核级304L不锈钢钎焊接头组织及耐腐蚀性能研究

核级304L不锈钢与BNi-7钎料真空钎焊接头存在晶间腐蚀行为,但工艺与钎缝耐腐蚀性能的关系尚未得到充分研究。为充分评估压水堆燃料组件结构件中不锈钢真空钎焊接头的晶间腐蚀和应力腐蚀敏感性,降低腐蚀失效风险,采用定量金相方法分析了钎缝中的化合物相含量,采用硫酸-硫酸铁法和双环动电位再活化(DL-EPR)法评价了钎缝耐晶间腐蚀性能,并采用高温高压水应力腐蚀裂纹扩展试验评价了钎缝的耐应力腐蚀性能。结果表明,钎缝中化合物相含量越高,耐晶间腐蚀性能越好。且钎缝在高温高压水中存在明显的应力腐蚀开裂行为,但其与钎焊工艺的关系尚需进一步试验研究。     

316NG不锈钢和合金800在含有硫酸盐及氯化物的高温水中应力腐蚀破裂(英文)

用SSRT方法研究秦山PWR核电站主管道焊接的316不锈钢和蒸汽发生器传热管Incoloy-800合金的应力腐蚀破裂行为,应变速率均为4.2×10~(-60)/s。316SS的试验温度为315℃,介质为模拟离子交换树脂热分解产物的酸性硫酸盐溶液(几个ppm至1000ppmSO_4~(2-);Incoloy-800的试验温度为270℃,介质为模拟离子交换树脂的热分解产物的酸性硫酸盐溶液(几个ppm至1000ppm SO_4~(2-))以及硫酸盐和氯化物组合的溶液(1000ppm SO_4~(2-),2～1000ppm Cl~-)。结果表明,316SS在上述介质中对穿晶应力腐蚀破裂敏感,Incoloy-800合金在上述介质中对应力腐蚀破裂不敏感。     

316NG不锈钢和合金800在含有硫酸盐及氯化物的高温水中应力腐蚀破裂

用SSRT方法研究秦山PWR核电站主管道焊接的316不锈钢和蒸汽发生器传热管Incoloy-800合金的应力腐蚀破裂行为,应变速率均为4.2×10~(-6)/s。316SS的试验温度为315℃,介质为模拟离子交换树脂热分解产物的酸性硫酸盐溶液(几个ppm至1000ppmSO_4~(2-));Incoloy-800的试验温度为270℃,介质为模拟离子交换树脂的热分解产物的酸性硫酸盐溶液(几个ppm至1000ppm SO_4~(2-))以及硫酸盐和氯化物组合的溶液(1000ppm SO_4~(2-),2～1000ppm Cl~-)。结果表明,316SS在上述介质中对穿晶应力腐蚀破裂敏感,Incoloy-800合金在上述介质中对应力腐蚀破裂不敏感。     

核级304L不锈钢钎焊接头组织及耐腐蚀性能研究

核级304L不锈钢与BNi 7钎料真空钎焊接头存在晶间腐蚀行为,但工艺与钎缝耐腐蚀性能的关系尚未得到充分研究。为充分评估压水堆燃料组件结构件中不锈钢真空钎焊接头的晶间腐蚀和应力腐蚀敏感性,降低腐蚀失效风险,采用定量金相方法分析了钎缝中的化合物相含量,采用硫酸 硫酸铁法和双环动电位再活化（DL EPR）法评价了钎缝耐晶间腐蚀性能,并采用高温高压水应力腐蚀裂纹扩展试验评价了钎缝的耐应力腐蚀性能。结果表明,钎缝中化合物相含量越高,耐晶间腐蚀性能越好。且钎缝在高温高压水中存在明显的应力腐蚀开裂行为,但其与钎焊工艺的关系尚需进一步试验研究。     

316NG焊接接头在含氯离子的高温高压水中应力腐蚀开裂研究

通过慢应变速率拉伸(SSRT)试验和高温电化学相结合的方法,研究外加电位对奥氏体不锈钢316NG焊接接头在含氯离子的高温高压水中应力腐蚀开裂(SCC)倾向的影响。试验结果表明:退火态316NG焊接接头SCC敏感性随外加电极电位升高而增大,且存在一个介于+50~+100 mV[相对标准氢电极(vs.SHE)]之间的SCC临界电位;低于该电位时,SCC敏感性较小,无明显沿晶开裂,仅断口边缘处存在少量穿晶开裂,随电极电位变化不明显;高于该临界电位时,SCC敏感性急剧增加,并出现明显的沿晶开裂。此外,高温Ar和腐蚀性低(电极电位≤50 mV)的环境中,焊接接头的断裂为力学主导的塑形开裂,其与焊接接头的硬度分布密切相关,硬度越低,越容易断裂;强腐蚀性(电极电位50 mV)环境中,焊接接头的断裂为腐蚀主导的脆性开裂;显然,焊缝及热影响区的SCC敏感性高于母材。     

