Ship Propeller Research Articles

Numerical prediction of cavitation phenomena on marine vessel: Effect of the water environment profile on the propulsion performance

Abstract Energy-saving and emission reduction are crucial since shipping activity due to the global maritime trade has increased exponentially. Several agreements have been engaged to optimize ship energy efficiency composed of ship design and shipping operation planning. However, most up-to-date studies focused on speed and route optimization. The interaction analysis between speed and route efficiency below varied environmental conditions is limited. To attain energy and cost efficiency, a study of cavitation on the propeller that considers the ocean environmental condition will be discussed in this work. Although researchers have previously observed cavitation phenomena, the predictability of simulations is not yet such that problems can be eliminated. Since the multiphase flow of water and vapor is sensitive to environmental conditions, it leads to varying observation accuracy. Thus, the current paper proposes a new performance indicator of the ship propeller under cavitation predicted by computational fluid dynamics (CFD). CFD-based simulation to observe the propeller cavitation was used to model the Zwart cavitation and Kunz cavitation models under two turbulence models of K−ε at different flow conditions and operating environments. Initial validation tests between experimental and numerical simulation show good agreement with a mean error of 4.7% in the Zwart model and 3.7% in the Kunz model, where the k−ε turbulence model provides an almost higher relative error. It is revealed from the result that the increase in temperature causes the rise in the cavitation problem. It is revealed from the result that the increase of temperature causes the increase in cavitation problem.

Passive suppression of radiation of low-frequency underwater noise

Using an exact solution of the problem of spherical wave diffraction on a fluid sphere, it has been predicted [Godin and Baynes, J. Acoust. Soc. Am. 143, EL67–EL73 (2018)] that passive, broadband suppression of radiation of low-frequency sound can be achieved by placing an acoustically compliant object in the vicinity of the source. In underwater acoustics, the compliant object can be realized as an air-filled bladder. One of the promising applications of the technique is to suppress the noise generated by ship propellers. If implemented on a wide scale, the passive noise suppression technique would help to decrease the noise levels due to commercial and recreational vessels and reduce the detrimental effects of anthropogenic noise on marine life. This paper extends the earlier theoretical treatment of the problem to more realistic models of the compliant object such as a gas volume surrounded by a stretchable membrane (a balloon) and a thin shell. The effect of the finite depths of low-frequency noise source and the compliant object on noise levels in the far field will be discussed.

Experimental and Numerical Study on Ice Blockage Performance of Propeller in Cavitation Flow

Cavitation greatly affects the ice blockage performance of propellers in polar areas, while the combined effect of cavitation and ice blockage on propellers has rarely been considered. In this work, the propeller model test in the cavitation tunnel and the viscous flow CFD numerical simulation based on RANS were conducted. In the cavitation tunnel test, the ice blockage model was simulated by a water-insoluble rectangular solid block, and the ice blockage was measured by the distance between the solid block and the propeller. The thrust and torque in tests and simulations were discussed under the uniform flow and ice blockage scenarios, as well as the variation of cavitation excitation force, pressure distribution of blades, cavitation characteristics and vortex intensity with advance coefficient when σn = 1.5, L/D = 0.15 in an ice blockage environment. The research shows that the numerical simulation results based on overlapping grids are in good agreement with the model test results, and the mean hydrodynamic errors are within 5%. In the uniform flow test, when the advance coefficient is small, the thrust and torque of the propeller will experience a sharp drop due to the influence of heavy cavitation. In the ice blockage test, the thrust and torque increase with the decrease of ice-propeller spacing, and the ice blockage becomes more serious as the cavitation grows. The propeller oscillates violently due to the cavitation excitation force, and the oscillation frequency increases with the increase of the advance coefficient. The cavitation is generated in the low-pressure area of the suction surface, and the cavitation shape captured in the present numerical simulation is consistent with the experimental phenomenon. Since the cavitation reduces the contact area between the water and the blade, the vortex strength will be reduced for the attachment of cavitation, and the vortex strength increases with the increase of the advance coefficient. This study will explore more hydrodynamic regularities with ice-class propellers in an ice blockage environment when cavitation occurrs, and provide technical support for the design of propellers of polar ships.

