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• The network-like LPSO phase in a Mg-11Y-1Al alloy decomposed into needle-like LPSO phase and polygonal Mg 24 Y 5 phase. • The in-situ oxidation of the needle-like LPSO phase facilitated the formation of a thicker and continuous Y 2 O 3 oxide scale. • The accelerated oxidation along the needle-like LPSO phase is attributed to divert the parabolic growth kinetics to a slower oxidation rate. In this study, we investigated the oxidation of the Mg-11Y-1Al alloy at 500 °C in an Ar-20%O 2 environment. Multiscale analysis showed the network-like long-period stacking ordered (LPSO) phase transformed into needle-like LPSO and polygonal Mg 24 Y 5 phases, leading to the formation of a high-dense network of needle-like oxides at the oxidation front. These oxides grew laterally along the oxide/matrix interfaces, forming a thicker, continuous scale that effectively blocked elemental diffusion. Hence, the preferential oxidation along the needle-like LPSO is believed to accelerate the formation of a thicker and continuous oxide scale, further improving the oxidation resistance of the Mg-11Y-1Al alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

Oxide growth and hydrogen pickup have been measured for Zircaloy-4 in environments of different pH. The metal-oxide interface has been studied at high-resolution over sufficient lengths to show meaningful trends in the behaviour of the ZrO and oxygen-saturated metal layers (suboxides). It was found that suboxide width decreases rapidly at transition, then increases more slowly. The suboxide width is a constant proportion of the dioxide width prior to transition, but drops then increases after transition. Density functional theory was used to predict suboxide stability under different pressures. The behaviour of the suboxide can be explained on the assumption that the dioxide layer is the protective layer until it is bypassed by the environment percolating into the crack network. The relation between hydrogen pickup rate and the oxide behaviour suggests that hydrogen moves across the Zircaloy-4 oxide with no net charge, and that the role of the constrained environment within the cracked fossil oxide requires investigation. • Metal-oxide interface has been characterized analytically with nm-resolution over tens of microns. • Suboxide width decreases rapidly at transition, then increases more slowly. • Zirconium dioxide remains protective until it is bypassed by the environment percolating into the crack network. • Hydrogen appears to move across the Zircaloy-4 oxide with no net charge. [ABSTRACT FROM AUTHOR]

Abstract: Atom-probe tomography was employed to characterise specimens containing the oxidised part of a grain boundary from a 304 stainless steel coupon specimen exposed to simulated PWR primary water. A TEM foil containing part of the same oxidised grain boundary was also extracted and characterised for comparison. Surface and grain boundary oxide compositions were identified and Ni enrichment was observed around the oxides. The data provides novel information on the role of the minor impurities and the formation of early-stage oxides. These studies were conducted as part of a broader investigation of the mechanisms underlying stress corrosion cracking in stainless steels. [Copyright &y& Elsevier]

Abstract: FIB 3-dimensional (3D) sequential sectioning has been used to characterize environmental degradation of 304 stainless steels in pressurized water reactor (PWR) simulated primary water. In particular, the effects of cold work and applied stress on oxidation have been studied in detail. It was found that a description of the oxidation behaviour of this alloy is only complete if it is treated statistically, since it can suffer from high variability depending on the feature described. [Copyright &y& Elsevier]

Abstract: Zirconium alloys have been long used for fuel cladding and other structural components in water-cooled nuclear reactors, but waterside corrosion is a primary limitation on both high fuel burn-up and extended fuel cycle operation. Understanding the processes that occur at the metal/oxide interface is crucial for a full mechanistic description of the oxidation process. In this paper we show that reliable quantification of the oxygen content at the metal/oxide interface can be obtained by Electron Energy Loss Spectrometry (EELS) if enough care is taken over both the preparation of Transmission Electron Microscopy (TEM) samples and the methodology for quantification of the EELS data. We have reviewed the accuracy of theoretically calculated inelastic partial scattering cross-sections and effective inelastic mean-free-paths for oxygen and zirconium in oxidized Zr-alloy samples. After careful recalibration against a ZrO2 powder standard, systematic differences in the local oxygen profile across the interface in different zirconium alloys were found. The presence of a sub-stoichiometric oxide layer (a suboxide) was detected under conditions of slow oxide growth but not where growth was more rapid. This difference could arise from the different corrosion resistances of the alloys or, more likely, as a result of the transition in oxidation behaviour, which refers to a sharp increase in the oxidation rate when the oxide is a few microns thick. [Copyright &y& Elsevier]

