Materials Transactions, Vol. 50, No. 10 (2009) pp. 2410 to 2417 #2009 The Japan Institute of Metals Temperature Dependence of Corrosion of Ferritic/Martensitic and Austenitic Steels in Liquid Lead-Bismuth Eutectic Yuji Kurata and Shigeru Saito Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan Corrosion tests of ferritic/martensitic (F/M), austenitic stainless and Si-added austenitic steels were conducted at 450 to 600 C for 2000 h or 3000 h in oxygen-saturated lead-bismuth eutectic (LBE) to clarify temperature effect on corrosion behavior. While the corrosion depth is small at 450 C because of oxide film formation with low growth rate, it increases at 500 C due to additional grain boundary corrosion/internal oxidation and node formation. At 550 C, extensive grain boundary corrosion/internal oxidation is observed in F/M steels. Ferritization characterized by selective dissolution of Ni and Cr, and LBE penetration occurs in austenitic stainless steels, and. Corrosion attack becomes very severe for most steels at 600 C. LBE penetration follows grain boundary corrosion/internal oxidation in F/M steels and ferritization advances deeply in and. The compound corrosion layer of oxidation, dissolution and LBE penetration often peels off. Addition of Si to austenitic steels is useful to improve corrosion resistance. [doi:10.2320/matertrans.m2009173] (Received May 12, 2009; Accepted July 6, 2009; Published August 26, 2009) Keywords: corrosion, lead-bismuth, ferritic/martensitic steel, austenitic stainless steel, temperature dependence, oxidation, selective dissolution 1. Introduction Lead-bismuth eutectic (LBE) is a promising candidate material of core coolants and high-power spallation targets of accelerator driven systems (ADSs) for transmutation of radioactive wastes and of coolants of fast reactors (FRs). However, compatibility of materials with liquid LBE is one of critical issues to develop the ADSs and the FRs. The study on the compatibility of materials with liquid LBE has been conducted in Russia since the early stage of nuclear reactor development and summarized in some papers. 1,2) The recent extensive results of LBE corrosion research, which has received significant attention and been performed in the worldwide, are compiled in ECD/NEA handbook. 3) It is considered that temperature, oxygen concentration, types of steels, flow rate of LBE and temperature difference between high and low temperature parts in loop tests have great influence on corrosion behavior in liquid LBE. Many experimental results on effect of temperature have been reported. 3 10) It is pointed out that corrosion mechanism in LBE changes from protective oxidation to dissolution above 500 C while it depends on oxygen concentration and types of steels. 4 10) However, there are unclear points to make a quantitative prediction about corrosion in LBE because the obtained experimental data are still insufficient and scattered. Phenomena such as oxidation, grain boundary corrosion/ internal oxidation, penetration of LBE and dissolution of elements occur in LBE corrosion. Dissolution of elements into LBE is the most aggressive and often called liquid metal corrosion. It is not necessarily clear whether these phenomena occur simultaneously and in a mixed way or one of the phenomena occurs dominantly under a certain condition. It will change depending on conditions such as temperature and oxygen concentration. Furthermore, it is plausible that some elements contained in steels have influence on the change. We have conducted static corrosion tests of ferritic/ martensitic (F/M), austenitic stainless and Si-added austenitic steels in oxygen-saturated LBE to understand basic corrosion behavior of various steels in LBE and reported results at 450 and 550 C. 10) In this study, we performed additional corrosion tests at 500 and 600 C to investigate temperature dependence of corrosion in detail. The purpose of this paper is to clarify temperature effect on corrosion in liquid LBE from analyses of experimental data from 450 to 600 C focusing on describing characteristics of corrosion at each temperature. 2. Experimental 2.1 Materials and specimens Table 1 shows chemical compositions of materials used in the experiment. 2.25Cr-1Mo steel, and are F/M steels.,, and are austenitic stainless steels. and are steels developed for use in sulfuric acid plants and their characteristic is high Si content of about 5%. Corrosion specimens were rectangular plates with the size of 15 mm 30 mm 2 mm, and a hole of 7.2 mm diameter was made for installation at the upper part of the specimen. The surface of corrosion specimens was polished using emery papers up to 600 grit. 2.2 Corrosion test apparatus and procedure The static corrosion test apparatus in liquid LBE was described in detail elsewhere. 