Сравнительная стойкость против кавитационной эрозии аустенитных сталей различного уровня метастабильности

Introduction. Reliability-critical components of equipment working in contact with high-speed liquid media (for example, turbine blades of hydroelectric power stations, pump impellers, ship propellers) are subjected to one of the types of wear – cavitation erosion. The current study aims to select and scientifically substantiate the type of coating and its structural-phase state for the effective protection of parts from cavitation erosion. Research methods. The study carries out a comparative analysis of differences in the cavitation erosion resistance of characteristic austenitic steels, in the form of bulk material (316L) and coatings (E308L, 60Cr8TiAl), used for protection against cavitation Arc surfacing, i.e. MMA and MIG, is used for depositing the coatings. The tests are carried out on an original installation for evaluating the cavitation resistance of materials with applying ultrasound and the electrical potential difference. Results and Discussion. The results show that the 60Cr8TiAl has a higher resistance to cavitation erosion than that of E308L and 316L by 4 and 10 times, respectively. The structural factors that determine the resistance to cavitation erosion damage are identified to analyze the reasons for the differences in material resistance. Firstly, a strong dependence of the cavitation erosion resistance of austenitic steels on the intensity of the deformation martensitic transformation, developing under the influence of cavitation, is confirmed. This structural transformation contributes to an increase in cavitation resistance of the surface layer. In metastable austenitic steel, a deformation martensite (α′) is formed in the surface layer during the initial test period. This causes an increase in hardness, dissipation of the energy of external action, and the appearance of compressive stresses that prevent the occurrence of microcracks. Subsequently, additional hardening of the previously formed dispersed crystals of α′-martensite occurs. In 60Cr8TiAl, these effects are significantly stronger than that of E308L and 316L due to the higher level of metastability of austenite and formation of carbon deformation martensite.

Underwater Sound Characteristics of a Ship with Controllable Pitch Propeller

The time-dependent spectral characteristics of underwater sound radiated by an ocean vessel has complex dependencies on ship machinery, propeller dynamics, hydrodynamics of ship exhaust and motion, as well as ship board activities. Here the underwater sound radiated by a ship equipped with a controllable pitch propeller (CPP) is analyzed and quantified via its (i) power spectral density for signal energetics, (ii) temporal coherence for machinery tonal sound, and (iii) spectral coherence for propeller amplitude-modulated cavitation noise. Frequency-modulated (FM) tonal signals are also characterized in terms of their frequency variations. These characteristics are compared for different propeller pitch ratios ranging from 20% to 82% at fixed propeller revolutions per minute (RPM). The efficacy and robustness of ship parameter estimation at different pitches are discussed. Finally, analysis of one special measurement is provided, when ship changes speed, propeller pitch and RPM over the duration of the measurement. The 50% pitch is found to be a crucial point for this ship about which tonal characteristics of its underwater radiated sound attain their peak values, while broadband sound and associated spectral coherences are at a minimum. The findings here elucidate the effects of pitch variation on underwater sound radiated by ships with controllable pitch propellers and has applications in ship design and underwater noise mitigation.

가상 자유 항주를 이용한 KCS 선형의 정수 중 선회 및 변침 성능 해석

To understand physical phenomena of ship maneuvering deeply, a numerical study based on computational fluid dynamics is required. A computational method that can simulate the interaction between the ship hull, propeller, and rudder will provide informative local flows during ship maneuvering tests. The analysis of local flows can be applied to improve a physical model of ship maneuvering that has been widely used in maneuvering simulations. In this study, the numerical program named as WAVIS that has been developed for ship resistance and propulsion problems is extended to simulate ship maneuvering by free-running tests. The six degree-of-freedom of ship motion is implemented based on Euler angles and the overset technique is applied to treat the moving grid of ship hull and rudder. The propulsion force due to a propeller is calculated by a panel method that is based on the lifting-surface theory. The newly extended code is applied to simulate turning and zig-zag tests of KCS and the comparison with the available experimental data has been made.

