FR0200443
A STUDY ON CORROSION MECHANISM OF FBR STRUCTURAL MATERIAL IN SMALL SODIUM LEAK UNDER INSULATOR
T. FURUKAWA (1), D. PI AT (2), S. ROSANVALLON (2) and C. LATGE (2)
(1) Japan Nuclear Cycle Development Institute, O-arai Engineering Center (JNC-OEC) 4002 Narita-cho, O-arai-machi, Higashi-lbaraki-gun, Ibaraki, 311-1393, JAPAN (2) Commissariat a I'Energie Atomique, Centre de Cadarache (CEA-Cadarache) 13108 Saint Paul Lez Durance Cedex, FRANCE
Key words: Corrosion, Sodium leak, Structural material
ABSTRACT was thermodynamically stable, and no direct In order to clarify the corrosion mechanism influence on the steel was observed. of FBR structural material by small sodium Based on these results, a number of cor- leak under mineral wool insulator in the sec- rosion tests were further carried out at OEC ondary or ancillary circuits of Fast Breeder (JNC) in order to obtain the corrosion behav- Reactors, sodium leak test has been carried ior below the melting point (about 873K) of out in the FUTUNA loop at Cadarache (CEA). the main product, Na4Fe03. It was concluded The mock-up was a 316L stainless steel pipe that the weight loss at 873K or below could 12-inch in diameter covered with the insula- be predicted by the time dependence proper- tor. The test lasted for 240 hours at a leak ties based on the diffusion law. rate of 0.1cc/min at 793K in atmosphere. The corrosion of the structural material has INTRODUCTION been extensively observed under the periph- As a coolant of Fast Breeder Reactors ery of the massive leakage products. The cor- (FBRs), sodium reacts with oxygen and mois- rosion mechanism has been estimated based ture in air at elevated temperature. And it is on the results of material analyses and ther- known that structural material is damaged by modynamic data. It was found that the main its by-products. Therefore, it is essential to corrosion mechanism could be similar to that clarify the corrosion phenomena prevailing prevailing in Na2O+Na environment, namely under the sodium leak condition to estimate the 'NaFe double oxidization type corrosion'. the structure integrity. Na-Si complex oxide, which was identified in Research on the corrosion mechanism of the outer region of the corrosion product structural materials in leakage sodium has formed from the reaction between the main been actively carried out since 1980's. elements of the insulator and leakage sodium, Newman [1], [2] has conducted corrosion tests oooo in various sodium compounds on the assump- between Europe and Japan. tions of the sodium-water reaction in the steam generator and of the sodium leak in the sec- EXPERIMENT ondary circuits. In addition, corrosion formula The testing mock-up is shown in figure 1. has been proposed in the latter. Knights et The material of the 12-inch diameter pipe was al. [3] have performed experiments under cor- made of type 316L stainless steel. On the rosive environments generated by the sodium- lower side of the orifice, the analyzing plate water reaction, and suggested one corrosion for X-ray examination which consisted of the mechanism. Aoto [4] has proposed two cor- same material, was fitted. The heater was rosion mechanisms based on thermodynamic installed inside the pipe. The insulator of 250 study to explain the corrosion behavior ob- mm thickness was directly placed around the served in sodium leak tests corresponding to pipe corresponding to that of European FBRs. the secondary circuit condition and the sodium Its chemical composition is given in table 1. leak incident which occurred in Japanese pro- The mock-up was set up in a sodium leak test totype Fast Breeder Reactor 'Monju' in 1995. facility called FUTUNA-2 (figure 2). The test Two corrosion models based on the findings lasted for 240 hours at a leak rate of 0.1 cc/ have been put forward [5], [6]. min at 793K in atmosphere (table 2). In order to describe the corrosion behav- After the test, measurement of the pipe ior of the structural materials under small so- thickness, X-ray analyses of the leakage prod- dium leak in the secondary or ancillary circuits ucts and microstructural observations of the of European FBRs, in connection with the leak pipe were performed. before break (LBB) research, Commissariat a I'Energie Atomique (CEA) has conducted RESULTS studies since 1989 [7]. Until now, it has been TEMPERATURE BEHAVIOR confirmed that leakage sodium reacts with the The temperature behaviors in the vicinity insulator directly covering the sodium pipe, of the orifice throughout the test period are and that sodium leak can continue in certain illustrated in figure 3. conditions to proceed as massive leakage According to the output of the thermo- product is formed. The corrosion of the struc- couple set up in the orifice, the temperature tural material has been observed under the rise which was apparently resulted from the periphery of the products to a great extent. interaction between sodium and oxygen, had
Based on the investigation, they have estab- been observed starting from leak initiation (To) lished electrochemical model for the phenom- ena. Insulator Orifice (
Table 1 Chemical Composition of Insulator dium and the insulator, was seen from T.+38 (mass%) hours to T0+40 hours. SiO2 CaO AI2O3 MgO 42% 37% 10.7 4% POST-TEST OBSERVATION Table 2 Test Condition The photographs of the pipe after the test Temperature 793K are shown in figure 4. Although discoloration Leak Rate 0.1cc/min owing to high temperature heating was ob- Leak Time 240h served on the surface of the insulator, leak- age sodium was not observed (figure 4(a)). Green product was deposited on the pipe re- Ventilation moved from the insulator, with a dimension of Test cell about 400 mm wide and 90 mm high (figure 4(b)). From this finding, it was considered that most of the leakage sodium was held within the product. The cross-section of the product is shown in figure 4(C). Semi-lustrous white Heater product was observed inside the green prod- uct, and black one at the interface of the white layer and the pipe.
