High Temperature Water Environments Research Articles

Optimal Selection of the Reference Potential Probe Point in DCPD Real-Time Monitoring of Crack Growth Rate

Quantitatively monitor the crack growth rate of material stress corrosion cracking (SCC) in an autoclave that simulates a high-temperature and high-pressure water environment, and the direct current potential drop (DCPD) method is the main method. Since the DCPD method tests micro-nano-voltage drop signals, the monitoring signal is weak and easy to be interfered by the environment. To reduce and balance the error caused by the temperature drift and other factors to the monitoring accuracy, it is very important to reasonably select the position of the reference potential probe point. In this study, genetic algorithm (GA), finite element method (FEM), and experimental analysis are used to optimize the position of the reference potential probe point of the compact tensile (CT) sample. Finite element method is used to analyze the electric potential field of the compact tensile sample, a mathematical model of the measurability and crack independence of the reference potential difference are constructed, genetic algorithm is used to find the optimal reference potential difference (RPD) probe point position, and finally, the crack monitoring experiments are performed to evaluate the feasibility of algorithm optimization results. The results indicate that the RPD measured at the current input point and the upper right position of the CT sample can provide the maximum compensation for the potential on both sides of the crack and make the performance of the monitoring signal optimal.

Roles of permeated hydrogen in the oxidation process of Ni-based alloy in high temperature water environment

The present research proposes the synergistic roles of H involved in the oxidation process of a Ni-based alloy in high temperature water environment, using an in-situ H-permeating method. The roles of H were differentiated according to the modification on the microstructure during oxidation in high-temperature water, as H is situated at multiple locations during permeating the material. The nature of the oxide was found to be modified significantly, however, it exhibited possibly opposing impacts on the reliability of the materials. The integrity of surface oxide film and matrix/oxide interface were degraded, yet grain boundary was protected by permeated H.

Mechanical and metallurgical considerations on the effects of ppb-level chloride on stress corrosion cracking of low alloy steels in high-temperature water

This paper studies the effects of very low-level chloride (< 5 ppb) on the stress corrosion cracking (SCC) growth of low-alloy pressure vessel steel in oxygenated high-temperature water. Chloride levels as low as 3 ppb increase SCC susceptibility. Besides the chemical reason, this paper provides perspectives on the underlying mechanical and metallurgical contributions. A substantial variation was observed from three high-sulphur steels. Sulphur is not the only contributor to SCC. The synergy of material, environmental, and mechanical parameters controlled the crack-tip strain rate and crevice chemistry that govern the SCC response in the high-purity high-temperature water environments at intermediate K.

Acceleration of Environmentally Assisted Cracking Initiation of the Type 316L Steel in High-Temperature Water and Hydrogenated-Steam Vapor Environments

Environmentally assisted cracking of solution-annealed Type 316L austenitic stainless steel with two different surface treatments (polished versus ground) was investigated using the constant extension rate tensile test methodology in high-pressure water at 350°C and low-pressure H2-steam vapor at 350°C, 400°C, 440°C, and 480°C, while maintaining electrochemical corrosion potentials in the NiO stability regime. Flat tapered specimens were used to indentify the threshold stress for cracking (∼400 MPa) under these environmental conditions. Intensive oxidation and typical intergranular cracks were observed to initiate from the polished surface at 400°C and 440°C, whereas wavy cracks initiated on the ground surface. The results indicate that the ultrafine-grained layer formed adjacent to the ground surface effectively suppressed intergranular crack initiation under these test conditions. The slow loading under H2-steam vapor at 400°C under the oxidizing condition (NiO) was found to be a suitable high-accelerated test to study early stages of the cracking.

Corrosion fatigue model of austenitic stainless steels used in pressurized water reactor nuclear power plants

Low cycle fatigue behavior of 316LN stainless steel (SS), 304 SS and 308L weld metal was investigated in air and high-temperature pressurized water environment. The effects of mechanical factors and environmental factors on fatigue lives were considered. The fatigue mean curve and design curve for nuclear-grade austenitic SSs in air are obtained by fitting fatigue data. The corrosion fatigue model for nuclear-grade austenitic SSs in high-temperature pressurized water environment is proposed based on environmental fatigue correction factor method, which mainly considers the effects of strain rate, temperature and dissolved oxygen concentration.

