PWR Primary Water Research Articles

Effects of dissolved hydrogen and surface condition on the intergranular stress corrosion cracking initiation and short crack growth behavior of non-sensitized 316 stainless steel in simulated PWR primary water

The effects of dissolved hydrogen (DH) and inner surface finish condition of the hollowed specimen on stress corrosion crack (SCC) initiation and small crack growth behaviors of 316 stainless steel were investigated in high temperature water with various DH levels. The drilled specimen showed higher surface hardness and more enhanced SCC initiation and short crack growth behavior than honed specimen but both showed a similar trend in the water chemistry with different DH levels. Fractographic examinations showed mixed modes of transgranular and intergranular cracking. The influences of DH and surface condition on SCC behaviors are discussed.

Fatigue life prediction of 316 stainless steel in simulated PWR primary water by time domain analysis based on short fatigue crack growth behavior

Fatigue life prediction of 316 stainless steel in simulated PWR primary water by time domain analysis based on short fatigue crack growth behavior

Effects of dissolved hydrogen on general corrosion behavior and oxide films of alloy 690TT in PWR primary water

The effect of dissolved hydrogen (DH) on the general corrosion behavior and oxide films of Alloy 690TT is investigated in simulated primary water at 330 °C. With increasing DH, the structure of oxide film significantly changed and the corrosion rate decreased. At DH = 5 cm3/kg H2O, the oxide layer was thick, and consisted of outer Ni oxide layer and inner Cr2O3 layer. Under the conditions of DH = 35 and 100 cm3/kg H2O, the oxide films grew thinner and composed of outer polyhedral spinel oxide particles such as NiCr2O4 or NiCrFeO4 and an intermediate metallic Ni-rich layer, with inner Cr2O3 layer. The general corrosion rate significantly decreased by about 72% as DH concentration increased from 5 to 35 cm3/kg H2O. In the range of 35–65 cm3/kg H2O, the corrosion rate slightly decreased with increasing DH concentration. However, no further changes were observed in the range of 65–100 cm3/kg H2O.

Effects of Dissolved Gas and Cold Work on the Electrochemical Behaviors of 304 Stainless Steel in Simulated PWR Primary Water

The effects of dissolved gas and cold work on the electrochemical behaviors of Type 304 stainless steel were investigated in simulated pressurized water reactor primary water environment by potentiodynamic polarization and electrochemical impedance spectroscopy. Results reveal that both dissolved gas and cold work have a significant influence on anodic passivation properties and corrosion resistance of the oxide film. In Ar deaerated water and oxygenated water environments, the corrosion potential of stainless steel and Pt is controlled by the dissolved oxygen concentration, and the corrosion resistance of the inner barrier layer is much higher. However, in H2 deaerated water, the corrosion potential is depressed by the H2/H2O redox potential and the inner oxide film formed in H2 deaerated water is less stable. In addition, the passive region of polarization curves on 304 stainless steel in oxygenated and Ar deaerated water is controlled by metal dissolution instead of water oxidation, and the passive cur...

A mechanistic study of the temperature dependence of the stress corrosion crack growth rate in SUS316 stainless steels exposed to PWR primary water

Complementary analytical transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) were used to study the influence of temperature on the crack growth rate (CGR) in SUS316 stainless steels. An Arrhenius-type temperature dependence of the CGR has been observed between 250 °C and 320 °C. However, stress corrosion cracking (SCC) CGRs were found to decrease between 320 °C and 360 °C, which cannot be explained in terms of a single operating mechanism. High-resolution characterization has produced direct evidence that the SCC CGR in SUS316 is subjected to, at least, two rate-controlling processes: thermally activated diffusion and mechanical response to external stress and internal strain. While diffusion of metallic and non-metallic species at the crack flanks are enhanced at higher temperatures (350 °C and 360 °C), mechanical response-based mechanisms appear to dominate at lower temperatures (320 °C and 340 °C). Higher strain concentrations and dislocation densities around the crack tip were found at low temperature, potentially leading to accelerated crack growth and a peak in the CGR at ∼320 °C. It is suggested that phenomena occurring near the crack tip can be potentially very different at high and low temperatures.

