Contribution of research to the understanding and mitigation of environmentally assisted cracking in structural components of light water reactors

Environmentally assisted cracking (EAC) is a potential threat to the safety and integrity of water-wetted components in operating water-cooled nuclear power plants. Two forms of EAC are commonly distinguished, depending on the form of loading contributing to damage: stress corrosion cracking (SCC) and corrosion fatigue. A number of instances of in-service degradation due to EAC have occurred in operating plants worldwide, often leading to unplanned plant outages. Understanding the causes of EAC is essential to minimise the loss of plant availability due to its occurrence and to avoid the possibility of catastrophic failure, for example, if a crack grew to a critical size in a major pressure boundary component. This paper will describe some examples of these phenomena in the main materials of construction of pressure boundary and other critical components in pressurised and boiling water reactors (BWRs). Over the last several decades, substantial research programmes have been carried out in a number of laboratories worldwide, aimed at further understanding of the processes leading to EAC to manage occurrences in plant and minimise future failures. Selected areas of research on EAC in light water reactor environments are discussed. Corrosion fatigue in low-alloy pressure vessel steels was the subject of considerable attention in the 1980s and early 1990s because of its potential threat to pressure vessel integrity and the publication of data, suggesting that there is a major influence of environment on fatigue crack growth in some laboratory tests. The author’s research provided insight into the conditions under which the major environmental effects occur and contributed to the development of an ASME Code Case for pressurised water reactor (PWR) conditions which provided a means of screening based on steel sulphur content and loading conditions. More recently, the research focus in this area has moved to austenitic stainless steels, again providing support to Code Case development and furthering mechanistic understanding. A recent review of knowledge gaps for EPRI provides a basis for future research on environmentally assisted fatigue and will inform the development of new assessment methodologies. A key area of the current study concerns differences in loading conditions between specimens in laboratory tests and plant components subject to transient loading. In the case of SCC, stainless steels have shown the greatest propensity to cracking in BWRs, while Alloy 600 has been a major cause of in-service failures in PWRs, both on the primary side, as recognised by Coriou in the early 1960s, and in secondary environments where a number of different corrosion-related failure processes have been identified. High-strength alloys, such as Alloy X-750 used for fastener applications, have also caused failures in both reactor types. For austenitic materials, SCC susceptibility is enhanced by irradiation, resulting in failures in core internals components. Ferritic stainless steels also undergo SCC under some specific circumstances but are generally more resistant than the lower chromium austenitic materials.     

核级不锈钢高温水腐蚀疲劳机制及环境疲劳设计模型

通过模拟核电高温高压循环水腐蚀疲劳实验,研究了国产核级不锈钢的环境疲劳损伤行为与失效机制;评价了影响不锈钢高温高压水疲劳寿命的环境和载荷等因素,建立了一个植入环境损伤效应的疲劳设计模型,给出了便于工程应用的核级不锈钢的环境疲劳设计曲线.     

高温高压水环境中腐蚀产物膜的研究现状

高温高压水是轻水核反应堆冷却系统的主要服役环境,反应堆压力容器、管道及蒸汽发生器等构件材料在高温水中的环境损伤是影响核电安全的重要因素.材料在高温水中形成的腐蚀产物膜是影响其服役稳定与环境失效的关键.本文介绍了高温高压水环境中不锈钢和镍基合金腐蚀产物膜的形貌、结构、影响因素及形成机制，并对当前研究中存在的主要问题进行了讨论.     

催化裂化装置膨胀节失效原因与措施

针对大连石化公司催化裂化装置膨胀节的失效，利用电子能谱(EDX)、SEM电镜扫描等手段对失效膨胀节进行检测分析，得出膨胀节失效的主要原因是膨胀节的工作温度正处于其敏化温度，膨胀节在各种残余应力和硫酸、连多硫酸等特定腐蚀介质共同作用下发生的应力腐蚀开裂导致的失效，并首次提出采用填料函结构来补偿冷热变形的解决方案，使用效果极好.      

