Simulated Pressurized Water Reactor Environment Research Articles

Short communication: “Effect of dissolved hydrogen on the early corrosion behavior of 316 stainless steel in simulated PWR environments”

In simulated pressurized water reactor environments, the corrosion test on 316 stainless steel was conducted for 40 to 4000 s, with 25 and 50 cc/kg-H2O dissolved hydrogen concentrations (NH and HH). It was found that the external Fe-rich oxide particles and the inner Cr-rich layer were simultaneously formed at the beginning of corrosion. However, the first formed Cr-rich layer was mainly Cr(OH)3 in the NH condition, while Cr2O3 in the HH condition. As exposure time increased, fraction of Cr2O3 increased due to transformation from Cr(OH)3 in both conditions. In addition, for HH condition, partial transformation to FeCr2O4 was observed.

The single effect of microstructure, residual strain and geometric structure on the stress corrosion cracking behavior of scratched alloy 690TT

Scratched area on alloy 690TT were treated by relief annealing (RA) and solution annealing plus thermal treatment (SATT), respectively, and then detailed characterization and immersion tests in simulated pressurized water reactor environment were carried out. RA reduces residual strain but not changes microstructure stratification, which causes stress corrosion cracking (SCC). SATT eliminates both strain and microstructure stratification, preventing SCC. Meanwhile, the independent effects of geometric structure, strain and microstructure on SCC behavior were investigated. Specifically, geometric structure is necessary but not sufficient for crack initiation. While the residual strain mainly enhances propagation, and the microstructure promotes both initiation and propagation.

Using Direct Current Potential Drop Technique to Estimate Fatigue Crack Growth Rates in Solid Bar Specimens under Environmental Assisted Fatigue in Simulated Pressurized Water Reactor Conditions

The direct current potential drop (DCPD) technique may be used in crack propagation tests to measure the crack growth rate (CGR). Potential probes attached to the specimen allow the variation of the crack length to be estimated. In this research, the DCPD technique using one single potential probe was applied to solid bar specimens (i.e., without any initial notch or crack) subjected to low-cycle fatigue testing in a simulated pressurized water reactor (PWR) environment. This particular analysis had two associated difficulties, the first one being the fact that crack initiation sites are not known beforehand, and the second one consisting in the experimental difficulties and conditioning factors associated with the simulation of the PWR environment. Nine solid bar specimens were tested to fatigue failure under different strain amplitudes and frequencies, while also measuring the corresponding DCPD signal during the fatigue process. It was observed that the initiation of multiple cracks was detected by the DCPD measurements. Moreover, as fatigue continued, one of the cracks became dominant and progressed to cause the specimen failure. The DCPD technique allowed the average CGR of the dominant crack to be estimated. Finally, the obtained average CGRs were validated by comparing them with average CGRs derived from striation spacing measurements, obtained from scanning electron microscopy (SEM) and from literature values gathered in the NUREG/CR-6909 document.

Localised electrochemical processes on laser powder bed fused 316 stainless steel with various heat treatments in high-temperature water

Laser powder bed fusion (LPBF) is an advanced additive manufacturing technology for stainless steel components fabrication, and a comprehensive understanding of electrochemical behaviour of the LPBF stainless steels is critical for expanding their applications in nuclear and other high-temperature water environments. In the present study, comparison of microstructure and high-temperature electrochemical behaviours between LPBF 316 and wrought 316 in simulated pressurized water reactor environment was made. Three heat treatments, stress relieving (SR), solution annealing (SA) and hot isostatic pressing (HIP), were used to heat treat the LPBF 316. The results showed that both the yield strength and impact energy of the SA and HIP treated LPBF 316 were lower than that of the SR treated sample, whilst the tensile elongation of the SA and HIP treated samples was higher than SR treated sample. These changes were found to be due to the disappearance of the cellular structures, decrease in the dislocation density and the occurrence of recrystallization during the treatments. Electrochemical impedance spectroscopy measurements at 288 °C and their interpretation with the Mixed-Conduction Model indicated that corrosion rate of the heat-treated LPBF 316 variants is significantly lower than that of the wrought 316. In addition, SR material exhibits marginally lower corrosion rates than SA and HIP ones. Microstructure examination after high-temperature water exposure revealed an inhomogeneous inner oxide layer on LPBF 316 in contrast to the thicker and more uniform inner oxide layer on wrought 316. The localised nature of electrochemical processes is suggested to be induced by the nano-precipitates in the LPBF samples. • High-temperature electrochemical behaviour of LPBF 316 was investigated • LPBF 316 has a significantly lower corrosion rate than wrought 316 • Heat-treated LPBF samples formed an inhomogeneous inner oxide layer in PWR water • Localised electrochemical processes are induced by nano-precipitates in LPBF 316 s

