Invar® oxidation in CO2 Kinetics and mechanism of formation of a wüstite layer
Résumé
In this article, corrosion of Invar® in a static carbon dioxide atmosphere 2104PCO2105Pa has been studied between 1163 and 1263 K. At the beginning, after a short initial deceleration for weight gains Δm/S <0.5 mg cm−2, oxidation kinetics were linear up to weight gains of about 4.0 mg cm−2, and only wüstite Fe1−x O was formed with a constant rate r (mg cm−2 s−1) where R is the gas constant and t the time (s). Reaction mechanism is similar to that of the pure iron in analogous conditions, with the same rate limiting step i.e. external reaction of CO2 with wüstite and outward diffusion of ions Fe2+ (not limiting). For weight gains Δm/S higher than 4 mg cm−2, the limiting step changes, with an increase of the reaction rate and an internal oxidation. The origin of this mechanism change lies in the microcracks appearing in the oxide during its growth. Then, wüstite is no longer bound to the substrate; outward diffusion of ions Fe2+ stops and a topotactic transformation converts wüstite into magnetite.