Transmission electron microscopy of oxide development on 9Cr ODS steel in supercritical water

Oxide layers formed on 9Cr oxide dispersion strengthened ferritic steel alloys during exposure to 600 °C supercritical water for 2- and 4-weeks were examined using cross-sectional transmission electron microscopy. A focused ion beam insitu lift-out technique was used to produce site-specific samples with electron transparent areas up to 8 μm by 10 μm. The oxide layers consist of several sub-layers: an Fe-rich outer oxide, a Cr-rich inner oxide, and a diffusion layer, extending beyond the oxide front into the metal. An evolution of the oxide layer structure is seen between 2 and 4 weeks, resulting in the development of a band of Cr₂O₃ at the diffusion layer/metal interface from the previously existing continuous mixture of FeCr₂O₄ ‘fingers’ and bcc metal. It is believed that transport in this Cr₂O₃ layer at the diffusion layer/metal interface becomes the rate-limiting step for oxide advancement, since this change in oxide structure also corresponds to a decrease in corrosion rate.     

Comparative study and imaging by PhotoElectroChemical techniques of oxide films thermally grown on zirconium and Zircaloy-4

The oxidation of iron and chromium that are present as impurities in zirconium metal or as alloying elements in Zircaloy-4 was investigated with PhotoElectroChemical techniques (PEC), highlighting the chemical nature, the size and the lateral distribution of Fe and Cr-containing phases in thin zirconia scales formed during the oxidation of pure zirconium and Zircaloy-4 at 470 °C in oxygen. In the case of zirconium, iron and chromium impurities led to the formation of oxides distributed in a homogeneous way in the zirconia scale, while in the case of Zircaloy-4 these elements, present in the form of intermetallic particles in the substrate, led to the formation of localised haematite Fe₂O₃, rhomboedric solid solution (Fe_xCr_1−x)₂O₃ and chromia Cr₂O₃ phases. These phases were accurately studied via the measurement of their respective band-gap (Fe₂O₃: 2.2 eV, (Fe_xCr_1−x)₂O₃: 2.6 eV and Cr₂O₃: 3.0 eV). It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale at a micron scale.     

Corrosion behavior of Al-surface-treated steels in liquid Pb-Bi in a pot

Corrosion tests were performed in oxygen-saturated liquid Pb-Bi at 450 °C and 550 °C in a pot for 3000 h for Al-surface-treated steels containing various levels of Cr contents. The Al surface treatments were achieved using a gas diffusion method and a melt dipping method. Al₂O₃, FeAl₂ and AlCr₂ produced by the gas diffusion method exhibited corrosion resistance to liquid Pb-Bi, while the surface layer produced by the melt dipping method suffered a severe corrosion attack. Fe₄Al₁₃ and Fe₂Al₅ produced by the melt dipping method disappeared during the corrosion test at 550 °C and only FeAl remained.     

Oxidation and creep failure of alloy 617 foils at high temperature

The microstructure of thermally grown oxides (TGO) and the creep properties of alloy 617 were investigated. Oxidation and creep tests were performed on 100 μm thick foils at 800-1000 °C in air environment, while the thickness of TGO was monitored in situ. According to energy dispersive X-ray (EDX) mapping micrographs observation, superficial dense oxides, chromia (Cr₂O₃), which was thermodynamically unstable at 1000 °C, and discrete internal oxides, alumina (α-Al₂O₃), were found. Consequently, the weight of the foil specimen decreased due to the spalling and volatilization of the Cr₂O₃ oxide layer after an initial weight-gaining. Secondary and tertiary creeps were observed at 800 °C, while the primary, secondary and tertiary creeps were observed at 1000 °C. Dynamic recrystallization occurred at 800 °C and 900 °C, while partial dynamic recrystallization at 1000 °C. The apparent activation energy, Q_app, for the creep deformation was 271 kJ/mol, which was independent of the applied stress.     

Characterization of oxide films formed on alloy 182 in simulated PWR primary water

The oxide film, formed on the alloy 182 due to the exposure to simulated Pressurized Water Reactor primary water conditions, is characterized using X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy and Raman spectroscopy. As a consequence of the low redox potential for nickel, loosely bound oxide film consisting mainly of chromium oxide, is formed on the surface of the alloy. The oxide film is found to have a double-layer structure, with an inner layer rich in Cr₂O₃, outer layer rich in FeCr₂O₄. Thin Ni(OH)₂ and Fe₃O₄ clusters were observed on top of the oxide film. The morphology and thickness of the layers critically depend on the exposure times and surface treatments prior to the exposure.     

