目的 研究锆合金表面Cr涂层在空气和氮气气氛下的高温氧化性能及失效行为,揭示涂层结构与性能退化机制。方法 采用多弧离子镀工艺在锆合金上制备15 μm厚Cr涂层,并在Setaram Setsys Evolution型热重分析仪中开展1 200 ℃恒温氧化实验。通过监测质量变化并结合扫描电子显微镜（SEM）、能量色散谱仪（EDS）和X射线衍射仪（XRD）分析氧化前后试样的微观形貌、元素分布及物相变化。结果 两种环境下涂层表面均反应生成Cr2O3,但氮气环境同时发生氮化反应形成CrN层;空气环境氧化后的试样表面出现颗粒状团聚物,而氮气环境中则出现蠕虫状团状物;氧化动力学曲线显示,氮气环境下试样的质量增重幅度相对降低了6.32%;结构分析表明,试样在空气环境氧化后形成了Cr2O3层、残余Cr层、Cr-Zr扩散层的三层结构,而氮气环境则以Cr2O3层和残余Cr层为主,结合XRD以及EDS元素结果表明,这两层结构之间存在一层CrN薄层;随着反应的进行,N原子扩散到Zr基体内部形成ZrN层,抑制了Zr、Cr间的元素扩散,阻碍了Zr-Cr金属间化合物的形成,对镀铬锆合金的服役性能产生积极正面的影响。结论 Cr涂层显著提升Zr-4合金的抗氧化性能。该研究为Cr涂层在事故容错燃料的工程应用提供了一定的实验依据及理论支撑。
Abstract
Chromium coatings on zirconium alloys are widely explored to mitigate the high-temperature degradation of nuclear fuel cladding under severe reactor transient conditions, particularly in the course of the Loss-of-Coolant Accident (LOCA). Given that nitrogen is not only the most abundant gas in the atmosphere (accounting for about 78% of the air by volume) but also commonly present in nuclear reactor environments, its potential effects cannot be overlooked. To clarify the effects of different atmospheric environments (air/N2) on the phase evolution, interfacial chemistry and oxidation kinetics of Cr-coated zirconium alloys at elevated temperatures and to provide experimental and theoretical support for the application of accident-tolerant fuel (ATF) coatings in nuclear reactors, the work aims to investigate the response of Cr-coated zirconium alloy to air versus N2 atmospheres under identical thermal histories at 1 200 ℃. A homogeneous and adherent chromium coating with a thickness of approximately 15 μm was fabricated on a Zr-4 substrate by multi-arc ion plating. Then, the coated sample was subject to isothermal oxidation in a Setaram Setsys Evolution thermogravimetric analyzer (TGA) at 1 200 ℃, during which the mass change was continuously monitored with high precision to derive oxidation kinetic curves. The microstructures, chemical compositions and phase constituents of the samples before and after the oxidation exposure were characterized respectively by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The experimental results indicated that the Cr-coated zirconium alloy sample underwent oxidation in both air and nitrogen environments at 1 200 ℃, with chromium oxide (Cr2O3) forming as the oxidation product. Nevertheless, compared to the air environment, the Cr coating in the nitrogen atmosphere also underwent nitridation, forming a CrN layer. Significant differences were observed in the surface morphology and structure after oxidation: air exposure led to the development of granular aggregates uniformly distributed across the oxide scale, whereas N2 treatment produced worm-like clusters and sinuous ridges. The results of the oxidation kinetics curves indicated that oxidation in air exhibited a rapid initial mass gain, which subsequently transitioned into a parabolic trend. In contrast, the overall mass gain in N2 was lower (6.32% less than in air). Cross-sectional analysis further revealed distinctly different layered architectures dependent on the atmosphere. After oxidation in air, a well-defined three-layer structure was formed: (i) an outer Cr2O3 scale acting as the primary diffusion barrier; (ii) an intermediate layer of residual chromium; and (iii) an inner interdiffusion zone of Cr-Zr at the coating/substrate interface. In contrast, exposure to N2 resulted in a seemingly simpler bilayer structure. The structure primarily consisted of an outer Cr2O3 scale and an inner residual Cr layer. However, combined XRD and EDS elemental analysis results confirmed the presence of a thin CrN interlayer between these two layers. As the reaction proceeded, the diffusion of nitrogen atoms into the zirconium substrate led to significant nitrogen enrichment adjacent to the interface, simultaneously creating a localized zone of oxygen depletion. This phenomenon was consistent with the formation of ZrN at the interface. The presence of nitride phases effectively suppressed the interdiffusion of Zr and Cr, thereby inhibiting the formation of Cr-Zr intermetallic compounds. Consequently, the high-temperature oxidation resistance of the Zr-4 alloy was significantly enhanced, which exerted a positive effect on the service performance of the Cr-coated zirconium alloy. In summary, the Cr coating significantly improves the oxidation resistance of the Zr-4 alloy. All the findings of this study provide critical insights for optimizing chromium coatings for accident-tolerant fuel cladding, thereby advancing their prospects for practical implementation.
关键词
事故容错燃料 /
Cr涂层 /
Zr合金 /
高温氧化失效 /
氮气环境 /
扩散
Key words
ATF /
Cr coating /
Zr alloys /
high-temperature oxidation failure /
nitrogen atmosphere /
diffusion
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