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Chloride induced stress corrosion cracking of candidate canister materials for dry storage of spent fuel
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Nuclear Engineering and Design 238 (2008) 1227–1232
Chloride induced stress corrosion cracking of candidate canister materials for dry storage of spent fuel M. Mayuzumi a,b,∗ , J. Tani b , T. Arai b a
Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan b Central Research Institute of Electric Power Industry, 2-6-1, Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan
Received 17 January 2005; received in revised form 12 February 2007; accepted 20 March 2007
Abstract Susceptibility to chloride induced stress corrosion cracking (ESCC) of candidate canister materials, UNS S31260 and UNS S31254 stainless steels (SS), was investigated by a constant load test in air at temperatures of 343 and 353 K with relative humidity (RH) of 35%, and at 373 K without controlling RH. UNS S31260 and UNS S31254 SS did not fail until 37,700 h at 353 K with RH = 35%, where UNS S30403 SS failed within 250–500 h. The same tendency also was obtained at 343 K, suggesting the superior ESCC resistance of UNS S31260 and UNS S31254 SS. Even rust was not observed on the specimens tested at the temperature of 373 K. To explain the higher ESCC resistance, the pitting potential was measured in the saturated synthetic sea water at temperatures from 303 to 353 K, since ESCC is usually associated with localized corrosion such as pitting and may be closely related to the corrosion resistance. The pitting potentials of UNS S31260 and UNS S31254 SS were much higher than that of UNS S30403 SS. Thus, it was concluded that the superior ESCC resistance is attributable to the higher resistance of UNS S31260 and UNS S31254 SS to pitting corrosion. The critical relative humidity for ESCC, under which no ESCC occurs, is equal to or higher than 15% at temperatures < 353 K judging from ESCC behavior of UNS S30400 SS. © 2007 Elsevier B.V. All rights reserved.
1. Introduction In the dry storage of spent nuclear fuels using concrete casks, stainless steel canisters act as the important barrier to encapsulate spent fuels and radio-active materials. According to the storage concept, the decay heat of the spent fuels is removed through canister wall by air cooling. Hence the canister wall contacts directly with air containing sea salt particles and might be contaminated by chlorides, since the facility will be located in a coastal area and the expected service life is a long period of 40–60 years. Stainless steels are widely used as structural materials for chemical plants, nuclear power plants, etc., because of the superior general corrosion resistance, mechanical properties, and weldability (Hasegawa, 1975). However, it is well known that
∗ Corresponding author at: Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan. Tel.: +81 3 5734 2914; fax: +81 3 5734 2914. E-mail address: [email protected] (M. Mayuzumi).
0029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2007.03.038
austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in certain environments under a tensile stress. SCC induced by sea salt particles, chlorides, for example, is observed on various components in chemical plants built in the coastal area (Nakahara and Takahashi, 1985; Nakamura et al., 1985; Kawamoto, 1988). This type of SCC is called as external SCC (ESCC) or atmospheric SCC since the cracking starts from out side of the equipment in air. ESCC manifests itself as inter-granular cracking or transgranular cracking depending on the material condition, sensitized or not, and temperature. Inter-granular SCC (IGSCC) was usually observed in sensitized parts of stainless steel components at around ambient temperature. On the other hand, trans-granular SCC (TGSCC) observed regardless of the material conditions at relatively high temperature of >327 K. For environmental condition, a certain degree of relative humidity (RH) is necessary to moisten the chlorides adhered on the stainless steel surface. The relative humidity for ESCC easy to occur (RHp ) is dependent on the type of chlorides. Shoji et al. (1986) reported, for example, RHp values of 60% for NaCl and 30% for MgCl2 , respectively. Another important factor of ESCC, tensile
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stress, is mostly derived from the residual stress of welding or cold working. The storage canister has several welding lines in the wall and rid which probably have high residual tensile stresses. Contamination by sea salt particles also is well expected during the long service life of canisters as mentioned previously. Thus susceptibility to ESCC should be evaluated carefully for candidate canister materials. Hence, the purpose of this study is to evaluate ESCC susceptibility of two candidate materials, UNS S31260 and UNS S31254 stainless steels (Nakayama et al., 2001; Tsuruta et al., 2001) and environmental factors affecting ESCC behavior of stainless steels (SS). 