Corrosion of Steels in Lead-Bismuth Flow. Minoru TAKAHASHI*, Masatoshi KONDO*, Naoki SAWADA* and Koji HATA**

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1 Corrosion of Steels in Lead-Bismuth Flow Minoru TAKAHASHI*, Masatoshi KONDO*, Naoki SAWADA* and Koji HATA** *Tokyo Institute of Technology, Res. Lab. for Nucl. Reactors, -1-1 O-okayama, Meguro-ku, Tokyo , Japan, ** Nuclear Development Corporation, 6-1 Funaishikawa, Tokaimura, Ibaraki , Japan, Abstract-Steel corrosion test in a lead-bismuth flow was conducted to investigate the corrosion behaviors of twelve steels some of which were the candidates of core and structural materials of FBR and window material of ADS. The weight loss in 1, hr-test was lower in the steels with higher chromium content, although there was exception in some steels. Single or multiple oxide layers where chromium was enriched were formed on the steel surfaces. The outer oxide layer was in general thicker and easily broken into small pieces with clacks than the inner stable oxide layer. The existence of the thin and stable inner oxide layer might be important for the steels to be more corrosion-resistant. The contents of Cr and Si were effective to form the thin and stable inner oxide layer and as a result to suppress corrosion. I. INTRODUCTION For the development of lead-bismuth cooled FBR[1] and ADS target system, one of the key issues is the compatibility of core, structural, and window materials with lead-bismuth. Thus, corrosion tests have been conducted to investigate the characteristics of corrosion of various steels in a lead-bismuth () flow. For the protection of the steels, it is required to form stable oxide films on the steel surfaces. In the first corrosion test[], weight losses due to corrosion were measured after removing adhered to test pieces together with oxide films in a hot sodium pool. Therefore, the oxide films could not be analyzed by SEM/EDX. Thus, in the next corrosion test[3], the adhered was removed in a hot glycerin pool in order not to remove oxide films. However, severe erosion occurred in some specimens, and oxide films were not observed possibly because oxygen potential was controlled to be too low by injection of Ar-steam-H mixture into. In the present study, the oxygen potential in Pb -Bi was controlled properly not to have erosion, and oxide films were observed and analyzed by SEM/EDX without removing the oxide films. Twelve types of steels were exposed to the flow simultaneously, and the corrosion behavior and the conditions of oxide films were compared with each other for the steels. At the same time, the effects of Cr and Si contents on the corrosion were investigated experimentally. II.EXPERIMENTAL APPARATUS AND PROCEDURE II.A Experimental apparatus and test materials The forced convective circulation loop[4] shown schematically in Fig.1 was used for the present corrosion test. It had high and low temperature sections, which was required for the material corrosion test. Table 1 presents the major specification of the loop. The working fluid was 45%Pb-55%Bi alloy that has the melting point of 398 K. Corrosion test section Heater EM F Electro-magnetic flow meter Ar gas line Cooler Oxygen concentration monitor EM P Electro-magnetic pump O (H ) supply Magnetic iron t rap O (H ) supply Vapor trap Expansion tank Dump tank Ar gas Ar gas Vapor tr ap Demister Fig.1 Schematic diagram of corrosion test loop Table 1 Specifications of corrosion test loop inventory in loop. m 3 Volume in dump tank.66 m 3 Maximum test temperature 83 K Maximum system pressure.4 MPa Maximum flow rate 6 l/min Maximum velocity in test section m/s Heater and cooler power kw Structural material in hot region 9Cr-1Mo steel Structural material in cold region SS316 Twelve steels were tested here. The test pieces were 15.5 mm wide and mm thick. Figure shows the test