Strengthening effect of Cr2O3 thermally grown on alloy 617 foils at high temperature

Alloy 617 has been selected for the intermediate heat exchanger (IHX) of the very high temperature gas-cooled reactor (VHTR) for the economic production of electricity and hydrogen. In this work, the strengthening effects of Cr₂O₃ thermally grown on alloy 617 foils at 800 and 900 °C were investigated. A micro-tensile test system was used for in situ measurement of tensile strain in the foils and superficial thermally-grown Cr₂O₃. Each foil was heated until the thermally-grown Cr₂O₃ reached a predetermined thickness; then, a load was applied to measure the tensile response. As the Cr₂O₃ layer thickened on the surface of the metal foils, the strengths and stiffnesses of the foils were enhanced. We assumed that there was no interaction between the substrate and the superficial chromia, and the strength of Cr₂O₃ itself was measured. At 800 °C, the Cr₂O₃ was brittle and the strength was governed by crack initiation. At 900 °C, the Cr₂O₃ was much more ductile, and strain hardening was observed for even the smallest thickness. The strength was maintained even after crack initiation was observed on the surface.     

Influence of tempering temperature upon precipitate phases in a 11%Cr ferritic/martensitic steel

The effect of tempering temperature on the precipitate phases in a 11%Cr ferritic/martensitic steel normalized at 1050 °C for 1 h and tempered for 2 h at temperatures ranging from 600 to 780 °C has been investigated using transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that tempering temperature does not affect the existences of niobium-rich carbonitrides, (Nb_0.7V_0.2Cr_0.1)(C,N) and (Nb_0.55V_0.35Cr_0.1)(C,N), vanadium-niobium-rich carbonitride (V_0.45Nb_0.45Cr_0.1)(C,N), chromium-rich carbonitride (Cr_0.83V_0.12W_0.05)₂(C,N) and chromium-rich carbide (Cr_0.7Fe_0.25W_0.05)₂₃-C₆, whilst the precipitations of vanadium-rich carbonitrides, (V_0.65Nb_0.2Cr_0.15)(C,N) and (V_0.55Nb_0.25Cr_0.2)(C,N) are dependent on tempering temperature, which were detected only at the higher tempering temperatures of 750 and 780 °C. No coarsening was occurred during the temperings for the niobium-rich and spherical vanadium-rich carbonitrides. There was a low coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides with increasing the tempering temperature from 600 to 700 °C and 650 to 780 °C, respectively, and a high coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides at the tempering temperatures 750 through 780 °C and 650 °C, respectively. The compositions show an increase in vanadium and a decrease in niobium and chromium contents for the niobium-rich carbonitrides, and a decrease in niobium and an increase in vanadium and chromium contents for the vanadium-niobium-rich carbonitrides, and an increase in vanadium and a decrease in tungsten contents for the chromium-rich carbonitrides. The chromium-rich carbides show an increase and a decrease in their iron and chromium contents, respectively, with increasing the tempering temperature from 650 to 780 °C.     

Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment

Advanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr₂O₃ oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations.     

The general corrosion behavior of ENiCrFe-7 weld overlay cladding material in deoxygenated high-temperature and high-pressure water

The general corrosion behavior of Alloy ENiCrFe-7 in deoxygenated high-temperature and high-pressure water was investigated. The results showed that the precipitates of Alloy ENiCrFe-7 included niobium carbide and Al-Ti-O compounds, and the weight gain increased fast firstly before 2250 h, then the weight gain slowed down. There were obvious large particles spread on denser oxide film after 3000 h exposure. Ni was present at a single chemical metallic Ni state, Feⁿ⁺ content of the outer layer was close to 60%, which was much higher than that of the matrix. The oxide film consisted of an inner layer and an outer layer, the inner layer was mainly composed of Cr₂O₃ and the outer layer was mainly composed of Fe₃O₄ and FeCr₂O₄. Finally, it is found that the preferential corrosion location of pitting was niobium carbide precipitates by in same site observation, while Al-Ti-O compounds was not dissolved in deoxygenated high-temperature and high-pressure water for 1500 h exposure. The size and number of the pitting was not significantly changed with increasing exposure time.     