Microstructural characterization of the synergic effects of dynamic strain ageing and hydrogen on fracture behaviour of low-alloy RPV steels in high-temperature water environments

Tensile tests in high-temperature air with pre-charged hydrogen and elastic plastic fracture mechanics tests in hydrogenated high-temperature water (HTW) at 250 and 288 °C on low-alloy reactor pressure vessel (RPV) steels revealed a clear but moderate reduction of ductility and fracture resistance, respectively. The observed behaviour is a consequence of synergistic effects between hydrogen embrittlement (HE) and the dynamic strain ageing (DSA), in which the HE was amplified by a high DSA susceptibility. The deformation microstructures in the vicinity of the crack tips in air and HTW of two RPV steels with high DSA susceptibility were characterized in detail. These investigations support the idea that the environmental degradation of fracture resistance in hydrogenated HTW was mainly due to the plasticity localization by the interaction between DSA and hydrogen in RPV steels. Synergistic effects of DSA and hydrogen lead to heterogeneous distribution of dislocations and formation of dislocation cells inside bainitic laths.

Amorphization and structural modification of the oxide film of Ni-based alloy by in-situ H charging in high temperature high pressure water environment

It is often assumed only limited amount of amorphous phase could generate in the oxide film of Ni-based alloy with H-participation. However, in this study, the formation of a remarkable amorphous layer on top of the oxide film of Ni-based Alloy 600 (Ni-15.5Cr-9.4Fe, wt.%) after oxidizing in high-temperature high-pressure water environment by in-situ H-charging condition is observed. A synergy of the film porosity and PH2O/PH2 is proposed as a plausible mechanism. Cavity layer appeared at the oxide and matrix interface, and the H-distribution are also discussed.

Study on the Rate of Elastic-plastic Crack Propagation of Heterogeneous Metal Welded Joints in Nuclear Power

In the high-temperature water environment of nuclear power, austenitic stainless steel, nickel-based alloy materials and welded joints composed thereof are the key locations for environmental cracking (EAC) problems such as stress corrosion cracking (SCC). In order to understand the crack propagation behavior and expansion rate of welded joints under the condition of abrupt and heterogeneous material properties of welded joints, this paper uses the extended finite element technique (XFEM) in the large nonlinear finite element software ABAQUS, based on the elastoplastic material model. Under the condition of welding residual stress field, the effects of different initial crack length and strength mismatch ratio on the crack growth rate of stress corrosion crack under constant load were studied. The results show that the SCC crack growth rate increases with the increase of the initial crack length. When the crack length of the initial welded joint is constant, different mismatch ratio has a certain effect on the crack growth rate. In the low ratio range(less than 1), the crack growth rate also increase if the strength mismatch ratio increase. but the crack growth rate is gradually reduce in high ratio range(more than 1). This indicates that the mismatch ratio of 1 is favorable for crack propagation, and the larger the mismatch ratio, the crack propagation is hindered.

Stress corrosion cracking of simulated heat-affected zone in a CF8A weld in high temperature water

In this study, thermal simulation was performed to conduct thermal cycles on CF8A cast austenitic stainless steel substrates with different ferrite contents to simulate the microstructures that form near a weld fusion boundary. The effects of ferrite content and morphology on the mechanical properties of CF8A were investigated. The stress corrosion cracking (SCC) susceptibility of the specimens was evaluated with slow strain rate tensile (SSRT) tests in a simulated boiling water reactor (BWR) coolant environment.The results indicated that an increase in the ferrite content of the CF8A base metal led to a minor increase in hardness. Thermal simulation caused a marked increase in ferrite content, especially in the high ferrite CF8A substrate. The yield strengths of the tested samples increased obviously with increasing ferrite content, but the ductility decreased. The ultimate tensile strengths of the samples reached a plateau when the ferrite number (FN) was greater than 18. The applied thermal simulation greatly improved the impact toughness of high ferrite CF8A in comparison with the original substrate. The results indicated that the samples with high ferrite content had a shorter rupture life but greater fracture strength than those with low ferrite content strained in high temperature water. Moreover, cleavage-like fracture was found in all of the samples that suffered from SCC. The SCC cracks were observed to be more likely to propagate along the δ/γ interfaces of a CF8A. Furthermore, crack initiation and growth inside the γ of low ferrite CF8A contributed to a great loss in strength in a high temperature water environment.

