Molten NaNO3-KNO3 Research Articles

Corrosion Behavior of FeCoNiCrAl High-Entropy Alloy in Molten NaNO3-KNO3

Molten nitrates are the main heat transfer fluid (HTF) for concentrated solar power (CSP) systems. However, due to the instability of molten nitrates at high temperatures, the corrosiveness of the molten nitrates poses high requirements for the structural material used in CSP. Therefore, it is urgent to develop highly corrosion-resistant materials. In this study, the corrosion behavior of FeCoNiCrAl high entropy alloys (HEA) in molten NaNO3-KNO3 (60 wt% to 40 wt%) under argon at 600°C is investigated by mass loss and electrochemical methods. The results show that the FeCoNiCrAl HEA experienced severe mass loss during the 100 h immersion due to the high oxygen partial pressure and the galvanic corrosion effect. The corrosion products of FeCoNiCrAl HEA in the melt consist of Fe2O3, Cr2O3, FeCr2O4, and NaFeO2. After immersion for 100 h, an outer layer dominated by porous iron oxides and an inner more compact Cr-rich layer are formed. Furthermore, both of the oxide layers are gradually thickened with the extension of the corrosion time, and the process is manifested by the increased value of the oxide layer resistance Rox and charge transfer resistance Rt in the electrochemical impedance spectra. At the same time, compared with the Rt of 316L stainless steel, it can be seen that with the extension of corrosion time, the Rt of FeCoNiCrAl HEA is larger and shows better corrosion resistance in the same corrosive environment. In addition, FeCoNiCrAl HEA shows a higher corrosion potential and a lower corrosion current density than 316L in molten nitrates at 600°C.

Numerical simulation of natural convection and heat transfer in a molten pool with embedded cooling tubes

This study described the natural circulation and heat transfer of a molten pool in a specifically designed core catcher conceived for a pressurized water reactor. In addition to external cooling, the core catcher features internal cooling tubes embedded in the molten pool. To investigate the coolability in such a configuration, first, a full-scale core catcher simulation is conducted to give a preliminary study under a real SA situation. Results illustrated that cooling efficiency can be remarkably enhanced due to the inner tubes. Then a test facility of the 2D slice with the geometrical scaled factor of 1:4 has been developed, and molten salt (NaNO3–KNO3) experiments will be implemented in the near future. This study also performed a pre-test simulation using molten NaNO3–KNO3 as a stimulant to study the heat transfer and flow characteristics of the salt pool. The melt convection in the test section was represented by a two-dimensional mesh with a WMLES turbulence model using the FLUENT code. The simulation captured the heat transfer enhancement by the cooling tubes as expected, and the results would help decide the proper test matrix and improvement of instrumentation required to obtain the necessary data for code validation.

Influence of partial components removal on pyrolysis behavior of lignocellulosic biowaste in molten salts

Using molten NaNO3–KNO3–NaNO2 for biowaste pyrolysis provides a promising way to synergize the utilization of biowaste and fluctuating solar energy. However, the diversity of lignocellulosic biowaste and violent decomposition complicated the interactions between natural constituents. In this study, the pyrolysis behavior of woody and agricultural herbaceous lignocellulosic biowaste in molten salts was comprehensively investigated at 300 °C. Through the selective removal of partial hemicellulose or lignin, the specific role of the original structure on the products formation characteristics was further estimated. The results showed that more volatiles were released from the hemicellulose-enriched agricultural herbaceous biowaste than cellulose-enriched woody biowaste. The yields of water and organic liquids from hemicellulose or lignin removal were higher than that of raw biowaste. Specifically, hemicellulose dominated the generation of nitrogenous substances in the liquid products. As for the gas product, its yield from the hemicellulose removal sample was higher than raw biowaste but it decreased after removing lignin. Lignin was the primary contributor to the combustible CO, H2, and CH4, while the production of CO2 was more affected by hemicellulose. The lignin would compete with hemicellulose and be superior to the latter in the reaction with molten salt.