Effect of temperature and dissolved hydrogen on oxide films formed on Ni and Alloy 182 in simulated PWR water

Alloy 182 is a nickel-based weld metal, which is susceptible to stress corrosion cracking in PWR primary water. It shows a peak in SCC susceptibility at a certain temperature and hydrogen concentration. This peak is related to the electrochemical condition where the Ni to NiO transition takes place. One hypothesis is that the oxide layer at this condition is not properly developed and so the material is not optimally protected against SCC. Therefore the oxide layer formed on Alloy 182 is investigated as a function of the dissolved hydrogen concentration and temperature around this Ni/NiO transition. Exposure tests were performed with Alloy 182 and Ni coupons in a PWR environment at temperatures between 300 °C and 345 °C and dissolved hydrogen concentration between 5 and 35 cc (STP)H2/kg. Post-test analysis of the formed oxide layers were carried out by SEM, EDS and XPS. The exposure tests with Ni coupons showed that the Ni/NiO transition curve is at a higher temperature than the curve based on thermodynamic calculations. The exposure tests with Alloy 182 showed that oxide layers were present at all temperatures, but that the morphology changed from spinel crystals to needle like oxides when the Ni/NiO transition curve was approached. Oxide layers were present below the Ni/NiO transition curve i.e. when the Ni coupon was still free of oxides. In addition an evolved slip dissolution model was proposed that could explain the observed experimental results and the peak in SCC susceptibility for Ni-based alloys around the Ni/NiO transition.

Intergranular oxidation of Ni-based Alloy 600 in a simulated PWR primary water environment

Oxidation testing of Alloy 600 was conducted in PWR simulated primary water to obtain a clear understanding of intergranular cracking. The degree of intergranular oxidation was highly dependent on the grain boundary character. Random high-angle grain boundaries were deeply oxidized, and nano-crystalline Cr2O3 oxides formed. However, oxygen diffusion was significantly suppressed on special boundaries. Metallic Ni was segregated as pure Ni nodules, indicating that the selective oxidation of Cr preferentially occurred. Intergranular oxidation and the resultant embrittlement of the grain boundaries are believed to be major factors affecting the occurrence of intergranular cracking in this alloy.

The effects of cold rolling orientation and water chemistry on stress corrosion cracking behavior of 316L stainless steel in simulated PWR water environments

Stress corrosion cracking behaviors of one-directionally cold rolled 316L stainless steel specimens in T–L and L–T orientations were investigated in hydrogenated and deaerated PWR primary water environments at 310 °C. Transgranular cracking was observed during the in situ pre-cracking procedure and the crack growth rate was almost not affected by the specimen orientation. Locally intergranular stress corrosion cracks were found on the fracture surfaces of specimens in the hydrogenated PWR water. Extensive intergranular stress corrosion cracks were found on the fracture surfaces of specimens in deaerated PWR water. More extensive cracks were found in specimen T–L orientation with a higher crack growth rate than that in the specimen L–T orientation with a lower crack growth rate. Crack branching phenomenon found in specimen L–T orientation in deaerated PWR water was synergistically affected by the applied stress direction as well as the preferential oxidation path along the elongated grain boundaries, and the latter was dominant.

290℃および320℃でのニッケル基合金の応力腐食割れ感受性に及ぼすPWR一次系模擬水中溶存水素濃度の影響

290℃および320℃でのニッケル基合金の応力腐食割れ感受性に及ぼすPWR一次系模擬水中溶存水素濃度の影響

Effects of long-term thermal aging on the stress corrosion cracking behavior of cast austenitic stainless steels in simulated PWR primary water

The stress corrosion cracking (SCC) behavior of cast austenitic stainless steels of unaged and thermally aged at 400 °C for as long as 20,000 h were studied by using a slow strain rate testing (SSRT) system. Spinodal decomposition in ferrite during thermal aging leads to hardening in ferrite and embrittlement of the SSRT specimen. Plastic deformation and thermal aging degree have a great influence on the oxidation rate of the studied material in simulated PWR primary water environments. In the SCC regions of the aged SSRT specimen, the surface cracks, formed by the brittle fracture of ferrite phases, are the possible locations for SCC. In the non-SCC regions, brittle fracture of ferrite phases also occurs because of the effect of thermal aging embrittlement.

