[en] Highlights: • Oxidation resistant SiC-ZrB₂ interlayer is successfully deposited on graphite. • Adherent Y₂O₃ coating over SiC-ZrB₂ surface shows good interfacial bonding. • Three-fold enhancement in the thermal cycle life of Y₂O₃ coating is demonstrated. • SiC-ZrB₂ show active to passive oxidation behavior by forming glassy SiO₂, ZrO₂ and ZrSiO₄. • Tensile residual mismatch stresses are relieved by the formation of cracks in topcoat. The SiC-ZrB₂ composite interlayer coating is developed by pack cementation over high density graphite (HDG) for the deposition of plasma sprayed yttria (Y₂O₃) topcoat for U-Zr (Uranium-Zirconium) melting application. The performance of Y₂O₃ coating with interlayer for enhancing its durability are investigated by performing thermal cycling experiments at 1450 and 1550 °C. The interlayer morphology with infiltrated and diffused coating of SiC with entrapped ZrB₂ particles reduces the differential thermal mismatch stresses and offers passive oxidation protection for HDG. The role of composite SiC-ZrB₂ interlayer enhancing threefold increase in the life of Y₂O₃ coating and its mechanisms at high temperature are provided.

[en] Nitrogen as an alloying element has been reported to improve the localized corrosion resistance, particularly pitting, crevice and intergranular corrosion of austenitic stainless steels in chloride containing aqueous environments. The authors highlight the corrosion behaviour of nitrogen-alloyed (up to 0.56 wt%) austenitic stainless steels and compared the studies carried out by several groups. The influence of various metallurgical variables including cold working, thermal ageing, grain size and surface treatment on the pitting corrosion behaviour of the nitrogen-alloyed austenitic stainless steels is addressed. The mechanism by which nitrogen enhanced the corrosion resistance is elucidated with the help of results obtained using electrochemical and surface analytical techniques. The role of nitrogen on the formation of passive films and the semiconducting nature of passive film with nitrogen addition, are discussed to understand the corrosion resistance offered by the passive films with increasing nitrogen addition. The differences in the localized corrosion behaviour between various nitrogen alloyed stainless steels are highlighted. (author)

[en] Aqueous reprocessing of plutonium-rich mixed oxide fuels require fluoride as a catalyst in boiling nitric acid for an effective dissolution of the spent fuel. The corrosion behavior of the candidate dissolver materials zircaloy-4 (Zr-4) and commercial pure titanium (CP-Ti grade 2) in boiling 11.5 M HNO_3 + 0.05 M NaF has been established. High corrosion rates were obtained for Zr- 4 and CP-Ti in nitric acid containing fluoride ions. Complexing the fluoride ions either with Al(NO_3)_3 or ZrO(NO_3)_2 aided in decreasing the corrosion rates of Zr-4 and CP-Ti. High corrosion resistance is claimed as one of the principal property of the amorphous alloy when compared to the crystalline alloy. Thus Ni_6_0Nb_4_0 and Ni_6_0Nb_3_0Ta_1_0 amorphous ribbons were prepared and exposed in boiling 11.5 M HNO_3 and 11.5 M HNO_3 + 0.05 M NaF. In nitric acid these alloys did not show any sign of corrosion attack. XPS analysis confirmed that the passivity was due to the formation passive films of thickness ≈3 nm enriched with Nb_2O_5 and of ≈1.5 nm enriched with both Nb_2O_5 and Ta_2O_5 on the respective surfaces of the ribbons. In boiling 11.5 M HNO_3 + 0.05 M NaF, severe corrosion attack was observed on Ni_6_0Nb_4_0 ribbon, due to the instability of the oxide/metal interface. The Ni_6_0Nb_3_0Ta_1_0 amorphous ribbon exhibited corrosion resistance of at least an order of magnitude higher than that for Ni_6_0Nb_4_0 ribbon