Corrosion behaviors of US steels in flowing lead-bismuth eutectic (LBE)

Corrosion tests of several US martensitic and austenitic steels were performed in a forced circulation lead-bismuth eutectic non-isothermal loop at the Institute of Physics and Power Engineering (IPPE), Russia. Tube and rod specimens of austenitic steels 316/316L, D-9, and martensitic steels HT-9, T-410 were inserted in the loop. Experiments were carried out simultaneously at 460 °C and 550 °C for 1000, 2000 and 3000 h. The flow velocity at the test sections was 1.9 m/s and the oxygen concentration in LBE was in the range of 0.03-0.05 wppm. The results showed that at 460 °C, all the test steels have satisfactory corrosion resistance: a thin protective oxide layer formed on the steel surfaces and no observable dissolution of steel components occurred. At 550 °C, rod specimens suffered rather severe local liquid metal corrosion and slot corrosion; while tube specimens were subject to oxidation and formed double-layer oxide films that can be roughly described as a porous Fe₃O₄ outer layer over a chrome-rich spinel inner layer. Neglecting the mass transfer corrosion effects by the flowing LBE, calculations based on Wagner’s theory reproduce the experimental results on the oxide thickness, indicating that the oxide growth mechanism of steels in LBE is similar to that of steels in air/steam, with slight modification by dissolution and oxide dissociation at the liquid metal interface.     

Structural study of the oxidation process and stability of NbOxNy coatings

In the present work, we study the oxidation behaviour of NbON multilayer films. The films were deposited by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayered coatings with a period of λ = 10 nm. Three different films with increasing duty cycles have been deposited.Rutherford backscattering spectroscopy (RBS) was used to study the chemical composition variations at different annealing temperatures (as-deposited, 400 °C, 500 °C and 600 °C) combined with X-ray diffraction (XRD) to identify the crystalline phases formed. At 400 °C, for all films a very thin layer starts to form at the surface with enhanced O concentration. The composition of the deeper part of the samples remains unchanged. At 500 °C, the oxide scale grows, encompassing about half the film thickness. At 600 °C, the process is finished and a single layer is formed with reduced Nb and increased O concentration. Fourier-transformation infrared spectroscopy (FTIR) results confirmed the increase of this surface oxidation, while XRD revealed that crystallization of Nb₂O₅ occurs at 600 °C.     

Behaviour of chromium steels in liquid Pb-55.5Bi with changing oxygen content and temperature

The behaviour of protective oxide layers on P122 steel and its welds and of ODS steel in liquid Pb_44.5Bi_55.5 (LBE) is examined under conditions of changing temperatures and oxygen concentrations. P122 (12Cr) and its welded joints are exposed to LBE at 550 °C for 4000 h with oxygen concentrations of 10⁻⁶ and 10⁻⁸ wt% (p(O₂) = 8.1 × 10⁻²³ bar and 5.2 × 10⁻²⁷ bar) which change every 800 h. It is found that like in case of constant oxygen concentration of 10⁻⁶ wt% a protective spinel layer (Fe(Fe_1−xCr_x)₂O₄) was maintained on P122 and also on its welded joint. Two experiments with exposure times of 4800 h are conducted on ODS steel, both with temperatures changing from 550 to 650 °C and back every 800 h, one experiment with 10⁻⁶ the other with 10⁻⁸ wt% oxygen in LBE. Both experiments show strong local dissolution attack after 4800 h which is in agreement with the behaviour of ODS in LBE at a constant temperature of 650 °C. However, dissolution attack is less in LBE with 10⁻⁸ wt% oxygen (p(O₂) = 3.0 × 10⁻²⁵ bar).     

Development of an in situ Raman spectroscopic system for surface oxide films on metals and alloys in high temperature water

In order to directly analyze the structure of oxide films on metals and alloys in high temperature water conditions, an in situ Raman spectroscopic system has been developed, utilizing Ar-laser with sapphire/diamond window assemblies as a part of an Alloy 690 autoclave. The performance of the developed in situ Raman spectroscopic system was verified using high purity NiO, NiFe₂O₄, Cr₂O₃, and NiCr₂O₄ powders. Obtained reference Raman spectra in room temperature air environment agreed well with published results. The developed Raman system has been applied for the characterization of the surface oxide films of a nickel-base Alloy 600 in typical primary water conditions of pressurized water reactors (PWRs); water with 1000 ppm boron and 2 ppm lithium at a pressure of 18 MPa and temperatures ranging up to 350 °C. In situ Raman spectra were collected for Alloy 600 in PWR water conditions at different temperatures up to 350 °C, while the specimens were heated and then cooled. In this study, NiO, Cr₂O₃, and NiCr₂O₄ phases were observed for Alloy 600 in PWR water conditions, in reasonable agreement with earlier results of ex situ studies.     

