Flow-accelerated Corrosion Research Articles

Hydrodynamic parameters affecting flow-accelerated corrosion in elbows

Flow-accelerated corrosion (FAC) is a serious degradation mechanism affecting carbon-steel piping in the secondary coolant systems of nuclear power plants. Many researchers have investigated the correlation between wall thinning and hydrodynamic parameters; however, their relationship remains unclear. In this study, the hydrodynamic parameters influencing FAC in elbow sections were investigated for identifying the locations highly susceptible to FAC. Accelerated FAC tests and computational fluid dynamics (CFD) analyses were conducted on a specimen containing elbow sections. The results revealed that the side of the elbow pipe was the most susceptible to FAC because it experienced the most severe wall thinning. The velocity and vorticity vectors obtained from the CFD analysis were decomposed into axial, circumferential, and radial components within a cylindrical coordinate system to evaluate their effects. We found that all velocity and vorticity components contribute to the rate of wall thinning induced by FAC. Therefore, a combination of velocity and vorticity components should be used as hydrodynamic parameters to characterize FAC-induced wall thinning. Specifically, the circumferential velocity and axial vorticity were identified as dominant parameters influencing FAC in the elbows.

Effects of fluid dynamics parameters on flow-accelerated corrosion at elbow of carbon steel pipeline

Flow-accelerated corrosion (FAC) has always posed a significant threat to the safe operation of the secondary circuit in nuclear power units. In this study, we investigated typical carbon steel elbow pipe sections susceptible to FAC failure using fluid dynamics software to analyze the hydrodynamic characteristics at varying inlet velocities (2 m s−1, 4 m s−1, and 6 m s−1). The distribution of the FAC rate was monitored in real time using an array electrode. The results revealed that the outermost side of the elbow pipe section was the most susceptible location to FAC. By comparing different fluid dynamic parameters with the FAC rate, we identified radial velocity as an effective parameter for characterizing the FAC rate. Additionally, we established an empirical formula for predicting flow-accelerated corrosion in elbow pipe sections using the least squares method. The implications of this research are pertinent to the design and operation of pipelines in nuclear power plants.

Effect of Fusion Boundary Microstructure on Flow-Accelerated Corrosion Cracking.

Flow-accelerated corrosion (FAC) preferentially attacks the downstream heat-affected zone of the root-pass weld in steam pipe systems. A detailed characterization identifies the fusion boundary as the initiation location for the attack. Alloying elements are found depleted along the weld fusion boundary, and multiple welding thermal cycles and repetitive austenite-to-ferrite phase transformations result in an increased proportion of grains with Goss {110}<001> texture along the fusion boundary. The synergistic effects of chemical segregation and the Schmid factor may contribute to the preferential initiation of FAC cracks along the root weld fusion boundary, making it the weakest link for FAC attack in steam pipe girth welds.

Noise-robust pipe wall-thinning discrimination system using convolution recurrent neural network model

Pipe wall-thinning is a phenomenon whereby the thickness of pipes in a nuclear power plant decreases over time owing to extended operational years. This thickness reduction is caused by various long-term thermal aging mechanisms such as Flow-Accelerated Corrosion, Liquid Droplet Impingement Erosion, cavitation, and flashing. Reducing the thickness of the secondary system pipes to the point of rupture can lead to severe human casualties and significant economic losses. Consequently, domestic power plant operators regularly manage the power plant pipes. Ongoing research focuses on the identification and management of pipe wall-thinning. However, previous studies have encountered problems in accurately judging the decrease in pipe wall-thinning in the presence of noise. To overcome this, Convolutional Neural Network (CNN) models based on image feature analysis have been used. This approach allows for the differentiation of pipe wall-thinning feature from small-sized noise in a single image. However, there were difficulties in making accurate judgments for large-sized noise that resembled the feature of pipe wall-thinning. This paper aims to analyze the limitations of the current pipe wall-thinning evaluation methods and to achieve accurate pipe wall-thinning discrimination through time-series analysis of continuous pipe wall-thinning data. The proposed method employs a Convolutional Recurrent Neural Network (CRNN) model, integrating the Recurrent Neural Network(RNN) model with the CNN model. The image feature of the pipes, extracted using CNN, are utilized as inputs for the RNN. This enables the observation of how the image features of the pipes change over time. This feature differentiates from the time-series feature of noise that occurs suddenly. Through this method, the paper proposes a new approach for effectively identifying the gradual decrease in pipe wall-thinning, enabling precise assessment of pipe wall-thinning progression.