10,11) Specimens were soaked in liquid stagnant LBE of the apparatus. Components contacting liquid LBE were made of quartz in the apparatus. Corrosion tests were conducted at 450, 500, 550 and 600 C. Exposure times were 3000 h at 450 and 550 C, and 2000 h at 500 and 600 C. 7 kg of fresh LBE (45Pb-55Bi) was used in the corrosion test at each temperature. The LBE was melted in Ar cover gas with purity of 99.9999%. Pb was formed on the surface of liquid LBE and corrosion tests were made in oxygen-saturated LBE. xygen concentration under the saturated condition was estimated to be 3:2 10 4 mass% at 450 C, 6:3 10 4 mass% at 500 C, 1:2 10 3 mass% at 550 C and 2:0 10 3 mass% at 600 C using the equation in the literature. 1)
Temperature Dependence of Corrosion of Ferritic/Martensitic and Austenitic Steels in Liquid Lead-Bismuth Eutectic 2411 Table 1 Chemical compositions of materials tested in the static corrosion experiment (mass%). C Si Mn P S Cr Ni Mo Fe V W Ti 2.25Cr-1Mo steel 0.10 0.34 0.44 0.009 0.002 2.18 0.02 0.92 Bal. 0.01 <0:01 0.095 0.10 0.01 <0:005 0.003 7.72 <0:02 <0:01 Bal. 0.18 1.95 0.005 0.067 0.31 0.80 0.035 0.007 12.21 0.12 0.02 Bal. 0.07 <0:01 0.058 0.50 1.54 0.026 0.004 14.14 15.87 2.29 Bal. 0.03 0.01 0.22 0.04 0.69 1.22 0.026 0.001 16.83 10.79 2.06 Bal. 1 0.010 4.80 0.60 17.58 19.08 0.356 Bal. 2.14 2 0.014 5.76 0.59 0.021 0.001 13.59 15.72 1.02 Bal. 1.04 1 : Trademark of the Sandvik Corporation 2 : Trademark of Allegheny Ludlum Company 2.25Cr 1Mo 2.25Cr 1Mo D for 100 µ m per 600d D for 100 µ m per 600d 450 C, 3000h xide film G.B. corrosion/ Internal oxidation Node 500 C, 2000h 0 10 20 30 40 50 Corrosion Depth, D/ µ m Fig. 1 Corrosion depths of various steels in oxygen-saturated LBE at 450 and 500 C. Test specimens were rinsed in silicone oil or glycerin at 170 C after the corrosion tests to remove adherent LBE on the surface of the specimens. However, the LBE could not be removed completely. The cut specimens were plated with copper and molded into resin to protect corrosion films during polishing. Analyses were made using an optical microscope and a scanning electron microscope (SEM) with energy dispersion X-ray (EDX) equipment. The corrosion depth was measured using an optical microscope and a SEM. The corrosion depth is the sum of oxide film thickness, grain boundary corrosion/internal oxidation depth, node thickness and thickness of the ferrite layer formed on the surface of austenitic stainless steels. The corrosion depth is the average of about 5 measurements obtained at the places where corrosion is deep. 3. Results and Discussion 3.1 Corrosion behavior at 450 and 500 C Figure 1 shows corrosion depths of various steels in LBE at 450 and 500 C. The line of the corrosion depth, which is calculated for 3000 h or 2000 h from the corrosion depth of 100 mm per 600 days assuming a linear rule, is indicated as a tentative guideline in this figure. The corrosion depth is represented as the sum of the oxide film thickness, the depth of grain boundary (G.B.) corrosion/internal oxidation and the thickness of nodes. Formation of the oxide film and slight G.B. corrosion/internal oxidation occurred in the corrosion test at 450 C for 3000 h in oxygen-saturated LBE. Slight G.B. corrosion/internal oxidation is found in 2.25Cr-1Mo steel and which are F/M steels with relatively low Cr contents. The corrosion depths at 450 C for 3000 h are small compared with the guideline of 100 mm per 600 days. xide nodes appear on some steels in addition to oxide film formation and G.B. corrosion/internal oxidation in the corrosion test at 500 C for 2000 h. Corrosion depths at 500 C become larger for most steels than those at 450 C. Figure 2 depicts optical micrographs of cross-sections of specimens after the corrosion test at 500 C for 2000 h. xide nodes are found in and. The corrosion depth seems to decrease at 500 C with increasing Cr content in steels as shown in Figs. 1 and 2. Corrosion depths of and containing Si of about 5% are small. Figure 3 depicts EDX analysis of the cross-section of 2.25Cr-1Mo steel after the corrosion test at 500 C for 2000 h. It is found that the formed oxide film has a duplex structure composed of an outer oxide layer of Fe and an inner oxide layer of Fe and Cr. It is well known that the oxide film formed on steels in LBE is composed of an external Fe 3 4 layer and an internal Fe-Cr spinel layer. 4,12,13) Therefore, the oxide film in Fig. 3 is
