Experimental determination of flow accelerated corrosion in bend and straight sections of carbon steel primary coolant feeder pipes in pressurised heavy water reactor system

Abstract

Coolant systems in nuclear power plants undergo degradation by various corrosion processes. The carbon steel feeder pipes for the primary heat transport system of pressurised heavy water reactors (PHWRs) undergo extensive wall thinning by a degradation process, known as flow accelerated corrosion (FAC). Flow accelerated corrosion occurs under certain conditions of flow, water chemistry, pipe geometry and material composition. In PHWRs, the outlet feeders carry the heavy water (D₂O) coolant, which flows at velocities in the range of 10 to 20 m s^?1. The requirement of high flow velocity at high temperature and pressure to simulate the exact service hydrodynamic conditions, and the long time required to enable unambiguous observations regarding the extent of FAC, makes it difficult to undertake this study in the laboratory. This paper presents the design of an FAC module that enables the relatively rapid measurement of FAC to be undertaken in simulated PHWR primary heat transport conditions. The results of corrosion rate measurements and the attempts to explain the results using a computation fluid dynamics model are presented in this paper.     

Role of magnesium ions in reducing high temperature aqueous corrosion of carbon steel

Carbon steel is used as the primary heat transport system piping material in pressurized heavy water reactors. The carbon steel surfaces corrode during the high temperature operation. Enhanced wall thinning of the piping in locations of high velocity and neutron activation of corrosion products pose serious operational difficulties. Magnesium ion modified water chemistry resulted in significant reduction in the corrosion and corrosion release of carbon steel. The changes induced by magnesium ions in the various processes at the metal–oxide, oxide–solution interfaces, and subsequent restructuring of the oxide were identified by in situ electrochemical impedance spectroscopy.     

Thin layer activation for probing flow accelerated corrosion of carbon steel

Carbon steel feeder elbows and feeder piping of the primary coolant system of pressurized heavy water reactors undergo significant wall thinning due to flow accelerated corrosion. The rate of corrosion in certain locations exceeds the design corrosion allowance. Corrosion rate is dependent on several parameters like pH, temperature, dissolved oxygen, chromium content in material, pipe geometry, etc. Thin layer activation, utilizing the ⁵⁷Fe(p,n)⁵⁷Co reaction, enabled rapid online measurement of wall thinning in a simulated straight feeder pipe. Significant velocity and pH dependence was observed for corrosion rates and the same were compared with the predictions by Sanchez-Caldera model.     

Effect of a chemical decontamination procedure on the corrosion state of the heat exchanger tubes of steam generators

In the present work, a comprehensive study of the general corrosion state of the heat exchanger tubes originating from different steam generators (SGs) of the Paks NPP (Hungary) was carried out. While the passivity of the inner surface of 25 stainless steel specimens was investigated by voltammetry, the morphology, chemical and phase compositions of the oxide-layer formed on the surface were analyzed by SEM-EDX, Conversion Electron Mössbauer spectroscopy (CEMS), and X-ray diffraction (XRD) methods. The systematic studies have revealed that corrosion characteristics (corrosion rate, thickness and chemical composition) of the surfaces observed in the long run are strongly dependent on the decontamination history of steam generators. Specifically, some adverse effects (general attack, formation of “hybrid” layer with accelerated corrosion rate and great mobility) have been detected with applying the AP-CITROX decontamination procedure. Process restrictions and modifications to minimize corrosion damages should be defined, and there is ample data available now for utilities to select a CITROX based process for particular application.     

Characterisation of the oxide layer on carbon steel during hot conditioning of primary heat transport systems in heavy-water reactors

The corrosion layer formation on carbon steel during hot conditioning of primary heat transport systems of pressurised heavy-water reactors has been characterised using ex-situ methods (gravimetry, electron microscopy, X-ray diffractometry). In addition, the electric and electrochemical properties of the corrosion layers have been followed in-situ by voltammetry and electrochemical impedance spectroscopy during exposure to simulated hot conditioning water chemistry. The corrosion layer formed has been found to be a bilayer oxide of the inverse spinel type. The impedance data have been quantitatively interpreted using the Mixed-Conduction Model for oxide films allowing for the estimation of certain kinetic parameters at the compact layer/electrolyte interface. The obtained results point out to the fact that the electric and electrochemical properties of the carbon steel are determined by the processes in a thin n-type semiconductor layer and at its interface with the electrolyte.     