Three-dimensional atomic characterization of oxidation in stainless steels under simulated nuclear reactor conditions has been achieved with atom-probe tomography (APT). By combining high spatial resolution and high chemical sensitivity, the ingress of oxygen and other trace elements has been studied on an “atom-by-atom” basis, together with their interaction with microstructural defects. APT has revealed how the highly dislocated microstructure in cold-worked stainless steels accelerates the oxidation and the formation of oxide precursors beyond the oxide–metal interface. [Copyright &y& Elsevier]

• Valence mapping of oxidized second phase particle oxidation in zirconium alloys has been successfully achieved by EELS. • Valence change has revealed that Cr and Zr oxidize preferentially, while Fe remains metallic for longer. • Cr oxidation is complete prior to transition. • Fe only oxidises when in full contact with the environment (via water in cracks or nanopores) after transition. The evolution of Cr and Fe within the oxide formed on Zircaloy-4 in high-temperature pure water and a high-pH aqueous environment has been followed in detail by electron energy loss spectroscopy (EELS). The oxidation states of Cr, Fe and Zr have been determined as a function of exposure time and location in the oxide film. Much Cr and all the Fe diffuse out of the original Laves phase and precipitate within cracks or on the outer oxide surface. Cr oxidation is complete prior to transition; Fe oxidation occurs mainly after transition. The different hypotheses concerning the mechanisms by which the alloying elements influence oxidation and hydrogen pickup are examined in the light of this information. The mechanisms which agree with these observations include: doping of the bulk ZrO 2 by low levels of alliovalent cations; doping ZrO 2 grain boundaries with alliovalent cations; injection of holes into near-surface oxide by islands of Fe 2 O 3 ; reaction of water with islands of metallic Fe (and Ni) on surfaces of internal cracks around transition; control of transition by mechanical properties [ABSTRACT FROM AUTHOR]

• Higher residual stress/strain and dislocation density in alloy 600 HAZ than in base. • Thicker oxide and deeper intergranular oxidation in alloy 600 HAZ than in base. • Longer SCC length and higher cracking engagement in alloy 600 HAZ than in base. • Stress-assisted oxidation contributes to enhanced SCC growth in alloy 600 HAZ. • The impact of local mechanical parameters on SCC in the HAZ is highlighted. The impact of welding-induced mechanics on the oxidation and stress corrosion cracking (SCC) behavior of Alloy 600 in a pressurized water reactor (PWR) primary water was investigated using an Alloy 600-Alloy 152 M weldment. The microstructural analysis found a 0.05 mm-wide composition transition zone from welding. The Alloy 600 heat-affected zone (HAZ) grain size matches the Alloy 600 base (600B). Deformation hardening initially decreases and stabilizes from the fusion boundary (FB) to the 600B Comparing the microstructural characteristics of the specimen from the HAZ at about 0.5 mm from the FB with specimen 600B shows identical composition, but the former exhibits higher Vickers hardness (HV), kernel average misorientation (KAM), dislocation density, and residual stresses. Results from both short- and long-term oxidation tests indicate that the specimen from the HAZ exhibits a greater thickness of the inner oxide film, a higher number of local oxidation sites, and an increased maximum depth of intergranular (IG) oxidation compared to the 600B specimen. Multiple sets of SCC test results demonstrate that the crack growth rate (CGR) in the specimens from the HAZ is higher than that in the base, with the former showing longer IG cracks and a greater proportion of IG cracking. [ABSTRACT FROM AUTHOR]

