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.