Vitrification of high chrome oxide nuclear waste in iron phosphate glasses

A simulated high level waste (HLW) containing 4 mass% chrome oxide, whose overall composition is representative of the high chrome oxide wastes at Hanford WA USA, was easily vitrified in a phosphate glass at temperatures ranging from 1150 °C, for waste loadings of 55 mass%, to 1250 °C for waste loadings of 75 mass%. Even at these high waste loadings, these wasteforms had an excellent chemical durability. The best chemical durability was achieved when the O/(Si + P) atomic ratio was between 3.5 and 3.8. These wasteforms were also resistant to crystallization although trace amounts of crystalline Cr₂O₃ were present in wasteforms containing more than 70 mass% HLW. It is concluded that up to 45 mass% of the total HLW at Hanford, especially that containing as high as 4.5 mass% chrome oxide, could be directly vitrified into an iron phosphate glass, that meets all of the current chemical durability requirements by simply adding 25-35 mass% P₂O₅ to the waste and melting the mixture at 1150-1250 °C for a few (<6) hours.     

Oxidation behavior of modified SUS316 (PNC316) stainless steel under low oxygen partial pressure

Oxidation behaviors of modified SUS316 (PNC316) and SUS316 stainless steels were investigated under the low oxygen partial pressure of 10⁻³¹−10⁻²² atm at 600-800 °C. Oxygen uptake by these materials parabolically increased with time, and the kinetic rate constants depended on both oxygen partial pressure and temperature. Thus, semi-empirical equations of the parabolic rate constants were obtained to be 2.70×10⁴exp(−109/RT)P_O2^0.279 for PNC316 and 9.23×10⁴exp(−98/RT)P_O2^0.313 for SUS316. For the duplex layer formed under the low oxygen partial pressure, the inner layer consisted of such oxides as Cr₂O₃ and FeCr₂O₄, while the outer layer consisted of non-oxidized α-Fe. Furthermore, oxidation along the grain boundaries was observed for samples oxidized for a long time. From the point of view of fuel cladding chemical interaction evaluation at high burn-up fuel for fast reactors, it is interesting that formation of non-oxidized α-Fe was observed under the low oxygen partial pressure.     

Identification of precipitate phases in a 11Cr ferritic/martensitic steel using electron micro-diffraction

The precipitate phases in a 11Cr ferritic/martensitic steel normalized at 1050 °C for 1 h followed by tempered at 750 °C for 2 h have been studied through electron micro-diffraction combined with EDX analysis. Two niobium-rich carbonitride phases, (Nb_0.75V_0.15Cr_0.1)(C,N) and (Nb_0.5V_0.4Cr_0.1)(C,N), with the same f.c.c. structure were identified. Three vanadium-rich carbonitride phases were identified to be (V_0.65Nb_0.15Cr_0.2)(C,N), (V_0.45Nb_0.45Cr_0.1)(C,N) and (V_0.55Nb_0.25Cr_0.2)(C,N) with the same f.c.c. structure. Chromium-rich carbonitride M₂(C,N) and chromium-rich carbide M₂₃C₆ phases with a hexagonal and a f.c.c. crystal structure as well as a typical chemical formula of (Cr_0.8V_0.2)₂(C,N) and (Cr_0.7Fe_0.25W_0.05)₂₃C₆, respectively, were also identified in the steel sample. The size of each kind of precipitate phase was examined. The morphologies and precipitation sites along with the amount of observed precipitate phases were described and discussed.     

