热老化后核级主管道不锈钢的点蚀行为及机理

The primary coolant pipe as the pressure boundary of the coolant system plays an important role to the safety of nuclear power plant. Thermal aging and localized corrosion are two main failures of primary coolant pipe during working, and usually, the localized (pitting) corrosion resistance of pipes will be worsened by the massive nanophases precipitated in steel during thermal aging. However, the mechanism of pitting induced by thermal aging is still lacking understanding now. The conventional pitting theory of MnS inclusion is unsuitable for the stainless steels used for nuclear power plant including very less inclusions. Moreover, Cr-depleted theory cannot accurately explain the mechanism of pitting induced by massive nano-precipitated phases too. Therefore, pitting corrosion behavior and mechanism of thermally aged cast stainless steel for the primary coolant pipe of nuclear power plant will be investigated by the micro-electrochemical technique, transmission electron microscopy (TEM), atomic force microscopy (AFM) technology and numerical simulation method in this project. The effect of size, amount and style of nanoparticles on the pitting corrosion and the pitting behavior of thermally aged steel in the simulated environment of nuclear power plant working will be studied. The mutual effects of nanoprecipitates on pits initiation and growth in the process of pitting will be investigated and the physical model of pitting of thermally aged stainless steel will be established. The group effect of nanoprecipitates on pitting corrosion will be analyzed and the relationship between thermal aging and pitting will be detected by the model in this project. This not only is helpful for revealing the size effect of pitting initiation caused by micro composition segregation but also can enrich the pitting corrosion theory of stainless steels and provide the scientific guidance to evaluate systematically the performances of stainless steel for primary coolant pipe.

核电主管道作为冷却剂系统的承压边界对核电站安全发挥着重要的作用。热老化和局部腐蚀是其服役失效的两种主要形式，热老化过程中产生的大量纳米析出相会加剧局部（点）腐蚀。然而，其机理目前还缺乏足够的了解，现有MnS点蚀理论不适合夹杂物含量极低的核级不锈钢的点蚀行为，贫铬理论也未能很好解释大量纳米粒子诱发点蚀的机理。本项目针对上述问题，通过（微）电化学、透射电镜、原子力显微镜和数值模拟等手段对热老化核级铸造不锈钢点蚀行为及机理进行研究。分析纳米析出相粒子尺寸、数量及类型对点蚀行为的影响规律以及核电站模拟环境中热老化不锈钢的点蚀行为；研究纳米粒子在点蚀萌生、发展过程中相互影响的问题；探讨纳米析出粒子诱发点蚀的“集体效应”行为；建立热老化不锈钢点蚀物理模型；分析纳米析出相对点蚀行为影响的内在本质，揭示微观结构变化而导致材料点蚀萌生的尺寸效应，丰富不锈钢点蚀理论内容，为核电主管道性能全面评估提供科学依据。

热老化脆化是奥氏体-铁素体双相不锈钢在一定温度服役时（例如压水堆核电站一回路主管道）不可避免的材料性能退化现象，不仅引起冲击韧性的降低还会造成腐蚀性能尤其是点蚀性能的下降。虽然国内为对热老化机理及对力学性能的影响进行了大量研究，但是直至本项目开始实施却鲜有关于热老化对点蚀性能影响的系统深入研究。本项目主要针对核电主管道不锈钢长期服役过程中因热老化产生的纳米析出相恶化点蚀性能的现象及现有理论在解释上述现象时遇到的困难，对热老化后的核电主管道不锈钢及典型双相不锈钢的点蚀行为及机理进行了研究。通过微观结构表征，在450℃长时间热老化的SAF2205及SAF2507不锈钢试样中发现了文献中鲜有报道的成分特殊的析出相，此析出相显示异常高的Mo元素含量。相同温度下，热老化时间对Z3CN20.09M不锈钢点蚀性能的影响呈抛物线变化规律，7000h前点蚀性能下降明显，时间继续延长点蚀性能降低缓慢。热老化对试样在模拟核电一回路主管道冷却剂中的点蚀性能影响不明显。环境温度对含有0.5 mol/L NaCl的模拟核电一回路主管道冷却剂溶液中热老化试样的点蚀性能影响显著，而对模拟核电一回路主管道冷却剂溶液中热老化试样的点蚀性能影响不明显。两种溶液中点蚀性能显著降低的临界温度分别为40℃和95℃。热老化温度对三种不锈钢点蚀性能的影响呈现出不同的规律，对于Z3CN20.09M不锈钢，随着温度升高点蚀性能一直降低，而对于SAF2205及SAF2507不锈钢则先降低后缓慢升高。相同α'相与基体的Cr含量差值时，随着α' 析出相尺寸增大，界面结合能降低，这促进点蚀发生。大量富铬纳米析出相周围的贫铬区相互接触、链接，形成较大范围的贫铬区域，创造了点蚀发生的成分条件和结构尺度条件，表现出纳米析出相诱发点蚀的“集体效应”。研究结果不仅可为核电管道安全服役评价提供依据，还可丰富现有点蚀理论。