2. Experimental 2.1. Material Fifty millimetres thick weld joints of UNS S31260 SS and UNS S31254 SS were prepared together with 2 mm thick plates of UNS S30400 SS, UNS S30403 SS and UNS S31603 SS. Table 1 shows chemical composition of the test materials. From the weld joints, tensile specimens with a gage section of 1.5 mm (UNS S31260 SS) or 2 mm (UNS S31254 SS) thick, 5 mm wide, and 30 mm long were machined so as to contain the weld part in the gage section. Tensile specimens of the same shape (2 mm thick) also were cut from UNS S30400 SS and UNS S30403 SS. After machining, the specimens were polished by emery papers to #600, degreased with acetone, rinsed with de-ionized water, and then attached to a loading apparatus that uses a spring to apply stress. Plate specimens of 11 mm long, 11 mm wide, and 2 mm thick also were machined from the test materials for electrochemical measurements of pitting potential. The test specimens were cut from both the weld part and the matrix for UNS S31260 SS and UNS S31254 SS. 2.2. Test procedure ESCC test was conducted by a constant load method using a spring for loading (Mayuzumi et al., 2003). Applied stress (σ ap ) ranged from 0.5σ y to 1.75σ y of the SS (where σ y : 0.2% proof stress). To deposit chlorides simulating sea salt particles on the gage section of specimen, droplets (10 l each × 5) of synthetic sea water were put on the gage section by a micro-pipette and dried. The resultant surface chlorine concentration was higher than 10 g m−2 as Cl on the gage section. The loading appara-
tus with a specimen each was placed in constant temperature and humidity chambers kept at temperatures of 353 and 343 K with RH = 35% after chloride deposition. A series of tests were conducted at a temperature of 373 K without controlling RH for UNS S31260 SS and UNS S31254 SS after applying tensile stresses of 1.2σ y and 1.5σ y , respectively. Test specimens of UNS S30400 SS with the applied stress of 1.5σ y were placed in tight boxes together with saturated CaCl2 solution at temperatures of 333, 343 and 353 K to control the RH (the resultant measured RH = 16–20%). The test RH was determined after considering the result of a previous study (Shoji et al., 1986). The purpose of this test is to determine the critical relative humidity (RHc ) below which no ESCC occurs. In addition to ESCC test under the constant humidity and temperature condition, the dry and wet ESCC test where RH being changed periodically between 15% and 35% was conducted on UNS S30400 SS and UNS S30403 SS at 353 K to examine the effect of repetition of wet and dry conditions. Applied stress was 1.75σ y of each stainless steel and the test time was 3000 h. To discuss the ESCC test result, the pitting potential was measured in the saturated synthetic sea water at the temperature range between 303 and 353 K according to the Japan Industrial Standard (JIS G) 0577, the method of pitting potential measurement for SS. After ESCC tests, the surface of some specimens was observed by scanning electron microscopy (SEM) to determine cracking or corrosion morphologies. 3. Results and discussion 3.1. ESCC susceptibility of the candidate materials Fig. 1 shows the relationship between applied stress and failure time for a reference material, UNS S30403 SS, obtained by the same constant load method (Mayuzumi et al., 2003). The failure time changed depending on the applied stress, the larger the applied stress the smaller the failure time. For example, failure times of 250 and 400 h were obtained for the applied stress of 500 and 150 MPa, respectively. To examine the statistical aspect of ESCC failure time, nine specimens were tested at the applied stress of 1.75σ y . The average failure time was 588 h with the standard deviation of 188 h. Fig. 2 shows ESCC test results of the candidate canister materials at 353 K together with the applied stress versus SCC rupture time relationship of UNS S30403 SS shown in Fig. 1. No ESCC failure of UNS S31260 SS and UNS S31254 SS was observed for more than 37,700 h in the applied stress ranges from 300
Table 1 Chemical composition of test materials (wt%)
UNS S31260 UNS S31254 UNS S30400 UNS S30403 UNS S31603
C
Si
Mn
P
S
Ni
Cr
Mo
Cu
W
N
0.01 0.013 0.06 0.020 0.019
0.41 0.51 0.61 0.67 0.65
0.45 0.55 0.95 1.34 1.23
0.024 0.023 0.028 0.032 0.034
0.001 0.001 0.011 0.002 0.002
6.88 17.84 8.12 9.69 12.07
25.67 19.84 18.10 18.13 17.51
3.33 6.12 – – 2.08
0.49 0.62 – – –
0.40 – – – –
0.23 0.19 – – –
M. Mayuzumi et al. / Nuclear Engineering and Design 238 (2008) 1227–1232
Fig. 1. Relationship between ESCC failure time and applied stress for UNS S30403 stainless steel at a temperature of 353 K with relative humidity of 35% (Mayuzumi et al., 2003).
Fig. 2. ESCC test results on UNS S31260 and UNS S31254 stainless steels at a temperature of 353 K with relative humidity of 35%.
to 800 MPa for UNS S31260 SS, and 200 to 600 MPa for UNS S31254 SS. Thus both the candidate materials had rupture times larger than that of UNS S30403 SS by 80 times at the applied stress level of 500 MPa. Almost the same relationship was obtained at the test temperature of 343 K as shown in Fig. 3. Since the minimum rupture time of the reference material is 400 h in this case (Mayuzumi et al., 2004), the candidate materials have rup-
Fig. 3. ESCC test results on UNS S31260 and UNS S31254 stainless steels at a temperature of 343 K with relative humidity of 35% (*: UNS S30403 reference data (Mayuzumi et al., 2004)).