The study for direct SCC fabrication in STS 304 pipe

ABSTRACT

Stress corrosion cracking is one of the aging phenomena for the major structure components in nuclear power plant. During the operation of a power plant, stress corrosion cracks are initiated and grown especially in dissimilar weldment of primary loop components. Among the three factors (susceptible material, residual stress, and corrosive environment) which make the SCC, the residual stress becomes a critical factor for stress corrosion crack when it is difficult to improve the material of the components and their environment under operating conditions. In this study, stress corrosion cracks were artificially produced on STS (stainless steel) 304 pipe itself by control of welding residual stress. The instrumented indentation technique and 3D finite element method (FEM) analysis (using ANSYS 12) were used to evaluate the residual stress values in the GTAW area. As the result of both FEM analysis and experiment, the stress corrosion crack was quickly generated and could be reproduced, and controlled by welding residual stress. Also non-destructive evaluation signals by Acoustic Emission will be discussed for the initiation and growth of SCC.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

核反应堆关键焊接结构应力腐蚀裂纹失效评估

以沸水堆堆芯围板焊接热影响区应力腐蚀开裂为例进行了裂纹扩展评估研究.采用有限元模拟与试验检测数据验证相结合的方法对焊接热影响区焊接残余应力场及应力腐蚀开裂扩展过程进行模拟.通过有限元方法计算了应力腐蚀开裂裂纹尖端应力强度因子,再根据应力腐蚀裂纹扩展试验数据获得裂纹尖端应力强度因子与裂纹扩展速率的关系,最终预测了沸水堆堆芯围板的使用寿命,解决了重大构件基于应力腐蚀开裂失效的安全评估问题.该研究过程为重大构件的腐蚀疲劳裂纹失效的安全使用提供了可借鉴的方法.     

A comparative study on corrosion failure analysis of hearing aid devices from different markets

The vulnerability of hearing aid devices to corrosion is critical due to their exposure to various kinds of ionic contaminants from the human body, such as sweat, sebum, and so forth, and harsh climatic conditions such as high temperature, humidity, and atmospheric pollutants. The device failure rate will vary depending upon the type of geographical location at which the device is used and root cause failure analysis is a crucial tool to understand the effect of geographical location on corrosion failures. In this study, field failed hearing aid devices from Europe, the United States, and Japan markets were investigated using a scanning electron microscope and elemental dispersive spectroscopy to locate failure mechanisms and causes. Information from the analysis was used for statistical analysis to compare the performance of the devices in the three markets based on failure percentage and failure probability for different parts and components. Solder terminals, battery contacts, light-emitting diodes, and wireless-link coil showed consistent and high failure probability across all three markets, whereas a higher failure rate for microphones was found in Europe and the United States market as compared to the Japanese market. The majority of the components corrosion failures occurred in the presence of high chloride ions from human sweat and the atmospheric conditions, whereas potassium hydroxide from the leakage of Zn–air battery was found as the additional cause for microphone failure.     

堆内构件304L焊接件在除氧和氯离子环境中的应力腐蚀开裂研究

采用高温高压慢应变速率拉伸试验方法(SSRT),研究了在除氧环境下不同浓度氯离子对304L焊接件在模拟一回路高温高压硼锂水介质中氯致应力腐蚀开裂的影响。结果表明:当氯离子浓度为1 mg/L时,其各项力学性能指标与惰性氮气空白样一致,表明在低浓度氯离子除氧条件下,304L焊接件没有应力腐蚀敏感性。随着氯离子含量(1~50mg/L)的增加,304L焊接件应力腐蚀敏感指数变化呈增加趋势。断口侧面没有观察到明显的裂纹,氯离子浓度越低,断口缩颈现象越明显,表明主要是机械断裂。氯离子浓度较高时,棱角部分出现微裂纹源,可能与棱角部分应力更为集中相关。所有样品的断裂位置都在焊缝和热影响区,在焊缝和热影响区发生了巨大的形变,离断口越近,变形越严重。焊接部位是304L堆内构件中薄弱环节,应该成为应力腐蚀开裂和其他性能检测的重点部位。     

The use of failure analysis as a preventative quality tool

The materials and processes used to fabricate electronic components have significant effects on their ul-timate reliability. By examining a small number of samples, high risk types of materials and poor work-manship can be flagged, either excluding them from production use,or using them to drive corrective actions. The tools of failure analysis are well suited to this approach, even though the sample components have technically not failed at all. Both nondestructive and destructive types of analysis can be used. By focusing on comparative device analysis between available suppliers, it is possible to find the best in class components for use in high- reliability applications such as automotive electronic modules.     