Supporting Predictive Maintenance of a Ship by Analysis of Onboard Measurements

Ship hull and propeller fouling has a severe impact both on fuel costs and on greenhouse gas emission, thus, it is an important subject to consider when developing a maintenance plan to enhance energy efficiency and reduce gas emissions from ships. This paper presents a two-step approach for evaluating the actual propulsive performance of a ship through analyzing continuous onboard measurement. First, the onboard monitored data are corrected for the wind and wave effects using fast and transparent empirical methods. Second, the corrected data are filtered based on hydrodynamic criteria. A case study is presented to demonstrate the effectiveness of the methodology. Subsequently, the processed data are analyzed by mathematical tools to derive an engine power–rpm curve to represent the actual propulsive performance of the ship. By comparing the derived curve against the nominal curve for engine design, it is possible to identify the deterioration of the ship’s condition, which is the key task in developing the identification capability that is needed in a proactive maintenance procedure. This methodology is expected to support the maritime sector in combating climate change by reducing the Greenhouse Gas (GHG) emissions from ship operations.

Engine propeller matching analysis on Fishing Vessel using inboard engine

Fishing vessels which are used for one day operations usually use outboard engines as their driving motor. However, to improve the performance of propulsion systems, and consequently the performance of ships, inboard engines are now being used. In this study, the propulsion system of a ship was modified by upgrading from an outboard to an inboard motor. Meanwhile, the study aims to obtain an optimal interaction between the propulsion system and the hull shape of the ship. It was conducted by calculating the ship's resistance using the van Oortmerssen method, and validating the result using the CFD method. Furthermore, thrust and torque calculations were performed to obtain the characteristics of the ship's propellers, and the results were validated using the CFD method. The result obtained from the calculation of the ship's resistance was a New Fishing Vessel engine power of 11 HP. 4 types of B-Series propellers characterized based on the size of their pitch, including 14.00 inch, 14.25 inch, 14.50 inch, and 14.75 inch were, analyzed using the engine propeller matching analysis. The results show that the propeller with the pitch size of 14.75 inches was the best, as it had a power of 100%, speed of 25.35%, and efficiency of 32%. Therefore, it was chosen as the new propeller for New Fishing Vessels.

The effect of propeller cap angle and fin size of PBCF on propeller performance

The propeller was one of the main propulsors in ships that consumed a lot of fuel due to the viscous force acting in the ship. Therefore many studies have been carried out to increase the performance of ship propellers. One of the ways is using Energy Saving Device (ESD) to improve propeller performance. One of the ESD is Propeller Boss Cap Fins (PBCF). PBCF was used to improve the propeller performance by reducing the hub vortex during propeller rotation. In this study, we compare the propeller performance before and after PBCF installation without changing propeller dimensions. The variations of PBCF fins were made in 0,1D, 0,2D, and 0,3D with a combination of the angle of the propeller cap were 15° and 20°. The propeller performance is carried out with the computational fluid dynamics (CFD) method. It was made in the open water assumption to reduce of wall effect. B-series propeller is used in the present study. The validation of propeller performance is made using empirical results. This study aimed to determine the effect of the installation of PBCF on the performance of B-Series propellers on the Ro-Ro 1000 GT ship. The results show that variation with a fin diameter of 0,2D and a propeller cap of 20° can increase propeller efficiency by 0.03%. It can be concluded the addition of PBCF can improve propeller performance.

Resistance and wake distortion optimization of JBC considering ship-propeller interaction

In this paper, the self-propulsion performance of a full formed hull is optimized by the in-house hull form optimization software OPTShip-SJTU 2.1. In the field of hull form optimization, complex hull optimizations such as hull-propeller interaction optimization have not launched a full investigation. To break through this limitation, the hull-propeller interaction module is brought into the software. We conduct a hull-propeller interaction design optimization of Japan Bulk Carrier (JBC) with an actual propeller (AP) based on CFD considering ship resistance and propeller wake distortion at Fr = 0.142. Hull form deformation module is applied to change the stern shape of the JBC. The in-house CFD software naoe-FOAM-SJTU is applied to calculate the hull resistance and wake fraction. The dynamic overset grid method is used to achieve the interaction between hull and propeller and obtain the high-fidelity flow field during the optimization processing. A multi-objective genetic algorithm (NSGA-II) is applied to obtain the optimized hull forms based on the Kriging surrogate model. The influences of propeller for the flow field around stern are also studied. Results indicate: (I) OPTShip-SJTU 2.1 can optimize the coupled hydrodynamic performance in complex hull form optimization. (II) The hull resistance and wake fraction have a strong positive correlation under the deformation setting in this paper. (III) Contraction of hull line at the stern can suppress the flow separation around the stern which is benefit for reducing the pressure resistance and improving the propeller wake distortion. (IV) It is necessary to consider the interaction between propeller and hull together in hull form optimization. The interactions between the shape of stern, propeller performance, and flow field around stern are discussed preliminary, which provide several references for the subsequent research.