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ture times larger than S30403 SS by 50 times at the stress level. Although no ESCC failure was observed for both the candidate materials, corrosion did occur on the surface of the ESCC specimens irrespective of applied stress. Hence, the surface of specimens tested at 353 K with applied stress of 1.2σ y for UNS S31260 SS and 1.5σ y for UNS S31254 SS was observed for the corrosion morphology by SEM at the test time of 10,000 h after removing the corrosion product and the simulated sea salt particles from the gage section. Small trans-granular cracks were observed on the UNS S31254 SS specimen surface. The maximum crack length was around 400 m with the depth larger than 50 m as shown in Fig. 4. Thus UNS S31254 SS was not immune to ESCC, although it has a superior ESCC resistance. Similar incipient cracks also were observed on the gage section of the UNS S31260 SS specimen. However, the corroded area was larger than UNS S31254 SS and the cracks were not much sharp as those of UNS S31254 SS. To determine the incubation time for crack initiation, ESCC tests up to 27, 120, 500 and 1000 h were conducted at the same test condition on both the materials. Fig. 5 shows the surface morphology of UNS S31254 SS specimens at the indicated test time. Small incipient cracks were observed even after 27 h of holding time, suggesting that the incubation time is very short, less than 100 h in this case. However, it is also clear that the ESCC propagation rate is very small for UNS S31254 SS, since the crack lengths are almost the same between the test time of 1000 h (Fig. 5d) and 10,000 h (Fig. 4). Almost the same tendency was obtained for UNS S31260 SS specimens. Neither ESCC initiation nor rust was observed on both the materials after 20,000 h of holding time in the test at 373 K without controlling RH. This result clearly shows that no concern should be paid on ESCC failure of SS canisters at the temperature range higher than 373 K. 3.2. Pitting potentials in synthetic sea water From the ESCC test results, it can be concluded that UNS S31260 SS and UNS S31254 SS have superior ESCC resistance to UNS S30403 SS. To discuss the cause of the higher resistance, pitting potential of the SS was measured in saturated synthetic sea water, since a lot of field and experimental data (Nakahara and Takahashi, 1985; Kawamoto, 1988; Mayuzumi et al., 2003) suggested an important roll of pitting corrosion to initiate ESCC. Fig. 6 compares pitting potentials of the candidate canister materials with reference SS. Pitting potentials of UNS S30403 SS and UNS S31603 SS were lower than 400 mV versus SCE at 303 K, and lower than 0 mV versus SCE at 353 K. On the contrary, the pitting potential of UNS S31260 SS was larger than 900 mV versus SCE up to 333 K, and that of UNS S31254 SS was larger than 900 mV versus SCE up to 343 K. These values will correspond to the potential where oxygen generation occurs from water (Matsuno and Asakura, 1977) at each temperature, since no pit was observed on the specimen surface. At 353 K, the difference in the pitting potentials became small, although the pitting potentials of the candidate materials were still larger than those of the reference materials.
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Fig. 4. Surface morphology of UNS S31254 after the test at a temperature of 353 K with relative humidity of 35% for 10,000 h (applied stress = 1.5σ y (555 MPa)).
Fig. 5. Change in surface morphology of UNS S31254 with holding time during the test at a temperature of 353 K with relative humidity of 35% (applied stress = 1.5σ y (555 MPa)). (a) 27 h, (b) 120 h, (c) 500 h and (d) 1000 h.
Thus one important reason of the high resistance to ESCC of UNS S31260 SS and UNS S31254 SS would be attributable to the high pitting corrosion resistance of the alloys. 3.3. Effect of repetition of wet and dry conditions
Fig. 6. Comparison of the pitting potentials of various stainless steels in saturated synthetic sea water at a temperature range between 303 and 353 K.
The test results described above were obtained at the constant temperature and humidity condition. However, in actual storage, the canister surface would contact various types of air, low temperature with high humidity or low humidity, and relatively high temperature with high or low humidity. Thus wet and dry conditions should be expected alternately on the surface of canisters in actual storage. Hence, the wet and dry ESCC test was done at the test temperature of 353 K and the result was compared with the constant temperature and humidity test result. During the dry condition of RH = 15%, chlorides will be dried and harmless, and ESCC process will progress during the wet con-
M. Mayuzumi et al. / Nuclear Engineering and Design 238 (2008) 1227–1232
Fig. 7. Comparison of ESCC rupture times between the constant humidity test and the wet and dry test for UNS S 30400 and UNS S30403 stainless steels at a temperature of 353 K (applied stress = 1.75σ y (515 and 497 MPa, respectively)).
dition of RH = 35%. And further concentration of chlorides also might be expected during the transient from the wet to the dry conditions. Fig. 7 shows the test result. Average failure times of UNS S30400 SS and UNS S30403 SS were 346 and 588 h, respectively, in the constant humidity condition (Mayuzumi et al., 2003). However, no failure was observed on UNS S30400 SS and UNS S30403 SS in the wet and dry ESCC test until 3000 h. The total time of wet condition exceeded 1100 h for both the SS without failure during the wet and dry ESCC test, suggesting the mitigative effect of the repetition of wet and dry conditions on ESCC. There would be two possible causes to explain the test result. In general, one of the harmful effects from the repetition of wet and dry conditions will be to provide the process which condensate chlorine on the SS surface to the level high enough for ESCC easy to occur. However, such the condensation process would not be necessary in this case since the surface chlorine concentration of higher than 10 g m−2 is high enough to cause ESCC in UNS S30400 SS or UNS S30403 SS. Another possible cause of the mitigative effect could be explained by considering Oshikawa’s work (1995) which showed that once the growth of an active pitting was stopped, reactivation of the pitting became very difficult even the specimen was placed in an environment suitable for pitting corrosion. The same situation also will be applicable to ESCC if one could assume that the initiation process of ESCC is controlled by the generation and growth of pitting as discussed in the previous section. Thus it was concluded that the repetition of the wet and dry conditions did not necessarily accelerate ESCC, and the ESCC test in the constant humidity condition should give us data with a certain degree of conservativeness.