In Situ X-Ray Diffraction Measurement Method for Investigating the Oxides Films on Austenitic Stainless Steel in Simulated Pressurized Water Reactor Primary Water

Synchrotron x-ray diffraction analytical techniques have been used to investigate the structure of oxide films formed on Type 316L (UNS S31603) austenitic stainless steel in simulated pressurized water reactor primary water. An in situ technique for investigating the layer structures of oxide films has been developed using this measurement method. The observed layer structures of the oxide films changed depending on the dissolved hydrogen concentration (DH) in PWR primary water. In two cases, where DH = 5 cm3/kg (H2O) or 30 cm3/kg (H2O), a (NixFe(1−x))Fe2O4-type spinel oxide was observed as the outer oxide, and a FeCr2O4-type spinel oxide was detected as the thin inner oxide. When DH = 30 cm3/kg (H2O), the Fe:Ni ratio in the (NixFe(1−x))Fe2O4 outer spinel oxide was much larger than when DH = 5 cm3/kg (H2O). In addition, sequential in situ measurements when the hydrogen concentration varied from 5 cm3/kg (H2O) to 30 cm3/kg (H2O) also demonstrated that the oxide layer structure seemed to adjust its characte...

Advanced microstructural characterization of the intergranular oxidation of Alloy 600

Oxidation of solution-annealed Alloy 600 has been performed in a hydrogenated steam environment, which is considered to simulate PWR primary water exposure. FIB and analytical TEM characterization, coupled with detailed SEM analyses, provided unequivocal evidence of localized grain boundary migration and intergranular oxide penetrations. These localized migrations were associated with pronounced Cr and Fe depletions, highlighting the role of the grain boundaries for Cr diffusion for intergranular oxidation. These findings are discussed in relation to the early stages of intergranular oxidation, diffusion-induced grain boundary migration and subsequent SCC initiation.

Characterization of oxide films formed on Alloy 600 and Alloy 690 in simulated PWR primary water by using hard X-ray photoelectron spectroscopy

Oxide films on Alloy 600 and Alloy 690 formed during exposure to the simulated primary side water of a pressurized water reactor were characterized using hard X-ray photoelectron spectroscopy. The specimens were immersed in a solution containing 500 ppm B + 2 ppm Li with or without dissolved hydrogen at 320 or 360 °C for 24 h. Photoelectrons generated in the oxide films with thickness up to 25 nm and in the substrate alloy underneath the oxide films could be observed simultaneously without any destructive techniques such as sputtering. The oxide films were composed of an inner oxide layer of mainly Cr and a covering hydroxide layer of Ni and Cr, with needle-like oxides distributed on the outermost surface. In addition, alloyed Cr was depleted from the substrate alloy directly underneath the oxide layer. Even though the Ni-based alloys examined were mainly composed of Ni, almost no Ni oxide was present in the oxide films formed in solution with dissolved hydrogen. Most of Ni was incorporated as hydroxide. However, Cr was incorporated both in the hydroxide and the inner oxide layers.

The Effects of Metallurgical Factors on PWSCC Crack Growth Rates in TT Alloy 690 in Simulated PWR Primary Water

Abstract Primary water stress corrosion cracking growth rates (PWSCCGRs) in highly cold-worked thermally treated (TT) Alloy 690 have been recently reported as exhibiting significant heat-to-heat variability. Authors hypothesized that these significant differences could be due to the metallurgical characteristics of each heat. In order to confirm this hypothesis, the effect of fundamental metallurgical characteristics on PWSCCGR measurements in cold-worked TT Alloy 690 has been investigated. The following new observations were made in this study: (1) Microcracks and voids were observed in or near eutectic crystals of grain boundary (GB) M23C6 carbides (primary carbides) after cold rolling, but were not observed before cold rolling. These primary carbides with microcracks and voids were observed in both lightly forged and as-cast and cold-rolled TT Alloy 690 (heat A) as well as in a cold-rolled TT Alloy 690 (heat Y) that simulated the chemical composition and carbide banded structure of the material previously tested by Paraventi and Moshier. However, this was not observed in precipitated (secondary) M23C6 GB carbides in heavily forged and cold-rolled TT Alloy 690 heat A and a cold-rolled commercial TT Alloy 690. (2) From microstructural analyses carried out on the various TT Alloy 690 test materials before and after cold rolling, the amount of eutectic crystals (primary carbides and nitrides) M23C6 and TiN depended on the chemical composition. In particular, the amount of M23C6 depended on the fabrication process. Microcracks and voids in or near the M23C6 and TiN precipitates were generated by the cold rolling process. (3) The PWSCCGRs observed in TT Alloy 690 were different for each heat and fabrication process. The PWSCCGR decreased with increasing Vickers hardness of each heat. However, for the same heats and fabrication processes, the PWSCCGR increased with increasing Vickers hardness due to cold work. Thus, the PWSCCGR must be affected not only by hardness (or equivalently the cold working ratio) but also by grain size, microcracks, and voids of primary M23C6 carbides, etc., which in turn depend on chemical composition and the fabrication process.