Corrosion sample streamlining to enhance the performance of the rotating cage testing setup

Computations of the flow inside the rotating cage, a widely used testing methodology for flow-accelerated corrosion studies, are presented in this study. The main objectives here are to elucidate the flow behaviour inside the cage geometry that is recommended by the American Society for Testing and Materials (ASTM) and to investigate the effects of streamlining the shape of the cuboid sample recommended by ASTM. The streamlined sample profile considered here comprises a semi-circular leading edge, a triangular trailing edge and a flat mid-section with uniform thickness. It is found that the low value of the ratio of radius of rotation to sample length in the standard geometry makes it difficult to completely eliminate flow separation. Nonetheless, the flow is attached to the streamlined samples at locations of interest to corrosion measurements which inflicts more shear on the samples’ surfaces compared to the flow around the conventional blunt cuboid samples normally used. Moreover, we discuss the unsteadiness of the samples’ wakes and how the flow recirculation behaves differently, depending on both the sample shape and the flow Reynolds number.

IDENTIFICATION OF WAVY FLOW SUB-REGIMES USING POWER SPECTRUM OF LASER DOPPLER VELOCIMETRY SIGNALS

The formation of a slug happens when the roll waves on the interface of two-phase flow undergo constructive interference. Slug flowing with high momentum results in flow-accelerated corrosion (FAC) leading to cracks in pipes. The leakages from such cracks are hazardous to industry. In order to avoid such hazards, a detailed analysis is required for the transition of smooth stratified flow to slug flow. This transition happens through different wavy flow sub-regimes. Previous studies in literature used visual observation and image processing to determine sub-regimes of wavy flow. However, visual observation alone is not sufficient to differentiate between the different sub-regimes of wavy flow. In the present study the power spectrum of the velocity signals measured using laser Doppler velocimetry (LDV) is used in addition to visual observations to develop a precise wavy-flow sub-regime map. Experiments are conducted in a two-phase flow test rig by varying liquid superficial Reynolds number in the range of 1685 and 6830 and gas superficial Reynolds number between 393 and 9824. The transition to wavy flow is observed much earlier in the present study compared to that reported in literature based solely on visualization methods. The present paper successfully utilized power spectrum analysis to accurately and precisely categorize wavy flow sub-regimes, including the creation of a flow subregime map demarcating "3D large-amplitude waves (3DLA)," "2D small-amplitude waves (2DSA)," "3D small-amplitude waves (3DSA)," and "2D large-amplitude waves (2DLA)." The modified flow-pattern map developed based on power-spectrum analysis of velocity signals and the flow visualization is compared with the flow-pattern maps available in literature.

Effect of turbulent flow on the oxide properties of carbon steel in alkaline water

The purpose of this study is to investigate the changes in the morphology, chemical state, defect density, and impedance of oxides formed on SA 106 Gr.B carbon steel by turbulent water flow at the micro and atomic levels. A compact magnetite layer composed of multi-faceted particles is formed in stagnant water, while a porous magnetite layer composed of particles without a certain shape is formed in turbulent water. Ferrous ions are preferentially dissolved from the oxide in turbulent water, which increases the vacancies in the oxide lattice. An increase in the concentration of point defects is shown to be comparable to an increase in the corrosion rate of carbon steel. The flow-accelerated corrosion behavior is also well correlated with the electrochemical impedance parameters including the charge transfer resistance and capacitance. Our study provides a further understanding of carbon steel corrosion under turbulent water flow conditions.