Early oxidation behavior of Ni-based Alloy 690 in simulated pressurized water reactor environment with varying surface conditions

The composition and morphology of a passive film formed on Ni-based alloy 690 with different surface conditions exposed to high-temperature, high-pressure aerated and deaerated deionized water vapor with different time were characterized by using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) in this study. Shot peened, mechanical polishing and electro polishing were used to obtain different surface conditions. The film thickness remained constant after different exposure treatments, while the film layered structures were different. On specimens exposed to aerated water for 2 min and deaerated water for 1 h, Ni-rich oxide was identified in the outmost oxide films while Cr-rich oxide existed in inner layers. On specimens exposed to aerated water for 1 h, Cr-rich oxide with participating of Ni- and Fe-oxide was identified in all layers, while on specimens exposed to deaerated water for 2 min, Cr-rich oxide in all layers with participating of Ni-oxide was identified. Large oxide particles with a low density were found. The oxide particles were the biggest and least dense for electro polished specimen, whilest smallest and most dense for shot peened specimen. Oxidation for the same time, the surface was oxidized more pronounced in aerated water than in deaeated water.

Environmentally assisted cracking in the fusion boundary region of a SA508-Alloy 52M dissimilar weld joint in simulated primary pressurized water reactor environments

Environmentally assisted cracking behavior in both parallel and perpendicular directions to the fusion boundary (FB) of SA508-Alloy 52M weld joint were investigated. The grain boundaries of dilution zone adjacent to the FB are more prone to cracking than the FB. Strength mismatch and galvanic corrosion promote crack initiation along the FB or the partially mixed zone/low alloy steel interface with the loading direction vertical to the FB. In addition, the FB hinders crack growth perpendicular to the FB, most likely because severe pitting decreases the crack tip stress/strain and oxidation along the FB increases the crack growth distance.

Corrosion behavior of Fe-based candidate accident tolerant fuel cladding alloys in spent fuel pool environment ― Effect of prior corrosion

The corrosion behavior of Fe-based candidate accident tolerant fuel (ATF) cladding alloys, such as FeCrAl and FeNiCrAl alloys, in simulated spent fuel pool (SFP) water was investigated. Specimens with different prior corrosion histories, such as fresh, normal-serviced (immersed in simulated pressurized water reactor (PWR) environment), and accident-suffered (PWR water immersion followed by high-temperature steam exposure) were used. All Fe-based alloys with various prior corrosion histories show weight loss during the SFP immersion test. In the fresh and normal-serviced conditions, all alloys experience relatively small weight loss in SFP water. In the accident-suffered condition, low-Al FeNiCrAl alloy shows large weight loss. Overall, high-Al FeNiCrAl alloy shows comparable corrosion resistance to FeCrAl alloy in SFP water for all prior corrosion histories. The corrosion behavior of the alloys in SFP water is significantly affected by the pre-formed oxides, such that Cr2O3, Al2O3, Fe-rich and Cr-rich spinel oxides offer better corrosion resistance than Fe2O3.