Effect of overload on the oxidation behavior of CF8A austenitic stainless steel in a high-temperature water environment

The oxidation behavior of CF8A austenitic stainless steel was investigated under constant load and overload tests in a high-temperature water environment. Compared with the constant load condition, the protective performance of the oxide film formed on the matrix under the overload condition was seriously degraded. The main manifestation is the transformation of the inner oxide from Cr2O3 to non-stoichiometric spinel (Ni,Fe)(Fe,Cr)2O4 under the overload condition. This is due to an increase in the chemical potential of the matrix and a change in the orientation between the oxide films and the matrix after overloading.

Effects of dendrite axis and fusion boundary on stress corrosion cracking of ER 308 L/SS 304L welds in a high-temperature water environment

This study investigated the effects of dendrite axis and fusion boundary (FB) on the stress corrosion cracking (SCC) of 308 L/304 L welds by using an alternating current potential drop (ACPD) technique to measure the SCC growth rates in a simulated boiling water reactor environment. The results show that the ferrite/austenite interfaces are more susceptible to SCC in the SS 308 L welds. Furthermore, the angle between the applied load and the dendrite axis affects the SCC growth rate, and as this angle approached 90°, the SCC crack growth rate increases. The SCC cracks in the heat affected zone (HAZ) are mainly intergranular and some cracks propagate transgranularly. Furthermore, the primary crack in the HAZ propagates along the FB; when reaching FB, some secondary cracks are arrested at the FB. The residual strain caused by weld shrinkage is the crucial factor for the increase in SCC susceptibility in HAZ.

Mean Stress Effect on Fatigue Properties of Type 316 Stainless Steel in Pressurized Water Reactors Primary Water Environment

The mean stress effect on the fatigue life of type 316 stainless steel was investigated in simulated pressurized water reactor (PWR) primary water and air at 325 °C. The tests in air environment have revealed that the fatigue life was increased with application of the positive mean stress for the same stress amplitude because the strain range was decreased by hardening of material caused by increased maximum peak stress. On the other hand, it has been shown that the fatigue life obtained in simulated PWR primary water was decreased compared with that obtained in air environment even without the mean stress. In this study, type 316 stainless steel specimens were subjected to the fatigue test with and without application of the positive mean stress in high-temperature air and PWR water environments. First, the mean stress effect was discussed for high-temperature air environment. Then, the change in fatigue life in the PWR water environment was evaluated. It was revealed that the change in the fatigue life due to application of the mean stress in the PWR water environment could be explained in the same way as for the air environment. No additional factor was induced by applying the mean stress in the PWR water environment.

High temperature and high pressure flowing water corrosion resistance of multi-arc ion plating Cr/TiAlN and Cr/TiAlSiN coatings

In order to improve the corrosion resistance of zirconium alloy in high temperature and high pressure water environment, Cr/TiAlN coating and Cr/TiAlSiN coating were prepared on the surface of Zr-4 alloy by multi-arc ion plating (MAIP). The corrosion resistance of the two coatings in the autoclave was compared by various analytical methods:The film-based bonding force of the coating was detected by an automatic scratch test; the microstructure of the coating was observed by scanning electron microscope (SEM); the porosity and the quantity of large particles on the surface were analyzed by Image J software; the phase of the coating was analyzed by x-ray diffractometer (XRD); the type and content of the coating were detected by energy dispersive spectrometer (EDS). The experiment result shows that the surface of the two coatings is relatively flat, and the surface of the Cr/TiAlSiN coating is finer than that of the Cr/TiAlN coating, and the number of large particles is small and the density is high. The critical loads that the Cr/TiAlN coating and the Cr/TiAlSiN coating can withstand are 34 N and 29 N respectively. Under the experimental conditions, part of the Cr/TiAlN coating peeled off and localized corrosion occurred; A large amount of ZrO2 was detected on the surface of the sample, the coating was destroyed and the protective effect was lost. While, the Cr/TiAlSiN coating is almost intact, it effectively preventing the substrate from being corroded. In conclusion: the coating quality of multi-arc ion plating Cr/TiAlSiN coating is better than that of Cr/TiAlN coating, and the corrosion resistance is better. It can protect the zirconium alloy matrix effectively. However, the residual stress is generated due to the presence of Si in the Cr/TiAlSiN coating, which resulting in a low film-based bonding force.