Oxidation of Zircaloy-4 during in situ proton irradiation and corrosion in PWR primary water

Abstract

模擬PWR1次系環境にて冷間加工を加えたAlloy600，Alloy690に生成する酸化皮膜の電気化学的性質

模擬PWR1次系環境にて冷間加工を加えたAlloy600，Alloy690に生成する酸化皮膜の電気化学的性質

Influence of surface roughness on the corrosion behavior of Alloy 690TT in PWR primary water

The purpose of this work is to investigate the effect of surface roughness on the corrosion behavior of Alloy 690TT steam generator tube material in simulated primary water at 330°C. The surface roughness was controlled in the range of 710–25nm by mechanical grinding and polishing method. Surface hardness gradually decreased with a decrease in the roughness value. Polyhedral oxide particles were formed on the ground surfaces, whereas the mixed oxides of a polyhedral and strip-like type were observed on the polished surface. The corrosion rate decreased by about 64% as the roughness values decreased from 710 to 150nm. However, no further changes were observed in the range of 150–25nm.

Semiconducting behavior and bandgap energies of oxide films grown on alloy 600 under PWR simulated primary water conditions with different dissolved hydrogen contents

Alloy 600 samples were oxidized under different simulated PWR primary water conditions, with varying contents of dissolved hydrogen. The photoelectrochemical techniques, used to characterize the semiconducting behavior and bandgap energies of oxide films have shown that the scale has several components. The first ones, exhibiting photocurrents at photon energies lower than 2.5eV behave as n-type semiconductors, whereas the analyze of the photocurrents measured at higher energies suggests that other components are either n-type or insulating, depending on the hydrogen partial pressure tested: <0.01, 0.3 and 6.5bar (<1, 30 and 658kPa). Furthermore, a novel approach developed recently by our group allowed us determination of the number of semiconducting components in the scales and assessment of the bandgap energy for each of them. These results revealed that the composition, as well as the semiconducting properties of the scales, were influenced by the hydrogen partial pressure used in the corrosion test.

Dielectrostriction and Intergranular Stress Corrosion Cracking of Alloy 600 in PWR Primary Water

Analysis of six sources of stress in oxides formed along grain boundaries of Alloy 600 in high temperature, pressurized water indicates that dielectrostriction is likely to represent a significant source of tensile stress and can significantly contribute to IGSCC.

Microstructural characterization on intergranular stress corrosion cracking of Alloy 600 in PWR primary water environment

• We examine PWSCC cracks of Alloy 600 through microscopic equipment. • Oxygen diffuses into the grain boundaries from the external primary water. • Cr oxides are precipitated on the crack tips and the attacked grain boundaries. • The oxide structure inside a crack consists of double (inner and outer) layers. • The penetrated oxygen strongly affects the PWSCC behaviors of Alloy 600. Stress corrosion cracks in Alloy 600 compact tension specimens tested at 325 °C in a simulated primary water environment of a pressurized water reactor were analyzed using microscopic equipment. Oxygen diffused into the grain boundaries just ahead of the crack tips from the external primary water. As a result of oxygen penetration, Cr oxides were precipitated on the crack tips and the attacked grain boundaries. The oxide layer in the crack interior was revealed to consist of double (inner and outer) layers. Cr oxides were found in the inner layer, with NiO and (Ni,Cr) spinels in the outer layer. Cr depletion (or Ni enrichment) zones were created in the attacked grain boundary, the crack tip, and the interface between the crack and matrix, which means that the formation of Cr oxides was due to the Cr diffusion from the surrounding matrix . The oxygen penetration and resultant metallurgical changes around the crack tip are believed to be significant factors affecting the PWSCC initiation and growth behaviors of Alloy 600.