COUPLED RANSE AND LIFTING LINE THEORY FOR ESTIMATION OF SHIP PROPULSION FACTORS

Advances in Computational Fluid Dynamics (CFD) techniques through the development of the Reynolds-Averaged Navier-Stokes Equations (RANSE) have assisted in estimation of resistance and propulsion characteristics of ships to a reasonable level of accuracy. The aim of this paper is to test and demonstrate the capabilities of the coupled RANSE and Lifting Line theory for undertaking ship resistance, propeller open-water and self-propulsion simulations. Further, parametric studies for generation of numerical propeller design sheets and optimisation of propulsive efficiency using the coupled simulation approach has been discussed. Commercial CFD solver “M/s Flowtech - Shipflow” has been used for the study. Initially, some benchmark experimental/numerical model results are validated with the results of the CFD simulations and then, further parametric analyses have been undertaken with the KRISO Container Ship and the KP505 Propeller. The numerical propeller series and the preliminary study methodology for optimization of location of propeller disc behind the ship’s hull are being proposed as an effective concept/feasibility design stage tool for estimation of ship propulsion characteristics.

МАТЕМАТИЧНЕ МОДЕЛЮВАННЯ РОБОТИ СУДНОВИХ ГВИНТІВ ПРИ РІЗНИХ РЕЖИМАХ МАНЕВРУВАННЯ

In the study of ship maneuvers, in the development of ship's autopilots and simulators, the availability of adequate mathematical models of the ship’s propulsion complex, which would describe with high accuracy the dynamics of the ship for the widest ranges of phase coordinates, plays a decisive role. Mathematical models of ship dynamics consist of mathematical models of inertial and non-inertial forces and moments on the ship's hull. The latter include hydrodynamic and aerodynamic forces and moments on the ship's hull, forces and moments caused by the operation of propellers, rigging, ship rudders. For the adequacy of the general mathematical model of the ship's dynamics, the adequacy of the mathematical models of all components of non-inertial forces and moments on the ship's hull is necessary. Mathematical models of non-inertial forces and moments on a ship's hull are usually empirical and made by processing data from experimental tests in ship model basin or in full-scale sea trials. In particular, this applies to the forces and moments caused by the operation of the propeller. In the mathematical modeling of the indicated forces and moments, the rectilinear movement of the vessel, or movement at small values of the drift angles, is usually considered. In this case, only the longitudinal action of the propeller is usually considered, and the transverse component of the force and the moment on the ship's propeller are unreasonably neglected. In addition, well-known mathematical models usually contain coefficients and functions that are difficult to determine using tables and graphs, which is inconvenient for numerical modeling. This work is intended to somewhat eliminate the indicated disadvantages. In particular, it was investigated how the curvilinear movement of the ship affects the operation of the propeller, analyzed the existing mathematical models, and new effective mathematical models of the longitudinal and transverse components of the force and moment caused by the operation of the ship's propeller in dimensional and dimensionless forms were proposed. In particular, representations are obtained for the dimensionless coefficients of the propeller stop and the moment on the propeller shaft, the wake fraction coefficient, the thrust-deduction factor, and skew of the flow on the propeller. It is shown how these parameters change for all possible values of the drift angle and angular velocity. For a number of commercial vessels of various types, technical characteristics are given, and parameters for constructing mathematical models of the dynamics of the propeller are calculated.

Ship operation in brash ice: results of investigations

The paper gives a review of the studies concerned with operation of vessels in brash ice. Recently, the ice conditions have received an ever increasing attention of the researchers related to the fact that shipping in the Arctic regions and freezing seas, as well as in inland waterways has been scaled up. One of the important fields of brash ice studies is specifics of sailing under these conditions and primarily determination of the ship ice resistance. The paper shows that theoretical methods combined with physical modeling in ice basins are used for determination of the ship ice resistance under brash ice conditions. The paper traces the evolution of theoretical models utilized for calculations. It is mentioned that the models are mainly based on loose material mechanics. A rapidly developing computer modeling of ship motion in brash ice based on discrete element method is considered. Physical modeling techniques used for modeling brash ice in ice basin are described, and challenges of experimental investigations are discussed. It is pointed out that experimental studies in ice basin can provide valuable data not only about ship ice resistance but also about the mechanisms giving rise to ice channels filled with brash ice. The paper describes the methods for studying operation of ship propellers in brash ice conditions. It is concluded that further research into brash ice is needed.

