Flow-accelerated Corrosion Research Articles

The effect of flow velocity and material microstructure on FAC behavior and mechanism in simulated PWR conditions

In this study, combined with room-temperature electrochemical and hydrodynamic numerical simulation techniques, rotating disc tests in a high-temperature, high-pressure water environment were conducted to investigate the mechanisms of flow velocity and microstructure of SA106B on Flow Accelerated Corrosion (FAC). It has been demonstrated that the microstructure of SA106B influenced the corrosion potential and consequently the corrosion resistance. The normalization treatment changed the pearlite morphology from a segregated band to a diffuse island, which shifted the corrosion potential of SA106B in a positive direction, increased the polarization resistance, greatly reduced the local corrosion degree, increased the corrosion resistance, and noticeably slowed down the FAC rate of SA106B. On the other hand, the FAC rate increased linearly with an increase in flow rate, and its slope was related to the corrosion potential. The difference between the FAC rates of the as-received and normalized SA106B strongly depended on the flow velocity, and became more pronounced as the flow rate increased. When the flow velocity approached 4.34 m/s, the decrease degree of FAC rate of normalized SA106B compared to that of as-received SA106B increased dramatically, which may be related to the thickening and cracking reduction of Fe3O4 film on the normalized SA106B.

The semi-analytic solution to the magnetic field of frequency-band-selecting eddy current testing for local corrosion flaw in large diameter pipeline.

Local corrosion flaws (LCFs) widely exist in large diameter pipelines (LDPs) (e.g. steam, oil/gas pipeline) due to flow-accelerated corrosion, environmental corrosion, and material aging. Frequency-band-selecting eddy current testing (FBS-ECT), as an improved eddy current technique (ECT), has rich harmonic components in exciting signals as a pulsed eddy current technique (PECT), as well as more effective utilization of electromagnetic energy by selecting a frequency band. It is expected that FBS-ECT has promising applications in detecting and evaluating the LCFs in LDP. Therefore, a solution to the magnetic field of FBS-ECT for LCF detection in LDP is of great concern to reveal the features of the FBS-ECT signal and the parameters of the LCF. Extending our previous work, the semi-analytic expression of the magnetic flux density for FBS-ECT in regions 1 and 2 (where the magnetic sensor is located) is deduced according to the superposition principle and TREE, the computation accuracy is validated by numerical simulation; next, a semi-analytic solution to the magnetic field of FBS-ECT for LCF in LDP is proposed; then, the sampling distance in FBS-ECT is optimized by considering the magnitude of △Bz and the sensitivity of △Bz with the LCF depth; finally, a case study is carried out to verify the optimal strategy and find the mapping relationship between the radius/depth and the FBS-ECT signal. The proposed semi-analytic solution for the magnetic field of FBS-ECT is not only helpful to understand the theoretical relationship of the magnetic field and LCFs with higher computation efficiency but also lays a theoretical foundation to promote FBS-ECT to apply in the characterization of other flaws in metal components.

A Future with Machine Learning: Review of Condition Assessment of Structures and Mechanical Systems in Nuclear Facilities

The nuclear industry is exploring applications of Artificial Intelligence (AI), including autonomous control and management of reactors and components. A condition assessment framework that utilizes AI and sensor data is an important part of such an autonomous control system. A nuclear power plant has various structures, systems, and components (SSCs) such as piping-equipment that carries coolant to the reactor. Piping systems can degrade over time because of flow-accelerated corrosion and erosion. Any cracks and leakages can cause loss of coolant accident (LOCA). The current industry standards for conducting maintenance of vital SSCs can be time and cost-intensive. AI can play a greater role in the condition assessment and can be extended to recognize concrete degradation (chloride-induced damage and alkali–silica reaction) before cracks develop. This paper reviews developments in condition assessment and AI applications of structural and mechanical systems. The applicability of existing techniques to nuclear systems is somewhat limited because its response requires characterization of high and low-frequency vibration modes, whereas previous studies focus on systems where a single vibration mode can define the degraded state. Data assimilation and storage is another challenging aspect of autonomous control. Advances in AI and data mining world can help to address these challenges.

