In-situ Measurements of Isothermal Wüstite Transformation of Thermally Grown FeO Scale Formed on 0.048 mass% Fe by Synchrotron Radiation in Air

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Isothermal wüstite transformation of thermally-grown FeO scale formed on 0.048 mass% Fe at 948 K for 180 s and annealed at 673, 723 K, and 773 K in air was studied in situ by synchrotron radiation. Wüstite transformation initiated right at the beginning of isothermal heating, either by the formation of a new phase of meta-stable iron-rich wüstite (at 673 and 723 K) or by direct transformation to magnetite (at 773 K). At 673 K, iron-rich wüstite replaced the parent wüstite thoroughly, and then transformed to magnetite and metal iron. At 723 K, formation of the iron-rich wüstite started as well, but the reaction was interrupted and was replaced by a gradual enrichment reaction of iron in both the parent and the iron-rich wüstite phases. Consumption of the parent wüstite and generation of magnetite accompanied. Wüstite transformation to generate magnetite and metal iron at 673 and 723 K was only possible for the iron-rich wüstite. At 673 and 723 K, incubation time was needed to initiate transformation of meta-stable iron-rich wüstite to magnetite and metal iron. Reaction to form the iron-rich wüstite from the parent wüstite was much easier. Tranformation of the iron-rich wüstite to magnetite and metal iron is considered a root cause for the formation of the lamellar structure and the magnetite seam in the scale. The transformation can well be interpreted from the analogy of the formation of the pearlite structure in steels.