锅炉高压蒸发器管泄漏原因分析

高压蒸发器管发生泄露。采用化学成分、金相、拉伸与能谱等方法对其材质进行鉴定,结果表明材质不是蒸发器管腐蚀泄露的原因。腐蚀形貌与射线检查结果表明:泄漏点主要发生在焊缝附近的热影响区,通过现场工况调查并结合未泄露管化学清洗前后形貌、酸洗模拟试验与内壁沉积物分析,表明内壁腐蚀特征与停用时积水情况有明显对应关系,酸洗过程不会对金属基体造成明显腐蚀损伤,腐蚀主要发生在化学清洗前;采用停用腐蚀模拟试验对现场工况进行还原,其结果与现场实际管子的腐蚀特征基本一致。因此,基建期间部分管子内局部有积水和污物导致发生停用氧腐蚀是本次蒸发器管泄露的主要原因。     

Mechanism of improved corrosion resistance of type 304l stainless steel,nitric acid grade,in nitric acid environments

Materials for critical components in nuclear fuel reprocessing plants are required to have low corrosion rates and long designed life because access for repairs is not possible. Stainless steel type 304L, nitric acid grade (NAG), is the new material suitable for such applications. It has guaranteed low corrosion rates and is not susceptible to intergranular corrosion (IGC) in nitric acid environments. The corrosion behavior of type 304L stainless steel, NAG, and type 304L stainless steel, commercial purity (CP), in nitric acid environments is investigated in detail. Studied are: microstructural mapping in the three directions (longitudinal, long transverse, and short transverse), effect of sensitization heat treatment, resolution annealing and sensitization heat treatment for the as-received and cold-worked samples of the two varieties on the resultant microstructures. The anodic polarization characteristics along the three directions for both varieties in 1N HNO₃ are compared. The susceptibility of both varieties to end grain corrosion in 9N HNO₃ + 1 g Cr⁺⁶/liter boiling solution is assessed, and microstructural examination of the exposed sample is carried out to compare the degree of end grain corrosion. Their susceptibility to IGC due to segregation of impurity elements to grain boundaries is also compared. It is shown that controlled microstructure (fine grain size, retained cold work, and discrete precipitation at grain boundaries) along with controlled chemical composition is responsible for improved corrosion resistance of the NAG variety. The NAG variety has much less susceptibility to corrosion along the long- and short-transverse directions and, therefore, less susceptibility to end grain corrosion. The means and consequences of controlling chemical composition are also discussed.     

Predicting crack growth rate vs. temperature behaviour of Type 304 stainless steel in dilute sulphuric acid solutions

The coupled-environment fracture model for intergranular stress corrosion cracking of Type 304 stainless steel in boiling water reactor primary (water) heat transport circuits containing relatively pure water has been extended to incorporate the effects of sulphuric acid additions to the coolant and to include thermal activation of the crack tip strain rate. These extensions allow comparisons to be made between theoretically estimated and experimentally determined crack growth rates (CGRs) over a considerable temperature range after calibration at a single temperature. The model predicts, in agreement with experiment, that the CGR passes through a maximum with increasing temperature at a temperature of about 180 °C. The temperature dependence of the CGR is attributed to the competing effects of temperature on the thermally activated processes that occur at the crack tip and the properties (including ECP and conductivity) of the external environment.     

某核电站上充泵转轴的腐蚀原因

某电站上充泵马氏体不锈钢转轴在化学清洗后表面出现局部腐蚀,为了查明局部腐蚀发生的原因,开展了现场检查和模拟试验。结果表明:在操作不当的情况下,马氏体不锈钢的表面易发生缝隙腐蚀;在模拟酸性溶液中,马氏体不锈钢的自腐蚀电位远远低于奥氏体不锈钢的,存在严重的电偶腐蚀倾向。在缝隙腐蚀和电偶腐蚀的共同作用下,马氏体不锈钢的泵轴出现了局部腐蚀。     

Long-term trends in the corrosion state and surface properties of the stainless steel tubes of steam generators decontaminated chemically in VVER-type nuclear reactors