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Fig. 8. ESCC test result on UNS S30400 stainless steel at a temperature of 353 K with relative humidity of 16–20% (applied stress = 1.5σ y (440 MPa)).
is dependent on the type of chlorides and the RHc of MgCl2 , for example, is less than 20% and that for sea salt particles also is less than 20%. This results, however, was obtained by a relatively short test time of 2 weeks, and unfortunately they did not show the exact values of RHc . Hence some tests using UNS S30400 SS were made to obtain RHc for sea salt particles by controlling RH with saturated CaCl2 solution placed in a tight box together with specimens. As shown in Fig. 8, no ESCC was observed on both solution annealed and sensitized UNS S30400 SS under the test condition of RH = 16–20% at test temperatures of 333 and 343 K. Small incipient cracks, however, were observed on the specimen surface with a cracking ratio of 50% at 353 K. Thus RHc is larger than 16% for sea salt particles up to 343 K, although further study is necessary for RHc at the higher temperature. When controlling RH by saturated CaCl2 , the RH changed from the initial value of 20% to the final value of 16% during the test period. Thus it could be assumed that the small incipient cracks initiated and grew during the initial short period of the test time when RH was relatively high at the test temperature of 353 K. This is because the failure time is less than 200 h for the applied stress if RH is
3.4. Critical relative humidity for ESCC ESCC is a continuum of the atmospheric corrosion and some degree of dampness (critical relative humidity: RHc ) will be necessary to moisten the sea salt particles or the metal surface and to cause wet corrosion. According to Shoji et al. (1986), RHc
Fig. 9. ESCC test results on various stainless steels at a temperature of 353 K with relative humidity of 15% (applied stress: 1.5σ y for UNS S30400, UNS S30403 and UNS S31254; 1.2σ y for UNS S31260).
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suitable to ESCC initiation and propagation (Mayuzumi et al., 2003), and the actual crack sizes were very small considering the long test period of 3000 h in this test. To determine the RHc at 353 K, an additional series of ESCC tests were conducted at RH = 15% by a constant temperature and humidity chamber. Fig. 9 shows the test result. No specimen was suffered from ESCC after the holding time of 3000 h, and even rust was not observed on both UNS S31260 SS and UNS S31254 SS. From the test results shown in Figs. 8 and 9, it can be concluded that the RHc is at least 15% up to a temperature of 353 K. The RHc value of 15%, however, was determined by the test result on ANS S30400 SS having smaller resistance to ESCC than the candidate canister materials, UNS S31260 SS and UNS S31254 SS. Hence, the RHc of 15% would be a very conservative criterion as can be estimated also from the fact that even rust was not observed on the specimens of the candidate canister materials. 4. Conclusions (1) UNS S31260 SS and UNS S31254 SS did not fail until more than 37,700 h in the condition of RH = 35% at a test temperature of 353 K where UNS S30403 SS failed within 250–500 h. Almost the same tendency was obtained at 343 K with RH = 35%. This result suggests superior ESCC resistance of both the candidate materials. (2) The pitting potentials of UNS S31260 SS and UNS S31254 SS were much higher than that of UNS S30403 SS in saturated synthetic sea water. This result could explain the superior ESCC resistance of both the candidate materials, since ESCC initiation and/or propagation are usually associated with localized corrosion such as pitting. (3) The repetition of wet and dry conditions does not necessarily promote ESCC of S30400 SS and S30403 SS when the surface chlorine concentration is high enough to cause ESCC in the constant temperature and humidity test at the same test temperature.
(4) The critical relative humidity below which no ESCC to occur is equal to or higher than 15% for sea salt particles up to 353 K for all test materials. References Hasegawa, M., 1975. Sutenresuko Binran (Stainless Steel Handbook). Nikkankogyo Shinbunsha, Tokyo, pp. 1–50. Kawamoto, T., 1988. An investigation into the actual condition of external stress corrosion cracking (ESCC) of austenitic stainless steels. Boshoku-Gijutsu (presently Zairyo-to-Kankyo) 37, 30–33. Matsuno, T., Asakura, S., 1977. Denki-kagaku. Dai-nippon Tosho, Tokyo, p. 52. Mayuzumi, M., Arai, T., Hide, K., 2003. Chloride induced stress corrosion cracking of type 304 and 304 L stainless steels in air. Zairyo-to-Kankyo 52, 166–170. Mayuzumi, M., Ishiyama, N., Tani, J., Arai, T., 2004. Effect of test temperature and applied stress on chloride induced stress corrosion cracking of austenitic stainless steels. In: Proceedings of the 51th Japanese Conference on Materials and Environment. Japan Society of Corrosion Engineering, Tokyo, pp. 187–190. Nakahara, M., Takahashi, K., 1985. Field experiences of ESCC in chemical plants. In: Proceedings of the JSCE Materials and Environments. Japan Society of Corrosion Engineering, Tokyo, pp. 217–220. Nakamura, T., Yamamoto, K., Kagawa, N., 1985. External SCC of austenitic stainless steels in atmosphere containing sea salt particles. Boshoku-Gijutsu (presently Zairyo-to-Kankyo) 34, 346–354. Nakayama, G., Hirano, T., Kobayashi, S., Sakaya, T., 2001. Corrosion resistance of various spent fuel container materials contaminated with sea salt particles under heat transfer condition in air. In: Proceedings of the 48th Japanese Conference on Materials and Environment. Japan Society of Corrosion Engineering, Tokyo, pp. 143–146. Oshikawa, W., Itomura, M., Tsujikawa, S., Shinohara, T., 1995. Corrosion behavior of SUS430 stainless steel in a corrosion sensor made form SUS430/Ag coupling. In: Proceedings of the 42th Japanese Conference on Materials and Environment. Japan Society of Corrosion Engineering, Tokyo, pp. 437–440. Shoji, S., Ohnaka, N., Furutani, Y., Saitoh, T., 1986. Effects of relative humidity on atmospheric stress corrosion cracking of stainless steels. Boshoku-Gijutsu (presently Zairyo-to-Kankyo) 35, 559–565. Tsuruta, T., Matsunaga, K., Abe, I., Kobayashi, T., 2001. A study on corrosion resistance of spent fuel container materials contaminated with sea salt particles under heat transfer condition in air. In: Proceedings of the JSCE Materials and Environments. Japan Society of Corrosion Engineering, Tokyo, pp. 159–162.