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。     

国产690合金与321不锈钢异种金属焊缝的应力腐蚀行为研究

利用慢应变速率试验,采用非标准的漏斗状试样,对国产690合金与321不锈钢异种金属焊接部位(包括690合金热影响区、焊缝、321不锈钢热影响区)在100 mg/L Cl~(-1)除O_2条件下和100 mg/L Cl~(-1)饱和O_2条件下的应力腐蚀行为进行研究。并通过慢应变速率应力-位移曲线和断口形貌对微观组织、氯离子、氧含量对于材料的应力腐蚀(SCC)的影响进行分析。结果表明:690合金热影响区在100 mg/L Cl~(-1)除O_2条件下不易发生SCC,在100 mg/L Cl~(-1)饱和O_2条件下表现出一定的SCC倾向;321不锈钢热影响区在2种条件下均表现出明显的SCC倾向;690合金热影响区的粗大晶粒不利于塑性变形的晶粒间相互协调,导致了热影响区SCC的倾向增大。     

600合金在含Ti缓蚀物质碱性环境中的开裂行为

中间退火的600合金暴露在高温碱性溶液中有两种敏感的开裂形式，即晶间腐蚀（IGA）和晶间应力腐蚀（IGSCC）。这两种开裂模式的发生都有确定的电化学电位区。从电化学分析和表面分析方法得出的结论说明，在两种开裂发生的电位下金属表面发生了化学过程。这一特征为理解腐蚀开裂发生的机制提供了重要线索，并常常被用于研究能减缓600合金碱性开裂的物质。     

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。     

321-52M-690异种金属焊接接头的应力腐蚀开裂行为研究

采用慢应变速率拉伸(SSRT)和高温电化学相结合的方法,研究了外加电位对321-52M-690异种金属焊接接头在含Cl-高温高压水中应力腐蚀开裂(SCC)倾向的影响规律。结果表明,在300℃、50ppm Cl-环境下,焊接接头的SCC敏感性随电极电位(-700~+100mV)的升高而增大,且存在一个介于0~+50mV(vs.SHE)之间的临界电位Ecrit。当电极电位低于Ecrit时,焊接接头的SCC敏感性较小,SCC敏感性指数ISSRT基本在40%左右,断裂形式为外力主导的塑性开裂;当电极电位高于Ecrit时,ISSRT急剧增加至70%以上,断裂形式为腐蚀主导的脆性开裂。试样断裂位置均位于硬度最低的321母材处,表明在321/690异种金属焊接接头中321母材对SCC最为敏感,故进一步探讨了321不锈钢的应力腐蚀开裂行为和机理。     

Intergranular stress corrosion cracking initiation model of austenite stainless steels used in BWRs for optimized water chemistry control

A calculation model on intergranular stress corrosion cracking (IGSCC) initiation time of materials used in boiling water reactors (BWRs) has been developed to evaluate effectiveness of water chemistry control for mitigation of the IGSCC. The model was composed of four terms which determine passive film break time: (1) a chemical term based on electrochemical corrosion potential (ECP) and impurity concentration; (2) a mechanical term based on strain rate; (3) a material term based on sensitization; and (4) an irradiation term based on acceleration of corrosion by γ-rays and neutron irradiation. The contribution of the chemical term in the passive film break was calculated based on a deterministic local corrosion model. Then, the local corrosion model was modified by adding mechanical acceleration of the film rupture to treat the IGSCC phenomenon. The model could reproduce the behavioral tendency seen in the slow strain rate tensile test on high carbon contents with sensitization heat treatment (for example, 620°C × 24 h). Under BWR operating conditions, IGSCC initiation time could be extended by a factor of 5 by lowering the electric conductivity from 1.0 to 0.06 μS/cm. If the ECP was reduced below the critical potential by a mitigation method, the IGSCC initiation time was predicted to become sufficiently long for pipings and components.     

