Si-rich phases and their distributions in the oxide scale formed on 304 stainless steel (304SS) kinetically affects SiO2 volatilization for the formation of silicate-based Cs-bearing microparticles released during the Fukushima Daiichi Nuclear Power Plant accident. Therefore, we investigated Si-rich phases and their distributions in the oxide scale on 304SS oxidized in H2O–H2 mixtures over the temperature range 1000–1300 °C. SiO2 film stays at the base of the Cr2O3-type scale formed at low temperatures or high H2 contents in steam. Fe2SiO2 phase is non-uniformly distributed in the spinel-type scale formed under opposite conditions. Fe2SiO2 accumulation in the spinel-type scale is attributed to its thermodynamically metastable state in H2O–H2 mixtures. With increasing oxidation time, Fe2SiO2 phase was further oxidized into SiO2 phase.
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