Hydrodynamic design of the Separate Effect test facility for Flow-Accelerated Corrosion and Erosion (SEFACE) studies in liquid lead

Flow-accelerated corrosion and erosion (FACE) phenomena can be crucial for performance of structural elements in heavy liquid metal (HLM) cooled reactor systems. Existing experimental observations indicate that turbulent flow characteristic can affect FACE, but there is no quantitative data that can be used for model development and validation. Main recirculation pump impellers, which operate at high relative velocities and rotational flow conditions can be especially vulnerable to FACE. For comparison, the core internals operate at lower velocities and in axial flow conditions, but at higher temperatures and neutron fluence. Hence, systematic experimental data is needed to improve our knowledge on FACE phenomena. The Separate Effect Test Facility for Flow-Accelerated Corrosion and Erosion (SEFACE) is designed to obtain such experimental data including high relative velocities (up 20 ms−1) and high temperatures (400 to 550 °C) of liquid lead. This article focuses on the hydrodynamic design of SEFACE. The aim of the design is to achieve well defined flow conditions for experiments and ensure safe operation of the facility. First, we examine three design concepts (i.e., forced convection loop, rotating cylinder, and rotating disk) and motivate the choice of the rotating disk approach for SEFACE. Second, we discuss different design options, i.e., a confined rotor–stator test chamber and the unconfined rotating disk configuration. We used Reynolds-Averaged Navier Stokes (RANS) calculations to identify and solve the issues stemming from the high rotational speed. These include, for instance, lead free surface deformation, radial pressure buildup, and axial bending forces due to asymmetric test chamber. The CFD-derived torque and power predictions in rotor–stator and rotating disk systems are verified with selected empirical turbulent friction factor correlations or/and DNS calculations. We demonstrate that the developed hydrodynamic design of SEFACE solves identified issues and enables obtaining experimental data under well-defined flow conditions. The findings are deemed to also be applicable to the design of rotating disk-type FACE installations for other liquid mediums.

Assessment of flow-accelerated corrosion-induced wall thinning in SA106 pipes with elbow sections

A combination of flow-accelerated corrosion (FAC) tests and corresponding computational fluid dynamics (CFD) tests were performed to determine the hydrodynamic parameters that could help predict the highly susceptible location to FAC in the elbow section. The accelerated FAC tests were performed on a specimen containing elbow sections fabricated using commercial 2-inch carbon steel pipe. The tests were conducted at flow rates of 9 m/s under the following conditions: water temperature of 150 °C, dissolved oxygen <5 ppb, and pH 7. Thickness reduction of the specimen pipe due to FAC was measured using ultrasonic testing. CFD was conducted on the FAC test specimen, and the turbulence intensity, and shear stress were analyzed. Notably, the location of the maximum hydrodynamic parameters, that is, the wall shear stress and turbulent intensity, is also the same location with maximum FAC rate. Therefore, the shear stress and turbulence intensity can be used as hydrodynamic parameters that help predict the FAC-induced wall-thinning rate. The results provide a method to identify locations susceptible to FAC and can be useful for determining inspection priority in piping systems.