Novel atom probe tomography (APT) data of an intergranular stress corrosion crack tip has been acquired. Using APT for stress corrosion cracking research, very small, localized features and their distribution around the crack tip can be studied in 3D. This work details the development of a technique for the preparation of atom probe needles. Initial characterization via analytical transmission electron microscopy provides with a complementary analysis and accurately locates features that can be correlated with the reconstructed APT data. Ni enrichment and intergranular oxidation ahead of the crack tip have been studied with APT in 3D and with near-atomic resolution. [ABSTRACT FROM AUTHOR]

The microstructure of the surface oxide film formed on an Fe-9Cr Ferritic-Martensitic (F-M) steel after exposure to deaerated high-temperature steam at 600°C for 100 h has been analyzed in detail by advanced characterization techniques. The surface oxide film has been revealed to have a triplex structure, including an outer oxide layer, an inner oxide layer, and an internal oxide layer. Although the outer and inner oxide layers are continuous, the internal oxide layer has been proved to consist of interconnected metallic and chromite phases, which is a typical feature of internal oxidation. The formation mechanisms of each layer have been discussed, finding that, contrary to what the available space model suggests, an external oxidation is not the controlling oxidation mechanism of F-M steels in high-temperature steam. The higher resolution used in this study confirms that the controlling mechanism is internal oxidation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

It is well-known that steels always oxidize faster in the environments containing water vapour than in dry oxygen. Due to the difficulties in obtaining necessary experimental scale of observations, the mechanisms responsible for the steam-accelerated oxidation are still unclear. Through a combination of multiscale characterization techniques, the surface oxide film formed on an Fe-17Cr-9Ni stainless steel after exposure to high-temperature steam has been studied in detail. The characterization results obtained in this study reveal that the inner oxide layer actually consists of two phases Fe-Ni austenite and FeCr 2 O 4 oxide, which formed due to internal oxidation. The classic internal oxidation model underestimates the thickness of the inner oxide layer by one order of magnitude. This difference can be explained by the existence of fast diffusion channels in the inner oxide layer. This study provides direct evidence of a high density of nanopores in the oxide phase of the internal oxide layer, which can act as fast-diffusion channels if interconnected, and proposes their mechanisms of formation, a consequence of water dissociation-induced protons promoting the formation, migration, and clustering of both cation and anion vacancies. Image, graphical abstract [ABSTRACT FROM AUTHOR]

It is often assumed that internal oxidation cannot occur at temperatures below 400 °C. However, in the present work, internal oxidation was observed in a 20% cold-worked Fe-17Cr-12Ni stainless steel (SS) after exposure to simulated primary water of a pressurized water reactor at 340 °C and not in a similarly tested sample without prior cold-work. The formation of discrete Cr-oxide precipitates and the role of cold-work are discussed. The internal oxidation model is also proposed as a plausible stress corrosion cracking mechanism of Fe-17Cr-12Ni SS at that temperature. Unlabelled Image [ABSTRACT FROM AUTHOR]

Highlights • Surface oxide film on 316 L SS has triplex structure and the mechanisms are discussed. • Mechanisms controlling the surface oxidation and crack tip oxidation are similar. • Dislocation and stress can enhance oxidation. • The formation of Ni-rich zone ahead of crack tip and in the surface oxide is controlled by selective oxidation. Abstract Oxidation and stress corrosion cracking (SCC) of 316L stainless steel were studied in simulated pressurized water reactor primary water. Surface, crack flank, and crack tip oxides were analyzed and compared by high-resolution characterization, including oxidation state mapping. All oxides were found to have a triplex structure, although of different dimensions and composition, revealing the effects of local water chemistry and applied stress. The higher oxidation rate at the crack tip could be explained due to the existence of a higher dislocation density, higher level of stress and cation unavailability from the environment. The implications to SCC mechanisms are discussed. [ABSTRACT FROM AUTHOR]