Influence of δ phase precipitation on the stress corrosion cracking resistance of alloy 718 in PWR primary water

The negative influence of δ phase on the intergranular stress corrosion cracking (IGSCC) resistance of alloy 718 is commonly taken for granted. In addition, δ phase formed at low temperature (about 1023 K) do not present the same characteristics than the one formed at higher temperatures (from 1173 to 1273 K). The aim of the present study is then to understand how δ phase precipitation could enhance crack initiation in alloy 718, whatever the form of δ phase is. For that purpose, several heat treatments leading to δ phase precipitation were realized on two alloy 718 heats, one sensitive to IGSCC and the second not. Specific slow strain rate tensile tests carried out on thin tensile specimens in simulated PWR primary medium at 633 K conclusively prove that δ phase has no effect on the intrinsic sensitivity to intergranular crack initiation of tested heats.     

Segregation effects on the corrosion behaviour of a phosphorus-doped AISI type 304L stainless steel

The effect of grain boundary (GB) segregation on intergranular stress corrosion cracking (IGSCC) in hot water environments at 150°C and 250°C was studied in a P-doped AISI type 304L stainless steel. The extent of segregation was measured by an exposure test in boiling 5 N HNO₃ + 8g/L K₂Cr₂O₇ solution as well as by a potentiostatic etch test at +1325 mV (SHE) in 5 N H₂SO₄ solution. Although GB segregation was detected in all the aged specimens, IGSCC was shown by only the specimens aged for 550°C/1000 h. The results suggest that it is the GB chromium depletion, rather than the segregation of phosphorus at the GBs, that controls IGSCC of stainless steels in the hot water environments studied.     

Evaluation of stainless steel pipe cracking: Causes and fixes

This paper discusses (1) studies of impurity effects on susceptibility to intergranular stress corrosion cracking (IGSCC), (2) intergranular crack growth rate measurements, (3) finite-element studies of the residual stresses produced by induction heating stress improvement (IHSI) and the addition of weld overlays to flawed piping, (4) leak-before-break analyses of piping with 360° part-through cracks, and (5) parametric studies on the effect of through-wall residual stresses on intergranular crack growth behavior in large diameter piping weldments. The studies on the effect of impurities on IGSCC of Type 304 stainless steel show a strong synergistic interaction between dissolved oxygen and impurity concentration of the water. Low carbon stainless steel (Type 316NG) appear resistant to IGSCC even in impurity environments. However, they can become susceptible to transgranular SCC with low levels of sulfate or chloride present in the environment. The finite-element calculations show that IHSI and the weld overlay produce compressive residual stresses on the inner surface, and that the stresses at the crack tip remain compressive under design loads at least for shallow cracks.     

Materials aspects of BWR plant life extension

Considerable experience with plant equipment performance in nuclear power stations has indicated that the principal factors limiting the life of BWRs and PWRs are materials related. Specifically, for LWRs it is known that these materials issues generally include parameters related to stress corrosion cracking, corrosion fatigue, wear and radiation embrittlement. Not only do these parameters affect and limit the actual useful design life of plant components but also affect the plant's operating availability. In all these cases, the elimination or control of one or more of these critical parameters should improve the plants availability and significantly extend the useful service life.In the present paper, research performed to address the intergranular stress corrosion cracking (IGSCC) area is described. Specific emphasis is placed on Type 304 stainless steel which has suffered IGSCC in piping in the heat-affected-zone (HAZs) adjacent to the welds in the BWR primary system. Research has developed and qualified a number of techniques which address the three necessary conditions for IGSCC in BWRs: (1) sensitized microstructure, i.e., chromium depletion at the grain boundaries during welding; (2) over yield tensile stress; and (3) oxygenated (200 ppb) high temperature (288Another potential life-limiting IGSCC phenomenon for certain components, irradiation assisted stress corrosion cracking (IASCC) of stainless steel exposed to a high neutron flux, is also discussed. Unlike the IGSCC, IASCC results in intergranular cracking of annealed material at low stress. Fortunately, preliminary research has indicated that some of the techniques utilized for IGSCC control in piping as well as new controlled impurity level stainless steel alloys may reduce the future potential IASCC concern to an insignificant level.     