Geodesic equations for guided wave helical path separation for a pipe bend

The sections of pipe bends are susceptible to wall thinning by flow-accelerated corrosion due to the sudden change in the fluid flow direction and velocity. Compared to existing screening approaches, Guided Wave Tomography (GWT) has been demonstrated to provide more accurate information about the defect. In GWT an inversion problem is solved, in which a forward model is used to predict a synthetic dataset for a given physical model. Two-dimensional (2D) forward models are preferred for the inversion scheme due to the feasibility of combining these with tomographic algorithms. Since 2D models resemble the GW propagation of a plate, they do not consider the cyclic nature of the pipe. To overcome this limitation, a robust helical path separation was introduced in Huthwaite and Seher (2015) for a straight pipe. However, it relies on time-reversing the measured wave field ϕ(t) as a function of the propagation distance xs,r from the source to the receiver. Although ray-tracing and grid-based methods are capable of computing the distance xs,r for complex geometries such as the pipe bend, they are complex and time-consuming. Hence, in this paper a straight forward approach is proposed to compute xs,r based on the pipe geometry and the geodesic equations of a torus. Then the computed distance xs,r is used for removing the helical path trajectories. Compared to the distance obtained by using the Hilbert envelope, the results show that the geodesic distances preserve the information of the wavefront of interest, including the focusing effect and the scattered wave field from a Hann-shaped defect. The technique is applied to experimental data and demonstrated to separate the wavefront of interest.

In-situ electrochemical testing and fluid dynamics simulation of pipeline defects under flow accelerated corrosion

The pipeline defects of flow accelerated corrosion (FAC) on the corrosion behaviors and localized corrosion mechanisms on different locations of the depressions and the welded joint with weld reinforcements were investigated using wire beam electrode (WBE) techniques and fluid dynamics simulations. The results revealed that the swirling vortex inside the depression and the local jet impact by the fluid separation in the weld reinforcement can accelerate the localized corrosion. Also, the depressed upper and trailing edge regions and the weld toes on both sides of the weld reinforcement experience considerable pressure fluctuations, enhancing interfacial mass transfer processes and inducing cavitation for severe localized corrosion failure. Furthermore, the galvanic corrosion strength factor (g) indicated an enhanced susceptibility of localized corrosion in depression and welded joints with weld reinforcement.

Failure analysis of the collection pipeline at the outlet of air cooler in the distillation column

Severe leakage occurs in the collection pipeline at the outlet of air cooler in the distillation column, which leads to an unplanned shutdown. In order to reveal the root cause of the leakage, the corrosion process under oil–water two-phase flow in the horizontal-vertical pipeline was investigated by macroscopic observation, thickness measurement, microscopic analysis, and computational fluid dynamic (CFD) simulation. The results show that flow-accelerated corrosion (FAC) is the main reason for the leakage in the pipeline. The water volume fraction and wall shear stress are the crucial parameters in the corrosion process. The water volume fraction determines whether corrosion occurs by influencing the process of water wetting the wall of the pipeline. Fluid flow in the corrosion acceleration process is represented by the wall shear stress. The region of high water volume fraction and high wall shear stress is basically the same as the region of actual leakage. When the water cut inside the pipeline decreases by half, the corrosion risk of the pipeline is greatly reduced.

Effects of Cr and Mo contents on the flow-accelerated corrosion behavior of low alloy steels in the secondary side of pressurized water reactors

Carbon steels have been extensively used as structural materials in the secondary side of pressurized water reactors, which are susceptible to flow-accelerated corrosion (FAC). To overcome this issue, low-alloy steels containing 2.25Cr-1Mo (P22) have been introduced, as Cr and Mo are effective alloying elements in mitigating FAC. However, the behavior of these alloying elements in the secondary water chemistry is not well-understood, and therefore, there is a need to explore alternative materials that can address the issue of FAC. In this study, three model alloys were manufactured based on Ducreux's model on FAC rate to examine the effects of Cr and Mo on the corrosion behavior of low-alloy steels compared to that of commercial P22. Microstructure of P22 was ferritic/pearlite structure while the model alloys was ferritic/tempered bainite structure due to the existence of Mn and Si. The difference in microstructure led to the difference in hardness values, and elimination of Mo from the model alloys resulted in reduction in strain. The FAC rate was mainly influenced by the Cr content in the oxide layer rather than Mo. The passive film layer hinders the exposure of the inner layer to H2O in the solution, resulting in Cr3+ dissolution into the Fe oxide layer. The continuous passivation leads to the formation of two compact layers, one amorphous and one crystalline Cr, creating Fe oxide substitutes. It is also revealed that the solubility of Cr species is much lower than that of Fe species resulting in the enrichment of Cr in the outer layer, and thus the higher Cr content reinforces the passivity of the steels. The Fe-Cr alloy exhibited promising corrosion resistance, suggesting that it could be a potential substitute for Fe-Cr-Mo alloy.