Graphical abstract Chemical composition and crystallography of a migrated grain boundary. Highlights • DIGM was observed ahead of all examined SCC crack tips. • The extent of DIGM varied from boundary to boundary. • Different extent of DIGM leads to different retardation effect on SCC growth. • A method is proposed to quantify the role of DIGM on SCC crack growth. Abstract Crack tips prepared from Ni-based alloys after stress corrosion cracking testing in simulated pressurized water reactor primary water have been studied by high-resolution characterization. Diffusion-induced grain boundary migration (DIGM) was observed in all the cracks and its potential role on the crack propagation is discussed in detail. The extent of DIGM observed changed with grain boundary type and alloy composition, which could lead to different extent of retardation effects on the crack propagation. A method is proposed to quantify the role of DIGM on crack propagation based on the results obtained in this work. [ABSTRACT FROM AUTHOR]

Abstract The mitigating effect introduced by intergranular Cr carbides on the stress corrosion cracking propagation of a cold-worked Alloy 600 has been firstly examined through high-resolution 3-dimensional (3D) sequential sectioning. High-resolution transmission electron microscope (TEM) and transmission Kikuchi diffraction (TKD) are used to reveal the underlying mechanisms contributing to the mitigating effect. Previously reported mechanisms contributing to the increased stress corrosion cracking resistance are evaluated and discussed. A new mechanism based on grain boundary migration inhibition and crack path deviation is proposed. Graphical abstract Image 1 Highlights • Mitigating effect of intergranular carbides on SCC was observed in 3D. • Cold woking-induced preferential deformation around intergranular carbides was observed via TKD. • Previously reported mechanisms contributing to the mitigating effect were evaluated and discussed. • A new mechanism based on grain boundary migration inhibition and crack path deviation was proposed. [ABSTRACT FROM AUTHOR]

• Oxidaiton and SCC tests for the dilution zones (DZs) and base alloy 52 M overlay. • Thicker inner oxide layer in the DZs than in the base alloy 52 M. • Deeper oxidation penetration depth in the DZs than in the base alloy 52 M. • Thicker inner oxide layer corresponds to longer SCC band for alloy 52 M. • Deeper local oxidation penetration corresponds to longer local intergranular SCC in DZs. Compact tension (CT) specimens from submerged arc welding Alloy 52 M overlay are used to determine the stress corrosion cracking (SCC) resistance of the dilution zones and bulk overlay metal on the 52 M side in a simulated pressurized water reactor (PWR) primary water. SEM-EDS and X-ray fluorescence spectroscopy (XRF) results show three dilution zones (DZs) on the 52 M side. Oxidation test results show that the thickness of the compact oxide layer and maximum local oxidation penetration depth decrease for 1st DZ, 2nd DZ, and bulk 52 M, which are consistent with the average SCC growth rate and the number of local intergranular SCC cracks in these regions. The dilution zones with low Cr but high Fe content possess lower oxidation and SCC resistance than the bulk 52 M. [ABSTRACT FROM AUTHOR]

The mechanical response of oxidized grain boundaries in a nickel alloy used for nuclear applications has been investigated by performing microcantilever bend tests. It was found that whilst failure can proceed along the oxide-metal interface not all oxidized grain boundaries exhibit intergranular failure. The presence of an external surface oxide has been identified as playing a crucial role in influencing the mechanical response. By removing the surface oxide, using a focused ion beam, tests were performed on the same grain boundaries with and without a surface oxide layer, and showed that surface oxides can suppress or delay fracture. Taking into account the effect of the surface oxide on microcantilever tests, it was possible to present the most accurate parameterization of the local stress at failure of oxidized grain boundaries to date and to predict the experimentally observed behavior via realistic cohesive damage finite element simulations, which further underline the experimental observations. [ABSTRACT FROM AUTHOR]