Compositional and Bath Temperature Effects on Heat Transfer During Quenching in Molten NaNO3–KNO3 Salt Mixtures

The present study involved the assessment of cooling severity of molten NaNO3–KNO3 mixtures which are widely used as quench media for austempering and martempering operations. An Inconel probe instrumented with thermocouples was quenched in molten NaNO3–KNO3 binary mixtures of varying concentration maintained at different quench bath temperatures. The temperature data acquired at various locations in the Inconel probe during quenching was used to calculate the spatially dependent transient heat flux at the metal–quenchant interface. Two critical points corresponding to peak heat extraction rates during the nucleate boiling stage and transition from boiling to convection stage were identified for each quench medium. The variation of average heat flux and average surface temperature corresponding to these critical points was mapped with variation in bath temperature and composition of the quench medium. AISI 4140 steel probes were quenched in these quench media maintained at 300 and 350 °C. The average hardness values measured in steel probes agreed with the cooling performance of these quench media determined using Inconel probe. The degree of uniformity in heat transfer as indicated by the spatial variation of normalized heat energy decreased with the increase in the concentration of KNO3 in the quench medium. A mechanism of boiling heat transfer during quenching based on thermochemical decomposition of the salt was proposed.

Corrosion of Additively Manufactured CoCrFeMnNi High Entropy Alloy in Molten NaNO3-KNO3

Exposure testing was performed on CoCrFeMnNi equiatomic high entropy alloy (HEA) produced via directed energy deposition additive manufacturing in NaNO3-KNO3 (60–40 wt%) molten salt at 500 °C for 50 h to evaluate the corrosion performance and oxide film chemistry of the HEA. Potentiodynamic electrochemical corrosion testing, scanning electron microscopy, focused ion beam milling coupled with energy dispersive spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectroscopy were used to analyze the corrosion behavior and chemistry of the HEA/nitrate molten salt system. The CoCrFeMnNi HEA exhibited a higher passive current density during potentiodynamic polarization testing than steel alloys SS316L and 4130 and the high-Ni alloy 800 H in identical conditions. The oxide film was primarily composed of a (Mn,Co,Ni)Fe2O4 spinel with a vertical plate-like morphology at the surface. Cr and Ni were found to be totally depleted at the outer surface of the oxide and dissolved in high concentrations in the molten salt. While Cr was expected to dissolve into the molten salt, the high concentration of dissolved Ni has not been observed with traditional alloys, suggesting that Ni is less stable in the spinel when Mn and Co are present.

Molten salt synthesis of SrFeO3 nanocrystals

Pure perovskite compound SrFeO3 nanocrystals were successfully synthesized in molten NaNO3–KNO3 eutectic with Na2O2 from a mixture of strontium nitrate and ferric nitrate. The effect of metal precursors, salt medium, annealing temperature (mainly 400–800°C), and oxidizing properties of the melt on the phase composition, crystallite size and morphology of the resulting metal oxides were systematically studied by simultaneous TG/DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM) methods. The results showed that the formation of the SrFeO3 phase mainly depended on the nature of the metal precursor and salt medium. It was found that metal nitrates are the suitable precursors and NaNO3–KNO3 eutectic with Na2O2 is the suitable salt medium, which results in the formation of pure SrFeO3 nanocrystals at a much lower temperature of 400°C.

Dissolution Susceptibility of the Oxide Species Formed on Mild Steel During Its Oxidation in Molten NaNO<SUB>3</SUB>-KNO<SUB>3</SUB> Eutectic Mixture

Mild steel was oxidized in pure molten NaNO3-KNO3 eutectic mixture at temperatures of 300, 375, and 450 °C and in the presence of 0.05 molal KH2PO4, K2Cr2O7, or Na2O2 as additives. The dissolution susceptibility of the formed oxide species was evaluated in H2SO4 acid solution using the potential-time and current-time measurements under the open-circuit conditions. It was found that the rate of dissolution depended on the composition of molten nitrate bath, used for oxidation of mild steel, and its temperature. This was attributed to the effect of the previous conditions of the nitrate bath on the nature and composition of the oxide species formed on the metallic surface. The more resistant to dissolution in H2SO4 solution were those electrodes that were oxidized in nitrate melt at 450 °C in the presence of K2Cr2O7 or Na2O2 as additives.

