[en] Letter-to-the-editors

[en] Corrosion of a Type 304 steinless steel and a molybdenum-base TZM alloy (Mo-0.5Ti-0.08Zr-0.025C-0.025 oxygen) in refluxing mercury with and without about 0.004 mol fraction of potassium in mercury has been examined using small capsules made of the steel and the alloy at 600 and 700deg C to recognize the corrosion mechanisms. The wall of the austenite 304 stainless steel capsule was severely corroded due to selective dissolution of nickel and cromium, leading to a ferritic layer which is more stable than the austenitic phase. The corrosion products were deposited near the vapor-liquid interface of the mercury. In contrast, the TZM alloy showed no noticeable changes except for carburization of the surface after heating for 2000 h, probably due to the low solubility of molybdenum in mercury. (orig.)

[en] Quasi-equilibrium of stainless steel in a sodium environment is studied using specimens of three kinds of Fe-(5, 7 or 9)Cr-1Mo-0.1C(wt%) martensitic/ferritic steels and a maximum isothermal region in a sodium loop constructed of a commercial Type 316 stainless steel. The weight loss, the composition of metallic elements near the surface, and the average carbon content for the specimens exposed to sodium of 700deg C, 1-2 wppm oxygen level, and 4.0 m/s velocity are determined as a function of exposure time of the specimens. From the present results and the previous results at 650 and 600deg C, it may be concluded that the contents of iron, chromium and nickel in stainless steel in quasi-equilibrium are Fe-∝5Cr-<1Ni at 700deg C, Fe-∝6Cr-<1Ni at 650deg C, and Fe-∝7Cr-<1Ni at 600deg C. The nickel in stainless steel will directly dissolve in the sodium according to the solubility of nickel, while the iron and chromium will predominantly react with the sodium containing 1-2 wppm oxygen and dissolve in the sodium in the maximum isothermal region of the loop according to the reactions Fe(or Cr)[s]+Na⁺-O^2- cluster[1]→Na-Fe(or Cr)-O[1]. On the basis of the quasi-equilibrium compositions, it appears that the free energy change in the reaction for the formation of Na-Cr-O complex is more negative than that of Na-Fe-O complex, and that the buffering action of chromium for oxygen in sodium, which would lower the corrosion of stainless steel in sodium, is confirmed. (orig.)

[en] The methods of removing impurities in the liquid sodium for FBR and the methods of measuring the concentration of impurities are reviewed. The coexistence of liquid sodium with austenite stainless steel now in use for FBR is also reviewed. Cold traps or hot traps are equipped in bypasses in loop type Na equipments in order to continuously remove impurities in sodium, and sodium is handled in inert gas atmosphere to prevent mixing of impurities. The methods of measuring concentration are chemical analysis, electrochemical analysis, distributive equilibrium method, and C and H diffusion gas analysis process. The distributive equilibrium method is that utilizing distribution of nonmetallic elements between liquid sodium and solid metals. An example of the coexistence experiment of sodium at high temperature and at high flow rate is reviewed. An example of corrosion model in high temperature, high speed sodium is also reviewed. As an example, Westinghouse sodium loop for testing material is illustrated. Further, the corrosion model between iron and sodium in boundary layer by Weeks is illustrated. The corrosion rate Rsub(ss) of stainless steel can be computed by the equation Rsub(ss)=(7.93 x 10¹¹T/Xsub(Fe)) [C₀(Na)/C₀⁰(Na)]sup(2XFe) exp(-50,300/RT) (1-5 x 10^-5(L/D)²) mil/year at 650-760⁰C in the range of 3-30 ppm C₀(Na), in which (Na) is dissolved state of oxygen in sodium. (Iwakiri, K.)

[en] Letter-to-the-editor

[en] Section seven deals with the coexistence of vanadium alloys for FBRs with liquid sodium. The conclusion of the study in Argonne, the experiment on a series of Vanstar alloy developed by Westinghouse, and the experimental study on V-20Nb-10Mo, V-20Nb-5Cr and V-20Nb-5Al using a small Na-loop made by Research Institute for Metallic Material Technology are reviewed. The experimental study revealed that the coexistence of vanadium alloys with liquid sodium can be determined by the degree of embrittlement due to oxygen in vanadium alloys, and that the test of tensile strength after immersion in sodium and the analysis of nonmetallic elements in vanadium alloys are effective as means of the evaluation of coexistence. Further, the experiment showed that V-20Nb-5Cr was relatively better among the compared vanadium alloys. Section eight deals with the coexistence of new metals for CTR with liquid lithium. Method of eliminating impurities in liquid lithium and method of measuring concentration are reviewed. Corrosion was compared among various metallic materials in liquid lithium. The group IV-A, V-A and VI-A metals have good and reliable corrosion resistance. The effect of oxygen on the coexistence of V, V-20Ti, Nb and Ta with liquid lithium is reviewed. The effects of O, N and C on the coexistence of new metals for CTR with liquid lithium are reviewed. Attention is drawn to the need of development for controlling and monitoring the concentration of O, C and N in liquid lithium. (Iwakiri, K.)