) Sodium storage tank
Fig. 2 Overview of the test facility for 14 hours. The temperature eventually kept at818K. The temperatures of the three thermo- couples mounted on the pipe at a distance of 50 mm from the orifice, sharply fluctuated in the range between 793K and 973K before About 100mm from orifice these thermocouples failed. In this region, grooving corrosion with luster was observed 800
(ret. next chapter). The temperatures of other 700 thermocouples installed on the pipe were 1100 50mm from orifice maintained at approximately 818K during the 1000 900 In insulator, 75mmH test period, excluding T0+38 hours to T0+40 800 hours. Based on the data from the thermo- 700 couple placed in the insulator, the tempera- 600 48 96 144 192 240 ture fluctuation, which seemed to be due to Passage Time / h the chemical reaction between leakage so- Fig.3 Temperature behavior OGOO
Table 3 X-ray Results The result of X-ray analyses is summarized Sampling Positions XRD Results in table 3. Green product near Na4SiO4, NaAIO2 The green product was identified mainly surface Black product on as Na4Si04, which was formed by chemical NaFeO2, NaOH surface reaction between leakage sodium and the
White product near Na2O2, Na2O, NaOH, main element (SiO2) of the insulator. surface (SiO2)
2Na2O[s] + SiO2[sl] = Na4Si04[sl] = "342-9 kJ/mo1
The white product consisted of Na2O, which existed at low oxygen pressure, and
stable compounds (Na2O2 and NaOH) at at- mospheric pressure. However, Na-Si-0 com- plex oxide was not identified here. It seemed
that Na2O2 and NaOH were formed during storage because the sample was kept in the glove box in the presence of approximately Fig. 4 The photograph of the mock-up 200ppm oxygen for 10 days. Photographs of the pipe following water cleaning and of the pipe thickness are shown in figure 5 and 6. Three differential phenom- ena prevailed on the surface. The color of the inner region located under the white prod- uct was brown with partial gray, and the cor- rosion depth was less than 0.7 mm. The outer region located under the green product was blackish brown, and the depth was the same as that of the inner region. Concentric groov- ing corrosion of maximum depth 2.1 mm was Fig. 5 The appearance of the pipe after observed between the two regions. The width water cleaning was about 20mm, and the appearance was silver metallic with luster. There was an insig- I 2-2.5 D 2.5-3 D 3-3.5 m 3.5-4 • 4-4.5 D 4.5-5 -130 nificant amount of corrosion product on the Thickness (mm) -90 spot. Only NaFeO2 and NaOH were identi-
-50 fied in the product by X-ray analysis. In short, -10 it was considered that the chemical reaction 30 in Na-Fe-0 system occurred in the area. 70 The results of optical microscopy of the 110 cross section and scanning electron micros- T- -i- CM CM CO CO - (a) Grooving region formation with that of other oxides as shown in figure 9. Although Na2O does not oxidize the steel (iron), it is possible for oxidation to occur through the formation of Na-Fe com- plex oxide. In brief, the chemical reaction of Na2O and Fe might have prevailed in this area. Aoto et al. [5] has reported the same phenom- enon with regard to the corrosion test in (b) Blackish brown region (outside of grooving region) Na+Na2O environment, and named it the 'NaFe double oxidation type corrosion'. In short, Na and Na2O were considered to form in the region. The width of the grooving corrosion was greater than that of other cases which were reported by Bertrand et al. It was considered Fig. 7 Observation results of the pipe that the width might have a relationship with in the outer region, none was observed in the (a) Grooving region V -I (b) Blackish brown region grooving region. Furthermore, general corro- sion prevailed without ruggedness. ^mmm DISCUSSION CORROSION MECHANISM The feature of the corrosion mechanism Fig. 8 Observation results of the surface was understood based on