Reactivity of hydrogen peroxide towards Fe3O4, Fe2CoO4 and Fe2NiO4

The kinetics of CRUD oxidation by H₂O₂ has been studied using aqueous suspensions of metal oxide powder. Fe₃O₄, Fe₂CoO₄ and Fe₂NiO₄ were used as model compounds for CRUD. In addition, the activation energies for the reaction between H₂O₂ and the three CRUD models were determined. The rate constants at room temperature were determined to 6.6 (±0.4) × 10⁻⁹, 3.4 (±0.4) × 10⁻⁸ and 1.6 × 10⁻¹⁰ m min⁻¹ for Fe₃O₄, Fe₂CoO₄ and Fe₂NiO₄, respectively. The corresponding activation energies are 52 ± 4, 44 ± 5 and 57 ± 7 kJ mol⁻¹, respectively. The mechanism of the reaction is briefly discussed indicating that the final solid product in all three cases is Fe₂O₃. In addition to the experimental studies, the theoretical grounds for kinetics of reactions in particle suspensions are discussed. The theoretical discussion is also used to explain the somewhat unexpected trends in reactivity observed experimentally.     

Mechanical properties of Y2O3-doped W-Ti alloys

W and W alloys are currently considered promising candidates for plasma facing components in future fusion reactors but most of the information on their mechanical properties at elevated temperature was obtained in the 1960s and 1970s. In this investigation, the strength and toughness of novel Y₂O₃-doped W-Ti alloys manufactured by powder metallurgy were measured from 25 °C up to 1000 °C in laboratory air and the corresponding deformation and failure micromechanisms were ascertained from analyses of the fracture surfaces. Although the materials were fairly brittle at ambient temperature, the strength and toughness increased with temperature and Ti content up to 600 °C. Beyond this temperature, oxidation impaired the mechanical properties but the presence of Y₂O₃ enhanced the strength and toughness retention up to 800 °C.     

Corrosion behavior of Ni-based structural materials for electrolytic reduction in lithium molten salt

In this study, the corrosion behavior of new Ni-based structural materials was studied for electrolytic reduction after exposure to LiCl-Li₂O molten salt at 650 °C for 24-216 h under an oxidizing atmosphere. The new alloys with Ni, Cr, Al, Si, and Nb as the major components were melted at 1700 °C under an inert atmosphere. The melt was poured into a preheated metallic mold to prepare an as-cast alloy. The corrosion products and fine structures of the corroded specimens were characterized by scanning electron microscope (SEM), Energy Dispersive X-ray Spectroscope (EDS), and X-ray diffraction (XRD). The corrosion products of as cast and heat treated low Si/high Ti alloys were Cr₂O₃, NiCr₂O₄, Ni, NiO, and (Al,Nb,Ti)O₂; those of as cast and heat treated high Si/low Ti alloys were Cr₂O₃, NiCr₂O₄, Ni, and NiO. The corrosion layers of as cast and heat treated low Si/high Ti alloys were continuous and dense. However, those of as cast and heat treated high Si/low Ti alloys were discontinuous and cracked. Heat treated low Si/high Ti alloy showed the highest corrosion resistance among the examined alloys. The superior corrosion resistance of the heat treated low Si/high Ti alloy was attributed to the addition of an appropriate amount of Si, and the metallurgical evaluations were performed systematically.     

Microstructural Characterization of SCC Crack Tip and Oxide Film for SUS 316 Stainless Steel in Simulated PWR Primary Water at 320°C

Recent studies on stress corrosion cracking (SCC) behaviors of austenitic stainless steels in hydrogenated high-temperature water show that low potential SCC (LPSCC) can occur on cold-worked SUS 316 stainless steel (hereinafter, 316SS). In this study, oxide films and crack tips on cold-worked 316SS exposed to hydrogenated high-temperature water were characterized using analytical transmission electron microscopy (ATEM), grazing incidence X-ray diffraction (GIXRD) and Auger electron spectroscopy (AES) in order to study the corrosion and SCC behaviors of these films and crack tips. A double layer structure was identified for the oxide film after a constant extension rate tensile (CERT) test. The outer layer was composed of large particles (0.2–3 μm) of Fe₃O₄ and the inner layer consisted mainly of fine particles (~10 nm) of FeCr₂O₄. In addition, nickel enrichment was identified at the metal/oxide interface. Particles of Fe₃O₄ were also identified on the crack walls. These results indicate that the same electrochemical reactions had occurred inside and outside the crack. The crack tip area was filled with corrosion products of a chromium-rich oxide. In addition, nickel enrichment was observed at the crack tip. The formation of the nickel-enriched phase indicates that a selective dissolution reaction of iron and chromium occurred at the front of the LPSCC crack.