Evaluation method of corrosion lifetime of conventional stainless steel canister under oceanic air environment

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1 Available online at Nuclear Engineering and Design 238 (2008) Evaluation method of corrosion lifetime of conventional stainless steel canister under oceanic air environment Akio Kosaki Nuclear Fuel Cycle Backend Research Center, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba-ken , Japan Received 22 March 2005; received in revised form 12 February 2007; accepted 20 March 2007 Abstract Natural exposure and accelerated corrosion tests of conventional stainless steels for canisters of Types 304, 304L, and 316(LN) for concrete casks were conducted using several test specimens and 1/5 scale canister models. The welding residual stress of a full-scale model canister was also measured and the lifetime of sealability of canisters against corrosion evaluated. The maximum pitting rate and crevice corrosion rate of Type 304 were approximately 20 and 30 m/year. Many SCC in the 4 Point Bending (4PB) test specimens were found to initiate from the bottom of the corrosion area by pitting or crevice corrosion. The SCC propagation rates in Types 304 and 304L under natural conditions were around 1.2E 12 to 1.8E 11 m/s in the K (Stress Intensity Factor) range of MPa m 1/2, and that of the accelerated test (60 C, 95% RHS, filled with NaCl mist) around 1.0E 10 to 3.5E 9 m/s in the K range of MPa m 1/2. The SCC propagation rates under both natural and accelerated conditions were independent of K. The lifetime of sealability estimated from 1/5 scale models was longer than that from the small bending test specimens and has a safety margin as a structure Elsevier B.V. All rights reserved. 1. Introduction Corrosion integrity of canisters in the concrete casks used for spent fuel storage is very important because the canister serves to maintain sealability over the storage period of years. During the storage period, sea salts in the cooling air from the outer atmosphere is carried onto the outer surface of the canister. Sea salt accumulates and condenses on the canister surface over the long storage periods. The temperature of the canister containing the spent fuel is gradually reduced by the decay heat of the spent fuel, and the air in the concrete cask becomes humid and corrosive. In addition, the welded canisters are used with remaining tensile residual stress. Thus, the development of methods for evaluating canister lifetime is very important with regard to stress corrosion cracking (SCC), etc. This paper presents a study by CRIEPI to develop methods of evaluating the corrosion lifetime of concrete module canister. In this study candidate materials were assumed to be conventional stainless steels such as, 304L, and 316(LN) address: kosaki@criepi.denken.or.jp. stainless steels. These materials are used in the actual nuclear power plants. Accelerated corrosion tests were conducted in an environment filled with NaCl steam mist at 60 C to obtain test results within a short period of about 5 years. Natural exposure tests were conducted at Miyakojima Island, one of the most corrosive areas in Japan, to obtain natural corrosion behavior as an example. 2. Corrosion type of candidate materials for canister The authors assumed the corrosion of Types 304, 304L, and 316(LN) stainless steels in this study to be local corrosion in the atmosphere of the storage canister by sea salt, i.e. pitting corrosion, crevice corrosion, and SCC. These materials need to be estimated for corrosion lifetime through the corrosion rate. 3. Estimation methods of the lifetime of canister for pitting and crevice corrosion A qualitative comparison of materials can be obtained by comparison of each lower limit of electro-chemical potential for pitting and crevice corrosion (Tsujikawa et al., 1988) /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.nucengdes

2 1234 A. Kosaki / Nuclear Engineering and Design 238 (2008) However, in order to evaluate the quantitative lifetime of a canister for pitting and crevice corrosion, each corrosion rate of materials needs to be obtained and the time taken to propagate through the thickness of the canister calculated. In this study, the corrosion lifetime of the canister for pitting and crevice corrosion is obtained as follows: [Corrosion life time of canister for pitting or crevice corrosion] = [Time taken for the canister to reach a corrosive temperature, i.e. below about 100 C] + [Time to propagate through the thickness by corrosion](lifetime of propagation) (1) Note: [Incubation time for corrosion to initiate after reaching corrosive temperature] is neglected because this time is very short. [Time taken for the canister to reach a corrosive temperature, i.e. below about 100 C] can be obtained by simulation of timedependent temperature analysis at each position of the canister during storage. The maximum pitting rate in Types 304 and 304L stainless steels including the effect of sensitization of materials by welding heat were obtained by natural exposure tests at the Miyakojima Island test site. The maximum pitting rate over about a 6-year test period was approximately 20 m/year (0.8 mm/40 years) for flat plate test specimens (5 mm in thickness 100 mm in width 100 mm in length). In addition, the crevice corrosion rate in steel was obtained at the same Miyakojima Island site using crevice corrosion specimens (/ crevice) (Kosaki, 2002; Kosaki et al., 1997). The maximum crevice corrosion rate was around 30 m/year (1.2 mm/40 years). Thus, no pitting or crevice corrosion penetrates through the thickness of the canister (about 20 mm) during the storage period of around 40 years. However, this corrosion should be considered a trigger of SCC, because SCC initiation was found in this study to start from the bottom of the corrosion area by pitting or crevice corrosion under the corrosion products. Natural exposure test data in this study are not from an actual storage site under an actual temperature profile of the canister. Thus, similar natural data at a site of actual storage with an actual canister temperature profile should be obtained and the lifetime of the canister calculated, after the storage site is decided. 4. Evaluation methods of the lifetime of canister for SCC 4.1. Residual stress caused by welding (Types 304, 304L stainless steel) The SCC is considered as corrosion with local slip at the tensile crack tip. There is tensile residual stress around the weld line of the canister. Fig. 1. Examples of measured residual stress and measured positions. (a) X-ray measurement (No. 1); (b) strain gauge method (No. 1); (c) weld line and measured position.

