锆合金的碘致应力腐蚀开裂研究(英文)

研究了不同热处理状态的Zr-2和Zr-4合金在不同浓度的碘介质及实验温度下的应力腐蚀开裂(SCC)行为。并对不同织构取向的试样在350℃下进行了蠕变实验,蠕变实验的载荷值选择与SCC实验相对应的一系列典型载荷。用扫描电子显微镜观察了断口特征,用透射电子显微镜和光学显微镜检查了材料的显微组织,用X-光衍射仪测定了锆合金的织构,分析讨论了材料状态、实验温度、碘浓度以及蠕变对锆合金碘致应力腐蚀行为的影响。     

锆合金表面缺陷引起氢致延迟开裂行为的研究

核反应堆用锆合金构件在服役过程中会发生氢致延迟开裂（HIDC）而失效,构件表面的微缺陷是否会引起HIDC是值得研究的问题。本文采用真空电子束焊接方法制备表面有微缝隙缺陷的样品,以研究这类微缝隙缺陷在400℃过热蒸汽中腐蚀以及在350℃高压水中热循环处理过程中的行为。由于这类缺陷处会形成尖劈状的氧化膜并镶嵌在金属中,在氧化膜前端将形成应力集中和应力梯度区,引起氢的扩散、富集和氢化物析出,即使样品中原先没有残余应力,也没有受到外加应力的作用,也会发生HIDC导致裂纹扩展而开裂。因此,在设计和加工制造核反应堆堆芯中锆合金的各种结构件时,需要重视如何避免锆合金构件表面可能生成这种缺陷的问题。     

304奥氏体不锈钢在硝酸溶液体系中的电化学腐蚀行为

采用极化曲线测量法对304不锈钢在硝酸溶液体系中的电化学耐蚀性能进行了测试,分别研究了在硝酸溶液中添加硝酸盐、草酸、乙酸、柠檬酸等成分对304不锈钢电化学腐蚀行为的影响。结果表明,在硝酸溶液中,硝酸盐的加入能够抑制不锈钢的电化学腐蚀,而草酸能够显著增强溶液对不锈钢的电化学腐蚀能力,在硝酸和草酸溶液体系中加入1g/L柠檬酸后,自腐蚀电流由6.02μA/cm~2上升到22.8μA/cm~2,对电流腐蚀有较明显的促进作用。     

氢及氢化物对锆合金疲劳裂纹扩展速率的影响及估算

采用紧凑拉伸（CT）试样．研究了不同氢含量的Zr-4及Zr-Sn-Nb合金在室温下疲劳加载裂纹扩展（dα/dN）行为．用扫描电镜观察了断口形貌。结果表明，氢含量对疲劳裂纹扩展速率影响微弱，疲劳断裂受通常的裂纹萌生、稳态扩展和瞬间断裂机制控制。根据疲劳裂纹扩展机理．导出了裂纹扩展门槛值△Kth的关系式．得出了一个描述疲劳裂纹扩展速率油（dα/dN）与材料性能常数之间的关系式，该关系式可用于预测材料的疲劳裂纹扩展速率。用锆合金实验数据对（dα/dN）预测表达式进行验证．结果表明，预测值与实验值吻合较好。     

304奥氏体不锈钢在不同浓度硝酸中的电化学行为

采用动电位极化曲线测量、开路电位测量等技术,研究了304奥氏体不锈钢在不同浓度硝酸溶液中的电化学腐蚀行为,并对304奥氏体不锈钢在硝酸溶液中的电化学反应历程进行了探讨。结果表明：304奥氏体不锈钢在硝酸溶液中具备不锈钢典型的极化曲线特征,有多个钝化区和过钝化区;硝酸浓度升高促进不锈钢表面钝化膜的生成,使开路电位向正电位方向移动,降低了硝酸溶液对不锈钢的腐蚀倾向,同时,随着硝酸浓度的升高,不锈钢的点蚀电位升高,提高了不锈钢耐点蚀能力;在硝酸溶液中,不锈钢的腐蚀速率同时受到酸度和硝酸根浓度的影响,二者相互矛盾,导致硝酸浓度对腐蚀速率的影响呈不规律性。结果表明,在0.5 mol/L硝酸中,不锈钢的腐蚀速率最高。     