Prediction of flow-accelerated corrosion in single/dual elbow in pipelines considering geometric parameters and layout effect

Prediction of flow-accelerated corrosion in single/dual elbow in pipelines considering geometric parameters and layout effect

Post-hazard condition assessment of nuclear piping-equipment systems: Novel approach to feature extraction and deep learning

Over the past decade, the use of artificial intelligence techniques in the field of health-monitoring has gained significant interest, especially for structures such as building and bridges. However, applications to industrial systems such as equipment-piping systems in nuclear plants have not been explored. In this paper, it is shown that the existing techniques developed for buildings and bridges cannot be extended directly to equipment-piping systems as the response of such systems is governed by multiple localized modes unlike that in buildings and bridges. This paper proposes a new approach that consists of three key aspects: (i) a novel vector of degradation-sensitive features extracted from measured data, (ii) using a deep Artificial Neural Network (ANN) for diagnosis of degradation location and degradation severity, and (iii) consideration of uncertainty in degradation severity when training the ANN. Degradation in piping-equipment systems can occur due to flow-accelerated erosion and corrosion. These locations can potentially exhibit damage such as localized yielding or initiation of cracking due to an external event such as an earthquake. Moreover, such locations can at times go undetected by current inspection techniques. Therefore, a robust framework is needed for detection of degradation after a seismic event. This manuscript proposes a proof-of-concept framework, which utilizes data collected from sensors to generate a deep ANN database for predicting degraded locations and severity in a piping-equipment system. Degradation severity is classified as minor, moderate, and severe. In the suggested methodology, a novel vector of degradation-sensitive features is extracted from the sensor data to train the ANN. A simple piping-equipment system is selected to demonstrate feature extraction as a means to simplify pattern recognition, explore the design and parameters of an ANN, and develop a sensor placement strategy. The effectiveness of the proposed framework is demonstrated on a realistic primary safety system of a two-loop nuclear reactor. It is shown that the proposed post-hazard condition assessment framework is able to detect degraded locations along with the severity levels, including minor degradation, with considerably higher accuracy.

배열회수보일러(HRSG)에서 이상 유동(Two phase) FAC 고찰

Flow accelerated corrosion (FAC) is one of the major causes of HRSG tube damage in combined cycle power plants. Research on FAC from power generation facilities has been in progress for a long time, and related reports are easily accessible. However, most of them are data on single-phase flow FAC that occurs relatively frequently; and most of the rare cases in domestic are FAC that occurs in single-phase flow. In particular, there are almost no cases of two-phase flow FAC in domestic combined cycle power plants, so there are no related reports or data. Due to this, it is causing confusion in the preparation of countermeasures in the event of an FAC occurring in the field. Based on FAC-related data, this paper describes the factors that cause FAC in single-phase flow and two-phase flow, as well as the location, damage states and countermeasures. In addition, it is possible to clarify the classification of FAC according to the flow state by introducing the case of FAC that occurred recently in a domestic combined cycle power plant. Therefore, through this paper, it is possible to prepare a clearer countermeasure in case of FAC occurring in the combined cycle power plant.

Flow-Accelerated Corrosion Damage in a Steam Pipe Girth Weld

Flow-Accelerated Corrosion Damage in a Steam Pipe Girth Weld

The Influence of Steel Alloying on the Flow-Accelerated Corrosion of Steels in the Loops of NPPs with VVERs

The Influence of Steel Alloying on the Flow-Accelerated Corrosion of Steels in the Loops of NPPs with VVERs

Evaluation of flow accelerated corrosion and mechanical performance of martensitic steel T91 for a ternary mixture of molten salts for CSP plants

Thermal energy storage (TES) systems with molten salts involve different operating conditions causing corrosion problems in the materials due to the high operating temperature. In this work, the corrosion behavior was studied by means of flow accelerated corrosion (FAC) and static corrosion tests in contact with the ternary mixture (KLiNa)NO 3 at 550 °C. Both tests were performed by gravimetric tests (mass gain) and subsequently characterized by SEM and XRD. The results showed a substantial increase in mass gain due to the dynamic effect of the FAC test in relation to the stationary test. Consequently, the dynamic effect of the FAC test at high temperatures could cause stress corrosion cracking in the material, leading to a change in the mechanical performance of T91 steel. Therefore, the slow strain rate tensile test (SSRT) on molten salts was performed to evaluate the impact of temperature on the mechanical property performance of the material. A considerable reduction in the mechanical performance of the material was observed compared to testing the material in air under normal conditions. This research shows the real operating conditions under the corrosion effect of T91 steel and its relevance as a material for the solar thermal industry. • Corrosion of T91 steel in (KLiNa)NO 3 under FAC and static conditions are different. • Lithium compound oxides are formed due to oxidation and lithiumization phenomena. • There was a reduction in the mechanical performance of T91 steel under tensile tests.

