水化学对锆合金耐腐蚀性能影响的研究

The Effect of Water Chemistry on the Corrosion Behavior of Zirconium Alloys

经对包括 Zr- 4在内的 4种不同成分锆合金耐腐蚀性能的研究发现：高温水中添加了 0.01～ 0.1molLiOH后，腐蚀转折提早发生，转折后的腐蚀速率增加。这种现象随 LiOH浓度增大变得更加显著，但在成分不同的锆合金中有明显的差异，同时添加 H3BO3后又可以抑制 LiOH的加速腐蚀作用。氧化膜显微组织的观察结果表明：转折后的加速腐蚀过程与氧化膜中孔洞簇的出现有关，添加 LiOH以及改变合金成分后，通过影响孔洞簇的形成而对腐蚀转折及转折后的腐蚀加速产生作用。从分析氧化膜中 Li＋的浓度及有关腐蚀增重的结果来看，似乎 Nb可以部分抑制 Li＋进入氧化膜，而足够含量的 Sn可以缓解 Li＋的有害作用。因此，只有 Sn和 Nb含量都比较高的 3样品，在含 LiOH介质中的耐腐蚀性能最好，而含 Sn和 Nb较低的锆合金样品，在这种条件下的耐腐蚀性能还不如 Zr- 4。这些锆合金在不含 LiOH的 400℃过热蒸汽中腐蚀时，表现出耐腐蚀性能的变化规律，与含 LiOH高温水中的正好相反，说明了这两种条件下影响腐蚀性能的主导因素有着明显的不同。在靠近氧化膜 /金属界面处的氧化膜，无论在转折前或转折后，都存在亚稳的非晶、立方和四方结构的 ZrO2，没有观察到孔洞，在氧化膜中间层和表层已生成孔洞处，是单一的单斜结构 ZrO2，

The effect of water chemistry on corrosion behavior of zirconium alloys with 4 different composition including Zircaloy- 4 has been investigated.The breakaway phenomena were appeared earlier and the corrosion rate after breakaway was accelerated during testing in the static autoclave at 360℃ with different concentrations of aqueous LiOH solutions.The phenomena were much pronounced with the increase of LiOH concentration,but it could be suppressed to a certain extent after adding H3BO3 at the same time.The results based on the examination of the microstructure of oxide films show that the acceleration of corrosion rate after breakaway is correlated to the appearance of the pores in clusters in oxide films.The effect of water chemistry and alloy composition on the corrosion behavior is caused by the formation ability of pores in clusters in oxide films.According to the analyses of the relationship between lithium contents in oxide films and the thickness of oxide films for different composition specimens,it seems that the alloying element of Nb could suppress the penetration of lithium into the oxide films,and the sufficient of Sn contents might mitigate the detrimental effect of lithium existed in the oxide films.Therefore,only the 3＃ specimens containing sufficient Nb and Sn shows very good corrosion resistance in this water chemistry conditions,and those specimens containing comparative lower of Nb and Sn possess worse corrosion resistance even than that of Zircaloy- 4.The various regulation of corrosion resistance for these zirconium alloys in 400℃ superheated steam is contrary to that during test in aqueous LiOH solutions,it means that the main factors,which will dominate the corrosion resistance,are quite different in different water chemistry conditions.The oxide films near the oxide/metal interface,no matter before or after breakaway,possess some metastable phases including amorphous,cubic and tetragonal structure,and no pores existed.The oxide located at the pores region in the surface and middle part of oxide films is a single monoclinic structure.This difference of crystal structure of oxide films should be related to the results caused by the stress gradient distributed along the thickness of oxide films and the maximum stress formed at the boundary of oxide/metal interface.The appearance of the pores in clusters in the oxide films might be related to the condensation of vacancies under the application of stress and temperature.More experimental results are still needed for the explanation of the roles of lithium penetrated in oxide films and the composition of alloys played on the corrosion behavior of zirconium alloys.