At some VVER-type pressurized water nuclear reactors (Russian-type PWR) different versions of the so-called AP-CITROX method have been widely used for the chemical decontamination of the heat exchanger tubes of steam generators. In the period of 2000–2007, within the frame of a joint-project dealing with the comprehensive investigation of the corrosion state of the steam generators of the Paks Nuclear Power Plant, Hungary, effects of the AP-CITROX chemical decontamination procedure on the corrosion and surface characteristics of the heat exchanger tubes have been studied. These studies provide evidences that some adverse features (formation of a “hybrid” layer with accelerated corrosion rate and great mobility) can be detected after 1–3 years of applying the AP-CITROX procedure. The present work is a continuation of the above program and focused on the long-term trends in the corrosion state and structure of protective oxide-layer grown on the decontaminated surfaces. The results of electrochemical (voltammetric), surface analytical (SEM–EDX, CEMS) and mobility (ICP–OES) studies have revealed that (1) some beneficial changes in the corrosion characteristics, mobility and chemical composition of the inner surfaces of decontaminated heat exchanger tubes can be observed in the long run, and (2) the passivity of the oxide-layers formed on decontaminated surfaces of steel tubes exhibits favorable tendency after 4–7 years under normal operation conditions.     

反应堆一回路系统去污工艺及其对结构材料腐蚀的影响

采用化学去污工艺可降低反应堆一回路冷却系统周围辐射场.总结了近年来反应堆一回路冷却系统去污工艺和去污试剂对结构材料的腐蚀影响的研究成果,并建议后续研究方向.     

Effects of cooling rate and annealing treatment on sensitization of austenitic-ferritic duplex stainless steel

Duplex stainless steel has higher corrosion resistance and better mechanical properties than conventional type 300 series stainless steel. The corrosion behavior of duplex stainless steel is strongly dependent on the ratio, shape, size and distribution of austenite and ferrite phase in the microstructure. The relationship between the microstructure and the corrosion behavior of the duplex stainless steel was studied. For this purpose, the duplex stainless steel samples were solution heal treated at 1150°C followed by either cooling at various rates (water quenching, air coooling, furnace colling with door opened and door closed) to 820°C and then water quenching to room temperature, or quenching to room temperature and annealing heat treatment at 840°C for various lengths of time. A double loop electrochemical polentiodynamic reactivation (EPR) test was carried out to examine the effect of various cooling procedures or annealing treatment on the sensitization of duplex stainless steel. The grain size, shape, and distribution of the two phases were examined under microscope. From the test results, the relationships were discussed among heat treatment, electrochemical properties and microstructure.     

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。     

Characterization of Corrosion Product Layers from CO2 Corrosion of 13Cr Stainless Steel in Simulated Oilfield Solution

The influence of temperature and flow rate on the characterization and mechanisms of corrosion product layers from CO₂ corrosion of 13Cr stainless steel was carried out in simulated oilfield solution. Cyclic potentiodynamic polarization method as well as weight loss tests in autoclave were utilized to investigate pitting corrosion behavior at various temperatures. Weight loss tests were performed at 100 and 160 °C under dynamic and static flow conditions. At the same time, the significant pitting parameters such as E _corr, E _pit, E _pp, ∆E, and I _pass in cyclic polarization curves at various temperatures were analyzed and compared for revealing the pitting behavior of 13Cr stainless steel. The surface measurement techniques such as SEM, XRD, and XPS were used to detect the corrosion product layers. The results showed that both temperature and flow rate had significant effects on characterization of corrosion product layers or passive films formed on 13Cr stainless steel in CO₂ corrosion system. At high temperature, lots of pits were formed at the localized corrosion areas of metal surfaces. Corrosion rates under the condition of 5 m/s were higher than those under the static condition regardless of the test temperatures.     