Reliability-based inspection and maintenance planning of a nuclear feeder piping system

Inspection and maintenance (I&M) is essential to ensure the integrity of feeder pipes, which are parts of the primary heat transport system in a nuclear power plant. The pipes are subject to flow accelerated corrosion (FAC), which can compromise the integrity of the piping system and lead to high repair costs. We explore the opportunity for improving I&M strategies while ensuring that the system still maintains an acceptable level of reliability. To this aim, a reliability-based planning framework is proposed, in which every pipe in the system meets the minimum thickness requirement at a specified annual probability. With this planning framework we can a) evaluate the performance of any I&M strategy constrained to a fixed reliability criterion, without requiring this strategy to be specifically designed for such a criterion; and b) find an I&M strategy optimized for this reliability level using a heuristic description of the strategy space. We demonstrate the framework with a case study, where the wall thinning due to FAC is modeled as a Gamma process with uncertain parameters. We compare the expected life-cycle cost of multiple strategies for I&M of a feeder system with 480 pipes. The proposed approach is compared with an I&M strategy currently used by the industry, which highlights the efficiency of the proposed optimization method.

In depth analysis of corrosion mechanism of U-tube under conditions of differential aeration

• A two dimensional model for the study of concentration corrosion in U-pipe is proposed creatively. • Based on the knowledge of electricity, the discrete equation of corrosion potential after polarization is derived and solved. Through the analysis of the results, the corrosion rate of the inner wall is predicted and the mechanism of concentration difference corrosion is revealed. • It is a meaningful supplement to the traditional FAC(flow accelerated corrosion) theory. The corrosion mechanism of fluid in pipeline is very complex, and many mechanisms often work together. Among these mechanisms, differential concentration corrosion(DCC) mechanism is often ignored by researchers. Due to the difficulty of experimental method, a numerical simulation method is used to carefully study it. Firstly, the flow field inside U-tube was calculated through FLUENT, and the oxygen concentration distribution of the ionic conductive layer near the wall is got; secondly, the ionic conductive layer is extracted and discretized, and the natural corrosion potential and current density of the surface unit are calculated according to the oxygen concentration; finally, according to Kirchhoff's Second Law, the discrete equations of the corrosion potential of the surface ionic conductive layer are derived. By solving these equations, the corrosion potential of the unit after polarization is obtained, and the final corrosion current density distribution on the surface is got too. The results show that when DCC is not considered, the natural corrosion potential and corrosion current density of the pipeline wall with high oxygen concentration are higher and larger, while those with lower oxygen concentration have lower natural corrosion potential and smaller natural corrosion current, when DCC is considered, the cathodic polarization will occur at the position with higher nature corrosion potential, and the corrosion rate will be reduced; while the anodic polarization will occur at the position with lower nature corrosion potential, and the corrosion current will increase. These greatly change the distribution of the corrosion current and the corrosion potential tends to homogenize.

Advanced Prediction Model for Pipe Wall Thinning Tendency by Flow Accelerated Corrosion at Junction Piping

Flow Accelerated Corrosion (FAC) is a pipe wall thinning phenomenon to be monitored and managed in the power plants with high priority. Its management has been conducted with conservative evaluation of thinning rate, and residual lifetime of piping based on wall thickness measurements. However, noticeable case of the wall thinning occurred in branch and junction piping (tee tube). There is a problem to manage the wall thickness beneath reinforcing plate of the tee tube, because measurement of this area is difficult to be conducted with ordinary ultrasonic testing devices due to the presence of the reinforcing plate. In this study, numerical analysis for the tee tube was conducted, and the wall thinning profile due to the FAC was evaluated by calculating the mass transfer coefficient. It was found that a localized wall thinning distribution occurs in the area where the reinforcing plate of the tee tube exists. And this tendency is affected by Reynolds number. In previous studies, a model was introduced to predict this trend, but in this study, the model was improved to take into account the effect of the Reynolds number.

