Evgeniya V. Nikitinа

—The corrosion behavior of 16Kh12MVSFBR steel during high-temperature nitrogen treatment has been studied. The surface composition and structure of the steel are shown to change. Interactions between the electronegative alloying elements of the ferritic–martensitic steel (Mn, Cr) and the gas phase components (nitrogen, impurity oxygen) occur on the material surface. The formation of chromium and manganese nitrides and oxides of various stoichiometric compositions is detected. The corrosion rates of 16Kh12MVSFBR steel at temperatures of 650 and 800°C are 0.104 and 0.241 mm/year after holding for 12 h and 0.013 and 0.020 mm/year after holding for 84 h, respectively. The character of surface degradation of steel samples is continuous nonuniform, with clear localization of corrosion at grain boundaries in steel, which is associated with the formation of secondary phases along the grain boundaries. Conclusions are drawn regarding changes in the structure during high-temperature holding and the character of corrosion damage. Based on the results of thermodynamic modeling and X-ray fluorescence analysis, conclusions are made about the composition of the corrosion products of 16Kh12MVSFBR steel.

Molten chloride salt electrolytes are promising for use as a working medium for the implementation of high-temperature technologies. Alkali metal chlorides are an aggressive environment in relation to structural materials. One of the possible methods of reducing the corrosion damage of a structural material is the method of oxygen passivation of the surface of a metal or alloy by introducing a certain amount of oxygen-containing additives into the melt. The article considers the effect of oxygen-containing impurities (lithium oxide and lithium hydroxide) on the corrosive behavior of a metal material — an alloy of the composition iron–cobalt–nickel. To assess the corrosion resistance of materials, gravimetric analysis, micro-X-ray spectral analysis (XRSA) of the surface and cross-section sections, and X-ray phase analysis (XRF) of the sample surface were used. The dependences of the corrosion rate of the material on the concentration of oxygen-containing additives Li2O and LiOH are presented. Based on the data set of gravimetric, MRSA and XRF data, it was found that 29NC alloy samples in the LiCl–KCl–nLi2O salt melt are not susceptible to corrosion, but in the LiCl–KCl–nLiOH melt, the speed of the 29NC alloy increases significantly due to the interaction of the LiOH additive with the most electronegative component of the alloy — iron.

—The successful use of molten alkali metal fluorides as electrolytes for high-temperature plants requires the creation of reactors materials that have high corrosion resistance in melts with the compositions that are typical of a mixing molten salt reactor (MSR-M). This is one of the most important unresolved problems. One of the effective ways to decreases corrosion losses is to create a layer on the metal surface in order to protect the metal from the aggressive effects of the environment. In this work, molybdenum insulating coatings on 12Kh18N10T steel, which are deposited in molten salts of various compositions using various methods, are analyzed. Experiments are conducted to perform the electrodeposition of molybdenum coatings on an iron-based structural material (12Kh18N10T steel). The electrochemically deposited coatings are heterogeneous and easily flaking. The thicknesses of the molybdenum coatings deposited on steel in molten salts are 8.15 and 20.34 μm in the FLiNaK and LiCl–KCl melts, respectively. Corrosion tests have shown the inefficiency of the molybdenum coatings deposited from both chloride and fluoride melts. The corrosion rate of 12Kh18N10T steel in the FLiNaK/(FLiNaK + 5% CeF₃) melts at 650°C for a holding time of 100 h decreases in the following row: 12Kh18N10T + Mo (0.75/0.77 mm/year) > 12Kh18N10T (0.45/0.50 mm/year).

When spent nuclear fuel (SNF) is processed, the LiCl–KCl melt (0.49 : 0.51) is used in an inert atmosphere. All metal materials in this salt melt are extremely susceptible to corrosion; moreover, both the liquid (melt) and gas phases are saturated by decay products, which can be additional oxidizing agents increasing the aggressiveness of the environment, during the processing of SNF. The pyrochemical SNF technology includes operations such as soft chlorination, electrorefining, and metallization, which imply the presence of the chlorides of rare earth metals (REM) lanthanum, cerium, and neodymium and uranium(III, IV) chlorides in a melt. In this work, the corrosion behavior of 12Kh18N10T steel in a LiCl–KCl melt containing NdCl₃, CeCl₃, LaCl₃, UCl₃, and UCl₄ additives up to 2 wt % is studied. Corrosion tests for 100 h are carried out at a temperature of 500°C in an inert argon atmosphere. The presence of REM chlorides is found to significantly decrease the degradation of the steel. The addition of (REM)Cl₃ leads to the formation of an (REM)OCl compound on the sample surface, the thickness and continuity of which increases in the row LaCl₃ < NdCl₃ < CeCl₃. The formation of similar compound leads to the inhibition of the corrosion of 12Kh18N10T steel due to the salt passivation of the surface. The addition of UF₄ into the melt causes significant intercrystalline corrosion of 12Kh18N10T steel. The introduction of UF₃ into the melt leads to a decrease in the corrosion rate, which is associated with the predominant interaction of the trivalent uranium chloride with molecular oxygen dissolved in the melt, and to the formation of a nonstoichiometric compound with the crystal-chemical formula U₃O₇ on the sample surface according to electron-probe microanalysis.

