预形变、晶界碳化物和氢影响核电镍基合金应力腐蚀开裂的耦合作用机制

Stress corrosion cracking of Nickel-base alloys in high temperature water limits the long term and safe operation of nuclear power plants. Material microstructure, mechanical properties and environmental factors are key controlling factors for stress corrosion cracking. This work investigates the coupling effects of prior-deformation, grain boundary carbide and hydrogen on stress corrosion cracking of Nickel-base alloys in pressurized water reactor primary water. The mechanism and rate-determining steps for stress corrosion cracking of Alloy 690 are emphasized, and Alloy 600 is used as a reference alloy due to its high SCC sensitivity. Nickel-base alloys with various coverage fractions of grain boundary carbides with and without prior-deformation are prepared, which are used for measuring the oxidation, oxidation under stress, and stress corrosion cracking behavior in pressurized water reactor primary water at potentials for the normal condition (Ni stable region), a low dissolved hydrogen concentration condition (NiO stable region) and non-hydrogen condition, separately. This work aims at clarifying (1) combined effects of prior-deformation and hydrogen on oxidation and stress corrosion cracking, (2) combined effects of grain boundary carbides and hydrogen on oxidation and stress corrosion cracking, and (3) the elucidation of the element processes and controlling factors for stress corrosion cracking under the combined effects of prior-deformation, grain boundary carbides and hydrogen. The results can be applied for improving the prediction of stress corrosion cracking kinetics, the optimization of fabrication technology and the operating conditions.

压水堆核电站镍基合金在高温水中的应力腐蚀开裂制约其长期安全运行。材料微结构、力学性能和环境条件是影响应力腐蚀开裂的关键因素。本项工作研究晶界碳化物、预形变和氢对镍基合金在压水堆核电站一回路水中应力腐蚀开裂行为的耦合作用。以690类型镍基合金的应力腐蚀开裂机理和速率控制因素为主要研究目标，使用容易发生开裂的600类型合金为参照体系，制备有/无预形变、不同晶界碳化物覆盖度的镍基合金，测试其在压水堆一回路正常水（高溶解氢，Ni稳定区）、低溶解氢水（NiO稳定区）和无氢水中的氧化、应力作用下氧化和应力腐蚀开裂性能，明确 (1) 扩散氢与预形变对氧化行为和应力腐蚀开裂行为的影响；(2) 扩散氢与晶界碳化物对氧化行为和应力腐蚀开裂的影响规律；(3) 解析晶界碳化物、预形变和扩散氢共同作用下应力腐蚀开裂的基元过程和控制因素。结果可为预测应力腐蚀开裂动力学、优化加工工艺及运行条件提供理论基础。

压水堆核电站一回路水中构件中的氢可来源于水中的氢以及金属腐蚀产生的氢等。氢对腐蚀的作用往往与氢陷阱比如碳化物和缺陷有着耦合效应。本项目采用690合金，52M焊接金属以及不锈钢作为主要研究对象，分析了与热处理以及冷加工工艺有关的微结构、成分与微区成分以及固溶氢对高温水中氧化膜特性和应力腐蚀开裂性能影响的综合效果及规律。采用专用装置研究了扩散氢对690TT合金传热管腐蚀的影响，无氢试样外层氧化膜为富含铁和镍的尖晶石型氧化物颗粒，有氢试样外层氧化膜为片状或针状富铬氧化铬型氧化物，扩散氢影响了外层氧化物的形态和组成而加速690合金的氧化。设计装置已获批新型专利，适用于模拟蒸汽发生器传热管实际工况下的腐蚀行为测试。固溶氢加速316L不锈钢在高温水中的局部氧化行为，并与冷加工一起协同促进应力腐蚀开裂。高温水中未充氢308L中铁素体的耐蚀性显著由于奥氏体，但充氢后铁素体的耐蚀性显著降低，固溶氢加速应力腐蚀开裂。在52M对接焊焊接金属焊接稀释区出现Ni和Cr含量的降低以及Fe含量的升高，并发现在晶界出现Cr的显著贫化、Ni的显著富集以及少量的Fe元素富集，氢能通过加速Fe原子的扩散而导致合金-氧化物界面出现局部腐蚀凹陷，碳化物分布与晶界成分对应力腐蚀开裂起到主要作用。镍基合金堆焊层也得到了类似的结果。固溶氢对合金中不同元素扩散的差异导致对演化膜和应力腐蚀开裂行为产生显著的影响，基于裂纹尖端应变速率和准固态瞬态氧化动力学的协同作用，通过引入有氢存在时的氧化速率常数，对氢影响合金氧化动力学以及腐蚀开裂过程的综合效应进行了解析与分析。