the results of X-ray -200 and microstructure analyses. The main compound in the green product, Na4Si04, is thermodynamically stable, and Na2O2. hence it is not possible for the compound to -400 oxidize the steel (Fe). Therefore, the mate- O rial located under the compound was slightly corroded. The compound hindered oxygen -600 1/2 Na5FeO4 inward diffusion. In the mean time, sodium 2/3 NaiFeOs was continuously supplied from the orifice. As fe a result, the region containing the white prod- < 2Na2O uct was kept at low oxygen potential during -800 the test. 1 /2 Na4SiO4 Grooving corrosion was observed between the two regions. On the surface, Na-Fe com- -1000 plex oxide was noted but none of iron oxide. 200 400 600 800 1000 1200 The reason can be explained by the compari- Temperature / K son of the standard free energy of iron oxide Fig. 9 Comparison of the standard free energy of iron oxide formation the sodium leak ratio. Since this corrosion took Na-Si-0 Compound Insulator NarO place at Na2O highest activity, the moving rate was based on the leak rate (figure 10). CORROSION FORMULA ItoO+Na The chemical diagram of Na-Fe-0 system 3161. SS is shown in figure 11. Na4Fe03 is formed in Na2O and Na environment. It has been was Fig. 10 Reaction phenomenon under insulator confirmed that the compound melted at about 923K [8]. Moreover, the corrosion rate in Na+Na2O at temperature less than 923K was 20 lower than the value of the average line based Na2O2 on the results over 973K [5]. In other words, Fe2O3 the corrosion rate was limited by the diffusion Na3Fe509 step through layer of Na4Fe03, which was NaFeO DP Na5Fe04 2 present in solid state below the melting point -° -20 Na8Fe2O7 (about 923K). Na?0 The temperature of the surface was fluc- tuating between 793K and 973K before fail- -40 Fe ure. Therefore, it is necessary to determine 0 5 10 the metal loss prior to sodium leak detection a -'°9 Na20 for LBB estimation. For this reason, time de- Fig. 11 Chemical diagram in Na-Fe-0 system pendence tests were carried out at tempera- at 873K ture lower than the melting point of Na4Fe03 in Na+Na2O. 0.02 To obtain direct comparison with the re- Na(70%)-Na2O(30%) O:873K, A:823K sults reported in the paper [5], the tests con- o 0 5 ducted in JNC took into account the follow- 0.01 0-^AW = 0.004787t ' ings: 05 f AWA0.000797t A (1) The ratio of Na/Na O was 70mass%/ 2 $A—7,—~~~, • A 30mass%. This ratio was chosen because in 0 0 5 10 15 20 25 this case the mixture was liquid and the cor- Time / h rosion rate was very high. Fig. 12 Corrosion test results in Na+Na2O (2) The material used was carbon steel for comparison with reference [5]. based on the time dependence and that the (3) The tests were performed in argon em- rate was limited by the diffusion law. ploying the test equipment as described in reference [5]. CONCLUSIONS The results are indicated in figure 12: the (1) In order to obtain a better understanding average lines were based on parabolic law of the corrosion mechanism of the structural (see equations in the figure). It is clearly material by small sodium leak in the second- shown that metal loss could be predicted ary or ancillary circuits of FBRs, sodium leak oooo test was carried out together with material REFERENCES analyses. According to the results, it was clari- [1] Newman R.N., Progress in Nuclear Energy fied that the corrosion prevailed occurred in Vol.12 p.119-147 (1983) Na+Na2O. [2] Newman R.N., International Conference (2) It was proven that Na4Si04, which was the on Science and Technology of Fast Reactor main product of the reaction between leak- Safety, p.535-539 (1986) age sodium and the main compounds of the [3] Knights C.F. and Perkins R., AERE-R9521, insulator, was formed on the surface, and that United Kingdom Atomic Energy Authority - it decreased the oxygen potential in the prod- Harwell (1979) uct. 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