2412 Y. Kurata and S. Saito (a) (b) (c) 2.25Cr-1Mo µ 50 m (d) (e) (f) (g) Fig. 2 ptical micrographs of cross-sections of (a) 2.25Cr-1Mo steel, (b), (c), (d), (e), (f) and (g) after the corrosion test in oxygen-saturated LBE at 500 C for 2000 h. uter Inner 20µm SEI Pb Bi Cr Fe Fig. 3 EDX analysis of the cross-section of 2.25Cr-1Mo steel after the corrosion test in oxygen-saturated LBE at 500 C for 2000 h. considered to be composed of outer Fe 3 4 and inner Fe-Cr spinel. As shown in Fig. 4, the outer part of oxide nodes is Fe 3 4 and the inner part of oxide nodes is Fe-Cr spinel similarly. 3.2 Corrosion behavior at 550 and 600 C Corrosion depths of various steels in LBE at 550 C are shown in Fig. 5 together with those at 600 C. The guideline of 100 mm per 600 days is indicated in the same manner as Fig. 1. xide film formation, G.B. corrosion/internal oxidation and ferritization occur in LBE at 550 C while occurrence of these phenomena depends on types of steels. Ferritization is caused by selective dissolution of Ni and Cr in austenitic stainless steels and LBE penetration accompanies it. The corrosion depths of austenitic stainless steels, and become larger at 550 C due to ferritization than those of F/M steels in contrast with results at 450 and 500 C. The corrosion depth of is very small. When the corrosion
Temperature Dependence of Corrosion of Ferritic/Martensitic and Austenitic Steels in Liquid Lead-Bismuth Eutectic 2413 SEI 20µm Pb Bi Cr Fe Fig. 4 EDX analysis of the cross-section of after the corrosion test in oxygen-saturated LBE at 500 C for 2000 h. 2.25Cr 1Mo 550 C, 3000h 2.25Cr 1Mo D for 100 µ m per 600d * xide film G.B. corrosion/ Internal oxidation G.B. C/ I / LBE penetration Ferritization 600 C, 2000h D for 100 µ m per 600d *:Thinning is included in G.B.C/I/LBE penetration for 2.25Cr-1Mo steel. **:Thinning is included in ferritization for. ** 576 0 50 100 150 200 Corrosion Depth, D/ µ m Fig. 5 Corrosion depths of various steels in oxygen-saturated LBE at 550 and 600 C. depths at 550 C are compared with those at 500 C in Fig. 1, the corrosion depths of F/M steels at 550 C are a little smaller than those at 500 C. It is considered that this is caused by formation of oxide nodes at 500 C and scatter of experimental data. As shown in Fig. 6, formation of oxide film, G.B. corrosion/internal oxidation and ferritization are found after the corrosion test at 550 C for 3000 h. 10) The characteristic of LBE corrosion at 550 C is ferritization, which is recognized as a typical severe corrosion phenomenon for austenitic stainless steels such as and. Corrosion depths of various steels in LBE at 600 C are shown in Fig. 5 together with those at 550 C. At 600 C, LBE often penetrates into the place where G.B. corrosion/internal oxidation occurs in addition to oxide film formation. It is mentioned as G.B.C/I/LBE penetration in Fig. 5. The LBE penetration also advances deeply together with ferritization in austenitic stainless steels. Even if oxide films form, their protective function is often damaged at 600 C. The corrosion depths of steels except for are over the guideline of 100 mm per 600 days. Figure 7 depicts optical micrographs of cross-sections of specimens after the corrosion test at 600 C for 2000 h. The uneven surface is clearly found in 2.25Cr-1Mo steel. It is considered that the hollow places have been made due to LBE penetration and peeling off as described later. Thinning observed for 2.25Cr-1Mo steel is included in G.B.C/I/LBE penetration in Fig. 5. G.B. corrosion/internal oxidation followed by LBE penetration advances deeply in and. Formation of the ferrite layer accompanied by LBE penetration is also deep in and. Formation of the ferrite layer is not observed clearly in containing Si of about 5% after the corrosion test at 600 C for 2000 h. In
2414 Y. Kurata and S. Saito (a) (b) (c) 2.25Cr-1Mo (d) (e) (f) Ferrite layer Ferrite layer Fig. 6 ptical micrographs of cross-sections of (a) 2.25Cr-1Mo steel, (b), (c), (d), (e) and (f) after the corrosion test in oxygen-saturated LBE at 550 C for 3000 h. (a) (b) (c) 2.25Cr-1Mo 200 µ m (d) (e) (f) 200 µ m (g) (h) Fig. 7 ptical micrographs of cross-sections of (a) 2.25Cr-1Mo steel, (b), (c), (d), (e), (f) and, (g) and (h) after the corrosion test in oxygen-saturated LBE at 600 C for 2000 h. contrast, local ferritization is observed in containing Si of above 5% while the Cr content in is lower than that in. Figure 8 shows EDX analysis of the cross-section of 2.25Cr-1Mo steel after the corrosion test at 600 C for 2000 h. The LBE penetrates into the G.B. corrosion/internal oxidation layer beneath the surface oxide film. The mixed layer of oxides and LBE often peels off. Since oxygen supply is insufficient in the LBE penetrating under the surface oxide film, the inside LBE becomes aggressive due to decrease in oxygen concentration. The LBE with low oxygen concentration induces dissolution attack. 