Locating, trajectory planning and control of an underwater propeller cleaning manipulator

After a long time of navigation, marine fouling occurs on a ship's hull and propeller, leading to higher frictional resistance of the hull and lower effectual output of the propeller, and it also damages the hull and propeller. Currently, there are extensive studies on robots for underwater cleaning of ship hulls with large areas and relatively regular surfaces. However, robots for underwater cleaning of propellers with complicated spatial surfaces are still in demand. Motivated by this need, a solution for cleaning the propeller by the underwater manipulator is proposed in this study. Coarse registration and iterative closest point fine registration were adopted to improve the propeller's locating accuracy. Non-dominated sorting genetic algorithm-II was applied to optimize the trajectory of the B-spline joint to obtain an optimized trajectory that meets the specific requirements of the underwater manipulator. Finally, sliding mode control was adopted for high trajectory tracking. The integrated task of locating, trajectory planning, and control was simulated. The simulation results demonstrate the effectiveness of the above methods and the feasibility of the proposed scheme. This study serves as a guideline for further research in the analysis and system design of the underwater manipulator for cleaning propellers.

Development of deep learning-based detection technology for vortex-induced vibration of a ship's propeller

Vortex-induced vibration (VIV) resulting lock-in phenomenon which is the vibration frequency of the structure immersed in a fluid is locked in its resonance frequencies within a flow speed range is a potential cause of vibration fatigue and/or singing of the propellers of large merchant ships. A deep learning-based indirect VIV detection algorithm that works by vibration measurement data of a hull structure instead of a propeller during the sea trial phase is proposed in this study. RPM-frequency representations of the vibration signal by stacking the vibration frequency spectrum relative to various propeller RPM's, so-called waterfall chart of 2-dimensional data like an image, are measured and fed into the proposed convolutional neural network (CNN) architecture for VIV detection i.e. VIV frequency and RPM range. To generate the large data set, artificial data looks real, for learning, a method based on the modal superposition method instead of time-consuming fluid-structure interaction analysis is proposed. 10k set data for training, 1k set for hyper-parameter tuning, and 1k set for the test were generated. After the training, it showed a diagnosis success rate of 82% for the test set. To test the proposed VIV detection algorithm, investigations were carried out in a laboratory on a small-scale ship propulsion system designed in such a way that the vortex shedding frequency and the underwater natural frequency match each other. The proposed VIV detection algorithm was tested using the structural vibration signals measured at stationary structures in the air instead of the rotating propeller immersed in water. Lastly, the validity of the proposed algorithm was tested using the structural vibration signal measured at the hull structure of a crude oil carrier in which a propeller singing occurred during her sea trial.

A study on the efficient numerical analysis for the prediction of full-scale propeller performance using CFD

In Computational Fluid Dynamics (CFD) simulations, limited full-scale studies with ship propellers have been conducted due to the limitation of computational resources and computation time. Two methods for efficient numerical analysis of full-scale propeller performance are; (1) a method of using large dimensionless wall distances (y+) at a full scale and (2) a method of applying a virtual fluid at a model scale. Thus, the aim of this study is to investigate the effect of different wall y+ values in a real fluid and the virtual fluid concept to predict full-scale propeller performance using CFD. For these investigations, the commercial CFD tool, STAR-CCM+, was used to predict the propeller open water (POW) performance of the KCS propeller (KP505) in a model and full-scale. The findings of this research study support the use of a small value of wall y+ (i.e., y+<1) for the model scale simulations, but the effect of the wall y+ is negligible in full-scale. This study also demonstrates that the similarity requirements for the advance coefficient and Reynolds number could be satisfied simultaneously in full-scale by using the virtual fluid properties without any need to conduct more computationally demanding full-scale simulations with real fluid.

