in chromium. The chromia layer, therefore, plays no kinetic role and does not directly protect the underlying alloy. This result was demonstrated using a simple test that consisted in removing the chromia layer from the surface of samples partially oxidized and then to continue the thermal treatment: insofar as the kinetics continued without any change in rate, this proved that this surface layer of oxide did not protect the substrate. Based on previous work on many chromia-forming alloys, the possibility of a similar reaction mechanism is discussed. If the chromia layer is not the source of protection for a number of chromia-forming alloys, as is suspected, this might have major consequences in terms of industrial applications.
%K Chromia-Forming Alloys
%K Chromia Layer
%K Oxidation Protection
%K Inconel<sup>&#174
%K </sup>625
%K Kinetics
%U http://www.scirp.org/journal/PaperInformation.aspx?PaperID=136265