不锈钢堆焊层在Cl-溶液中的腐蚀性能研究

采用浸泡腐蚀试验方法,研究了不锈钢堆焊层材料在Cl^-溶液中的腐蚀情况,并通过金相显微镜、扫描电子显微镜、能谱分析观察表面形貌。研究表明,室温条件下堆焊层材料未发生任何腐蚀。在高温条件下,Cl^-的存在诱导了点腐蚀的发生,且随着Cl^-浓度的增加,点腐蚀加剧;较高浓度的Cl^-可导致缝隙内金属元素Cr的流失,缝隙腐蚀加深;应力腐蚀裂纹有沿晶开裂的特征,应力腐蚀敏感性随Cl^-浓度的增加有提高的趋势。     

二(2-乙基己基)磷酸酯从硝酸体系中萃取Mo(Ⅵ)

用低浓缩铀靶代替高浓缩铀靶辐照进行~(99)Mo的生产是一个必然的趋势,但采用低浓缩铀靶辐照后裂变体系的组成可能发生改变,从而影响~(99)Mo的分离提取过程。为此,本工作以低浓缩铀辐照后溶解的模拟溶液为研究对象,在U(Ⅵ)大量存在的情况下,考察了二(2-乙基己基)磷酸酯(P_(204))从硝酸体系中萃取Mo(Ⅵ)的行为,重点研究了不同Mo(Ⅵ)浓度下萃取时间、萃取剂浓度、硝酸浓度、温度、其他主要元素(Cs(Ⅰ),Zr(Ⅳ),Y(Ⅲ),Nd(Ⅲ),Al(Ⅲ))等因素对萃取的影响。实验结果表明,不同Mo(Ⅵ)浓度下,P_(204)-磺化煤油对硝酸体系中Mo(Ⅵ)的萃取行为相似;在相比为1时,φ=10%P_(204)-磺化煤油对Mo(Ⅵ)即有较好的萃取效果;硝酸浓度不大于2mol/L时分配比随着硝酸浓度的增加而减少,但硝酸浓度进一步增大时对萃取无显著影响;萃取反应的ΔH和ΔG均为负值,表明该萃取是一个常温下能自发进行的放热反应;溶液中U(Ⅵ)和本工作考察的其它主要元素存在及其浓度的改变不会显著影响P204对Mo(Ⅵ)的萃取行为,且采用P_(204)可将Mo(Ⅵ)与Y(Ⅲ)、Nd(Ⅲ)、Al(Ⅲ)选择性地分离。     

The first step for delayed hydride cracking in zirconium alloys

Two models for delayed hydride cracking (DHC) in zirconium alloys are distinguished by their first step:

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The loading of a crack induces hydride precipitation. The hydride is postulated to create a hydrogen concentration gradient, where the bulk concentration is greater than that at the crack tip. This concentration gradient is taken as the driving force for diffusion of hydrogen to the crack tip, and subsequent hydride growth. This model is called the precipitate first model (PFM).

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The tensile stress at the crack tip induces a gradient in chemical potential that promotes the diffusion of hydrogen to the crack tip. Hydrides form if the hydrogen concentration reaches the solubility limit for hydride precipitation. The mechanism is postulated to create a hydrogen concentration gradient, where the bulk concentration is lower than that at the crack tip. The gradient in chemical potential is taken as the driving force for diffusion of hydrogen to the crack tip, and subsequent hydride growth. This model is called the diffusion first model (DFM).

The second model, DFM, is developed. This model is shown to describe the main features of the experimental observations of DHC, without invoking new phenomena, such as reduction in the solubility limit for precipitation of hydride, as required by the PFM.     

Japanese Contributions to Fields of Predicting and Combating Environmentally Assisted Cracking (EAC) of Steels in LWR Water

Two major currents of the EAC relevant activities in Japan are reviewed in some chronological manner. One is the studies on the acceleration of fatigue crack growth in LWR. The work was first pioneered early in Japan, and extensive collaborative works have followed interacting with the international activities, which have yielded a clear views on the potential issues of the RPV structural steels used in the domestic NPPs. Another is the development of materials and techniques to combat the stress corrosion cracking of the structural materials in LWR water environment. Extensive collaboration among the government agencies, utilities, industries and academic societies lead the issues to a dramatic solution in rather short period. Both of those two aspects are stressed to have generic significance in the present and future nuclear technology.     