Enhancement of the ship propeller hydrodynamic performance by different energy-saving devices mounted at the upstream zone

Enhancement of the ship propeller hydrodynamic performance by different energy-saving devices mounted at the upstream zone

Scaling properties of the Ffowcs-Williams and Hawkings equation for complex acoustic source close to a free surface

We perform a scaling analysis of the terms composing the Ffowcs-Williams and Hawkings (FWH) equation, which rules the propagation of noise generated by a rigid body in motion. Our analysis extends the seminal work of Lighthill (Proc. R. Soc. Lond.A, vol. 211, 1952, pp. 564–587) and the dimensional analysis of classical sources (monopole, dipole and quadrupole) considering all the FWH integral terms. Scaling properties are analysed in light of perfect/imperfect similarity when laboratory-scale data are used for full-scale predictions. As a test case we consider a hydrodynamic example, namely a laboratory-scale ship propeller. The data, obtained numerically in a previous study, were post-processed according to the scaling analysis presented herein. We properly scale the speed of sound to obtain perfect similarity and quantify the error with respect to the imperfect scaling. Imperfect similarity introduces errors in the acoustic response related both to the linear terms and to the nonlinear terms, the latter of great importance when the wake is characterized by robust and organized vorticity. Successively, we analyse the effect of a free surface, often present in hydrodynamic applications. We apply the method of images to the FWH equation. The free surface may generate a frequency-dependent constructive/destructive interference. The analysis of an archetypal acoustic field (monopole) provides robust explanation of these interference effects. Finally, we find that imperfect similarity and the absence of a free surface may introduce errors when model-scale data are used to obtain the full-scale acoustic pressure. The error is small for microphones placed in the near field and becomes relevant in the far field because of the nonlinear terms.

Using the developed cavitation test to evaluate erosion resistance of cermet thermal sprayed coatings

Many machinery parts working in contact with a fast-flowing fluid flow (e.g. turbine blades of hydroelectric power plants, valves, pump impeller blades, ship propellers, cooling systems for various units, etc.) are subjected to such type of wear as cavitation erosion. An important objective is to eliminate or reduce cavitation erosion so as to achieve a considerable economic effect. This research uses a patented technique developed to evaluate the cavitation erosion resistance of cermet thermal spray coatings (WC–10Co4Cr and WC–20CrC–7Ni). These coatings were prepared using high velocity air fuel thermal spraying (HVAF). The aim of this study is to test a new technique for evaluating coating cavitation resistance, which differs from the standard one by specimen positioning relative to the testing liquid. In addition, scanning electron microscopy (SEM) was used to analyze the initial structure of the coatings prepared and study their behavior after cavitation exposure. The material volume loss criterion during the cavitation test was used to evaluate the coating resistance. The results of cavitation tests showed that the WC–20CrC–7Ni coating has a somewhat higher cavitation resistance than that of WC–10Co4Cr despite its slightly lower average hardness (850±90 HV0.5 versus 950±60 HV0.5). The study of coating surfaces and cross-sections showed that they feature by different erosion mechanisms. It can be concluded that the presence of defects (pores) in the coating structure is the main reason for reducing their cavitation erosion resistance. Therefore, the developed technique proved effective in obtaining experimental data to analyze cermet thermal spray coatings for cavitation wear.

Effect of plastic deformation on mechanical properties and corrosion resistance of nickel-aluminum bronze

PurposeNickel-aluminum bronze (NAB) has been widely used in ship propellers. It is always subjected to local micro-plastic deformation in service environments. This paper aims to study the influence of plastic deformation on the mechanical strength and corrosion resistance of NAB in 3.5 Wt.% NaCl solution.Design/methodology/approachScanning electron microscope and X-ray diffraction were used to analyze the microstructure of NAB alloy with different plastic deformations. Mechanical properties of the sample were measured by tensile experiment, and corrosion behavior was studied by electrochemical measurements and the long-term immersion corrosion test.FindingsResults showed that the plastic deformation caused lattice distortion but did not change the microstructure of NAB alloy. Microhardness and yield strength of NAB were significantly improved with the increase of deformation. The lattice distortion accelerated the formation of corrosion product film, which made the deformed alloy show a more positive open-circuit potential and an increased Rp. However, during the long-term immersion corrosion, the corrosion resistance of NAB alloys deteriorated with the increase of plastic deformation. This is because larger plastic deformation brought about higher internal stress in corrosion product film, which resulted in the premature peeling of the film and the loss of its protective effect on the alloy substrate.Originality/valueTensile plastic deformations were found to cause a decline in the corrosion resistance of NAB. And the mechanism was clarified from the evolution of corrosion products during the corrosion process.