Zr-Sn-Nb-Fe-V合金在过热蒸汽中的周期性钝化-转折行为

Periodic Densification-Transition Behavior of Zr-Sn-Nb-Fe-V Alloys During Uniform Corrosion in Superheated Steam

研究了2种高Nb、低Nb含量新型Zr-Sn-Nb-Fe-V合金在400℃、10.3 MPa过热蒸汽中的长期均匀腐蚀行为,发现2种合金均表现出周期性钝化-转折规律,对其演变行为及机制进行了深入研究。多次转折发生后,氧化膜显微组织仍呈规律性分层,柱状晶及缺陷带周期性出现。转折前,氧化膜/金属界面起伏强度及四方ZrO_(2)(t-ZrO_(2))等效厚度随时间延长而逐渐增大,转折时均快速减小;氧化组织再次钝化-转折时,2者演变规律均与初次转折相同。分析认为,转折时缺陷带的产生受到氧化膜/金属界面的粗糙起伏畸变及t-ZrO_(2)相变共同影响,周期性氧化规律的产生与起伏强度、t-ZrO_(2)等效厚度达到最大临界值的周期性密不可分。Raman光谱分析表明t-ZrO_(2)的280 cm^(-1)特征峰负偏移,定性地反映了t-ZrO_(2)中O空位浓度。O空位浓度在钝化阶段保持稳定,转折时迅速下降,2种合金的耐腐蚀性差异与O空位浓度差异相关。

Zirconium alloy is frequently used in the nuclear industry as a reactor fuel cladding material.Uniform corrosion of zirconium alloys has received much attention because it is one of the material's life-limited properties.To study their long-term uniform corrosion behavior,two types of Zr-Sn-Nb-Fe-V alloy claddings,one with a high niobium content and one with a low niobium content,were exposed to 400o C and 10.3 MPa of superheated steam for 800 d.Both alloys clearly exhibit periodic oxide densification-transition.Different quantitative methods were used to study the evolution and mechanism of multiple oxide features.The periodic layers of oxide morphologies are still observed even after two times of transitions.Periodically,columnar grains and the defect layer appear.The undulation intensity(related to the“cauliflower”morphologies)and the equivalent thickness of tetragonal zirconia(t-ZrO_(2))at the metal/oxide interface increase with corrosion time in pretransition oxides and decrease at transition time.The evolution of both features in the subsequent oxide densification-transition period is the same as initial densificationtransition cycle.The critical value of the interface undulation intensity is approximately 0.25 for both alloys.The critical thickness of t-ZrO_(2)is approximately 450 nm for low-niobium alloy and approximately 200 nm for high-niobium alloy.Both features periodically reach critical conditions.The evolution of undulation intensity provides an excellent explanation for the production of isolated and interconnected lateral cracks.Additionally,the transformation of t-ZrO_(2)to monoclinic zirconia(m-ZrO_(2))results in cracking of the oxide and produces interconnected,tiny equiaxed defects at the interface.Both lateral cracks and equiaxed defects are both important components of the defect layer.It is hypothesized that the synergetic effects of interface undulation and t-ZrO_(2)transformation affect the transition.The occurrence of periodic transitions is strongly correlated with the periodic behavior of oxide features reaching critical conditions.The volume of oxygen vacancy in t-ZrO_(2)was presumably evaluated by studying the Raman shift of the characteristic t-ZrO_(2)peak of 280 cm−1.As a result,the amount does not change considerably during oxide densification but decreases during the transition period.The Raman shift of t-ZrO_(2)in low-niobium alloys is approximately-1.2 cm^(−1)less than that in high-niobium alloys,indicating that the lowniobium alloys have a greater volume of oxygen vacancies.It is proposed that the difference in the corrosion behavior of two alloys is derived from the difference in the volume of oxygen vacancies.