Effect of nitrate on corrosion of austenitic stainless steel in boiling nitric acid solution containing chromium ions

The oxidation behavior of chromium and the corrosion behavior of austenitic stainless steel in boiling nitric acid solution containing highly concentrated nitrates were investigated using UV-visible spectroscopic measurements, Raman spectral measurements, immersion tests, and potentiodynamic polarization measurements. The oxidation rate measurement of chromium from Cr(III) to Cr(VI) was performed by 1 M boiling nitric acid solution containing each highly concentrated nitrates: Al(NO₃)₃, Nd(NO₃)₃, Ca(NO₃)₂, Mg(NO₃)₂, and NaNO₃ as a simulant of uranium nitrate in uranium concentrator in reprocessing plants. As a result, the rate of chromium oxidation was different depending on the added nitrates even at the same nitric acid concentration. In addition, the oxidation rate of chromium was increased with increasing the calculated partial pressure of nitric acid in consideration of the hydration of cation of nitrates. Furthermore, the corrosion rate of type 310 stainless steel was accelerated by the solution having a high chromium oxidation rate containing nitrates. These results indicated that the acceleration of the corrosion rate in the solutions depending on the oxidation rate of chromium, and the rate is affected by the salt-effect of nitrates.     

Redox behavior of chromium on the corrosion of austenitic stainless steel (R-SUS304ULC) in 8 M nitric acid solution

We investigated the relationship between the corrosion of austenitic stainless steel (R-SUS304ULC) and the redox behavior of chromium in nitric acid solution under three conditions – boiling at atmospheric pressure, boiling at reduced pressure, and non-boiling – to evaluate the effects of the boiling phenomenon. In the Cr(III) oxidation rate measurement, about 20% of the Cr(III) was oxidized to Cr(VI) under atmospheric-pressure boiling at 384 K after 168 h. Under non-boiling conditions at 373 K, Cr(VI) was not observed. However, about 3% of Cr(III) oxidation was observed at 373 K with reduced-pressure boiling. Thermodynamic calculations showed that the abundance of Cr(VI) in boiling nitric acid was increased over that in non-boiling nitric acid, even at the same temperature. These results show that Cr(III) is more likely to oxidize under boiling conditions than non-boiling conditions. On the other hand, in the corrosion tests, we found that the apparent reduction rate of Cr(VI) as a result of corrosion of R-SUS304ULC was more than 10 times greater than the Cr(III) oxidation rate. Therefore, the corrosion of R-SUS304ULC appears to be dominated by the rate of Cr(VI) reduction in nitric acid solution regardless of the boiling or non-boiling conditions.     

新锆合金碘致应力腐蚀断口研究

利用扫描电镜和EDXA对新锆合金碘致应力腐蚀的断口进行了形貌观察和成分分析。在断口表面发现腐蚀产物、二次裂纹、沿晶开裂和穿晶准解理开裂等应力腐蚀断裂特征,并观察到锆合金碘致应力腐蚀的“沟槽”特征。起裂区为沿晶开裂,在裂纹扩展阶段,开裂以穿晶为主。断口上腐蚀产物的成分主要是氧和锆,局部准解理开裂区域可检测到碘。去应力退火试样上发现了平行轧面的深沟,且沿晶开裂不明显。试验温度升高,断口上的腐蚀产物增多,而且沿晶开裂更容易。碘分压不仅影响腐蚀产物层的厚度,而且碘分压较高时沿晶开裂容易发生。     

固溶态控氮不锈钢在高温水中的应力腐蚀破裂

采用高温电化学测控和慢应变拉伸实验方法,研究了304NG超低碳控氮不锈钢（固溶态）在250℃高温水中的应力腐蚀破裂（SCC）与电极电位和水中Cl^-浓度的关系,并与316LN控氮不锈钢对比：结果表明,在不同环境下两种不锈钢的SCC敏感性随电极电位的升高而升高,并且存在一个临界电位Ec,当电位高于该Ec时,才发生SCC。该临界电位Ec随水中Cl^-浓度升高而下降,即发生SCC的环境范围扩大。304NG钢在含5mg／L Cl^-的250℃高温水中的Ec处于0～＋200mV标准氢电极（SHE）之间,更高时发生穿晶型SCC,表明该Cl^-浓度下只有在高氧（高电位）环境中才能发生SCC。当Cl^-浓度升高到50mg／L时,Ec降到-700mV（SHE）以下,表明该浓度下即使完好除氧（低电位）也可能发生SCC。316LN的SCC抗力高于304NG,其在含5mg／L Cl^-的高温水中的Ec位于＋300～＋400mV（SHE）之间,主要是穿晶型SCC。     

