锆-2合金在过热蒸汽中形成氧化膜的电子显微镜研究

ELECTRON MICROSCOPIC STUDY OF OXIDE FILMS FORMED ON ZIRCALOY-2 IN SUPERHEATED STEAM

应用电子显微镜研究了锆-2合金在613k和773k过热蒸汽中形成的氧化膜,对节状腐蚀也进行研究。黑色氧化膜中存在立方、四方和单斜三种晶体结构的氧化锆,随着温度升高,立方和四方的比例减少。当氧化膜变成白色后,完全转变成了单斜结构。从高分辩电子衍衬图象中可以看到黑色氧化膜的晶体是由～20nm大小的嵌锒块组成,在(100)和(101)晶面上存在一些晶面间距较大的地区,这可能与锡原子的聚集片有关。在氧化膜与金属界面处,节状腐蚀斑形似“花菜”向金属基体内生长,“花菜”表面留下波纹痕迹,这可能与节状腐蚀生长快的特性有关。在白色氧化膜中,可以观察到晶界上存在空洞(<10nm)。根据本实验结果,对作者过去提出的节状腐蚀形成模型作了进一步的讨论。

Electron microscopic techniques were adopted to investigate oxide films formed on Zircaloy-2 in superheated steam at 673 and 773K. The nodular corrosion appearing on black oxide films during oxidation transition was also investigated.Monoclinic,cubic,and tetragonal lattice structures in black oxide films formed both at 673 and 773K were identified.The proportion of cubic and tetragonal phases decr- eased with increase of temperature and single monoclinic lattice struc- ture was fouhd in white oxide films after oxidation transition.It may be seen from the high resolution lattice image that the oxide crystals consist of mosaics～20 nm in size,and a larger space on(100) and(010)planes in some places might be associated with the clusters of tin atoms.The feature of oxide/metal interface at nodules was characterized by protrusions,which looked like cauliflower with ripple marks on its surface.This might occur since nodules grew much faster than surrounding oxide.Pores(<10nm)existing on triple grain boun- daries in white oxide films were observed.The model for the forma- tion of nodular corrosion proposed by the author previosly is further discussed in the light of results obtained in the present work.