Investigation of solid particle erosion‐related failures in once‐through steam generators (OTSGs) in oil sands in‐situ production: The limitations of the API RP 14E guideline in OTSG design or operational decision‐making

AbstractThe required steam for steam‐assisted gravity drainage (SAGD) oil sands operations is generated using a once‐through steam generator (OTSG) that is fed with relatively poor quality process water. Industries have reported possible solid particle erosion‐related failure in the OTSG boiler tubes because of the transport of precipitated dense inorganic particles. However, the presence of other damage mechanisms, e.g. corrosion, flow accelerated corrosion (FAC), often masks the evidence of erosive wear. Also, industries set an upper limit operating velocity for the OTSG using the API RP 14E guideline, which provides no quantitative erosion rates to determine an operating envelope. This study presents a computational fluid dynamics (CFD) analysis of erosion damage in a SAGD OTSG boiler tube. The results revealed that API RP 14E may not be an effective decision‐making tool for operating the OTSG system. For example, a 10% increase in velocity, even at conditions below the API RP 14E threshold, showed a decrease in the failure time of the boiler tubes by 40%–50%.

Inhibiting flow-accelerated copper corrosion under liquid jet impingement by utilizing nanobubbles

Flow-accelerated corrosion can be very destructive, leading to rapid loss of component efficiency and eventual failure of the system; however, finding inexpensive, effective, and chemically benign inhibitors is still challenging. This paper studied for the first time the use of nanobubbles/ultrafine bubbles as a “green” functional nanostructured material for inhibiting flow-accelerated corrosion under the impingement of a turbulent liquid jet. We examined the corrosion inhibition of the copper specimen for 5 h in the flow regime at a Reynolds number of from 6.1 × 103 to 3.0 × 104, with adding ∼6 × 107 bubbles/mL of air-nanobubbles to the tested liquid of 0.25% CuCl2 solution at 40 °C. We measured weight losses, analyzed microstructure using SEM, and acquired maximum erosion depths and roughness curves of tested copper specimens. We found that nanobubbles mitigated flow-accelerated corrosion under jet impingement. The inhibition effectiveness was more significant at higher jet velocity and longer test time, with up to 43.9 ± 0.1% and 69.5 ± 1.8% based on weight losses and erosion depths, respectively. Nanobubbles play a role in reducing wall shear stress on the copper surface, likely by generating bubble mattresses. We suggest that nanobubbles inhibit the corrosion in highly erosive/corrosive turbulent conditions where most inhibitors could not work well.

Microstructural Understanding of Flow Accelerated Corrosion of SA106B Carbon Steel in High-Temperature Water with Different Flow Velocities.

All light or heavy water reactors fabricated with carbon steels suffer from flow-accelerated corrosion (FAC). The FAC degradation of SA106B with different flow velocities was investigated in terms of microstructure. As flow velocity increased, the major corrosion type changed from general corrosion to localized corrosion. Severe localized corrosion occurred in the pearlite zone, which can be the prior location for generating pits. After normalizing, the improvement in microstructure homogeneity reduced the oxidation kinetics and lowered cracking sensitivity, causing a decrease in FAC rates of 33.28%, 22.47%, 22.15%, and 17.53% at flow velocity of 0 m/s, 1.63 m/s, 2.99 m/s, and 4.34 m/s, respectively. Additionally, localized corrosion tendency was decreased by reducing the micro-galvanic effect and tensile stresses in oxide film. The maximum localized corrosion rate decreased by 21.7%, 13.5%, 13.8%, and 25.4% at flow velocity of 0 m/s, 1.63 m/s, 2.99 m/s, and 4.34 m/s, respectively.