Probability of crack-induced failure in BWR recirculation piping

The Lawrence Livermore National Laboratory (LLNL) has estimated the probability of double-ended guillotine break (DEGB) in the reactor coolant piping of Mark I boiling water reactor (BWR) plants. Two causes of pipe break are considered: crack growth at welded joints and the seismically-induced failure of component supports. For the former a probabilistic fracture mechanics model is used, for the latter a probabilistic support reliability model. This paper describes a probabilistic model developed to account for effects of intergranular stress corrosion cracking (IGSCC). The IGSCC model, based on experimental and field data compiled from several sources, correlates times to crack initiation and crack growth rates for Types 304 and 316NG stainless steel against material-specific ‘damage parameters’ which consilidate the separate effects of coolant environment (temperature, dissolved oxygen content, level of impurities), stress (including residual stress), and degree of sensitization. Application of this model to actual BWR recirculation piping shows that IGSCC clearly dominates the probability of failure in 304SS piping, mainly due to cracks that initiate within a few years after plant operation has begun. Replacing Type 304 piping with 316NG reduces failure probabilities by several orders of magnitude.     

Sensitization development in austenitic stainless steel piping

Pacific Northwest Laboratory and the Division of Engineering Technology of the US Nuclear Regulatory Commission are conducting a program to determine a method for evaluating welded and repair-welded stainless steel piping for light-water reactor service. Validated models, based on experimental data, are being developed to predict the degree of sensitization (DOS) and the intergranular stress corrosion cracking (IGSCC) susceptibility in the heat-affected zone (HAZ) of the SS weldments. The cumulative effects of material composition, past fabrication procedures, past service exposure, weldment thermomechanical (TM) history, and projected post-repair component life are being considered.     

The Influence of Dissolved Hydrogen on the Corrosion of Type 304 Stainless Steels Treated with Inhibitive Chemicals in High Temperature Pure Water

In order to assess the influence of dissolved hydrogen on the intergranular stress corrosion cracking (IGSCC) characteristics of Type 304 stainless steels treated with inhibitive chemicals, electrochemical corrosion potential (ECP) measurements and slow strain rate tensile (SSRT) tests were conducted in high temperature pure water. A number of thermally sensitized specimens were prepared and then pre-oxidized in a 288°C pure water environment with the presence of 300ppb dissolved oxygen for 360h. Most of the specimens were then separately treated with various inhibitive chemicals including powdered zirconium oxide (ZrO₂), powdered titanium oxide (TiO₂), and zirconyl nitrate [ZrO(NO₃)₂] via hydrothermal deposition at 150°C. Test environments with a dissolved oxygen concentration of 300ppb and various dissolved hydrogen concentrations at 288°C were created. Test results showed that the ECPs of the treated specimens were lower than that of the untreated one no matter what the dissolved hydrogen concentration was. In addition, IGSCC was observed on all specimens (treated or untreated) in all tested environments. However, the untreated specimen exhibited lower elongation, shorter failure time, and more secondary cracks on the lateral surfaces. It was therefore suggested that inhibitive chemicals such as ZrO₂, TiO₂, and ZrO(NO₂)₂ did provide a certain degree of enhancement in improving the mechanical behavior of the treated specimens and in prolonging IGSCC initiation times.     

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.     

Hydrogen and Hydrazine Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors, (VI) The Effect of Ammonia on Intergranular Stress Corrosion Cracking

Hydrogen and hydrazine co-injection into a boiling water reactor was considered as a new mitigation method of stress corrosion cracking (SCC). In this method, some amount of ammonia will be formed by the decomposition of hydrazine. The effect of ammonia on SCC susceptibility was studied over a wide range of electrochemical corrosion potentials (ECPs) in 288_C water by conducting slow strain rate technique SCC experiments (SSRTs). ECP was changed from _0:6V versus the standard hydrogen electrode (V(SHE)) to 0.1 V(SHE) by controlling dissolved oxygen concentration. Ammonia concentration was controlled to have values of 100 and 530 ppb. Similarly, sulfuric acid was injected to confirm the difference in the effect of injected chemical compounds on SCC susceptibility. The intergranular stress corrosion cracking (IGSCC) fraction, which was used as the index of SCC susceptibility, decreased with decreasing ECP for the case of no chemical injection. Sulfuric acid enhanced the IGSCC fraction. These data were in good agreement with literature data. On the other hand, ammonia at less than 530 ppb did not affect IGSCC fraction. It is expected that 51–280 ppb hydrazine and 0–53 ppb hydrogen will be injected into reactor water to mitigate SCC in BWRs. In the bottom region of the reactor pressure vessel, ECP and ammonia concentration will be _0:1 V(SHE) and 15–60 ppb, respectively. Under these conditions, ammonia did not affect SCC susceptibility. So SCC susceptibility will be mitigated by decreasing the ECP using hydrazine and hydrogen co-injection.