页岩气输送用转角弯头内腐蚀减薄原因分析

目的针对某页岩气输送平台用转角弯头发生严重内腐蚀减薄的问题开展原因分析,明确腐蚀类型及机理,指导防腐处理,提高转角弯头的服役安全性。方法以页岩气输送用转角弯头为研究对象,针对转角弯头内腐蚀减薄行为开展基础研究,通过宏观观察及尺寸测量分析内腐蚀的腐蚀形貌及分布,并进行理化检测、微观观察、物相分析,探究腐蚀产物,综合分析转角弯头内腐蚀减薄的原因。结果宏观分析发现,转角弯头内壁外弧侧与中性区过渡区域有壁厚发生突变而产生的腐蚀台阶,最大壁厚减薄率达63.4%。电子显微形貌与金相分析表明,弯头内壁的腐蚀坑呈纵深发展,逐层剥离,腐蚀产物疏松、形貌多样,且可观察到细菌形貌。腐蚀产物的能谱及XRD分析发现,管体内壁的腐蚀产物主要是Fe S、Fe2O3、Fe CO3等,内腐蚀可能与CO2、H2S、SRB等有关。结论弯头腐蚀减薄是硫酸盐还原菌(SRB)-CO2腐蚀协同作用的结果,SRB的存在对CO2腐蚀起催化作用。此外,Cl-对腐蚀产物膜的破坏和弯头外弧侧的冲刷加速了腐蚀作用。建议确定SRB细菌来源,以便有效投放杀菌剂,同时对管线内壁定期进行清理,避免菌落长期附着于管体内壁。此外,建议添加多级气液分离装置,严格控制气相中的含水量。     

TP321不锈钢无缝钢管晶间腐蚀问题研究

对影响TP321不锈钢无缝钢管耐晶间腐蚀性能的因素进行分析,从化学成分、金相组织、工艺流程、热处理制度等方面研究可能导致耐蚀性降低的原因,并采取措施消除不利影响。通过控制化学成分、调整热处理制度以及优化脱脂工艺等一系列改进措施,使TP321不锈钢无缝钢管晶间腐蚀试验一次合格率稳定在95%以上水平。     

激光熔覆含SiC金属陶瓷涂层显微组织特征

采用连续ＣＯ２激光器在钢表面进行熔覆含ＳｉＣ金属陶瓷涂层。对不同工艺条件下激光熔属层组织结构、成分与硬度进行了研究，对试验结果和凝固过程进行了传热分析。     

Korrosionsuntersuchungen an Reaktorwerkstoffen in strömender Natriumschmelze bei hohen Temperaturen

Research on corrosion of reactor materials in flowing sodium melt at high temperatures Corrosion tests on materials for fuel elements of a high-capacity sodium-cooled reacto, in sodium flowing at a rate of 0.5 metres per second at temperatures ranging from 500 to 600°C have been carried out in a circulation apparatur's of high-grade steel, containing some 80 litres of sodium melt. The electromagnetically circulated sodium is fine-cleaned internally, the purity being measured in a shunt circuit. The degree of sodium corrosion of high-grade austenitic steels and nickel-based alloys is relatively low and obeys different laws from those known from the reaction with vanadium-based alloys, which is greatly influenced by low oxide contents in the sodium. The results are in broad agreement with the corrosion rates measured in the USA., France and Britain.     

A case study of a pipe line burst in the Mihama Nuclear Power Plant

On August ninth, 2004, at the Mihama nuclear power plant in Japan, a carbon steel pipe line carrying hot water under high pressure burst, killing five workers employed in maintenance job nearby. The major cause for the burst was presumed to be the pipe wall thinning due to so‐called erosion‐corrosion which had been revealed to be the cause of similar accident in 1986 at the Surry nuclear power plant in USA. However, the opinion of this author is that the wall thinning must have been caused through differential flow velocity corrosion accompanied with passivation, the origin of which was irregularities in the fluid flow velocity as well as in the pipe wall temperature. A rationale is presented that is consistent with this hypothesis. A countermeasure to this type of corrosion is also proposed.     

Evaluation of an EMAT–EC dual probe in sizing extent of wall thinning

An electromagnetic acoustic transducer (EMAT)–eddy current (EC) dual probe is developed to assess wall thinning. With a view to developing compatible EMAT and EC operating modes, probe’s capability to size width and depth of wall thinning is quantitatively evaluated. Experiments carried out on austenitic stainless steel plates with various groove sizes show that the EMAT mode of the dual probe functions best when thinning is shallow but wide, whereas the EC mode functions best when thinning is deep but narrow. Mode complementarity indicates that this dual probe offers greater reliability in wall thinning monitoring.