Evaluation of SCC susceptibility of supermartensitic stainless steel using slow strain rate tests

Abstract

The susceptibility to chloride stress corrosion cracking (Cl-SCC) of supermartensitic stainless steel (SMSS) was evaluated at different temperatures through slow strain rate tests. In order to evaluate the Cl-SCC susceptibility the reduction in area and the time to failure ratio were considered. It is clear that all specimens tested in the chloride solution exhibited a generally high resistance to chloride SCC. However, according to the slow strain rate tests results, at lower temperature (5°C) the susceptibility to Cl-SCC increases compared to the other temperatures studied; also an increase in the corrosion rate was observed as the temperature decreased according to electrochemical impedance spectroscopy measurement. The corrosion mechanism observed on the material under dynamic stress was a combination of high activation resistivity with a diffusive process on the metallic interface given by a thin film from a protector layer of chromium oxides. This mechanism generates a high corrosion resistance and therefore good performance to chloride induced SCC.     

Stress corrosion cracking for 316 stainless steel clips in a condensate stabilizer

In one of the gas processing facilities in Abu Dhabi, UAE; a case of 316L stainless steel material failure occurred in the fractionating column due to stress cracking corrosion twice in a cycle of less than 2 years. This paper studies the stress corrosion cracking behavior of the 316L stainless steel in an accelerated corrosion environment and compares it with a higher corrosion resistant nickel alloy (Inconel 625). The experimental work was designed according to ASTM G36 standard, the samples were immersed in a boiling magnesium chloride medium which provided the accelerated corrosion environment and the tested samples were shaped into U‐bend specimens as they underwent both plastic and elastic stresses. The specimens were then tested to determine the time required for cracks to initiate. The results of the experimental work showed that the main mode of failure was stress corrosion cracking initiated by the proven presence of chlorides, hydrogen sulfide, and water at elevated temperatures. Inconel 625 samples placed in the controlled environment showed better corrosion resistance as it took them an average of 56 days to initiate cracks, whereas it took an average of 24 days to initiate cracks in the stainless steel 316L samples. The scanning electron microscopy (SEM) micrographs showed that the cracks in the stainless steel 316L samples were longer, wider, and deeper compared to the cracks of Inconel 625.     

乏燃料贮存环境中Al-B4C复合材料的腐蚀行为研究

通过实验室环境模拟沸水堆(BWR)和压水堆(PWR)乏燃料的贮存环境对不同表面处理试样开展全浸腐蚀实验。采用SEM和EDS观测腐蚀微观形貌和分析腐蚀产物,采用XRD测试腐蚀试样的物相,并通过测试试样干重得到腐蚀动力学曲线。结果表明,材料中的Al和微量元素Mg,Fe是引起腐蚀的主要因素;抛光处理试样在PWR贮存液中时,因Fe的影响出现4种微观腐蚀形貌,耐蚀性较差,而在BWR贮存液中时生成Al(OH)₃保护膜,耐蚀性较好;阳极氧化处理试样在PWR和BWR贮存液中均腐蚀增重,腐蚀产物能封闭氧化膜孔洞,抗腐蚀性能最好。     

注水泵密封环腐蚀原因分析

海上平台所用注水泵由于长期服役,导致注水泵出口叶轮环磨损严重,出现腐蚀断裂现象,通过对渤海海上平台的注水泵密封环进行拆解,发现腐蚀部位均发生在固定螺丝处;本文从理化性能及腐蚀因素方面对注水泵密封环腐蚀原因进行了全面分析。     

不锈钢截止阀波纹管组件腐蚀开裂失效分析

目的针对某炼油厂波纹管截止阀中双层不锈钢304波纹管组件发生开裂,造成截止阀失效的现况,通过失效分析,寻找腐蚀开裂的原因。方法对失效开裂的不锈钢截止阀双层波纹管组件进行外观检查,采用金相显微镜和直读光谱仪分别对失效组件的金相组织和化学成分进行分析,用电子显微镜观察组件断口形貌与特征。结果波纹管组件外层管壁断口上可以观察到解理面和解理台阶,并且能看到腐蚀产物的存在,这是奥氏体不锈钢发生应力腐蚀断裂的典型特征;内层管壁断口上有韧窝存在,属于机械断裂。金相组织和化学成分分析表明,波纹管组件使用的不锈钢材质合乎设计与使用要求。导热油介质检测结果显示,导热油中含氯55 mg/kg,总硫含量350 mg/kg,有害离子含量较高。结论双层波纹管组件的内层管壁和外层管壁的失效机制不同:外层管壁是由Cl~-导致的应力腐蚀开裂;内层管壁是由于外层管壁失效引起波纹管组件失稳,造成抗压强度和寿命急剧降低,在应力的作用下出现韧性断裂。建议降低导热油中有害离子含量,使用耐蚀性更好的材质。     

水环泵叶轮腐蚀失效分析

应用电子探针和金相技术检测了腐蚀失效的水环式真 空泵叶轮,化验了水环泵的水样,分析了CO2对叶轮的腐蚀机制,认为腐蚀原因是电化学 腐蚀及Cl-点蚀,同时提出了预防措施.     