Nuclear Engineering and Design 238 (2008) 1227–1232
Chloride induced stress corrosion cracking of candidate canister materials for dry storage of spent fuel M. Mayuzumi a,b,∗ , J. Tani b , T. Arai b a
Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan b Central Research Institute of Electric Power Industry, 2-6-1, Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan
Received 17 January 2005; received in revised form 12 February 2007; accepted 20 March 2007
Abstract Susceptibility to chloride induced stress corrosion cracking (ESCC) of candidate canister materials, UNS S31260 and UNS S31254 stainless steels (SS), was investigated by a constant load test in air at temperatures of 343 and 353 K with relative humidity (RH) of 35%, and at 373 K without controlling RH. UNS S31260 and UNS S31254 SS did not fail until 37,700 h at 353 K with RH = 35%, where UNS S30403 SS failed within 250–500 h. The same tendency also was obtained at 343 K, suggesting the superior ESCC resistance of UNS S31260 and UNS S31254 SS. Even rust was not observed on the specimens tested at the temperature of 373 K. To explain the higher ESCC resistance, the pitting potential was measured in the saturated synthetic sea water at temperatures from 303 to 353 K, since ESCC is usually associated with localized corrosion such as pitting and may be closely related to the corrosion resistance. The pitting potentials of UNS S31260 and UNS S31254 SS were much higher than that of UNS S30403 SS. Thus, it was concluded that the superior ESCC resistance is attributable to the higher resistance of UNS S31260 and UNS S31254 SS to pitting corrosion. The critical relative humidity for ESCC, under which no ESCC occurs, is equal to or higher than 15% at temperatures < 353 K judging from ESCC behavior of UNS S30400 SS. © 2007 Elsevier B.V. All rights reserved.
1. Introduction In the dry storage of spent nuclear fuels using concrete casks, stainless steel canisters act as the important barrier to encapsulate spent fuels and radio-active materials. According to the storage concept, the decay heat of the spent fuels is removed through canister wall by air cooling. Hence the canister wall contacts directly with air containing sea salt particles and might be contaminated by chlorides, since the facility will be located in a coastal area and the expected service life is a long period of 40–60 years. Stainless steels are widely used as structural materials for chemical plants, nuclear power plants, etc., because of the superior general corrosion resistance, mechanical properties, and weldability (Hasegawa, 1975). However, it is well known that
∗ Corresponding author at: Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan. Tel.: +81 3 5734 2914; fax: +81 3 5734 2914. E-mail address: [email protected] (M. Mayuzumi).
0029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2007.03.038
austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in certain environments under a tensile stress. SCC induced by sea salt particles, chlorides, for example, is observed on various components in chemical plants built in the coastal area (Nakahara and Takahashi, 1985; Nakamura et al., 1985; Kawamoto, 1988). This type of SCC is called as external SCC (ESCC) or atmospheric SCC since the cracking starts from out side of the equipment in air. ESCC manifests itself as inter-granular cracking or transgranular cracking depending on the material condition, sensitized or not, and temperature. Inter-granular SCC (IGSCC) was usually observed in sensitized parts of stainless steel components at around ambient temperature. On the other hand, trans-granular SCC (TGSCC) observed regardless of the material conditions at relatively high temperature of >327 K. For environmental condition, a certain degree of relative humidity (RH) is necessary to moisten the chlorides adhered on the stainless steel surface. The relative humidity for ESCC easy to occur (RHp ) is dependent on the type of chlorides. Shoji et al. (1986) reported, for example, RHp values of 60% for NaCl and 30% for MgCl2 , respectively. Another important factor of ESCC, tensile
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M. Mayuzumi et al. / Nuclear Engineering and Design 238 (2008) 1227–1232
stress, is mostly derived from the residual stress of welding or cold working. The storage canister has several welding lines in the wall and rid which probably have high residual tensile stresses. Contamination by sea salt particles also is well expected during the long service life of canisters as mentioned previously. Thus susceptibility to ESCC should be evaluated carefully for candidate canister materials. Hence, the purpose of this study is to evaluate ESCC susceptibility of two candidate materials, UNS S31260 and UNS S31254 stainless steels (Nakayama et al., 2001; Tsuruta et al., 2001) and environmental factors affecting ESCC behavior of stainless steels (SS). 