2 section. Two flow channels above and below the test pieces were 13 mm wide, mm high and 45 mm long. The test pieces of these steels were mounted in a cylindrical holder made of Mo as shown in Figs. and 3, and the holder was inserted in the test section of the corrosion test loop. Test piece Stopper 7mm Test pieces F34mm F34mm (51mm) 5mm 45mm 35mm Fig. 4mm 15.5mm 13mm Test section Flow channel 13mm x mm.mm 6 mm Test piece holder Fig.3 Test pieces and flow channels in a molybdenum holder Table Chemical components of test steels Cr Mo W Si others SCM F8H Ti-.1Cu STBA NF STBA SUH C ODS Ti-.3Y O Y HCM1A Cu HCM SUS Al SUS Ni SUS Chemical components of test steels are presented in Table. The surfaces of the specimens were polished to be smooth, and not oxidized. The steels that were more corrosion resistant according to the previous result[,3] were mounted in upstream region of the holder to prevent the effect of corrosion upstream on that downstream Thus, based on the previous result[3], the test pieces were mounted in the test pieces holder in the following order from upstream: (1)HCM1A, ()SUH3, (3)STBA6, (4)NF616, (5)SUS43, (6)SUS45, (7)SUS316, (8)F8H, (9)ODS, (1)SCM4. II.B Measurement of oxygen concentration in and experimental conditions It had been found that the as received had several ppm of oxygen, i.e., the in the dump tank was oxygen-saturated. Thus, the was kept at the temperature of 453 K in the dump tank for long time, and then slowly charged to the circulation loop. By this method, it was expected that the oxygen concentration in the circulation loop was nearly equal to that saturated at the temperature of 453 K. This process was the same as that in the first corrosion test[] where corrosion was not serious except for SUS316, and no appreciable erosion occurred possibly because oxide film was formed on the steel surfaces. An oxygen sensor made of a solid electrolyte conductor (ZrO -MgO) with the reference fluid of oxygen saturated bismuth was mounted at the outlet of the corrosion test section for the measurement of oxygen potential in a flow at 83 K. The electro-motive force of the oxygen sensor is given by the Nernst equation: 1 RT RT E = ln PBiref ln PO F, (1) where F is the Faraday consant. The oxygen potential in (1/)RTlnP O is given by GOx : C GOx = G + RT ln, () C s where G is the standard oxygen potential of PbO or Pb.45 Bi.55 O The oxygen concentration in, C is in the unit of wt.%, and C s is the saturated concentration or solubility of oxygen in. given by the empirical equation: logc s = / T (673K < T < 973K). (3) The relation between the electro-motive force E and the oxygen concentration in C is obtained. The oxygen concentration measured with the oxygen sensor was estimated to be 3.7x1-8 wt% and wt% for G of PbO and Pb.45 Bi.55 O 1.75, respectively. The measured oxygen concentration is plotted on the diagram of Gibbs free energy of oxide formation in Fig. 4. As the

3 oxygen concentration in the was slightly higher than the curve of Fe 3 O 4 formation potential, it was expected that the oxide film of Fe 3 O 4 was formed successfully on the steel surfaces and corrosion was suppressed. The test pieces were exposed to the flow at the conditions in Table. II.C Removal of adherent from specimen surfaces G o, G Ox, (1/)RT ln P O (kj/g atm O) PbO NiO H O (1/4)Fe 3 O 4 (1/3)Cr O C (wt.%) Temperature ( C) Oxygen potentials of oxides G o Oxygen potential in G Ox Fig.4 Diagram of oxygen potential; symbol of black triangle: G in Eq.() is estimated from oxygen potential of Pb.45 Bi.55 O 1.75, symbol of black square: that is estimated from oxygen potential of PbO Table Experimental conditions Exposure time 1, hrs temperature in test section 83 K temperature in low 673 K temperature sections of loop velocity in test section 1 m/s flow rate 3 l/min Oxygen 3.7x1 G of PbO) -8 wt% concentration in G of -8 wt% Pb.45 Bi.55 O 1.75 charge temperature 453 K In order to keep the oxide films formed on the test piece surfaces, glycerin was used for the removal of adhered in stead of sodium. After the exposure of test pieces in the flow, the test piece holder was immersed in a glycerin pool at the temperature of 453 K. Then, the holder was opened and the test pieces were taken out of the holder and washed again in glycerin. Finally, glycerin on the test piece surface was removed in a warm water at the temperature of 343K. II.D SEM/EDX analysis The test pieces were analyzed by the following method: (1) Surface observation The surface of all the test pieces had metallic luster before the test. The change of surface color was checked after the test. Also, the occurrence of erosion was inspected by the surface observation. () Measurement of weight loss In order to estimate corrosion rate, weight losses were measured. (3) SEM/EDX analysis Test pieces were cut at the spanwise center. The cross section was polished, and then observed and analyzed by SEM/EDX. III.A Surface observation III. EXPERIMENTAL RESULT The appearance of test piece surfaces in the molybdenum holder after 1, hr-exposure to the flow is shown in Fig.5. It can be seen that no appreciable traces of erosion existed on all of the steel surfaces. The colors of the steel surfaces changed to black from initial metallic cluster before the exposure. It was found from the observation that the corrosion was not so severe as that in the first test in the previous study[]. HCM1A SUH3 STBA6 NF616 SUS43 SUS45 SUS316 F8H ODS SCM4 SUS43 Fig.5 Appearance of test piece surfaces in the Mo holder III.B Weight loss