Corrosion behaviour of reactor coolant system materials in nuclear power plants

The materials used for the pressure‐retaining parts of reactor coolant system components in light water reactor nuclear power plants have to meet special requirements in terms of their mechanical properties, workability and in‐service performance. Corrosion issues play an important role in connection with plant operating conditions. While giving consideration to the specific service environment of the reactor whether a pressurized or boiling water reactor – the materials used for the individual components and the water chemistries employed in the various systems are selected such that metal loss due to general corrosion will remain very low. Thus the materials used in light water reactor plants exhibit a high general resistance to corrosion for their specified service conditions, material conditions and mechanical loads. However, under certain operating conditions other corrosion mechanisms may be found to induce damage. This paper uses data from the literature, published results of national and international research programs, information on damage which has actually occurred world‐wide and experience gained by Framatome ANP GmbH (former Siemens/KWU) in this field as a basis for discussing these mostly localised corrosion phenomena in terms of “classical” corrosion systems. Aspects associated with irradiation and its effects are not considered. Suitable remedial actions are, however, addressed wherever these are of relevance. The materials considered comprise unalloyed and low‐alloy steels, austenitic chromium‐nickel steels as well as high‐nickel steels and nickel‐base alloys which are exposed to the reactor coolant environment of boiling water reactor or pressurized water reactor plants, including materials investigated in corresponding water environments simulated in the laboratory.     

不当焊接引起的奥氏体不锈钢结构应力腐蚀破裂事例

针对某厂一套316L设备多次沿焊接热影响区(HAZ)发生的断裂事故,进行失效分析,认为该设备的断裂属于SCC,焊接接头的质量差是造成断裂的根本原因,并提出了提高焊接质量,防止SCC的办法.     

Effect of prior machining deformation on the development of tensile residual stresses in weld-fabricated nuclear components

Austenitic alloy weldments in nuclear systems may be subject to stress- corrosion cracking (SCC) failure if the sum of residual and applied stresses exceeds a critical threshold. Residual stresses developed by prior machining and welding may either accelerate or retard SCC, depending on their magnitude and sign. A combined x- ray diffraction and mechanical procedure was used to determine the axial and hoop residual stress and yield strength distributions into the inside- diameter surface of a simulated Alloy 600 penetration J- welded into a reactor pressure vessel. The degree of cold working and the resulting yield strength increase caused by prior machining and weld shrinkage were calculated from the line- broadening distributions. Tensile residual stresses on the order of +700 MPa were observed in both the axial and the hoop directions at the inside- diameter surface in a narrow region adjacent to the weld heat- affected zone. Stresses exceeding the bulk yield strength were found to develop due to the combined effects of cold working of the surface layers during initial machining and subsequent weld shrinkage. The residual stress and cold work distributions produced by prior machining were found to influence strongly the final residual stress state developed after welding.     

Pitting of carbon steel in the synthetic concrete pore solution

Pitting corrosion is a possible mode of failure of the carbon steel overpack of the Belgian supercontainer concept for the isolation of high-level nuclear waste (HLNW). However, no firm experimental data are currently available to estimate the probability of failure over the extended storage time (100,000 years). Extensive work shows that passivity breakdown results from the condensation of cation vacancies (CVs) at the metal/barrier layer (m/bl) interface, in response to the absorption of Cl⁻ into oxygen vacancies at the surface of the barrier oxide layer. The CVs migrate across the bl to the m/bl interface where they condense, leading to the separation of the bl from the metal. The resulting blister prevents the growth of bl into the metal and dissolution results in blister rupture, marking a passivity breakdown event. Stabilization via differential aeration produces a potentially damaging, stable pit. We review our work on passivity breakdown and the nucleation of pits on P355 QL2 carbon steel in high-pH aqueous media typical of concrete pore solution, with emphasis on the mechanistic aspects. We conclude that failure of the carbon steel overpack containing the HLNW over a storage horizon of 100,000 years is improbable.     

海上某油田井下潜油电泵的失效分析与控制建议

针对海上某油田油井潜油电泵发生的腐蚀穿孔情况,在充分了解该井维修历史的基础上和流体性质的基础上,通过材质、泵壳壁厚、力学性能以及电镜分析等方法最终得出了电泵失效穿孔的原因,针对腐蚀原因提出有针对性的控制建议。