Oxidation Behavior Of Alloys Simulating Irradiation Induced Segregation In Stainless Steels In Simulated Pressurized Water Reactor Primary Water

Austenitic stainless steels, commonly used as structural materials of nuclear power reactors because of their relatively excellent comprehensive properties, are susceptible to irradiation-assisted stress corrosion cracking(IASCC), which is a critical issue for the safe and economical operation of light water reactors. Radiation-induced segregation(RIS) at the grain boundaries in austenitic stainless steels, such as the depletion of Cr and enrichment of Ni and Si, is an important factor that contributes to IASCC. The model alloys with modified Cr、Ni、Fe、Si contents were made by additions of pure Ni, Fe and Si in 316 L SS. Studying the oxidation behavior of the model alloys as the simulation of RIS at grain boundary in high-temperature water could benefit further elucidating the effect of the chemical composition changes at grain boundary on IASCC. The oxidation behavior of 316 L SS with different surface conditions in simulated PWR primary water is also investigated.The present work investigates the morphologies, cross-section characteristics and chemical composition of the oxide films grown on 316 L SS and model alloys exposed to high-temperature water. Raman spectroscopy, X-ray photoelectron spectroscopy(XPS), scanning electron microscopy(SEM), and transmission electron microscopy(TEM) are used as the characterization methods. The main experimental results and conclusions are as follows.(1) After exposure to PWR primary water for 120 h, duplex oxide films formed on emery paper-polished and mechanically polished specimens. A compact oxide film monolayer was formed on electropolished 316 L SS. After exposure to PWR primary water for 500 h, duplex oxide films formed on emery paper-polished, mechanically polished, and electropolished 316 L SS. The duplex oxides consisted of an outer layer composed of large spinel oxide particles rich in iron and an inner layer composed of fine spinel oxides(Fex CryNizO4) rich in chromium. The inner layer oxide films were inhomogeneous featuring some scattered porous granular areas. These granular areas were Cr rich and their distribution was influenced by the surface conditions of the specimens. Electropolishing contributed to improved oxidation resistance of 316 L SS in simulated PWR primary water especially in the early oxidation stages.(2) The oxide films formed on 316 L SS and 9Cr-25 Ni after 1194 h exposure consisted of an outer layer composed of large oxide particles rich in iron and an inner layer composed of fine spinel-type oxides FexCry NizO4. The inner layer formed on 316 L SS with about 17 wt.% Cr is thinner than those formed on 9Cr-25 Ni with about 9 wt.% Cr. The Cr content in the inner oxide formed on 316 L SS is higher than that on 9Cr-25 Ni. A high Cr-content in the alloys favors the formation of a protective thin Cr-rich inner layer that hinders the further growth of the inner layer.(3) Ni enrichment was observed near the oxide/metal interface of 316 L SS and 9Cr-25 Ni. The porous granular areas in the inner oxide film formed on 9Cr-25 Ni affected the Ni-enrichment near the interface of the oxide/metal. Ni depleted near the oxide/metal interfaces for 9Cr-25Ni-5Si specimen, indicating the effect of Si on the oxidation kinetics of Ni.(4) High reactivity of Si and high solubility of silicon oxides in high temperature water contribute to the observed Si-depletion in the outer oxide layer and the local Si-enrichment near the metal/oxide interface of 9Cr-25Ni-5Si alloy after 1194 h immersion in deaerated high temperature water. The prior-oxidation of Si and the subsequent dissolution of the Si-bearing oxides in high temperature water contribute to the observed cavities in the outer oxide layer. High reactivity of Si in 9Cr-25Ni-5Si with water enhances the oxidation of other alloy elements to form a continuous iron-rich spinel outer layer after 114 h immersion in high temperature water. Dispersed polyhedral oxide particles were formed on 9Cr-25Ni-xSi surface after immersion for 500 h in hydrogenated PWR primary water. The effect of Si on IX the oxide film morphology in hydrogenated PWR primary water was much less significant than that in deaerated simulated PWR primary water. The oxidation behavior of 9Cr-25Ni-xSi in high temperature water is closely related to the immersion environment.(5) The effect of Si on the oxidation behavior of Fe-Cr-Ni in high temperature water indicates that irradiation induced Si-segregation would play an important role in irradiation assisted stress corrosion cracking.