Condition Monitoring of Nuclear Equipment-Piping Systems Subjected to Normal Operating Loads Using Deep Neural Networks

Abstract Various fields in engineering explore the applicability of deep learning within condition monitoring. With the resurgence of nuclear energy due to electricity and carbon-free power generation demand, ensuring safe operations at nuclear power plants is important. Nuclear safety systems can undergo vibrations due to operating loads such as pump operations, flow-induced, etc. Safety equipment-piping systems experience degradation over the course of time due to flow-accelerated erosion and corrosion. Undetected degradation at certain locations can be subjected to a buildup of cyclic fatigue due to operational vibrations and thermal cycles. A condition monitoring framework is required to avoid fatigue cracking and for early detection of degraded locations along with the severity of degradation. This study aims to propose a condition monitoring methodology for nuclear equipment-piping subject to pump-induced vibrations during normal operations by designing a novel feature extraction technique, exploring parameters and developing a deep neural network, incorporating uncertainty in degradation severity, conducting a thorough investigation of predicted results to analyze erroneous predictions, and proposing strategic recommendations for “safe” pump operating speeds, as per ASME design criteria. Even with nondestructive testing, the detection of fatigue in pipes continues to be a difficult problem. Thus, this novel strategic recommendation to the operator can be beneficial in avoiding fatigue in piping systems due to pump-induced vibrations. The effectiveness of the proposed framework is demonstrated on a Z-piping system connected to an auxiliary pump from the Experimental Breeder Reactor II nuclear reactor and a high prediction accuracy is achieved.

Flow-accelerated corrosion mechanism of overlap and non-overlap zones in multi-track laser-clad nickel-based alloy coating

Flow-accelerated corrosion mechanism of overlap and non-overlap zones in multi-track laser-clad nickel-based alloy coating

Flow accelerated corrosion of X65 steel gradual contraction pipe in high CO2 partial pressure environments

Flow accelerated corrosion (FAC) behavior of X65 steel gradual contraction pipe in high CO2 partial pressure environments was investigated by in situ dynamic high pressure electrochemical measurements with Ag/AgCl reference electrode in FAC loop and microscopic characterization analysis. The corrosion rates in dynamic high CO2 partial pressure environments are obviously enhanced compared with those in dynamic atmospheric pressure and static high pressure environments. The FAC mechanism at gradual contraction pipe in high pressure environments exhibits significant distinction from that in atmospheric pressure environments. The variation of corrosion rates is associated with the hydrodynamics at gradual contraction pipe. The presence of high CO2 partial pressure under fluid flowing accelerates rupture of corrosion product scales.

Shale gas flow carrying liquid in gathering pipeline with different inclination angles

Shale gas reserves are abundant and widely distributed. As a green and clean energy source, it has tremendous development potential. Nonetheless, the shale gas transmission process is often accompanied by liquids. The gas transmission will transform to liquid-carrying gas transmission and liquid will accumulate in low-lying areas. These will accelerate pipeline corrosion and perforation, threatening pipeline safety and causing significant economic losses. In this paper, computational fluid dynamics (CFD) can be used to establish a transient model of liquid-carrying gas flow. The flow characteristics of liquid-carrying gas in the up-dip shale gas gathering pipelines are studied. The critical flow velocities at 10°, 20°, 30° can be determined 8.25 m/s, 9.5 m/s, 11 m/s. The critical flow velocity can provide a reference for discharging liquid efficiently in engineering. Combining the flow process with corrosion studies, we identified the main cause and the sensitive areas of flow-accelerated corrosion. Finally, we proposed recommendations to prevent pipeline accelerated corrosion. Thus, the flow-accelerated corrosion of shale gas pipelines can be minimized.