4 Figure 6 shows the result of the weight losses. The result obtained in the first corrosion test at the same test conditions except for velocity in the previous study[,3] is shown for comparison in Fig.7. It is found that the present results of the weight losses of SUS43, SUS45, SCM4 and STBA6 were nearly equal to those in the previous experiment. This means that the data of the weight loss were repeatable and reliable. The weight loss of SUS316 was negative in the present experiment in spite that it was the highest among the test specimens in the previous study. This is possibly because penetrated into the porous layer which was formed by the selective dissolution of nickel into was not completely removed in the glycerin. The weight increased in STBA8, although it is not clear if the weight increase was caused by the formation of oxide layer or the adherence of and/or the penetration of. material from corrosion or the dissolution of Fe into Pb- Bi. Thus, the dependence of the weight loss on chromium content in the steels is examined in Fig.8. It is found that the weight loss was lower as the chromium content was higher in the order of SCM4, F8H, STBA6, ODS, SUS45 and SUS43, which confirmed the hypothesis. The weight loss of HCM1A was slightly higher than the decreasing line. There were weight increases in STBA8 and HCM1, which could be also attributed to the formation of thick oxide layers or penetration and/or adherence of. The weight loss of SUH3 was remarkably lower than those of the other steels. The silicon It was also found that the thickness of the test pieces increased by the exposure of them in the flow because of oxidation and/or penetration. The exception was SUH3, SUS45 and ODS. Weight loss(g/m) HCM1A SUH-3 STBA6 NF616 SUS43 Fig.6 Weight losses of test pieces during 1, hrexposure to the flow of test pieces Weight loss g/m 6 4 SUS43 SUS45 SUS45 SUS316 SCM4 F8H 9Cr1Mo (STBA6) ODS Exposure time: 959 hrs temperature: T=55 C Oxygen concentration C O =5.x1-7 wt% Fig.7 Weight losses of test pieces during 959 hrexposure to a flow in the first corrosion test in the previous study[,3] SCM4 SS316 HCM 1 STBA8 Weight loss(g/m) SCM4 F8H STBA8 NF616 STBA6 HCM1A ODS SUH3 HCM1 SUS45 SUS43 SUS Contents of Cr(wt %) Fig.8 Relation between weight losses and chromium content IIIC Results of SEM/EDX analysis The oxide films or layers formed on the steel surfaces were observed by means of the SEM/EDX analysis. The typical result is schematically illustrated in Fig.9. The two oxide layers A and B are observed between the right steel matrix and left epoxy resin. The outer layer A may be formed by the chemical reaction of oxygen in and metal elements of Cr and Fe diffused through the oxide layers from the steel matrix in the outer direction. On the other hand, the inner oxide layer B may be formed by the diffusion of oxygen from into the steel matrix. The layer B may serve as protection layer against both the penetration of Pb and Bi into the steel matrix and the dissolution of steel elements, particularly iron, into the melt. The previous result has been interpreted by the hypothesis that chromium in the steels enhances the formation of a chromium oxide film that protects the