• Ground surface has higher oxidation resistance than mechanically polished surface. • A fine-grained layer favors formation of dense oxide film with high Cr/Fe ratio. • Highly network Cr-bearing δ-ferrite improves the oxidation resistance. • Thermally activation enhanced self-diffusion of Cr in ferrite inhibits oxidation. • The ferrite-affected oxidation zone expands with the increase in temperature. The properties of the oxide films grown on 308 L SS cladding with various surface treatments and temperatures in simulated PWR primary water are studied. The thickness of the inner oxide layer increases with the decrease of the surface roughness. The high oxidation resistance of the ground surface is due to the thicker fine-grained layer with high dislocations density and subgrain boundaries in the near-surface. The ferrite-affected oxidation zone (FAOZ) expands with the increase in temperature, resulting from the thermally activated self-diffusion of Cr in ferrite. The effects of surface states, ferrite and temperature on oxidation performance of 308 L SS cladding are discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Zr alloys were oxidised for various times in an autoclave to simulate PWR conditions. [•] Isotopic tracers 18O and 2H were added to reveal active oxidation sites by NanoSIMS analysis. [•] Hydrides were present in all samples, even those with short oxidation times. [•] Porosity mediated transitions between corrosion regimes occur at critical oxide thicknesses. [Copyright &y& Elsevier]

• Oxidation and SCC of 308L SS cladding with or without hydrogen are studied. • Phase boundary (PB) has lower oxidation resistance than austenite and δ-ferrite. • Hydrogen-induced PB oxidation and preferential grain boundary oxidation. • Charged hydrogen decreases the oxidation resistance of δ-ferrite and austenite. • Charged hydrogen increases the SCC susceptibility of 308L SS cladding. Oxide film properties and stress corrosion cracking (SCC) behaviour are investigated in the 308L stainless steel cladding layer with and without being H-charged in a deaerated high temperature water. Phase boundary (PB) has lower oxidation resistance than austenite and δ-ferrite matrix. A compact and high Cr content-bearing inner oxide layer improves the oxidation resistance of the δ-ferrite phase without H charging. H-induced PB penetration oxidation and grain boundary (GB) preferential oxidation are observed in H-charged specimen due to the PB and GB act as H trapping sites. Charged H decreases the oxidation resistance of δ-ferrite and austenite. δ-Ferrite acts as a path for H diffusing and H flux facilitates mass transport, resulting in the poor oxidation resistance of δ-ferrite. The mechanism underlying the effect of H on the oxidation behaviours of δ-ferrite and austenitic phases, as well as SCC performance, are discussed. [ABSTRACT FROM AUTHOR]

• Oxidation and SCC behaviour of 309 L/308 L SS welds were studied in PWR primary water. • There was no indication of SCC for low alloy steel (LAS) after SSRT tests. • The oxidation resistance of 309 L SS was lower than that of 308 L SS. • 309 L SS was more susceptible to SCC than 308 L SS in PWR primary water. Oxidation resistance and stress corrosion cracking (SCC) susceptibility of A508Ⅲ-309 L/308 L stainless steel (SS) claddings were investigated in simulated pressurized water reactor (PWR) primary water following a microstructure characterization. Preferential oxidation of slip bands was observed in 309 L SS. The oxidation resistance of 309 L SS was found to be lower than that of 308 L SS due to the higher Cr- and ferrite-content in 308 L SS than in 309 L SS after cladding. There was no indication of SCC for low alloy steel (LAS) after slow strain rate tensile (SSRT) tests. The presence of type-I and type-II boundaries, composition dilution and less δ-ferrite content synergistically contributed to increasing the SCC susceptibility of 309 L SS cladding. The effect of chemical composition and microstructure on SCC performance of the SS claddings was discussed. [ABSTRACT FROM AUTHOR]