Effect of thermal aging and irradiation on the electrochemical and passivation behavior of 308L stainless steel in simulated pressurized water reactor environment

Effect of thermal aging and irradiation on the corrosion behavior of 308L stainless steel (SS) in simulated pressurized water reactor (PWR) environment was investigated by using electrochemical techniques and surface analysis. The results delineated that the corrosion susceptibility of 308L SS decreased first and then increased obviously after the thermal-aged, thermal-aged and irradiated process, respectively. It can be attribute to variation of the microstructure and composition of the passive film. Compared with the 308L SS, the thermal-aged ones performed a compacter passive film characteristic with oxide Cr2O3, Fe2O3 and hydroxide Ni(OH)3. Whereas, after the thermal-aged and irradiated process, the thickness of the passive film decreased obviously and the hydroxide Ni(OH)3 transform to the unstable NiOOH. As a result, performing higher corrosion susceptibility once more.

Early corrosion behaviour of irradiated FeCrAl alloy in a simulated pressurized water reactor environment

The early corrosion behaviour of an irradiated FeCrAl alloy in a simulated pressurized water reactor environment was investigated. The irradiated alloy showed a Cr-rich oxide layer, which was not observed in the as-received alloy. The corrosion induced the formation of a duplex-layer oxide structure consisting of an outer hematite α-Fe2O3 layer and an inner spinel Cr-rich oxide layer. In the initial corrosion stage, the smooth surface formed by ion sputtering and the formation of the Cr-rich oxide layer suppressed the corrosion. However, in the later corrosion stage, the irradiated alloys exhibited severe corrosion because of the presence of irradiation-induced defects.

Corrosion fatigue crack growth behavior of 316NG heat affected zone in simulated pressurized water reactor environment

The corrosion fatigue crack growth (FCG) behavior of the 316NG heat affected zone (HAZ) exposed to simulated pressurized water reactor (PWR) environment has been investigated. The results show that significant residual strains and lattice misorientations exist in the HAZ near the welding root. Z-phase precipitates are present in both the HAZ and annealed 316NG base metal. The FCG rate of the HAZ is much higher than that of the base metal. In PWR water, the corrosion FCG rate of the HAZ is strongly dependent on the cyclic loading frequency, but the effect of dissolved oxygen is negligible.

Role of residual ferrites on crevice SCC of austenitic stainless steels in PWR water with high-dissolved oxygen

The crevice stress corrosion cracking (SCC) susceptibility of austenitic stainless steels was evaluated in simulated pressurized water reactor (PWR) environments. To simulate the abnormal condition in temporary clamping devices on leaking small bore pipes, crevice bent beam (CBB) tests were performed in the oxygenated as well as hydrogenated conditions. No SCC cracks were found for SS316 in both conditions. SS304 also showed good resistance in the hydrogenated condition. However, all SS304 specimens showed SCC cracks in the oxygenated condition, indicating poor crevice SCC resistance. It was found that residual ferrites were selectively dissolved because of the galvanic corrosion coupled with the neighbouring austenite phase, resulting in SCC initiation in SS304. Crack morphologies were mostly transgranular assisted by the damaged δ-ferrite and deformation-induced slip bands.

Effect of Warm Forging on the Microstructure and Corrosion Behavior of Austenitic Stainless Steel 316LN

The as-received as well as deformed microstructure and electrochemical corrosion behavior of warm-forged and annealed 316LN austenitic stainless steel have been investigated by using electron backscatter diffraction (EBSD) and electrochemical corrosion test methods. An attempt has been made to understand the effect of warm forging and annealing on the microstructure and electrochemical corrosion behavior of 316LN stainless steel. As-received and deformed microstructures of a sample were observed by EBSD, and from micrographs, it was observed that the refinement of grain occurs after deformation. Microstructural refinement occurs after appropriate annealing of the forged sample because of the revision of alpha ferrite and strain-induced martensite into austenite, which is formed after forging. Annealing treatment after warm forging gives better mechanical strength and optimum corrosion resistance because of the formation of homogeneous and equiaxed strain-free austenite grains. After warm forging, around 85 % of grains show a low misorientation angle of 2.5°, which shows that a lot of dislocation generation occurred during warm forging deformation and that these dislocations aligned themselves to form subgrain boundaries also known as recovery during annealing dislocation. The corrosion rate drastically increases after deformation but decreases after the annealing of deformed samples, which indicates that after appropriate annealing the corrosion resistance and repassivation capacity of materials improved because of the formation of stable oxide of chromium. The value of corrosion current density (Ecorr) shifted toward the passive direction in the Tafel plot after annealing of deformed samples, which indicates that a more protective passive film of Cr2O3 formed on the surface of the annealed sample after warm forging. The surfaces of the samples after an immersion test in the FeCl3 solution have been observed by scanning electron microscopy, and from micrographs, it has been observed that the number of pits is reduced after the annealing of a warm-deformed sample as compared with undeformed samples. The pitting potential of the samples was calculated in a simulated pressurized water reactor environment using the Tafel potentiodynamic polarization test.

