Studies on electrochemical surface modification for molten salt blanket system in fusion reactor

This doctoral dissertation presents studies for materials for devices in molten fluoride salt blanket system in fusion reactor. When steel materials are employed as a structural material, its corrosion with the fluoride salt is a critical issue. To prevent it, many kinds of ceramics and coating processes have been discussed for compatibility with fluoride salts. However, there are many problems such large area coating over 1000m2, toughness related with peeling and crack, wastes after the coating process and healing of damage parts. In this dissertation, a surface modification method through an electrochemical process using molten fluoride salt itself was proposed to form robust functionally graded material layers at the structural material surface and to overcome these problems.First, several kinds of oxides and nitrides were thermodynamically considered for compatibility with fluoride molten salts. The thermodynamic consideration predicted that oxides dissolve into molten fluoride salt and that nitride have compatibility with molten fluoride salt. And prior to development of the surface modification process, compatibilities of oxides and nitrides in FLiNaK at 600 deg C were examined in immersion test over 1000 hours using bulk test specimens such Er2O3, Y2O3, Al2O3 and AlN. The results also demonstrated that nitride, AlN, indicated excellent compatibility with molten fluoride salt, FLiNaK. Secondly, coating processes were considered. To form robust graded compositional nitride layers using compositional elements from the structural material, an electrochemical process was proposed. In the process, the surface of structural material is electrochemically treated in molten fluoride salt including Li3N as a nitrogen source. Thirdly, because the experiments using molten fluoride salt have to be conducted in dry environment at high temperature over 500 deg C, an original experimental setup, which consists of a stainless steel reactor, a nickel crucible, thermocouples, heaters and electrodes, was designed and assembled for the experiments. It was installed in a dry Ar gas filled glove box. Aluminum rods were used as counter and reference electrodes. The specimen of 316 stainless steel (ss316) was bound tightly by a nickel wire at an end of nickel rod and it worked as working electrode. The potential standard was determined by the following equilibrium redox reaction of aluminum lithium alloy: Al + Li+ + e- = LiAl. The reaction can cause two half-reactions: oxidation at anodic reaction (loss of electron) and reduction at cathodic reaction (gain of electron). Temperature was controlled by a PID controller connected via cables to the heater. The electrodes were insulated by alumina tubes from the stainless steel reactor vessel. These electrode assembles were connected via cables to a potentiostat and a function generator. Data was recorded by a data logger connected with the potentiostat. Fourthly, cyclic voltammograms were measured using the experimental setup. From the results, the nitriding condition was decided. Fifthly, ss316 surface was treated in a binary eutectic mixture of LiF-KF (FLiK) including Li3N in a potentiostatic condition. The treatment was conducted at 1.0V with respect to lithium redox potential as the standard potential, ie, 1.0V vs. Li/Li+. For the treatment for 100 and 240 minutes, nitrogen was introduced into a depth of 35 and 65 um from the surface, respectively. When d[um] is definced as the depth of nitrogen introduced layer and t[min] is defined as the treatment time, it was found that d is approximately proportional to t, ie, d[um] = 0.3×t[min]. These specimens were analyzed using analytical methods such X-ray diffraction (XRD), electron probe micro analyzer (EPMA), electron energy dispersive X-ray spectrometry (EDX), X-ray photoelectron spectroscopy (XPS), and scanning electromicroscopy (SEM). It was revealed that chromium nitride CrN formed selectively. The composition ratios of the nitride layer and the bulk layer were evaluated as Cr17.5wt% - Fe70.8wt% - Ni11.3wt% - Mo0.4wt% and Cr17.0wt% - Fe71.9wt% - Ni9.9 wt % - Mo1.2wt%, respectively. It was also suggested that face-center cubic (fcc) structure transformed to body-centered tetragonal (bct) structure. These results would mean that while the metal composition ratio was mostly held, the phase transformation was caused. Formation of solid solution alpha-FenN (n>8) was also suggested. Although oxygen impurities were also expected to be introduced to the nitride layer, in fact, oxygen was not introduced into the layer. This means that nitrogen was mainly introduced in the layer through the treatment. Finally, considering the experimental conditions such as temperature, nitrogen concentration and specimen composition, nitride formation was theoretically analyzed based on combination of thermodynamics and electrochemistry. CrN, Cr2N, Fe2N and Fe4N were considered from composition of ss316. Potential-nitride formation diagram and potential-nitrogen ion concentration diagram were made. From discussions on formation of these nitrides based on these conditions, it was theoretically derived that CrN is most stable. This theoretical consideration was well in agreement with the experimental result. In conclusion, these results demonstrate availability of this nitriding method and will allow a guideline for optimization of this nitriding process in molten fluoride salt. This doctoral dissertation consists of five chapters and one appendix. Chapter 1 presents back ground and proposal of this work, ie, issues and problems on a molten salt blanket system in a fusion reactor. Chapter 2 presents thermodynamical discussion of compatibilities of several ceramics (metal oxides and nitrides). Compatibilities for those ceramics were evaluated based on thermodynamical theory. The prediction indicated that nitrides are compatible with molten fluoride salts. Chapter 3 presents the experimental descriptions and results. Introduction of nitrogen into SS316 specimen surface and formation of nitride layer were described. Chapter 4 presents theoretical explanation based on thermodynamics and electrochemistry. Formation of nitrides about iron and chromium is discussed. Chapter 5 presents conclusions. Appendix presents the results of immersion test using 4 bulk specimens of Er2O3、Y2O3、Al2O3、AlN at 600 deg C over 1000 hours. In recent years, nitrides have been focused on as fluorescent materials and magnetic materials. Especially, nitrogen solid solution of iron, alpha-FenN (n>8), is expected as a alternative material without Nd, rare-earth element. Nitriding technique established in this work will be able to be applied not only to blanket system in fusion reactor, but also to many kinds of industrial applications