Evaluation of Flow Accelerated Corrosion in Typical Recovery Boiler Environments of Energy Production Industries

Flow accelerated corrosion (FAC) is a steel degradation that occurs in heat recovery steam generators in the power industry. The mechanism of this corrosion comprises an electrochemical dissolution of the semi-protective magnetite layer (Fe3O4) that is formed within the pipes employed in the boilers. The FAC is influenced by different factors such as fluid velocity, pH and dissolved oxygen concentration. In this context, Rotating Cage tests were used to evaluate the influence of pH and dissolved oxygen content on FAC of A210, P11 and P22 steels. General corrosion was lower for P11 and P22 steels, an effect consistent with the presence of chromium in their compositions. General corrosion was most critical at pH 8.5. The corrosion intensity decreased for the three steels when the dissolved oxygen concentration increased, at 2 m/s. This behavior is coherent considering the most effective precipitation of Fe2O3 inside the pores of the Fe3O4 layer in a more oxidizing environment. For 3 m/s, the corrosion intensity increased for both materials when oxygen increased, showing the shear stress effect. These behaviors were more significant at pH 8.5. About pitting corrosion, the analyzes show localized attack in the majority of coupons, however with very low number of pits.

Analysis of Material Loss Behavior According to Long-Term Experiments on LDIE-FAC Multiple Degradation of Carbon Steel Materials

Recently, damage caused by liquid droplet impingement erosion (LDIE) in addition to flow-accelerated corrosion (FAC) has frequently occurred in the secondary side steam piping of nuclear power plants, and the damage-occurring frequency is expected to increase as their operating years’ increase. In order to scrutinize its causes, therefore, an experimental study was conducted to understand how the behavior of LDIE-FAC multiple degradation changes when the piping of nuclear power plants is operated for a long time. Experimental results show that more magnetite was formed on the surface of the carbon steel specimen than on the low-alloy steel specimen, and that the rate of magnetite formation and extinction reached equilibrium due to the complex action of liquid droplet impingement erosion and flow-accelerated corrosion after a certain period of time. Furthermore, it was confirmed at the beginning of the experiment that A106 Gr.B specimen has more mass loss than A335 P22 specimen. After a certain period of time, however, the mass loss tends to be the opposite. This is presumed to have resulted from the magnetite formed on the surface playing a role in suppressing liquid droplet impingement erosion. In addition, it was confirmed that the amount of erosion linearly increases under the conditions in which the formation and extinction of magnetite reach equilibrium.

The Influence of Rotating Cage Geometry and Baffles on the Wall Shear Stress Equation in Sweet/Sour Corrosion

Flow-accelerated corrosion has been studied by means of computational fluid dynamics (CFD) techniques and rotating cage (RC) autoclave tests according to ASTM G170. This study evaluated through CFD and laboratory tests the influence of RC geometry and baffles on the wall shear stress (τw) proposed by the aforementioned standard. The experimental were performed at 120 °C and 2 angular velocities (500 and 800 rpm) during 25 h in a solution with 600 mg/L of Cl- ions and 10-3 mol/L of sodium thiosulfate. Corrosion rates were obtained by ASTM G31 and the surfaces were evaluated by OM and XRD. Corrosion rate values and XRD analyses showed divergence between samples in different positions in the RC and between distinct regions in the same sample. The numerical simulations and the experimental tests showed a great influence of the baffles on τw. The conservatism in the values obtained with this technique is caused by excessive corrosion close to the samples edges and the non-uniformity of τw on the internal samples surfaces. Thus, the main conclusion of the study is related to the importance of better knowledge on experimental methodology and mathematical approximations for updating the criterion for the correct τw calculation on sample surfaces.

A New Understanding of the Flow-Accelerated Corrosion Mechanism of Hp-13cr Stainless Steel in an Extremely Aggressive Oilfield Environment

A New Understanding of the Flow-Accelerated Corrosion Mechanism of Hp-13cr Stainless Steel in an Extremely Aggressive Oilfield Environment

Estimating degradation growth rate and time of component replacement from limited inspection data using mixed-effects modelling

Predicting the progress of degradation of a specific engineering component can be highly uncertain, due to a lack of repeat observations over time. This paper illustrates how the mixed-effects modelling approach can be used to significantly reduce the uncertainty in the component specific predictions by leveraging the available information from the larger component population. The methodology is applied to the assessment of flow accelerated corrosion (FAC) of a large population of piping in the primary heat transport system of a nuclear reactor. The pipe wall thickness loss is modelled using the linear mixed-effects regression (LMER) approach, and applied at the group level based on pipe geometry. The results demonstrate how the mixed-effects approach provides a highly effective and consistent method for combining all of the valuable (and often highly expensive) inspection data into a unified framework to obtain more robust and credible estimates of degradation growth rates and times of replacement of all components in the population, including uninspected components and components with very limited data (e.g., one or two inspections).