Effect of zinc on the environmentally-assisted fatigue behavior of 316 stainless steels in simulated PWR primary environment

In a simulated pressurized water reactor (PWR) environment with 30 ppb of Zn and peak holding, the low-cycle fatigue (LCF) life of 316 SS increased about 3 times compared to that without them. For the specimens tested with Zn and strain holding, the crack tip oxide layer was enriched with Zn. Electrochemical analysis showed the oxide layer had higher impedance and less defect density, indicating less corrosion compared to oxide layers from non-Zn-injection conditions. Thus, the improvement in LCF life of 316 SS was attributed to crack tip oxide modification by the combined effect of Zn-addition and peak strain holding.

Stress corrosion cracking characteristics of CF8A austenitic stainless steels and interactions between multiple cracks in a simulated PWR environment

The stress corrosion cracking behaviors of CF8A austenitic stainless steels in a simulated pressurized water reactor environment are systematically investigated by slow-strain-rate tensile tests. Numerous transgranular stress corrosion cracking (TGSCC) cracks are identified upon the fracture of CF8A austenitic stainless steel surfaces. The specimens tested at lower strain rates showed higher fractions of TGSCC, a larger number of cracks, and smaller crack opening displacement. These phenomena are caused by anodic dissolution and crack interactions. The anodic dissolution is dominant at lower strain rates, and thus the crack-tip morphologies are changed by the diffusion of metal atoms and micro-pores. The crack interactions increase rapidly with increasing number of cracks and decrease of the crack-tip radius and crack spacing ratio. In addition, the inhomogeneity of the growth of interaction cracks results in the small cracks being shielded by the preferred growth cracks. On the contrary, the main cracks are accelerated by the stagnation of small cracks, eventually becoming critical cracks that lead to specimen fracture.

Evaluation of Grain Boundary Network and Improvement of Intergranular Cracking Resistance in 316L Stainless Steel after Grain Boundary Engineering.

For understanding the improvement of intergranular stress corrosion cracking (IGSCC) propagation in grain boundary engineering (GBE)-processed metals exposed to a simulated pressurized water reactor (PWR) environment, characteristics of the grain boundary network of 316L stainless steel before and after GBE were investigated and compared, including proportions both in length and in number of ∑3n boundaries, sizes, and topology of grain clusters (or twin-related domains), and connectivity of random boundaries. The term through-view random boundary path (TRBP) was proposed to evaluate the random boundary connectivity. A TRBP is a chain of end-to-end connected crack-susceptible boundaries that passes through the entire mapped microstructure. The work provides the following key findings: (I) the length fraction of ∑3n boundaries was increased to approximately 75% after GBE, but the number fraction was only approximately 50%; (II) a connected non-twin boundary network still existed in the GBE sample due to the formation of grain clusters; (III) the GBE sample exhibited a higher resistance to IGSCC; and (IV) as the twin boundary fraction increased, the number of TRBPs decreased and the normalized length of the minimum TRBP increased monotonically, leading to a higher resistance to IGSCC.