2. Experimental 2.1. Material Fifty millimetres thick weld joints of UNS S31260 SS and UNS S31254 SS were prepared together with 2 mm thick plates of UNS S30400 SS, UNS S30403 SS and UNS S31603 SS. Table 1 shows chemical composition of the test materials. From the weld joints, tensile specimens with a gage section of 1.5 mm (UNS S31260 SS) or 2 mm (UNS S31254 SS) thick, 5 mm wide, and 30 mm long were machined so as to contain the weld part in the gage section. Tensile specimens of the same shape (2 mm thick) also were cut from UNS S30400 SS and UNS S30403 SS. After machining, the specimens were polished by emery papers to #600, degreased with acetone, rinsed with de-ionized water, and then attached to a loading apparatus that uses a spring to apply stress. Plate specimens of 11 mm long, 11 mm wide, and 2 mm thick also were machined from the test materials for electrochemical measurements of pitting potential. The test specimens were cut from both the weld part and the matrix for UNS S31260 SS and UNS S31254 SS. 2.2. Test procedure ESCC test was conducted by a constant load method using a spring for loading (Mayuzumi et al., 2003). Applied stress (σ ap ) ranged from 0.5σ y to 1.75σ y of the SS (where σ y : 0.2% proof stress). To deposit chlorides simulating sea salt particles on the gage section of specimen, droplets (10 l each × 5) of synthetic sea water were put on the gage section by a micro-pipette and dried. The resultant surface chlorine concentration was higher than 10 g m−2 as Cl on the gage section. The loading appara-
tus with a specimen each was placed in constant temperature and humidity chambers kept at temperatures of 353 and 343 K with RH = 35% after chloride deposition. A series of tests were conducted at a temperature of 373 K without controlling RH for UNS S31260 SS and UNS S31254 SS after applying tensile stresses of 1.2σ y and 1.5σ y , respectively. Test specimens of UNS S30400 SS with the applied stress of 1.5σ y were placed in tight boxes together with saturated CaCl2 solution at temperatures of 333, 343 and 353 K to control the RH (the resultant measured RH = 16–20%). The test RH was determined after considering the result of a previous study (Shoji et al., 1986). The purpose of this test is to determine the critical relative humidity (RHc ) below which no ESCC occurs. In addition to ESCC test under the constant humidity and temperature condition, the dry and wet ESCC test where RH being changed periodically between 15% and 35% was conducted on UNS S30400 SS and UNS S30403 SS at 353 K to examine the effect of repetition of wet and dry conditions. Applied stress was 1.75σ y of each stainless steel and the test time was 3000 h. To discuss the ESCC test result, the pitting potential was measured in the saturated synthetic sea water at the temperature range between 303 and 353 K according to the Japan Industrial Standard (JIS G) 0577, the method of pitting potential measurement for SS. After ESCC tests, the surface of some specimens was observed by scanning electron microscopy (SEM) to determine cracking or corrosion morphologies. 3. Results and discussion 3.1. ESCC susceptibility of the candidate materials Fig. 1 shows the relationship between applied stress and failure time for a reference material, UNS S30403 SS, obtained by the same constant load method (Mayuzumi et al., 2003). The failure time changed depending on the applied stress, the larger the applied stress the smaller the failure time. For example, failure times of 250 and 400 h were obtained for the applied stress of 500 and 150 MPa, respectively. To examine the statistical aspect of ESCC failure time, nine specimens were tested at the applied stress of 1.75σ y . The average failure time was 588 h with the standard deviation of 188 h. Fig. 2 shows ESCC test results of the candidate canister materials at 353 K together with the applied stress versus SCC rupture time relationship of UNS S30403 SS shown in Fig. 1. No ESCC failure of UNS S31260 SS and UNS S31254 SS was observed for more than 37,700 h in the applied stress ranges from 300
Table 1 Chemical composition of test materials (wt%)
UNS S31260 UNS S31254 UNS S30400 UNS S30403 UNS S31603
C
Si
Mn
P
S
Ni
Cr
Mo
Cu
W
N
0.01 0.013 0.06 0.020 0.019
0.41 0.51 0.61 0.67 0.65
0.45 0.55 0.95 1.34 1.23
0.024 0.023 0.028 0.032 0.034
0.001 0.001 0.011 0.002 0.002
6.88 17.84 8.12 9.69 12.07
25.67 19.84 18.10 18.13 17.51
3.33 6.12 – – 2.08
0.49 0.62 – – –
0.40 – – – –
0.23 0.19 – – –
M. Mayuzumi et al. / Nuclear Engineering and Design 238 (2008) 1227–1232
Fig. 1. Relationship between ESCC failure time and applied stress for UNS S30403 stainless steel at a temperature of 353 K with relative humidity of 35% (Mayuzumi et al., 2003).
Fig. 2. ESCC test results on UNS S31260 and UNS S31254 stainless steels at a temperature of 353 K with relative humidity of 35%.
to 800 MPa for UNS S31260 SS, and 200 to 600 MPa for UNS S31254 SS. Thus both the candidate materials had rupture times larger than that of UNS S30403 SS by 80 times at the applied stress level of 500 MPa. Almost the same relationship was obtained at the test temperature of 343 K as shown in Fig. 3. Since the minimum rupture time of the reference material is 400 h in this case (Mayuzumi et al., 2004), the candidate materials have rup-
Fig. 3. ESCC test results on UNS S31260 and UNS S31254 stainless steels at a temperature of 343 K with relative humidity of 35% (*: UNS S30403 reference data (Mayuzumi et al., 2004)).
