U. Kamachi Mudali

Alumina coating, with high electrical resistivity, has been proposed over Inconel 600 rings for application in fission chamber neutron detectors, operating at 923 K in fast breeder reactor. For improving the thermal cycling life and adhesion strength of alumina coating, selection of the suitable coating technique and bond coat, with appropriate spray parameters was pursued. In the present study, Ni–50%Cr bond coat was deposited over Inconel 600 by the high velocity oxygen fuel technique, while the top alumina coat was deposited using air plasma spray process. A dense coating with minimum porosity and a stable insulation resistance (600 V DC) of ∼10¹¹ ohm at room temperature and ∼10⁸ ohm at 923 K was achieved. Thermal cycling studies on coated samples and rings showed good thermal cycle life tested up to 100 cycles. The coating exhibited good bond strength and only marginal decrease in bond strength was observed with increase in thermal cycling.

Transition metal nitride films have a unique combination of properties and finds application as protective coatings. The present paper deals with the degradation behaviour of HfN coating deposited by reactive magnetron sputtering on high-density (HD) graphite and niobium (Nb) crucibles with molten uranium. Uranium melting experiments were carried out with uranium shots placed in HfN-coated crucibles and heated to 1573 K under ultra high purity argon gas for 25 min. The study showed delamination of HfN coating deposited on Nb and HD graphite crucibles. The degradation of HfN-coated niobium was attributed to partial melting and surface oxidation. Uranium melting of HfN-coated HD graphite crucible led to coating spallation that could be attributed to thermal expansion mismatch and also resulted in the formation of uranium–carbon mixture of unusual morphology. The paper highlights the results of the present investigation.

Pyrochemical reprocessing utilizing molten chloride salt medium has been considered as one of the best options for the reprocessing of spent metallic fuels from future fast breeder reactors. Purification of molten salt is an important step where chlorine gas is purged in molten LiCl-KCl eutectic salt at 873 K. Materials for manufacturing of vessels and components for salt purification system should possess high corrosion resistance under such a highly corrosive environment. The present paper discusses the corrosion behavior of INCONEL Alloys 600 (UNS N06600), 625 (UNS N06625), and 690 (UNS N06690) and their welds in molten LiCl-KCl eutectic salt at 873 K under Cl2 bubbling. Characterization of the exposed surfaces was performed by using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy, and glancing incidence x-ray diffraction. The results of the present study indicated that N06600 and N06690 offered better corrosion resistance compared to N06625. Surface morphology of the exposed base metal and weld metal surfaces exhibited intergranular and interdendritic corrosion, respectively, on N06600 and N06625. SEM micrographs clearly indicated that N06690 exhibited uniform dissolution, while N06600 and N06625 exhibited localized attack. The paper highlights the results of the present investigation.

Critical components in reprocessing plants like continuous dissolver and centrifugal extractor demand austenitic stainless steels (SS) with good corrosion resistance coupled with high wear resistance in nitric acid medium. High nitrogen containing, impurity controlled, and vanadium-added 304LN1 (0.132% N), 304LN2 (0.193% N), and 304LN3 (0.406% N) SS with improved hardness were developed and corrosion studies were performed as per ASTM A262 Practice-A & C test and double loop electrochemical potentiokinetic reactivation test. The 304LN stainless steels were evaluated in forged, hot rolled, and hot rolled + heat treated conditions (948 K: 1, 10, 25, 50, 100, 260 h; 1,223 K: 1 h; 1,323 K: 1 h; and 1,473 K: 1 h). The study revealed good corrosion resistance for 304LN1 and 304LN2 SS as per ASTM A262 Practice-C test in sensitized condition (948 K/1 h). The degree of sensitization (DOS) and corrosion rates of high nitrogen 304L SS increased during sensitization heat treatment (948 K) done up to a certain aging duration, and thereafter desensitization was observed with decrease in corrosion rate and %DOS. Among the high nitrogen 304L SS, 304LN3 with 0.4% nitrogen content showed accelerated intergranular corrosion associated with grain droppings. Heat treatment of these high nitrogen 304L SS at solution annealing temperature gave encouraging results. This study increases the confidence in considering high nitrogen alloyed 304L SS with improved hardness for reprocessing applications by achieving adequate corrosion resistance with controlled alloying elements and impurities, and proper heat treatment/microstructure.