With increasing number of batches, corrosion time and the concentration of impurities also increase, but there is a lack of research to study the effect on corrosion behavior. When LiCl-KCl electrolyte is recycled, new electrolyte will affect the corrosion behavior of structural material in the initial period, but most corrosion studies only do one-time measurement of properties for selected corrosion period, so initial timely corrosion behavior is not clearly explained. This study concentrates on revealing the effect of impurities on corrosion behavior, and investigates initial corrosion behavior by timely measurement with electrochemical techniques.
YCl3 tested as a representative rare earth impurity dissolved in LiCl-KCl eutectic salt to assess the effect of fission product impurities on corrosion behavior. Observed the corrosion behavior under intensive conditions to overcome the 100 hour limitation by testing some cases at 873K.
Introduction
General Background
Shared storage facility at each multi-reactor site (four to eight reactors served by a single storage facility);
Pyroprocessing flowsheet
The remaining oxides are collected from metal ingots in the electrorefining process, and the dissolved chlorides are collected through an electrolytic winnowing process to be recycled. In the electrorefining process, uranium is separated from the metal product of the electrolytic reduction process and collected at the solid cathode while noble elements remain in the anode chamber. Uranium and a small amount of uranium salts deposit on solid cathode, then the salts remaining on cathode are distilled by cathode process to refine the uranium to pure ingot.
The electrolyte is composed of LiCl-KCl eutectic salt containing 9 wt% UCl8, and the concentration of UCl8 should be sufficient to prevent reduction of TRU or RE. During the process, transition metal fission products remain in the anode basket as it cannot be oxidized in the state of the electrorefining cell. The amount of uranium required for the chlorination process is proportional to the amount of slag.
Separation of cadmium from the TRU-cadmium intermetallic compound, conversion of the product to ingots, reduction of TRU from the salt before removal of the fission products from the salts. After the concentration reaches the limit, the electrolyte went to the electrowinning process, then the RE/TRU deposition on the LCC at the appropriate electrochemical potential range. Since the melting point of cadmium is lower than TRU, only the cadmium is distilled and the remainder is processed into ingots.
The TRU in the salt is first removed in the pumping process for reprocessing in the electrorefiner, then the rare earths in the salt are separated by the salt purification process. In the salt cleaning process, the fission products produced by the electrolytic reduction and electrowinning process, such as Cs, Sr, RE and a small amount of TRU, are separated from the salt. The salts are recycled in pyroprocessing, and the fission products are discarded in ceramic form for Cs and vitrified form for Sr, RE and TRU.
Mechanism of corrosion in molten chloride salt
Literature Review
Super alloy utilized in molten chloride eutectic salts
Effect of alloying elements on corrosion resistance
With the presence of chloride, even a small amount of oxygen or water can cause reactions with chlorides to form oxychloride, hydrochloric acid and chlorine gas. Although the inflow of oxygen or moisture is well blocked, oxide layer or ceramic containing oxygen reacts with chloride and will form corrosion products with materials forming alloys. In the initial phase of corrosion, metal chlorides are formed and LiCl-KCl eutectic salt is dissolved in gas form.
If there is an oxygen ion in the LiCl-KCl eutectic salt, the metal chloride gases react with it to form a metal oxide as a corrosion product. If an element of an alloy has a lower free energy than the other, the element forms corrosion products in the outer region and the other element in the inner region. The solubility of oxygen ion or moisture is closely related to the concentration in the atmosphere.
These impurities must be excluded to measure the corrosion behavior of samples to replicate the normal operating environment of the electrorefining process. Also, the oxide layer adhered to the surface of the sample must be loosened by special polishing.
Research Objective and Scope
Research Objective
Research Scope
Electrochemical Measurement
- Experimental methods
- Specimen preparation
- Electrolyte preparation with impurities
- Reference electrode preparation
- Experimental cell system for corrosion study
- Electrochemical techniques
- SEM-EDS analysis on microstructure
- Analysis method
Inconel 600 was measured hourly by measuring the polarization resistance and table slope within ± 5 mV of the corrosion potential to measure the change in corrosion rate with time. The measurement of corrosion rate by electrochemical experiments is based on the measurement of corrosion density intensity and polarization resistance. Potentiodynamic measurements can be used to derive the change in slope of the current (I) - power (E) graph.
12 summarizes the results of the corrosion current measured for 100 hours in the temperature range of 773 ~ 873K using YCl3 as an impurity in the LiCl-KCl eutectic salt environment. The environment of (C) shows a very high corrosion current density in the early part of the measurement, but the corrosion current density drops to similar level of the measurement results in other environments until about 25 hours. The corrosion rate is calculated by applying the material properties of the sample to the corrosion current density, as shown in the ASTM guidelines, so that it basically appears as the behavior of the corrosion current density.