Diagnosing nuclear power plant pipe wall thinning due to flow accelerated corrosion using a passive, thermal non-destructive evaluation method: Feasibility assessment via numerical experiments

Flow accelerated corrosion (FAC) in nuclear power plant pipes is one of the leading causes of accidents, fatalities, damage and outages. Current FAC identification methods employ expensive sensing technology and are “active” methods, where the response of the piping system to an externally-generated thermal, mechanical or optical excitation must be measured. As a result, these techniques require a disruptive and time-consuming setup. In this article, we propose a method that utilizes pipe surface temperature measurements to passively monitor for FAC-induced pipe wall thinning without the need for expensive equipment or post-installation setup time. This diagnostic method utilizes a simulation data-driven diagnostic model to estimate the amount of thickness reduction in a pipe based on changes in measured steady-state pipe temperatures. In order to reduce the computational burden of generating large, simulation-based datasets, the behavior of the insulation of the pipe was modeled using a suitably calibrated heat transfer boundary parameter. Additionally, global sensitivity analysis was performed to determine system parameter(s), such as the temperature of water flowing inside the pipe, which significantly affect the steady state pipe wall temperature and could cause errors in diagnosis. Two diagnostic models, one using only the change in steady-state temperature as an indicator for FAC-induced pipe wall thinning and the other using water temperature as an additional diagnostic model input were evaluated for their ability to estimate thickness reductions in a pipe using simulated pipe wall temperature data. For the numerical experiments conducted in this work, both models estimated wall thickness with errors within 0.5 mm, indicating that the proposed technique can potentially be used as a low-cost, first-pass method for FAC monitoring.

Flow accelerated corrosion and erosion\u2212corrosion behavior of marine carbon steel in natural seawater

In this work, flow accelerated corrosion (FAC) and erosion−corrosion of marine carbon steel in natural seawater were electrochemically studied using a submerged impingement jet system. Results show that the formation of a relatively compact rust layer in flowing natural seawater would lead to the FAC pattern change from ‘flow marks’ to pits. The increase of the flow velocity was found to have a negligible influence on the FAC rate at velocities of 5−8 m s−1. The synergy of mechanical erosion and electrochemical corrosion is the main contributor to the total steel loss under erosion−corrosion. The increase of the sand impact energy could induce the pitting damage and accelerate the steel degradation. The accumulation of the rust inside the pits could facilitate the longitudinal growth of the pits, however, the accumulated rusts retard the erosion of the pit bottom. The erosion and corrosion could work together to cause the steel peeling at the pit boundary. The steel degradation would gradually change from corrosion-dominated to erosion-dominated along with the impact energy increasing.

Microstructure and flow accelerated corrosion resistance of Cr coatings electrodeposited in a trivalent chromium bath

In this study, Cr coatings were electrodeposited on carbon steel substrates by a trivalent Cr salt. The microstructure of the Cr coatings and their flow accelerated corrosion (FAC) resistance in simulated secondary circuit environments (10 MPa and 150 °C) of nuclear power plants were investigated. The results showed that the as-deposited coating was amorphous and crack-free. Annealing at 200 °C resulted in the decrease of Cr (OH) 3 and the increase of Cr and Cr 2 O 3 on the coating surface , which improved the corrosion resistance of the coating. The subsequent FAC test showed that the Cr coatings significantly improved the FAC resistance of the carbon steel substrate. The coating annealed at 200 °C exhibited better FAC resistance. The Cr-rich oxide film formed on the coatings was mainly Cr 2 O 3 , with a small amount of Cr and CrO 2 . Due to the galvanic effects caused by pinhole defects, abnormally grown annular deposits formed on the coating surface. • Uniform and dense Cr coatings were electrodeposited from a trivalent Cr bath. • Annealing at 200 °C reduced Cr(OH) 3 on the coating surface and increased Cr and Cr 2 O 3 . • The Cr-rich oxide film formed in the corrosion was Cr 2 O 3 and a small amount of CrO 2 . • The interface between the coating and the substrate formed Cr-rich corrosion areas.