3,6,7) Figure 9 shows EDX analysis of the cross-section of after the corrosion test at 600 C. Although the oxide film with 10 mm thickness forms at the surface, the spot-like layer penetrated by LBE, the internal oxide layer and large pores are observed. It was reported that the line of pores and dispersed pores were often found beneath the surface oxide film in 8%Cr DS steel after corrosion tests in LBE at 500 to 650 C. 8) The line of pores and dispersed pores may be linked to migration of Fe atoms due to surface oxide formation or loss of Cr-carbides and Crsulfides. 8) It was also reported that sulfur segregation was found at the scale/alloy interface in Fe-18.5%Cr-5%Al and Ni-25%Cr alloys after high temperature oxidation. 14) For these reasons, mapping and line analysis for elements such as S, P and Mn were performed using EDX for specimen after the corrosion test at 600 C. Enrichment of S was not found near the surface oxide film and the internal oxide layer.
Temperature Dependence of Corrosion of Ferritic/Martensitic and Austenitic Steels in Liquid Lead-Bismuth Eutectic SEI 100µm Bi Fig. 8 Pb Cr Fe 2415 EDX analysis of the cross-section of 2.25Cr-1Mo steel after the corrosion test in oxygen-saturated LBE at 600 C for 2000 h. SEI 50µm Bi Fig. 9 Pb Cr Fe EDX analysis of the cross-section of after the corrosion test in oxygen-saturated LBE at 600 C for 2000 h. As shown in Fig. 10, the formed internal oxides are Cr-rich ones and peaks of S, P and Mn are not observed around them. Further investigation is needed to search for the cause of large pores. Figure 11 depicts EDX analysis of the cross-section of after the corrosion test at 600 C for 2000 h. Extensive ferritization occurs due to selective dissolution of Ni and Cr, and penetration of Pb and Bi advances deeply with the ferritization. Thinning is observed on account of partial loss of the compound layer of ferritization and LBE penetration in. Thinning observed for is included in ferritization in Fig. 5. Although formation of the surface oxide film is not clear in Fig. 11 due to observation with a low magnification, the formation of the oxide film of Cr and Fe was found in observation with a high magnification. Therefore, the oxide film, which often forms on austenitic stainless steels in the shape of the Cr-Fe oxide at 600 C in oxygen-saturated LBE, is not the protective barrier to prevent selective dissolution of Ni and penetration of LBE while similar indication is also pointed out for results of and at 550 C. When the result of is compared with that of containing Si of about 5%, the corrosion depth of is larger than that of as shown in Fig. 5. Furthermore, local ferritization occurs in at 600 C (Fig. 7). Since was considered to tend to produce a protective oxide film, EDX analysis was made around the oxide film of and specimens. Typical examples of EDX line scan are shown for and in Fig. 12. The uniform oxide film, which contains Si and Cr, forms on. n the other hand, the oxide
2416 Y. Kurata and S. Saito P S Cr Mn 5µm Analysis line Fig. 10 Line analysis of the cross-section of after the corrosion test in oxygen-saturated LBE at 600 C for 2000 h. SEI 500µm Bi Pb Cr Fe Ni Fig. 11 EDX analysis of the cross-section of after the corrosion test in oxygen-saturated LBE at 600 C for 2000 h. film on is thick and contains LBE inside the scale while it also contains Si and Cr. The fact that corrosion resistance of with Si content above 5% is lower than that of may be related to the low Cr content in compared with the Cr content in. 4. Conclusions Corrosion tests for various steels were performed at 450 to 600 C for 2000 h or 3000 h in oxygen-saturated LBE to study temperature effect on corrosion behavior. The main conclusions are as follows: (1) At 450 C, oxide film formation is a main phenomenon and the corrosion depth is small due to low growth rate. At 500 C, grain boundary corrosion/internal oxidation and node formation occur in some steels in addition to oxide film formation and the corrosion depth increases. (2) At 550 C, extensive grain boundary corrosion/internal oxidation is observed in F/M steels, 2.25Cr-1Mo steel, and. Ferritization caused by selective dissolution of Ni and Cr, and accompanied by LBE penetration occurs in and. containing Si of about 5% has high corrosion resistance.
