A. Ravi Shankar

Laser surface modification of plasma sprayed yttria stabilised zirconia coatings on type 316L stainless steel

Abstract Plasma sprayed yttria stabilised zirconia/NiCrAlY thermal barrier coatings proposed for molten LiCl–KCl salt environments contain pores and microcracks that may cause corrosion of the substrate on prolonged exposure. In the present work the as sprayed samples were laser remelted with various scan speeds of 1, 2·5 and 5 mm s−1 to reduce the segmented cracks. Microstructural inhomogeneities like pores and voids have been eliminated on laser remelting, however, segmented cracks were formed irrespective of scan speed. Distinct interface separating fine and coarse grains were observed at all scan speeds. The microhardness of the glazed surface improved and surface roughness was reduced. The beneficial non-transformable tetragonal phase was formed after laser remelting while the as sprayed coating consisted of insignificant monoclinic phase. An attempt was made to seal the pores of the zirconia coating by applying ZrO2 and ZrO2 + SiO2 powder.

Corrosion of Nickel-Containing Alloys in Molten LiCl-KCl Medium

Pyrochemical reprocessing utilizing molten chloride salt has been considered one of the best options for reprocessing of spent metallic fuels of future fast breeder reactors. Materials for pyrochem...

Corrosion Behavior of Candidate Materials in Molten LiCl-KCl Salt Under Argon Atmosphere

Pyrochemical reprocessing involves the use of molten LiCl-KCl (lithium chloride-potassium chloride) eutectic salt at 773 K for the recovery of uranium and plutonium from spent metallic fuel of fast breeder reactors. The materials selected for such corrosive environments should withstand high temperatures and at the same time offer good corrosion resistance. The present work discusses the corrosion behavior of candidate materials like 2.25Cr-1Mo steel (UNS K21590), 9Cr-1 Mo steel (UNS K90941), Ni-based alloy 600 (UNS N06600), Ni-based alloy 625 (UNS N06625), and Ni-based alloy 690 (UNS N06690) in molten LiCl-KCl eutectic salt at 873 K for various durations under ultrahigh-purity argon atmosphere. Corrosion behavior of partially stabilized zirconia (PSZ) coating on candidate materials also was evaluated. Weight-loss results indicated that the corrosion resistance of the materials increased in the following order: 2.25Cr-1Mo > 9Cr-1 Mo > Ni-based alloys > PSZ coating. PSZ-coated specimens showed better corro...

Microstructural Characterization and Corrosion Behavior of Activated Flux Gas Tungsten Arc-Welded and Multipass Gas Tungsten Arc-Welded Stainless Steel Weld Joints in Nitric Acid

AISI Type 304L (UNS S30403) austenitic stainless steels are widely used in spent nuclear fuel reprocessing plants, and welding is an indispensable tool used for joining these materials. In the pres...

Characterisation of pyrolytic graphite exposed to molten LiCl–KCl salt

Abstract Graphite based materials are proposed as one of the candidate materials for metallic fuel reprocessing involving high temperature molten chloride salt environment. Pyrolytic graphite (PyG) was deposited on graphite substrates by thermal chemical vapour deposition using methane gas to enhance their performance in molten chloride environments. The structural features of the PyG deposit were characterised using X-ray diffraction by measuring interlayer spacing, crystallite size and preferred orientation. The present study discusses the corrosion behaviour of PyG in molten LiCl–KCl eutectic salt at 873 K. The PyG samples tested in molten salt for 2000 h in controlled argon atmosphere exhibited negligible weight loss. The as deposited and corrosion tested samples characterised using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and atomic force microscopy showed no significant change in surface morphology. The study showed excellent corrosion resistance of PyG in molten salt under inert atmosphere.

Nanostructured coatings for corrosion protection in reprocessing plants

The main process medium in the reprocessing industry is highly oxidizing nitric acid ranging from dilute to concentrated solutions containing fission products and from room temperature to boiling conditions. Corrosion resistance of materials chosen for reprocessing plants is of prime importance for uninterrupted operation. Surface modification and coatings can significantly improve the corrosion resistance of materials. A number of surface modification and coating development works such as double oxide coating on Ti for reconditioning (DOCTOR); mixed oxide coated Ti anodes (MOCTAs); nanostructured Ti, TiO2, TiN, and ZrN; bulk metallic glasses (BMGs); and superhydrophobic (SHB) coatings for corrosion protection are being pursued in our laboratory. Nanostructured coatings developed on Ti-like DOCTOR and MOCTAs showed improved corrosion resistance and longer life. Nanostructured Ti, TiO2, and ZrN coatings deposited on type 304L stainless steel (SS) by magnetron sputtering technique and Zr-based bulk metallic Zr59Ti3Cu20Al10Ni8 alloy deposited on type 304L SS by pulsed laser deposition (PLD) technique showed improved corrosion resistance in nitric acid. SHB coating on 9Cr-1Mo and Ti lead to improved corrosion resistance and biofouling resistance of Ti. The surface modification and coating development carried out in our laboratory for corrosion protection in reprocessing plants are briefly highlighted.