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ture times larger than S30403 SS by 50 times at the stress level. Although no ESCC failure was observed for both the candidate materials, corrosion did occur on the surface of the ESCC specimens irrespective of applied stress. Hence, the surface of specimens tested at 353 K with applied stress of 1.2σ y for UNS S31260 SS and 1.5σ y for UNS S31254 SS was observed for the corrosion morphology by SEM at the test time of 10,000 h after removing the corrosion product and the simulated sea salt particles from the gage section. Small trans-granular cracks were observed on the UNS S31254 SS specimen surface. The maximum crack length was around 400 m with the depth larger than 50 m as shown in Fig. 4. Thus UNS S31254 SS was not immune to ESCC, although it has a superior ESCC resistance. Similar incipient cracks also were observed on the gage section of the UNS S31260 SS specimen. However, the corroded area was larger than UNS S31254 SS and the cracks were not much sharp as those of UNS S31254 SS. To determine the incubation time for crack initiation, ESCC tests up to 27, 120, 500 and 1000 h were conducted at the same test condition on both the materials. Fig. 5 shows the surface morphology of UNS S31254 SS specimens at the indicated test time. Small incipient cracks were observed even after 27 h of holding time, suggesting that the incubation time is very short, less than 100 h in this case. However, it is also clear that the ESCC propagation rate is very small for UNS S31254 SS, since the crack lengths are almost the same between the test time of 1000 h (Fig. 5d) and 10,000 h (Fig. 4). Almost the same tendency was obtained for UNS S31260 SS specimens. Neither ESCC initiation nor rust was observed on both the materials after 20,000 h of holding time in the test at 373 K without controlling RH. This result clearly shows that no concern should be paid on ESCC failure of SS canisters at the temperature range higher than 373 K. 3.2. Pitting potentials in synthetic sea water From the ESCC test results, it can be concluded that UNS S31260 SS and UNS S31254 SS have superior ESCC resistance to UNS S30403 SS. To discuss the cause of the higher resistance, pitting potential of the SS was measured in saturated synthetic sea water, since a lot of field and experimental data (Nakahara and Takahashi, 1985; Kawamoto, 1988; Mayuzumi et al., 2003) suggested an important roll of pitting corrosion to initiate ESCC. Fig. 6 compares pitting potentials of the candidate canister materials with reference SS. Pitting potentials of UNS S30403 SS and UNS S31603 SS were lower than 400 mV versus SCE at 303 K, and lower than 0 mV versus SCE at 353 K. On the contrary, the pitting potential of UNS S31260 SS was larger than 900 mV versus SCE up to 333 K, and that of UNS S31254 SS was larger than 900 mV versus SCE up to 343 K. These values will correspond to the potential where oxygen generation occurs from water (Matsuno and Asakura, 1977) at each temperature, since no pit was observed on the specimen surface. At 353 K, the difference in the pitting potentials became small, although the pitting potentials of the candidate materials were still larger than those of the reference materials.
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Fig. 4. Surface morphology of UNS S31254 after the test at a temperature of 353 K with relative humidity of 35% for 10,000 h (applied stress = 1.5σ y (555 MPa)).
Fig. 5. Change in surface morphology of UNS S31254 with holding time during the test at a temperature of 353 K with relative humidity of 35% (applied stress = 1.5σ y (555 MPa)). (a) 27 h, (b) 120 h, (c) 500 h and (d) 1000 h.
Thus one important reason of the high resistance to ESCC of UNS S31260 SS and UNS S31254 SS would be attributable to the high pitting corrosion resistance of the alloys. 3.3. Effect of repetition of wet and dry conditions
Fig. 6. Comparison of the pitting potentials of various stainless steels in saturated synthetic sea water at a temperature range between 303 and 353 K.
The test results described above were obtained at the constant temperature and humidity condition. However, in actual storage, the canister surface would contact various types of air, low temperature with high humidity or low humidity, and relatively high temperature with high or low humidity. Thus wet and dry conditions should be expected alternately on the surface of canisters in actual storage. Hence, the wet and dry ESCC test was done at the test temperature of 353 K and the result was compared with the constant temperature and humidity test result. During the dry condition of RH = 15%, chlorides will be dried and harmless, and ESCC process will progress during the wet con-
M. Mayuzumi et al. / Nuclear Engineering and Design 238 (2008) 1227–1232
Fig. 7. Comparison of ESCC rupture times between the constant humidity test and the wet and dry test for UNS S 30400 and UNS S30403 stainless steels at a temperature of 353 K (applied stress = 1.75σ y (515 and 497 MPa, respectively)).
dition of RH = 35%. And further concentration of chlorides also might be expected during the transient from the wet to the dry conditions. Fig. 7 shows the test result. Average failure times of UNS S30400 SS and UNS S30403 SS were 346 and 588 h, respectively, in the constant humidity condition (Mayuzumi et al., 2003). However, no failure was observed on UNS S30400 SS and UNS S30403 SS in the wet and dry ESCC test until 3000 h. The total time of wet condition exceeded 1100 h for both the SS without failure during the wet and dry ESCC test, suggesting the mitigative effect of the repetition of wet and dry conditions on ESCC. There would be two possible causes to explain the test result. In general, one of the harmful effects from the repetition of wet and dry conditions will be to provide the process which condensate chlorine on the SS surface to the level high enough for ESCC easy to occur. However, such the condensation process would not be necessary in this case since the surface chlorine concentration of higher than 10 g m−2 is high enough to cause ESCC in UNS S30400 SS or UNS S30403 SS. Another possible cause of the mitigative effect could be explained by considering Oshikawa’s work (1995) which showed that once the growth of an active pitting was stopped, reactivation of the pitting became very difficult even the specimen was placed in an environment suitable for pitting corrosion. The same situation also will be applicable to ESCC if one could assume that the initiation process of ESCC is controlled by the generation and growth of pitting as discussed in the previous section. Thus it was concluded that the repetition of the wet and dry conditions did not necessarily accelerate ESCC, and the ESCC test in the constant humidity condition should give us data with a certain degree of conservativeness.