Influence of nanoparticle dispersibility on hygrothermal properties of HNT-GFRP/BFRP nanocomposites

In the nanocomposite, the dispersion and bonding properties of the nanoparticles at the interfaces were major factors that were related to the physical properties of the final product. Nanoparticle aggregation mainly occurs in the dispersion process between nanoparticles and a polymer. In addition, re-aggregation phenomenon generates by large amounts of pores during curing process. In this case, it was necessary to break the physical bonds, such as the attraction between the initial nanoparticles, and achieve a smooth chemical bonding with the polymer. In this study, the effect of halloysite nanotube (HNT) dispersion on the interfacial bonding was compared and analyzed in [Formula: see text] high temperature water environment over 30 days by adding 0.5 wt.% HNT to glass fiber reinforced plastic (GFRP) and basalt fiber reinforced plastic (BFRP). As a result, the HNT contributed to disturbing the moisture absorption rate. It has great effect on thinner lamination part. Moreover, the thicker the lamination, the less the HNT re-aggregation occurred on the curing process. This phenomenon showed a uniform dispersion in the entire laminate area. The weight recovery rate by moisture absorption was high in the HNT-glass fiber (GF), because structural relationship between HNT and GF are larger than in epoxy resin.

Influence of fiber laminated thickness on the hygrothermal property of HNT-GFRP nanocomposites for the RFI process-based structures

Glass fiber–halloysite nanotube (GF/HNT) nanocomposites were fabricated with different numbers of fiber layers and were exposed to a high-temperature water environment to analyze the role of the HNTs regarding the relationship of interface between the fibers and the moisture absorption behavior. The sections of the GF/HNT nanocomposite plates were divided, and the change in the moisture absorption rate was measured. As a result, the ratio of the HNT to epoxy resin was not related to the water deterioration phenomenon according to the fiber-laminated thickness. However, in the curing process after the vacuum bag molding, the external pressure that was applied to each interface influences the flow characteristics and promotes the occurrence of HNT agglomeration, which interferes with interfacial crosslinking.

Structure of the Oxide Films Formed on Hydrogen-Charged and Uncharged 316L Stainless Steels in Oxygenated High Temperature Water

Austenitic stainless steels (SSs) are commonly used in pressurized water reactors (PWRs) nuclear power systems as key structural materials. The corrosion resistance are closely related to the passive film formed on the surfaces of austenitic alloys in high-temperature aqueous environments. It has been reported that hydrogen can affect corrosion, localized corrosion as well as stress corrosion cracking of materials in aqueous solutions. It is necessary to investigate the influence of hydrogen on the corrosion behavior of stainless steels material in high temperature water environments. In the present study, the oxide films formed on 316L SS with uncharged (316L) and hydrogen-charged (316L-H) in oxygenated simulated PWR primary water at 300℃were investigated. Oxygen was added in high temperature water to simulated occlude region condition or faulted water chemistry. The aim of this work is to clarify the effect of hydrogen in stainless steel on the corrosion behavior. The surface morphologies of the oxide films formed on 316L (uncharged) and 316L-H (charged) specimens after exposure to oxygenated PWR primary water at 300 oC for168 h were observed. The regular and irregular oxides particles homogeneous distributed on the outer surface of 316L specimen, and compact filamentary oxides in the sub-surface. On 316L-H specimen, large oxides dispersed and inhomogeneous distributed on the outer surface, and compact minor polyhedral oxide particles packed on the sub-surface. The number of the large oxides on the outermost surface of 316L specimen was more than that on the 316L-H sample. The cross-section of the oxide films formed on 316L specimens after exposed to oxygenated PWR primary water was analyzed by FIB-TEM. The interface front between the inner oxide layer and the base metal was wavy-shaped. There were compact Cr-rich oxides in the inner layer, and filamentary (Fe, Ni)-rich oxides in the outer layer. There was slightly Ni-enrichment near the inner oxides/matrix interface and Fe-enrichment beneath the inner oxides/matrix interface from EDS line scan profile. Some discontinuous cellular oxides distributed on the inner oxides/matrix interface on 316L-H after the exposure test, which was Fe-rich oxides identified by EDS maps and line scan. Similar to the 316L specimen, the inner layer oxide was compact and Cr-edriched, showing a slightly Fe-enriched beneath the inner oxides/matrix interface, but no Ni-enrichment close to the interface of inner oxides/matrix was observed. Figure 1