Temperature Dependence of Corrosion of Ferritic/Martensitic and Austenitic Steels in Liquid Lead-Bismuth Eutectic 2417 (a) Si Pb Cr Fe 5µm (b) Si Bi Cr Fe Analysis line 5µm Analysis line Fig. 12 Line analyses of the cross sections of (a) and (b) after the corrosion test in oxygen-saturated LBE at 600 C for 2000 h. (3) At 600 C, corrosion attack becomes very severe in most steels and the corrosion depth increases due to compound corrosion phenomena. Penetration of Pb and Bi follows grain boundary corrosion/internal oxidation in F/M steels. Ferritization accompanied by LBE penetration advances deeply in and. The compound corrosion layer of oxidation, dissolution and LBE penetration often peels off. While local ferritization is found in, addition of Si to austenitic steels is useful to improve corrosion resistance as shown in. Acknowledgement The authors are grateful to Dr. Futakawa for his assistance and careful comments at the early stage of the present study. REFERENCES 1) B. F. Gromov, Y. I. rlov, P. N. Martynov and V. A. Gulevsky: Proc. Heavy Liquid Metal Coolants in Nucl. Technol. HLMC 98, ctober 5 9, 1998, bninsk, Russia, (1999) pp. 87 100. 2) I. V. Gorynin, G. P. Karzov, V. G. Markov, V. S. Lavrukihin and V. A. Yakovlev: Proc. Heavy Liquid Metal Coolants in Nucl. Technol. HLMC 98, ctober 5 9, 1998, bninsk, Russia, (1999) pp. 120 132. 3) Handbook on Lead-bismuth Eutectic Alloy and Lead Properties, Materials Compatibility, Thermal-hydraulics and Technologies, (2007). http://www.nea.fr/html/science/reports/2007/nea6195-handbook.html 4) C. Fazio, G. Benamati, C. Martini and G. Palombarini: J. Nucl. Mater. 296 (2001) 243 248. 5) G. Benamati, C. Fazio, H. Piankova and A. Rusanov: J. Nucl. Mater. 301 (2002) 23 27. 6) L. Soler, F. J. Martin, F. Hernandez and D. Gomez-Briceno: J. Nucl. Mater. 335 (2004) 174 179. 7) F. J. Martin, L. Soler, F. Hernandez and D. Gomez-Briceno: J. Nucl. Mater. 335 (2004) 194 198. 8) G. Mueller, G. Schumacher, A. Weisenburger, A. Heinzel, F. Zimmermann, T. Furukawa and K. Aoto: Japan Nuclear Cycle Development Institute Report, JNC TY9400 2003-026 (2003). 9) T. Furukawa, G. Mueller, G. Schumacher, A. Weisenburger, A. Heinzel, F. Zimmermann and K. Aoto: J. Nucl. Sci. Technol. 41 (2004) 265 270. 10) Y. Kurata, M. Futakawa and S. Saito: J. Nucl. Mater. 343 (2005) 333 340. 11) Y. Kurata and M. Futakawa: Mater. Trans. 48 (2007) 519 525. 12) F. Barbier and A. Rusanov: J. Nucl. Mater. 296 (2001) 231 236. 13) L. Martinelli, F. Balbaud-Celerier, A. Terlain, S. Bosonnet, G. Picard and G. Santarini: Corros. Sci. 50 (2008) 2537 2548. 14) P. Y. Hou and J. Stringer: xid. Met. 38 (1992) 323 345.