5 Layer A Layer B ODS Resin Steel matrix Fig.9 Oxide layers formed on the steel surfaces µm Figures 1 through 1 show the results of the SEM/EDX analyses. In Fig.1(a), three oxide layers exist on the surface of HCM1A. The layers are thin and separated from each other. The outer layer is broken into some pieces. Iron is enriched in the outer layer, which suggests the diffusion of iron through the inner two layers and the dissolution of iron into. Chromium is enriched in the middle and inner layers. The inner layer sticks to the matrix stably. There are two oxide layers in ODS as shown in Fig.11. The outer layer where chromium is enriched is broken to small fragments. Chromium is enriched only slightly in the thick inner layer which sticks to the matrix stably. Three layers are observed in NF616 characterized by martensitic steel in Figs.1(a) through 1(c). There are some cracks in the outer layer where iron content is lower and chromium content is slightly higher than in the matrix. Iron content is poor and chromium and tungsten are enriched in the middle layer which may be effective together with the quite thin oxide layer close to the surface of the inner layer to protect the steel from corrosion. Fig.11 Oxide films and metal elements in ODS after 1, hr-exposure to the flow HCM1A Fig.1(a) Oxide films and content of Fe in NF616 At the surface of layer A and Layer B, W was enriched Fig.1 Oxide films and metal elements in HCM1A Fig.1(b) Oxide films and content of Cr in NF616 after 1, hr-exposure to the flow

6 SUS316 Fe Fig.1(c) Oxide films and content of W in NF616 after 1, hr-exposure to the flow Two oxide layers exist in F8H as shown in Fig.13. Chromium is enriched in the outer layer, and penetrates into the inner layer. Figures 14(a) and (b) show that SUS316 characterized by austenitic stainless steel has single oxide film where iron is very poor and chromium is rich. penetrates into the matrix appreciably and deeply. Chromium content is low in the penetration region compared with the matrix, although iron content is slightly higher. The Pb -Bi penetration may be partially caused by the selective dissolution of nickel into. The penetration may be the reason why the weight increased during the exposure of SUS316 to the flow. The outer oxide layer where chromium is enriched is broken to a lot of pieces in STBA6. The inner layer is very thin. Two oxide layers are observed in STBA8 in Fig.15. The thickness of the layers are nearly the same. There are some cracks particularly in the outer layer. F8H Fig.14(a) Oxide films and content of Fe in SUS316 SUS316 Cr Fig.14(b) Oxide films and content of Cr in SUS316 after 1, hr-exposure to the flow 1 3 STBA µm µm Fig.15 Oxide films and metal elements in STBA6 Fig.13 Oxide films and metal elements in F8H There are two layers in SUH3. The outer oxide layer is separated from the steel surface. The inner layer is thin and the surface is smooth without damage. It has been found by EDX analysis that silicon is rich in the thin layer.

7 Therefore, silicon oxide that is effective to protect the steel is formed on the surface. There are two oxide layers in HCM1. Chromium is rich and some crack exist in the outer layer. The inner layer has no serious damage. There appear thick outer layer and thin inner layer in SUS45 characterized by ferritic steel in Figs 19(a) and (b). Iron is poor and chromium is rich in the outer layer. There is some cracks in the inner layer. Thick outer layer and thin inner layer are observed in SUS43 in Fig.. The outer layer has some cracks. SCM4 characterized by low-alloy steel has only single layer with cracks. As the chromium content in the steel is low, chromium oxide that is effective for corrosion resistance may not be formed. Iron may dissolve faster into because of no effective oxide barrier on the steel surface HCM1 6 7 STBA8 Fig.18 Oxide films and metal elements in HCM1 SUS45 Fe Fig.16 Oxide films in cross section of STBA8 SUH3 Fig.19(a) Oxide films and content of Fe in SUS45 SUS45 Cr Fig.17 Oxide films in cross section of STBA8 Fig.19(b) Oxide films and content of Cr in SUS45