[en] AISI type 316L stainless steel is one of the candidate materials proposed to be used for manufacturing of components in pyrochemical reprocessing plants, handling molten LiCl+KCl. the stainless steel components are to be coated with stable ceramic coatings to provide enhanced corrosion resistance at high temperatures. Type 316L stainless steel was initially coated with FeCrAlY bond coat of 50 μm thick to provide good adhesion between substrate and ceramic coat. Over the bond coat Yttria stabilized zirconia ceramic coating of 300 μm thick was coated by plasma spraying. The surface morphology of plasma sprayed Yttria stablized zirconia coating applied over 316L SS indicated the presence of pores, and micro cracks. The presence of such pores and microcracks are undesirable as the molten salts may penetrate through these and corrode the underlying stainless steel substrate. This problem could be overcome by consolidation of the coated surface through laser melting. Laser consolidation of plasma coated surfaces was carried out using continuous wave CO₂ laser with 50 W power at a scan rate of 2000 μm/sec. Due to laser treatment, porosity was eliminated completely, however, the segmented crack morphology was observed in the laser treated region. The well defined segmented crack network was formed due to shrinkage and relief of thermal stresses during cooling. The laser parameters are to be fine tuned in order to eliminate these cracks. X-ray diffraction pattern of the as sprayed and laser melted surfaces indicated the presence of metastable tetragonal phase in the laser treated sample which is in good agreement with the literature. SEM examination after laser treatment of the coating shows the preferred well delineated grains of zirconia. This type of structures is desirable for pyrochemical reprocessing applications where molten salts are handled. The paper presents the results of the investigation. (author)

[en] Metal waste forms (MWF) are solid metallic waste categorized as high-level radioactive waste generated during pyro-reprocessing of spent metallic fuels and geologically disposed off. This waste form generated requires an excellent corrosion resistance in a geological repository environment. In the present work, Fe-7Cr-1Mo-12Zr MWF alloy was developed by casting in an arc melting furnace. The x-ray diffraction analysis revealed the formation of stable phases of α-Fe solid solution and the intermetallic phases of Fe₂Zr and Fe₂₃Zr₆. The corrosion behavior of Fe-7Cr-1Mo-12Zr alloy was investigated using electrochemical techniques in aggressive (highly acidic with Cl⁻) and nominal repository pH environment. Cyclic potentiodynamic polarization studies indicated the shift in corrosion potential toward cathodic potential in highly acidic solution with Cl⁻. The corrosion mechanism of pitting is associated with acidified pH medium with more Cl⁻, whereas uniform corrosion or transpassive dissolution is resultant of mild acidity or mild alkaline medium. The passive region is wider in pH 5 and 8 when compared to pH 1 and 3. Electrochemical impedance spectroscopic (EIS) analysis revealed the lowering of passive film stability in the highly acidic medium in the presence of Cl⁻. The role of passive oxide layers on the corrosion resistance of the Fe-7Cr-1Mo-12Zr MWF alloy is discussed.

[en] The corrosion behavior of Ni₆₀Nb₃₀Ta₁₀ metallic glass (MG) and its crystalline counterpart was studied in concentrated nitric acid. The crystallization of Ni₆₀Nb₃₀Ta₁₀ MG was carried out at onset crystallization temperature of 670 °C under vacuum. The TEM studied revealed the formation of FCC Ni in the amorphous matrix. The electrochemical impedance spectroscopy studied indicated high stability of the passive film formed on the metallic glass that exhibited higher corrosion resistance in the nitric acid environment when compared to the crystallized ribbon. (author)