Corrosion Behavior of Structural Materials in LiCl-KCl Molten Salt by Thermogravimetric Study

Spent metallic fuel from future fast-breeder reactors would be processed by non-aqueous pyrochemical reprocessing. Different unit operations of pyrochemical reprocessing demands various structural materials that are compatible with molten salt medium at high temperatures. In the present study, the corrosion behavior of the candidate materials stainless steel (SS) 410, alloy 600 and 9Cr-1Mo (9% chromium and 1% molybdenum) steel was investigated in molten lithium chloride–potassium chloride (LiCl-KCl) salt by thermogravimetric studies under inert and reactive atmospheres. The test specimens, along with molten salt, were isothermally heated at 500 and 600°C for 24 h in both atmospheres. Alloy 600 showed marginally higher weight gain than 9Cr-1Mo and SS410 under both atmospheres at 600°C. Higher weight gain was observed under reactive atmosphere than in inert atmosphere at 600°C. All of the samples investigated under both atmospheres at 500°C showed insignificant weight loss. The surface morphology and energy...

Electrochemical and surface studies on zircaloy-4 exposed to concentrated nitric acid medium

Zircaloy-4 has been considered as a candidate material for dissolver and evaporators in nuclear fuel reprocessing plants involving nitric acid of high concentrations at high temperatures. The present work deals with the electrochemical studies and surface analysis of passive films on zircaloy-4 and its welds in 11.5 M nitric acid medium. Potentiodynamic polarization technique was employed to study the corrosion behaviour of the alloy in both base and welded (Manual TIG and electron beam (EB) welding) conditions. The polarization curves for zircaloy-4 and its welds exhibited a distinct breakdown of passivity at 1.5 V (SCE) and repassivation, after showing a region of stable passivation. The characterization of passive films by SEM, XRD and LRS, grown at different potentials in 11.5 M nitric acid are presented. The surface morphology and phase analysis of passive films grown at higher potentials showed the presence of monoclinic ZrO2. The phases present in the passive film grown electrochemically in 11.5 M nitric acid at 2.0 V and by autoclaving are found to be similar. The presence of characteristic 1040 cm−1 Raman peak from the sample passivated at 1.2 V indicate that the oxide present consist of nanometer size grains of monoclinic zirconia.

Application of Zirconium Alloys for Reprocessing Plant Components

Zirconium is highly resistant to nitric acid environments and is considered as a candidate material for various applications in spent nuclear fuel reprocessing plants involving concentrated nitric acid medium under highly corrosive oxidizing conditions. Zirconium is insensitive to intergranular corrosion in nitric acid unlike stainless steels of Type AISI 304L (304L SS), which is commonly used in reprocessing plants. Also, unlike titanium and its alloys, zirconium is not affected by the vapor and condensate phases of boiling nitric acid. Zirconium and its alloys are thus considered candidate materials for various applications in spent nuclear fuel reprocessing plants involving nitric acid of high concentrations at high temperatures. This paper discusses the international experience on zirconium for reprocessing plant applications and the consideration of Zircaloy-4 (Zr-4) for reprocessing plant applications based on the research and development work carried out at Indira Gandhi Centre for Atomic Research on Zr-4 as well as manufacturing of dissimilar joints between Zr-4 and 304L SS and a mock-up dissolver for fast breeder reactor reprocessing.

Aluminum phosphate sealing to improve insulation resistance of plasma-sprayed alumina coating

ABSTRACT The insulation resistance of conventional atmospheric plasma-sprayed alumina coatings with 10–15% porosity is ∼1011 Ω. The presence of pores, lamellae boundaries, and other non-fillings dampens the insulation resistance of the coating. In the present study, aluminum phosphate was used to seal the surface of plasma-sprayed alumina coating and evaluate the effect of sealing on the insulation resistance and its thermal cycling response. Sealing was carried out with three concentrations of sealant (P/Al molar ratio of 3, 10, and 15). Characterization by X-ray diffraction and scanning electron microscopy revealed the primary sealing phase as aluminum metaphosphate and effective sealing of the pores by the aluminum phosphate phases. Insulation resistance is improved by two orders of magnitude after sealing the coated samples. Sealing with P/Al molar ratio 3 exhibited maximum insulation resistance of ∼1013 Ω at room temperature. Thermal cycling studies between 650°C and 200°C on the sealed samples showed deterioration in thermal cycling life after sealing.