冷变形316不锈钢在高温水中的应力腐蚀开裂行为

对不同冷变形量的核级316和316L不锈钢在高温水中的应力腐蚀开裂(SCC)行为进行了研究。通过试验,对溶解氧、氯离子和温度对裂纹扩展速率的影响进行了深入探讨和分析。试验结果显示,溶解氧和氯离子能明显加快材料的应力腐蚀开裂速率。当水化学条件一致时,325℃时的裂纹扩展速率较288℃时的裂纹扩展速率高。     

电感耦合等离子体质谱法测定核级海绵锆中痕量铀

为保障核级海绵Zr的质量,应用电感耦合等离子体质谱法(ICP-MS)测定海绵Zr中痕量U。样品经HNO3-HF快速溶解后,从同位素质量数的选择、内标同位素选择、基体的影响等方面进行分析。结果表明,以232Th作为内标,可有效校正基体效应及仪器漂移引起的干扰;在样品中加入浓度分别为0.20、1.00、1.50μg/g的U标准储备溶液,其加标回收率均在94%~99%之间,相对标准偏差最大为5.1%,合成相对标准不确定度为5.9%,方法的检出限为0.001μg/g。采用该方法对实际样品U含量进行测定,测定结果满足核级海绵Zr对U含量的控制要求。     

镍基合金825在超临界水中的腐蚀性能研究

研究了镍基合金825在550 ℃/25 MPa、600 ℃/25 MPa和650 ℃/25 MPa超临界水（SCW）中的应力腐蚀开裂敏感性,以及在超临界650 ℃/25 MPa、次临界290 ℃/15.2 MPa水中的均匀腐蚀性能。通过慢应变速率拉伸实验得到了相应的应力-应变曲线,结果表明,随温度的升高,825的机械强度和塑性逐渐下降;实验后试样的SEM图像表明,825在3种工况下的应力腐蚀开裂倾向大小关系为600 ℃＞550 ℃＞650 ℃。825在SCW条件下的腐蚀实验表明,其腐蚀增重大致符合幂函数生长规律;而其在次临界条件下的腐蚀增重变化却呈现出先减后增的特征。     

围板连接螺栓的辐照促进应力腐蚀裂纹研究

在压水堆(PWR)中,堆芯部件围板连接螺栓的材质是奥氏体不锈钢,长期处于强中子辐照区域.当辐照超过一定限值后,即使应力水平很低,但由于辐照过程中的缺陷、相转变、晶界偏析以及应力变化等原因,材料的塑性逐步丧失,辐照促进应力腐蚀裂纹(IASCC)随之发生.使用高纯度材料、控制水质和改进连接结构可以有效地减缓围板螺栓辐照促进应力腐蚀裂纹的产生.     

Fracture mechanics investigation regarding the effects of cracks on the structural integrity of a BWR-core shroud

As a consequence of core shroud intergranular stress corrosion cracking (IGSCC) detected in the course of inservice inspections, a fracture mechanics analysis was carried out to evaluate the effects of postulated cracks on the structural integrity. In this study, critical crack sizes and crack growth were calculated. Due to the comparatively low stress acting on the core shroud during normal operation, the residual stresses in the welds make up the major proportion of the tensile stresses responsible for IGSCC. In order to consider residual stresses of the lower core support ring welds, a finite element analysis was performed at MPA Stuttgart using the FE-code ANSYS. The crack growth computed on the basis of USNRC crack growth rates da/dt demonstrated that crack growth in depth direction increases quickly at first, then retards and finally comes almost to a standstill. The cause of this ‘quasi-standstill’ is the residual stress pattern across the wall, being characterized by tensile stresses in the outer areas of the wall and compressive stresses in the middle of the wall. Crack growth in circumferential direction remains more or less constant after a slow initial phase. As the calculation of stress intensity factors K_I of surface flaws under normal conditions demonstrated, a ‘lower bound’ fracture toughness value is only exceeded in the case of very long and deep surface flaws. It can be inferred from crack growth calculations that under the assumption of intergranular stress corrosion cracking, the occurrence of such deep and at the same time long flaws is unlikely, regardless of the initial crack length. Irrespective of the above, the calculated critical throughwall crack lengths, which were determined using a ‘lower bound’ fracture toughness value, demonstrated that even long throughwall cracks will not affect the component’s integrity under full load. Moreover, it can be concluded from the studies of crack growth that—assuming intergranular stress corrosion cracking—a sufficiently long period will elapse before a crack which has just been initiated reaches a relevant size. Therefore, it can be stated that these cracks will likely be detected during periodic inservice inspections.