[en] Pyrochemical reprocessing route is one of the best option for reprocessing of spent metallic nuclear fuel from future fast breeder in many countries, especially in the US (Integral fast reactor, IFR), Russia (Research Institute of Atomic Reactors, RIAR), Japan, Korea and India. This technology with intrinsic nuclear proliferation resistance is regarded as one of the most promising nuclear fuel cycle technologies of the next-generation. However, the selection of materials of construction for pyrochemical reprocessing plants is challenging because of the extreme environments, i.e., high radiation, corrosive molten salt (LiCl-KCl, LiCl-KCl-CsCl, KCl-NaCl-MgCl_2, etc.), reactive molten metals, and high temperature. Efforts have been made to develop compatible materials for various unit operations like salt preparation, electrorefining, cathode processing and alloy casting in pyrochemical reprocessing. Nickel and its alloy are the candidate materials for salt purification exposed to molten LiCl-KCl under Cl_2 bubbling, in air or ultra high purity argon environment. In the present study, the corrosion behavior of candidate materials like Inconel 600, Inconel 625, Inconel 690 exposed to molten LiCl-KCl eutectic salt environment at 500 to 600 °C have been carried out. The surface morphology of the exposed samples and scales were examined by SEM/EDX and XRD. The weight loss results indicated that Inconel 600 and Inconel 690 offer better corrosion resistance compared to Inconel 625 in air and chlorine environment. Higher corrosion of Inconel 625 is attributed to development of Mo rich salt layers. However, Ni base alloys exhibited a decreasing trend of weight loss with increasing time of exposure and weight gain was observed under UHP Ar environment. The mechanism of corrosion of Ni base alloys appeared to be due to formation of Cr rich and Ni rich layers of Cr_2O_3, NiO and spinel oxides at the surface and subsequent spallation. Based on the present studies, Inconel 690 followed by Inconel 600 and Inconel 625 are considered suitable for salt purification and electrorefiner applications. (author)

[en] The effect of cold work (CW) on the microstructure and corrosion fatigue (CF) resistance of 316 LN stainless steel (SS) using microscopic, electrochemical and surface analytical methods was investigated. The CF resistance increases with CW. The fractographs revealed that the crack growth zone was minimum for the base metal as compared to CW SS. Fatigue-tested sample revealed a slip offset was high for as-received, while not much significant difference for 5 and 20% CW SS at low mean stress levels. The presence of NH₄ improved passivity by the formation of nitrate ions that inhibits anodic dissolution.

[en] The AISI type 304L stainless steel (SS) was solution annealed at 1050 °C and cryo-rolled at liquid nitrogen (L-N₂) temperature (77 K) under severe plastic deformation (SPD). The thickness reduction through cryo-rolling plastic deformation was carried out up to 90% (designated as CR-90) of initial thickness. The cryo-rolling during rolling at liquid N₂ temperature (77 K), resulted in transformation induced plastic deformation of the austenite γ-phase into α’- martensite phase. The phase transformation was characterized by x-ray diffraction (XRD) and saturation magnetization (M_s) measurement. The microstructure was measured by using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD peak positions confirmed the martensite (α’) phase formation. Saturation magnetization (Ms) value of the cryo-rolled specimens shows increasing linearity with cryo-rolling, and transformed to bct martensite (α’). The measured corrosion rate of the cryo-rolled specimens as per ASTM A-262 practice-C test (Huey test) ranges from 6.6 mpy or 0.167 mm/yr (CR 0) to 12.9 mpy or 0.328 mm/yr (CR 90), indicating the effects of severe deformation of strain-induced martensite with the cryo-rolling. Further, the role of severe deformation of strain-induced martensite with the cryo-rolling on AISI type 304L SS and also influence on the corrosion resistance are investigated in detail. The deformation microstructure roles, i.e., the phases and volume percentage induced by the different percentage of the cryo-rolling that influences the corrosion resistance properties and corresponding corrosion resistance in nitric acid have been investigated.

[en] The issues of materials development and corrosion performance of various components, coating technology and corrosion monitoring for nuclear fuel reprocessing plants is presented. Development of special stainless steels, Ti and zircaloy based alloys, double oxide coating for electrolytic dissolver, dissimilar joints, noble metal coating, thermal barrier coatings, nanostructured Ti, TiO₂ and nitride coatings, amorphous metallic glass coatings, etc. for highly corrosive nitric acid environments are discussed. The results of dynamic nitric acid loop of type 304L SS and zircaloy-4 used for long term corrosion assessment of materials employed in reprocessing plants are also addressed. (author)