Comparison of oxide layers formed on the low-cycle fatigue crack surfaces of Alloy 690 and 316 SS tested in a simulated PWR environment

Abstract Low-cycle fatigue (LCF) tests were performed for Alloy 690 and 316 SS in a simulated pressurized water reactor (PWR) environment. Alloy 690 showed about twice longer LCF life than 316 SS at the test condition of 0.4% amplitude at strain rate of 0.004%/s. Observation of the oxide layers formed on the fatigue crack surface showed that Cr and Ni rich oxide was formed for Alloy 690, while Fe and Cr rich oxide for 316 SS as an inner layer. Electrochemical analysis revealed that the oxide layers formed on the LCF crack surface of Alloy 690 had higher impedance and less defect density than those of 316 SS, which resulted in longer LCF life of Alloy 690 than 316 SS in a simulated PWR environment.

The oxidation behavior of 316L in simulated pressurized water reactor environments with cyclically changing concentrations of dissolved oxygen and hydrogen

The oxidation behavior of 316L stainless steel in the simulated pressurized water reactor primary water with cyclic exposure to hydrogenated and oxygenated water conditions was investigated by contact electric resistance, electrochemical impedance spectroscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy techniques. The electrochemical resistance of the oxide film exhibits significant changes as the environment is shifted from hydrogenated water chemistry to oxygenated water chemistry and vice versa. However, in different hydrogenated periods (or oxygenated periods) with the same water chemistry, the electrochemical resistance of the oxide film is very similar. On the contrary, the oxide morphology and composition do not show obvious changes, and the oxide film thickness has a generally increasing trend. The reason for this phenomenon was also discussed.

Uniaxial low cycle fatigue behavior for pre-corroded 16MND5 bainitic steel in simulated pressurized water reactor environment

The effects of uniaxial tension properties and low cycle fatigue behavior of 16MND5 bainitic steel cylinder pre-corroded in simulated pressurized water reactor (PWR) were investigated by fatigue at room temperature in air and immersion test system, scanning electron microscopy (SEM), energy disperse spectroscopy (EDS). The experimental results indicated that the corrosion fatigue lives of 16MND5 specimen were significantly affected by the strain amplitude and simulated PWR environments. The compositions of corrosion products were complexly formed in simulated PWR environments. The porous corrosion surface of pre-corroded materials tended to generate pits as a result of promoting contact area to the fresh metal, which promoted crack initiation. For original materials, the fatigue cracks initiated at inclusions imbedded in the micro-cracks. Moreover, the simulated PWR environments degraded the mechanical properties and low cycle fatigue behavior of 16MND5 specimens remarkably. Pre-corrosion of 16MND5 specimen mainly affected the plastic term of the Coffin-Manson equation.

Investigation of Fatigue Crack Propagation Behavior on the Surface of Machining Notches of Alloy 690TT in Simulated Pressurized Water Reactor Water

The fatigue crack propagation behavior on the surface of machining notches of Alloy 690TT (thermally treated) was investigated in room-temperature air environment and in simulated pressurized water reactor (PWR) environment with saturated air at 325°C. The results showed that more than one crack formed on the notch surface and the crack propagation paths were transgranular with some branching. The grain size also had some effect on the crack propagation path but had no significant effects on the crack growth rates (CGRs). The CGR vs. a curve could be divided into two stages in air environment and three stages in simulated PWR environment. The corrosive environment promotes crack propagation in the macroscopic crack propagation stage. Different load frequencies express similar crack propagation process, but decreasing frequency can assist the corrosion fatigue crack nucleation and propagation on the notch surface.

Stress corrosion cracking behavior of cold-drawn 316 austenitic stainless steels in simulated PWR environment

The stress corrosion cracking (SCC) behavior of solution-treated and cold-drawn 316 austenitic stainless steels was investigated in simulated pressurized water reactor environment by slow strain rate tensile test. Both cold drawing and Cl addition in simulated primary environment can observably increase SCC susceptibility of 316 austenitic stainless steels. The oxides containing higher Cr concentration initially nucleate on crack tip, and then react with dissolved oxygen and H2O to form the Fe-rich oxides on crack flank during the subsequent crack propagation in abnormal water containing Cl. The deformation twins activated by cold-drawing provide paths to accelerate the oxygen diffusion during SCC.