Microstructural Evolution of 52M Weld Metal Near the Fusion Boundary and Oxide Films Formed in Simulated PWR Primary Water

Environmentally assisted cracking of structural materials such as dissimilar metal welds in high temperature water environments is strongly affected by the surface oxide film formed in the operating environments. In the present work, Nickel-base 52M weld metal samples are taken from two dilution zones and undiluted zone, and the microstructure was analyzed. Oxide films formed on these samples immersed in simulated PWR primary water at 325oC for 100h. The films are duplex-layered with the outer layer consisted of larger and scattered particles, and the inner layer consisted of small and dense particles. With increasing the distance from FB, the number of larger particles of outer layer decreased while the appearance of smaller particle of inner layer changed into needle-liked particles. With the increase of distance, the proportion of needle-liked particles increase, but the larger particles became less angular. The inner layer became thinner while the size of larger particles of outer layer became smaller with the distance from FB increases. The outer layer was Fe-rich while the inner layer was Cr-rich. The change of element concentration resulted in different oxidization behavior in PWR primary water. The formation of larger particles of outer layer can influence the appearance of inner oxide film. Figure 1

XPS Analysis of the Oxide Films Formed on Non-Charged and Hydrogen-Charged 316L SS in Deaerated High Temperature Water

Austenitic stainless steel components have been used in pressurized water reactors (PWRs) coolants. These austenitic stainless steels may absorb hydrogen from either the dissociation of dissolved hydrogen in PWR primary water or the corrosion processes at the steel surfaces during the immersion in PWR primary water. It is necessary to investigate the effect of hydrogen in alloys on the oxidation and resultant environmental degradation processes in high temperature water environments. In the present study, the oxide films formed in deaerated PWR primary water at 300℃on a cold worked 316L (316LCW) stainless steel with or without prior hydrogen-charging were characterized. Hydrogen-charging of 316LCW was performed by cathodic charging in sulfuric acid solution containing thiourea at room temperature. Prior to the immersion tests, the Immersion tests were performed in a 2-L Ti-autoclave with solution deaerated by purging N2 gas before heating to test temperature. The oxide films formed on 316LCW and hydrogen-charged 316L CW (316LCW-H) specimens after exposure to deaerated PWR primary water at 300 oC for168 h were observed. The XPS depth profiles of the oxide films formed on 316LCW and 316LCW-H specimens after exposed to simulated PWR primary water are compared. The thickness of the oxide film was estimated from the half height of oxygen concentration and the sputtering rate (0.1 nm/s), for determining the thickness of the oxide film that was about 180 nm on 316LCW and about 250 nm on 316LCW-H. The oxide films formed on both 316LCW and 316LCW-H were enriched in Fe and Ni in the outer layer, and enriched in Cr in the inner part of the oxide film. The Cr content in the inner oxide layer formed on 316LCW-H specimen was lower than that on 316LCW specimen. Figure 1

Environmental degradation of fracture resistance in high-temperature water environments of low-alloy reactor pressure vessel steels with high sulphur or phosphorus contents

The fracture behaviour of low-alloy reactor pressure vessel steels with various sulphur and phosphorus contents, different environmentally-assisted cracking (EAC) and temper embrittlement (TE) susceptibilities was evaluated by elastic plastic fracture mechanics tests in air and various simulated light water reactors environments. A moderate but clear reduction of fracture initiation resistance occurred in a) high-sulphur steel with high EAC susceptibility in oxygenated high-temperature water with aggressive occluded crevice environment and preceding EAC growth, and in b) high-phosphorus steel with high TE susceptibility, where the reduction of fracture resistance was most pronounced in hydrogenated high-temperature water.

Effect of dynamic strain ageing on environmental degradation of fracture resistance of low-alloy RPV steels in high-temperature water environments

Elastic plastic fracture mechanics tests on different low-alloy reactor pressure vessel (RPV) steels in air and various simulated light water reactor environments revealed a clear, but moderate reduction of the fracture initiation resistance of RPV steels with high susceptibility to dynamic strain ageing (DSA) in high-temperature water (HTW). In HTW, the reduction of fracture initiation resistance increased with decreasing loading rate and was most pronounced in hydrogenated HTW. The stronger reduction of fracture resistance in hydrogenated HTW was mainly due to the localization of plastic deformation by interaction between DSA and hydrogen.