[en] The corrosion of ferritic stainless steels using sodium at 650degC in a maximum isothermal region contained in a non-isothermal sodium loop constructed of a Type 316 stainless steel has been examined. Also, previous results on corrosion of austenitic stainless steels in sodium at 700degC in the same loop have been reproduced. The selective dissolution and absorption of nickel, the selective dissolution of chromium, and the resultant increase in iron in the surface of stainless steels in the loop mainly determine the corrosion loss of the stainless steel specimens. The austenitic steels hardly decarburize, but denitride. The ferritic steels decarburize and denitride and the denitriding is more remarkable than the decarburizing. The vanadium and niobium, carbide and nitride formers, in the ferritic steels inhibit the decarburizing to some extent, but barely inhibit the denitriding. The nitrogen in the steels rapidly diffuses to the grain boundaries, and rapidly dissolves into sodium, which will lower surface energy of the steels to enhance the dissolution of other elements. The dissolved N in sodium would then be transported to the free surface of the sodium adjacent to the argon cover gas of sodium and easily be released into the cover gas. This mechanism would cause the rapid dissolution of nitrogen into sodium and the enhancement of the corrosion rate of the steels containing nitrogen. (orig.)

[en] Sodium compatibility of six kinds of high-purity Fe-(5 to 15)Cr-1 Mo ferritic steels was examined by exposure to sodium in a high-velocity (4,0 m/s) region and a downstream low-velocity (0.02 m/s) region in a sodium loop system constructed of a Type 316 stainless steel. The sodium exposure was conducted at 600 and 650⁰C and 1-2 ppm oxygen in sodium for up to about 3000 h. The weight loss in the high-velocity increased with increasing the chromium content, but was less than that of a reference Type 316 stainless steel. The weight loss in the low-velocity was much smaller than that in the high-velocity or even revealed weight gain. The absorption of nickel and the selective dissolution of chormium were observed in the high-velocity, while the adsoprtion of both nickel and chromium was obseved in the low-velocity. The carbon transfer was dependent on the composition of the steel, the temperature and the exposed location; the extent of decarburization increased with decreasing the chromium content in the high-velocity while noticeable transfer of carbon was hardly observed in the low-velocity. Also, each of the ferritc steels in both velocity regions and at 600 and 650⁰C was compared for the sodium compatibility on the basis of carbon transfer in this report. (orig.)

[en] We conclude that multilocular cysts are 3 cm and over, or elevated types as in mucin producing pancreatic tumors must be neoplasm in the pathological study. But, we question whether or not we can diagnose the elevated type before the operation. So, for the purpose of making a clear guideline of diagnosis for elevated types, we examined 46 operating cases of mucin producing tumors (Kuroda type I) documented from 1985 to 1994. We examined existence, height and properties of elevated lesions, 25 cases of elevated type and 21 cases of flat type. There is a significant difference of height with US and CT, and existence of enhancement with CT. As a result, we have obtained the following guidelines. The height of an elevated lesion with US is 4 mm and over, and the height of that with CT is 3 mm and over. It is enhanced with CT. Therefore, we find it possible to diagnose elevated types with US and CT. (author)

[en] To obtain fundamental data on the compatibility of high-chromium ferritic steels in sodium, high-purity Fe-0.1C-1Mo-5, 9 or 13Cr ferritic steels were prepared by vacuum melting. Test specimens of these steels which were normalized and tempered and a reference type 316 stainless steel (316 ss) were exposed to two sodium-velocity regions for periods up to 10.8 Ms in a sodium loop system which had a direct resistance main heater and was made of SUS 316. The test temperature, the maximum temperature, of the loop system in this work was 873 K, the oxygen content of sodium was 1 - 2 ppm, and the sodium velocities were about 4.0 and 0.02 m/s. The specimens exposed to the high sodium-velocity region revealed that corrosion loss at a zero downstream position of the three kinds of ferritic steels was smaller than that of the reference 316 ss ; about one fifth for the 5 and 9 %Cr steels and one half for the 13 %Cr steel. The surface analysis showed deposition of Ni that dissolved at upstream for all the ferritic steels, deposition of Cr for the 5 %Cr steel, and selective dissolution of Cr for the 9 and 13 %Cr steels. The ferritic steels without Ni and with less amounts of Cr than the reference 316 ss would result in their smaller corrosion loss than the 316 ss. Transfer of carbon, nitrogen and oxygen was not remarkable, except the carburization of the 5 %Cr steel. The specimens of the three kinds of ferritic steels which were exposed to the low sodium-velocity region revealed much smaller corrosion loss than that in the high velocity region, deposition of both Ni and Cr, and no transfer of carbon, nitrogen and oxygen except for slight carburization of the 13 %Cr steel. (author)