8 SUS Fig. Oxide films in cross section of SUS43 SCM4 5µm Fig.1 Oxide films in cross section of SCM4 III.D Thickness of oxide layers The thickness of the oxide layers and the occurrence of penetration obtained from the SEM analyses are listed in Table 3. In general, the outer layers were thicker than the inner layers except for ODS and HCM1A. IV. DISCUSSION From the fact that chromium was enriched in the oxide layers, it is suggested that chromium diffused to the surface and react chemically with oxygen in to form the oxide layers. Once the chromium oxide layer was formed, iron could not be dissolved easily into the because of the barrier of the chromium oxide layer. However, iron sometimes diffused across the barrier to the surface and iron oxide was formed on the surface. Simultaneously, oxygen diffused from the into the steel matrix across the oxide layer, and reacted chemically with metal elements such as chromium and silicon. This oxygen diffusion might produce a thin and stable oxide layer close to the surface of the steel matrix. As this inner layer was not broken easily, the formation of the inner layer might be mo re important to have a corrosionresistant steels. Table 3 Thickness of oxide layers and penetration obtained from SEM analysis Thickness (µm) Outer layer A Inner layer B SCM F8H STBA STBA NF SUH ODS HCM1A HCM SUS SUS SUS penetration As the oxide layers thickness were thinner in general and more stable as chromium contents were higher in steels, the corrosion rate might be lower with chromium content through oxide layer thickness. In other word, it is expected that the steels which have higher Cr contents have thinner oxide layer and consequently more corrosion-resistant in a flow. The content of Si was also effective to form the thin and stable inner oxide layer and as a result to suppress corrosion. V. CONCLUSION For the development of core and structural materials of FBR and window material of ADS compatible with coolant, steel corrosion behavior was investigated by means of 1, hr-corrosion test in a lead-bismuth flow. Twelve steels were tested simultaneously in a corrosion test loop. Conclusions are as follows: 1) The weight loss was lower in the steels with higher chromium content in general, although there was exception in some steels. ) Single or multiple oxide layers where chromium was enriched were formed on the steel surfaces. The outer oxide layer was thicker and easily broken into small pieces with clacks than the inner stable oxide layer. 3) The existence of a thin and stable inner oxide layer might be important for the steels to be more corrosion-resistant. The content of Si was effective to form the thin and stable inner oxide layer and as a result to suppress corrosion.

9 ACKNOWLEDGMENTS The authors thank Dr. A. Otsubo, Mr. T. Iguchi, Dr. Y. Qi, Prof. H. Sekimoto and Prof. Yano for their valuable discussion, and Mr. M. Imai, Dr. S. Yoshida, Mr. A. Yamada and Dr. S. Qiu for their experimental assistance. The authors acknowledge the supply of the test material F8H of Japan Atomic Energy Research Institute (JAERI) and Koukan Keisoku K.K., and the supply of the test material ODS of Japan Nuclear Cycle Development (JNC). This study was a part of the Project of Technical Development of Innovative Reactor System supported financially by the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES 1. S.Uchida, H. Osada, Y. Kasahara, M. Takahashi and K. Hata,., A Feasibility Study on the Lead-bismuth Cooled Direct Contact Boiling Water Fast Reactor, Proc. of 11 th Int. Conf. Nuc. Eng., ICONE (3).. M. Takahashi, T. Suzuki and H. Sekimoto, Corrosion of Steels in a Flowing Nonisothermal Leadbismuth, Trans. of ANS, 85, 3 (1). 3. M. Takahashi, H. Sekimoto, K. Ishikawa, T. Suzuki, K. Hata, S. Qiu, S. Yoshida, T. Yano, and M. Imai, Experimental Study on Flow Technology and Steel Corrosion of Lead Bismuth, Proc. of 1 th International Conf. Nucl. Eng., ICONE1-6 (). 4. M.Takahashi, N. Sawada, H. Sekimoto, M. Kotaka, T. Yano, S. Uchida, T. Takahashi, K. Hata, and T. Suzuki, Design and Construction of Lead-bismuth Corrosion Test Loop and Test Plan, Proc. of 8 th Int. Conf. Nucl. Eng., Baltimore, ICONE-857 ().