Synthesis of dispersed ZrO2 in the fluoride-doped molten NaNO3-KNO3 mixtures

Synthesis of dispersed ZrO2 in the fluoride-doped molten NaNO3-KNO3 mixtures

Characteristics and sintering behaviour of 3 mol % Y2O3-ZrO2 powders synthesized by reaction in molten salts

Yttria-stabilized tetragonal zirconia powders have been synthesized by reaction in molten salts at 450 °C. To obtain a Y2O3-ZrO2 solid solution powder, it was necessary to use yttrium salts (not yttrium oxide directly) in molten NaNO3-KNO3 eutectic. The agglomeration state of the powders depended on their washing conditions. An alcohol-washed powder containing soft agglomerates led to a fine-grained and high-density sintered body.

The thermal conductivity of the molten NaNO3-KNO3 eutectic between 525 and 590 K

Molten salts are one of the few remaining classes of fluids for which standardquality (±1% accuracy) data on thermal conductivity have not hitherto been available. We have therefore developed a new apparatus based on the transient hot-wire technique to obtain reference-quality measurements of the thermal conductivity of molten salts at high temperatures. Liquid metal-filled quartz capillaries served as insulated hot wires in our method, and in addition, a two-wire technique was used in order to obtain absolute values of the thermal conductivity. New data for the NaNO3-KNO3 eutectic between 525 and 590 K are reported in this paper and comparisons with other recent measurements are shown.

硝酸塩系溶融塩中におけるステンレス鋼の溶出挙動

硝酸塩系溶融塩中におけるステンレス鋼の溶出挙動

Corrosion of plasma nickel-containing coatings in molten electrolytes

The self-fluxing alloys PN-KhN90S2R2 and PN-KhN80S3R3 sprayed by the plasma method increase the corrosion resistance of 40Kh and 3 steels at 773–1223°K in molten NaNO3-KNO3 by 4–10 times, in Na2SO4,-K2SO4 by 3–12 times, and in (NaPO3)n or (KPO3)n by 20–80 times. In NaCl-KCl the corrosion resistance of carbon steels and nickel-containing self-fluxing alloys is the same.

Apparatus for the precise measurement of the physical properties of liquids at elevated temperature and pressure

This report describes an instrument designed to measure the viscosity, surface tension, and density of liquids at elevated temperature and pressure. The theoretical principles, details of construction and operation, and response of the instrument are discussed in detail. As an example of the accuracy of the data generated by this instrument, the thermophysical properties of molten NaNO3–KNO3 have been measured over the temperature range 250–400 °C and are shown to be in excellent agreement with the recommended literature values.

Corrosion of Mild Steel in Molten Sodium Nitrate–Potassium Nitrate Eutectic

The corrosion (oxidation) of mild steel in molten NaNO3–KNO3 eutectic has been investigated at temperatures ranging from 250° to 450°c, for exposure periods up to 8 h. The results of both weight-gain and potential measurements revealed the formation of a passivating film consisting mainly of Fe3O4. The thickening of the oxide film proceeds according to a parabolic law, at a rate depending on the temperature of the melt. The activation energy of the process was estimated to be 71·3 kJ/mol. A break was observed in the parabolic plots at higher temperatures; this may indicate the beginning of the passivation process. The results have been discussed in the light of oxidation kinetics data for iron in both molten nitrates and air at higher temperatures.

Corrosion of Mild Steel in Molten Sodium Nitrate–Potassium Nitrate Eutectic

The corrosion (oxidation) of mild steel in molten NaNO3–KNO3 eutectic has been investigated at temperatures ranging from 250° to 450°c, for exposure periods up to 8 h. The results of both weight-gain and potential measurements revealed the formation of a passivating film consisting mainly of Fe3O4. The thickening of the oxide film proceeds according to a parabolic law, at a rate depending on the temperature of the melt. The activation energy of the process was estimated to be 71·3 kJ/mol. A break was observed in the parabolic plots at higher temperatures; this may indicate the beginning of the passivation process. The results have been discussed in the light of oxidation kinetics data for iron in both molten nitrates and air at higher temperatures.

Reduction of cadmium ions at stationary platinum micro-electrodes in fused alkali nitrates

Current-reversal chronopotentiometry was used to investigate the cathodic reduction of Cd2+ ions at platinum micro-electrodes in molten NaNO3-KNO3 at different temperatures. The high rate of the irreversible oxidation of cadmium metal by the nitrate solvent and consequently the difficulties to interpret quantitatively the data obtained are discussed.