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Fig. 8. ESCC test result on UNS S30400 stainless steel at a temperature of 353 K with relative humidity of 16–20% (applied stress = 1.5σ y (440 MPa)).
is dependent on the type of chlorides and the RHc of MgCl2 , for example, is less than 20% and that for sea salt particles also is less than 20%. This results, however, was obtained by a relatively short test time of 2 weeks, and unfortunately they did not show the exact values of RHc . Hence some tests using UNS S30400 SS were made to obtain RHc for sea salt particles by controlling RH with saturated CaCl2 solution placed in a tight box together with specimens. As shown in Fig. 8, no ESCC was observed on both solution annealed and sensitized UNS S30400 SS under the test condition of RH = 16–20% at test temperatures of 333 and 343 K. Small incipient cracks, however, were observed on the specimen surface with a cracking ratio of 50% at 353 K. Thus RHc is larger than 16% for sea salt particles up to 343 K, although further study is necessary for RHc at the higher temperature. When controlling RH by saturated CaCl2 , the RH changed from the initial value of 20% to the final value of 16% during the test period. Thus it could be assumed that the small incipient cracks initiated and grew during the initial short period of the test time when RH was relatively high at the test temperature of 353 K. This is because the failure time is less than 200 h for the applied stress if RH is
3.4. Critical relative humidity for ESCC ESCC is a continuum of the atmospheric corrosion and some degree of dampness (critical relative humidity: RHc ) will be necessary to moisten the sea salt particles or the metal surface and to cause wet corrosion. According to Shoji et al. (1986), RHc
Fig. 9. ESCC test results on various stainless steels at a temperature of 353 K with relative humidity of 15% (applied stress: 1.5σ y for UNS S30400, UNS S30403 and UNS S31254; 1.2σ y for UNS S31260).
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suitable to ESCC initiation and propagation (Mayuzumi et al., 2003), and the actual crack sizes were very small considering the long test period of 3000 h in this test. To determine the RHc at 353 K, an additional series of ESCC tests were conducted at RH = 15% by a constant temperature and humidity chamber. Fig. 9 shows the test result. No specimen was suffered from ESCC after the holding time of 3000 h, and even rust was not observed on both UNS S31260 SS and UNS S31254 SS. From the test results shown in Figs. 8 and 9, it can be concluded that the RHc is at least 15% up to a temperature of 353 K. The RHc value of 15%, however, was determined by the test result on ANS S30400 SS having smaller resistance to ESCC than the candidate canister materials, UNS S31260 SS and UNS S31254 SS. Hence, the RHc of 15% would be a very conservative criterion as can be estimated also from the fact that even rust was not observed on the specimens of the candidate canister materials. 4. Conclusions (1) UNS S31260 SS and UNS S31254 SS did not fail until more than 37,700 h in the condition of RH = 35% at a test temperature of 353 K where UNS S30403 SS failed within 250–500 h. Almost the same tendency was obtained at 343 K with RH = 35%. This result suggests superior ESCC resistance of both the candidate materials. (2) The pitting potentials of UNS S31260 SS and UNS S31254 SS were much higher than that of UNS S30403 SS in saturated synthetic sea water. This result could explain the superior ESCC resistance of both the candidate materials, since ESCC initiation and/or propagation are usually associated with localized corrosion such as pitting. (3) The repetition of wet and dry conditions does not necessarily promote ESCC of S30400 SS and S30403 SS when the surface chlorine concentration is high enough to cause ESCC in the constant temperature and humidity test at the same test temperature.
(4) The critical relative humidity below which no ESCC to occur is equal to or higher than 15% for sea salt particles up to 353 K for all test materials. References Hasegawa, M., 1975. Sutenresuko Binran (Stainless Steel Handbook). Nikkankogyo Shinbunsha, Tokyo, pp. 1–50. Kawamoto, T., 1988. An investigation into the actual condition of external stress corrosion cracking (ESCC) of austenitic stainless steels. Boshoku-Gijutsu (presently Zairyo-to-Kankyo) 37, 30–33. Matsuno, T., Asakura, S., 1977. Denki-kagaku. Dai-nippon Tosho, Tokyo, p. 52. Mayuzumi, M., Arai, T., Hide, K., 2003. Chloride induced stress corrosion cracking of type 304 and 304 L stainless steels in air. Zairyo-to-Kankyo 52, 166–170. Mayuzumi, M., Ishiyama, N., Tani, J., Arai, T., 2004. Effect of test temperature and applied stress on chloride induced stress corrosion cracking of austenitic stainless steels. In: Proceedings of the 51th Japanese Conference on Materials and Environment. Japan Society of Corrosion Engineering, Tokyo, pp. 187–190. Nakahara, M., Takahashi, K., 1985. Field experiences of ESCC in chemical plants. In: Proceedings of the JSCE Materials and Environments. Japan Society of Corrosion Engineering, Tokyo, pp. 217–220. Nakamura, T., Yamamoto, K., Kagawa, N., 1985. External SCC of austenitic stainless steels in atmosphere containing sea salt particles. Boshoku-Gijutsu (presently Zairyo-to-Kankyo) 34, 346–354. Nakayama, G., Hirano, T., Kobayashi, S., Sakaya, T., 2001. Corrosion resistance of various spent fuel container materials contaminated with sea salt particles under heat transfer condition in air. In: Proceedings of the 48th Japanese Conference on Materials and Environment. Japan Society of Corrosion Engineering, Tokyo, pp. 143–146. Oshikawa, W., Itomura, M., Tsujikawa, S., Shinohara, T., 1995. Corrosion behavior of SUS430 stainless steel in a corrosion sensor made form SUS430/Ag coupling. In: Proceedings of the 42th Japanese Conference on Materials and Environment. Japan Society of Corrosion Engineering, Tokyo, pp. 437–440. Shoji, S., Ohnaka, N., Furutani, Y., Saitoh, T., 1986. Effects of relative humidity on atmospheric stress corrosion cracking of stainless steels. Boshoku-Gijutsu (presently Zairyo-to-Kankyo) 35, 559–565. Tsuruta, T., Matsunaga, K., Abe, I., Kobayashi, T., 2001. A study on corrosion resistance of spent fuel container materials contaminated with sea salt particles under heat transfer condition in air. In: Proceedings of the JSCE Materials and Environments. Japan Society of Corrosion Engineering, Tokyo, pp. 159–162.