不同温度FLiNaK熔盐对Hastelloy-N合金腐蚀的影响

Impact of temperature on the molten salt corrosion of Hastelloy-N alloys

国产Hastelloy-N合金与FLi Na K(Li F-Na F-KF:46.5-11.5-42 mol%)熔盐在有氩气保护的箱式炉中进行了相同时间(300h)、不同温度(600-900oC)的腐蚀实验。对腐蚀后的样品进行了失重测量,并用扫描电子显微镜/能量扩散光谱(Scanning Electron Microscope/Energy Dispersive Spectrometer,SEM/EDS)观测了腐蚀深度、表面形貌以及腐蚀区域的元素分布变化,同时对比熔盐中有水、氧等杂质参与的腐蚀,从而进行腐蚀机理研究。研究发现,对于经过去水、氧等杂质处理的熔盐的腐蚀,当腐蚀温度从600oC升高到900oC时,合金的腐蚀深度从8μm逐渐增加到40μm。合金单位面积的腐蚀失重从2 mg·cm-2增加到8 mg·cm-2,但增加速率逐渐下降。SEM/EDS测量结果显示,合金腐蚀区域均出现了Cr元素的流失现象,并伴有Mo元素的富集;合金微观形貌的变化表现为腐蚀后的晶界结构明显加宽并显现出来,但是当腐蚀温度升高到700oC时,合金表面晶粒细化明显,晶粒尺寸由腐蚀前的40μm细化到5-20μm不等,800oC和900oC的腐蚀使得晶粒重新粗化到40μm,且有Mo为主成分的析出物出现,尤其在晶界处更为明显。另外,当熔盐中有杂质(水、氧等)时,850oC的腐蚀使得腐蚀深度已经远大于100μm,同时使耐熔盐腐蚀的Mo元素也出现了脱溶现象,明显加剧了熔盐对合金的腐蚀作用。

Background： The demand for energy is growing with the rapid population growth and the expallsion of the economy. As one of the six most promising Generation IV reactors, Thorium Molten Salt Reactor （TM 5R） has been attracting increasing attention. Unlike the traditional Pressure Water Reactor （PWR）, operating temperature of TMSR is varying from 565 ℃ to 850 ℃ and the nuclear fuels are dissolved in the molten salt LiF-BeF2-ZrF4-UF4 （65-29.1-5-0.9 mol%）. Considering the causticity of the molten salt, there are more stringent demands on ttie loops materials, including good mechanical properties, low activity, good chemical compatibility with the molten salt, and resistance to corrosion. To ensure the safe operation of the reactor, it is necessary to investigate the behavior of loop material corrosion in high-temperature molten salt. Purpose： This study is to explore the corrosion mechanism based on the change of microstructure and weight of the alloys corroded with same corrosion time and different corrosion temperatures. Methods： Data for the weight loss of the alloys before and after corrosion were obtained by weighing experiment. To identify microstructure changes of the alloys after corrosion, Scanning Electron Microscope（Energy Dispersive Spectrometer （SEM/EDS） were applied to measure the micro-elements changes and obtain corrosive morphology. Results： After molten salt corrosion, corroded areas of all samples exhibited enrichment of Mc and Cr elements loss. Considering the Cr oxides and fluorides generated during etching in the saturation solubility of the molten salt, which will be deposited on the surface of the alloy to form a protective layer for certain inhitition to corrosion, resulting the corrosion weight loss increases but the increasing rate decreases with the temperature of corrosion rises. Be different from the un-corroded alloy, grain boundary structure of the alloy revealed significantly widened after corrosion, and the grain size occurs after the first refinement coarsening phenomenon as the corrosion temperature increases. Conclusion： Corrosive environment has a great impact on alloy corrosion, molten salt corrosion showed significant intergranular corrosion characteristics, and corrosion at different temperatures showed different corrosion characteristics due to the effect of temperature on the rate of diffusion of atoms.