Corrosion processes of candidate materials for MSR were investigated: nickel-chromium-molybdenum alloys of the "Hastelloy" type: C-2000, CrNi65 and steel 12Cr18Ni10Ti, nickel-chromium (Ni80Cr20), nickel-titanium (NITINOL), nickel-copper (Monel 404) in a melt based on LiF–NaF–KF with compositions characteristic of MSR at a temperature of 550 °C. The weight and depth indicators of corrosion of materials are determined.

The corrosion process of 12X18H10T steel in a KCl-Nacl melt with the addition of Li2O (0-11000 ppm O2-), as well as lanthanum, cerium and neodymium trichlorides, was investigated. The concentration interval of lithium oxide in the LiCl-KCl melt is determined in the range from 0 to 0.2 % by weight of Li2O, at which the corrosion rate decreases by 5–6 times. The concentration interval of lithium oxide in the LiCl-KCl melt is in the range from 0 to 0.2 % by weight of Li2O, at which the corrosion rate decreases by 5–6 times.

—Electrochemical diagnostics is carried out for nickel electrodes in the LiCl–KCl eutectic melt containing up to 0.2 wt % Li₂O, 10 vol % О₂, and 2% LaCl₃ at 500°C in an inert argon atmosphere. Both silver chloride and lithium dynamic electrodes are used as reference electrodes. The latter is certified by thermodynamic analysis and recording of cyclic voltammograms along with the potential decay curves under the chronopotentiometric conditions for which the following most optimum parameters of the galvanostatic pulse are selected: a pulse duration of 4 s at a current density of 0.6 A/cm². Under these conditions, the stability time of a lithium dynamic reference electrode is 59.5 ± 1.2 s. The electrochemical diagnostics of a nickel–molten salt electrolyte system with artificially added oxygen-containing impurities affecting the redox potential of the corrosion-active medium is carried out. The introduction of oxygen anions into the system shifts the nickel potential to the cathodic region, which decreases the corrosion activity of the medium. An increase in the oxygen concentration in the system shifts the nickel potential to the anodic region, indicating a more aggressive salt medium. The addition of lanthanum chloride to the melt affects the potential only if the system contains O₂, which is due to their interactions. The corrosion potentials of nickel obtained at both the silver chloride and lithium dynamic reference electrodes correlate to each other. This fact allows one to extend the applicability of the lithium dynamic reference electrode to any halide melts, including fluoride ones.

The problem of tailoring the structural materials for MSR is solved by continuously overcoming the shortcomings of widely used materials and finding new ones. The materials commonly used in engineering may not be applicable for MSR due to their high corrosivity. Experiments were carried out to determine the corrosion rate of stainless steel 12Cr18Ni10Ti with different concentrations of oxide ions (by adding lithium oxide to the melt in the concentration range from 0 to 0.8 wt.%) in a FLiNaK melt. The formation of a protective oxygen-containing layer with a thickness of 1 micron has been realized. The corrosion rate decreases by an order of magnitude at the concentration of oxygen anions in the melt, in the range from 0.2 to 0.4% by weight, which may indicate high-temperature passivation of the material due to modification of the composition of the fluoride melt and reduction in its corrosion activity. In addition, the corrosion type of stainless steel in fluoride melts changes from the intercrystalline and pitting that is usually harmful to reactor material structure to total corrosion when lithium oxide is added. This is due to the “healing” of individual corrosion defects formed on the surface of the studied material by oxygen-containing compounds.

The article is devoted to the study of light-emitting heterostructures based on GaP(As,N) dilute nitrides, monolithically grown on silicon substrates by plasma-assisted molecular beam epitaxy. Current-voltage characteristics and electroluminescence spectra of the grown heterostructures are obtained. For the first time, a unique effect is observed in GaP(As,N) dilute nitrides - the appearance of white electroluminescence when a reverse bias is applied. The result was obtained due to the original design of the light-emitting heterostructure and the unique properties of dilute nitride solid solutions.

Molten fluorides of alkali metals are considered a technological medium for molten salt reactors (MSRs). However, these media are known to be extremely corrosive. The successful implementation of high-temperature technological devices using molten alkali metal fluorides requires the selection of such structural materials that have high corrosion resistance in melts with compositional characteristic of MSRs. In this research, the corrosion behavior of 12Cr18Ni10Ti steel, the alloy Ni60Cr20Mo15, and the alloy Monel 404 (Ni50Cu50) was investigated in the LiF–NaF–KF eutectic melt, containing additions of CeF₃ and NdF₃ from 0 to 5 wt.% as imitator fluorides of actinides in an inert argon atmosphere at 550 °C for 100 h. Gravimetry, energy-dispersive X-ray (EDX) microanalysis of surfaces and cross-section of samples, and ICP-MS were used to establish the corrosion behavior of the investigated alloys. Corrosion resistance of the studied materials was found to decrease in a row from Monel 404 > Hastelloy C2000 > 12Cr18Ni10Ti. The addition of cerium fluoride into the melt resulted in the additional etching of the alloy surface. The addition of neodymium fluoride resulted in the formation of the point/inter-crystalline corrosion damages in the sample bulk. The samples of steel 12Cr18Ni10Ti were subjected to local cracking corrosion. The austenitic nickel-based alloys suffered specific local corrosion with formation of subsurface voids. Excellent corrosion resistance of the Monel alloy under the test conditions was found.