Case (C) shows a drastic drop in the corrosion potential from about 140 mV to -15 mV from the initial 25 hours. The lower the corrosion potential, the greater the tendency of the sample to oxidize and the faster corrosion occurs. The corrosion potentials of the three cases were generally in the order C > A > B, which is the reverse order of the corrosion rates.
16 are the SEM-EDAX results of the Inconel 600 specimen which was subjected to the corrosion test for 100 hours in the 873K molten salt of LiCl-KCl with or without impurities. Since LiCl-KCl eutectic molten salt is a main electrolyte used for electrorefining and pyroprocessing electroprocessing, it is very important to study the corrosion characteristics simulating the actual operating environment of the structural material. Since impurities have been identified to affect the corrosion rate of the pyroprocessing structural material, it is necessary to study the molten salt environment in which various impurities are added.
Result & Discussion
Verification of experimental measurement system
Prior to the corrosion experiment, comparison was made with existing research on molten salt and experimental equipment, in addition to specimens to verify future measurement and analysis methods. As a measurement technique, the stability of the interfacial state of the electrode is checked through the open circuit test provided by the Versastat 3F potentiometer and the accompanying Versastudio software. Multiple cyclic voltammetry is used to identify the phenomenon of oxidation and reduction at a specific voltage, the reliability of the reference electrode and the ability to detect impurities were evaluated.
The following is the result of multiple cyclic voltammetry measurements of LiCl-53.2 wt% KCl-5 wt% LaCl3 at 773K and comparison with the reference case. A tungsten wire was used as a working electrode and a counter electrode, and an Ag/AgCl electrode was used as a reference electrode. Since there is a difference in the reference electrode and the amount of LaCl3 is 5 times more than the reference case, the shape of peak and current are not exactly matched.
For a quantitative comparison of the CV curve with the reference case, taking into account the difference at the reference electrode, then Fig. study is Ag/1 wt% AgCl or Ag/0.3924 mol% AgCl in mullite. Assuming that the difference created by the membrane material is negligible, different Nernst equilibrium potentials result in a difference between the two reference electrodes.
The scan rate of this study and the reference case are 300 mV/sec and 250 mV/sec, respectively. This relationship and assumption shows that the ratio between the calculated LaCl3 concentration of this study (C0) and the reference case (C1) is the same. The calculated concentration ratio from the Randles-Servick equation is very similar to the actual ratio of 5.59, so the experimental cell is in good agreement with reference data.
![Fig. 11. Cyclic voltammograms for test cell on (a) [-2.5V, 0V], (b) [-2.3V, 0.5V] and reference voltammograms for (c) both La + /La and Li + /Li and (d) only La + /La](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10514008.0/36.892.106.759.174.720/fig-cyclic-voltammograms-test-cell-reference-voltammograms-la.webp)
Electrochemical measurement of corrosion behavior
The corrosion rate was calculated for each case using the measured corrosion current density as shown in Fig. Case (A) also exhibits significant deviations in corrosion potential as shown by relatively high corrosion rate deviations. Especially in the interval up to 15 hours, corrosion potential decreases very sharply, and the corrosion rate drops rapidly in that interval.
The corrosion rate also increases drastically from about 0.1 mm/year to 0.6 mm/year by 46 hours where the corrosion potential becomes less variable about -145 mV at 70 hours to -150 mV. If the surface of the sample during the relevant time period is analyzed and it is possible to determine which factors cause the rapid deterioration of the corrosion rate, this factor can be used as an important data to improve the corrosion resistance of the electrolytic refining furnace. In this process, the corrosion rate also drops rapidly from a very high level of more than 3.3 mm/year to a low level of 0.25 mm/year, so there appears to be a very weak initial surface corrosion component. to exemplify.
The corrosion potential and corrosion rate after 100 hours of corrosion test for each case are shown in Table 7. However, the initial corrosion behavior of cases (B) and (C) (873 K) showed that the corrosion rate was also higher at the point where initially the corrosion potential gradually decreased. It is reasonable to assume that the composition of the molten salt dissolves rapidly in the early stage of corrosion until a certain percentage of metal chloride from the sample is reached, rather than saying that a change in the material composition of the surface of the sample causes rapid corrosion.
Even case (A) with a temperature of 773K was about 21% higher in corrosion rate than the 873K case without impurities. The Gibbs free energy of YCl3 is higher than that of the LiCl-KCl eutectic salt at a temperature of 873K, but lower than that of the chloride compounds in the Inconel 600 components. Therefore, YCl3 can accelerate corrosion by becoming a springboard for chloride formation of Cr, Fe, Ni of Inconel 600, or yttrium can form yttrium compound with other metal components on the surface of Inconel 600.
SEM-EDAX microscopic analysis
SEM photomicrograph and EDAX composition map for Inconel 600, tested in LiCl-KCl eutectic salt at 873K for 100 hours. Relative concentration of composite materials without Cr from the result of EDAX on Inconel 600 tested in LiCl-KCl at 873K for 100 hours.
Conclusions