3 A. Kosaki / Nuclear Engineering and Design 238 (2008) Residual stress distribution around the weld line by automatic TIG welding was measured using Types 304 and 304L stainless steel test models (cylinders) with a diameter of 1300 mm (Kosaki et al., 2002). These test models had two types of joints simulating actual canisters. Joint A simulates the circumferential joint between the upper lid and the shell body as shown in Fig. 1c, and Joint B simulate the circumferential and longitudinal joints between bodies and that between the shell body and the lower lid as shown in Fig. 1c. Configurations of these two types of welding joints are similar to those shown in Fig. 9 of Section 4.5. The surface residual stress was measured by X-ray and the average residual stress through thickness was measured by the strain gauge method with incremental cutting and relaxation. Fig. 1a and b shows examples of measured residual stress. The dominant residual stress distribution around the welding line is the circumferential tensile stress and its tensile stress value is near the 0.2% proof stress (0.2% PS). Joint A between the upper lid and shell body shows tensile stress values near the 0.2% PS. Joint B between the shell body and lower lid shows the same result. The residual stress value of the welding surface measured by X-ray method is greater than the average value through the thickness by the strain gauge method with incremental cutting and relaxation. In light of these test results, tensile stress conditions for corrosion testing were set at two levels of stresses, i.e. about 0.5 PS and 1.0 PS, as the test parameters Methods of evaluating canister lifetime for SCC A qualitative comparison of materials can be obtained by comparison of each lower limit of electro-chemical potential for SCC, that is the same lower limit level as the re-passive electro chemical potential of crevice corrosion (Tsujikawa et al., 1980; Shinohara et al., 1985; Kosaki and Inohara, 1997). However, in order to estimate the quantitative lifetime of a canister for maintaining sealability, SCC incubation time and SCC corrosion rate of materials needs to be obtained and the time taken to propagate through the thickness of the canister calculated. In this study, the corrosion lifetime of the canister for SCC is obtained as follows: Fig Point Bending test specimen for SCC initiation test. dependent temperature analysis at each position of the canister during storage Lifetime of SCC initiation (Kosaki, 2002) Initiation time for SCC to propagate in Types 304, 304L, and 316(LN) steels were obtained both in the natural exposure test and the accelerated test using a small 4 Point Bending test specimen as shown in Fig. 2. The test piece was 1.5 mm in thickness 10 mm in width 65 mm in length, and loaded with 0.5 PS or 1.0 PS using insulated tool made of Titanium Alloy. Both the direct exposure and the under glass exposure tests are employed in natural exposure test at the Miyakojima Island site. The accelerated test conditions are as follows. Test temperature: 60 C, humidity: 95% RHS, environment filled with NaCl steam mist (at the surface of the test specimen is the saturated NaCl concentration). [Corrosion life time of canister for SCC] = [Time taken for the canister to reach a corrosive temperature, i.e. below about 100 C] + [Initiation time for SCC to propagate after reaching corrosive temperature](lifetime of SCC initiation) + [Time to propagate through the thickness by SCC after initiation](lifetime of SCC propagation) (2) [Time taken for the canister to reach a corrosive temperature, i.e. below about 100 C] can be obtained by simulation of time- Fig. 3. SCC initiation time of (natural exposure test (direct)). (a) Base metal; (b) weld.

4 1236 A. Kosaki / Nuclear Engineering and Design 238 (2008) Fig Point Bending test specimen for SCC propagation test. Fig. 4. SCC initiation time of (accelerated test). (a) Base metal; (b) weld. After the corrosion tests, the loading stress onto the test specimens was measured and the loading tools removed. And the section of the test specimen was cut to observe the existence of SCC crack by an optical microscope and an electron microscope. Figs. 3 and 4 show examples of obtained data and summarized data are shown in Table Lifetime of SCC propagation (Kosaki, 2002, 2004) The SCC propagation rates in Types 304, 304L, and 316(LN) stainless steels were obtained both in natural exposure tests and accelerated tests using small 3 Point Bending test specimens as shown in Fig. 5. The test piece is 10 mm in thickness 15 mm in width 90 mm in length with two types of pre-cracks induced by fatigue (through crack and half elliptical surface crack). These were loaded with 0.4 PS or 0.8 PS using an insulated tool made of titanium alloy. The natural exposure method and the accelerated test conditions are the same as those in Section 4.3. After the corrosion tests, the loading stress onto the test specimens was measured and the loading tools removed. After including post-crack by fatigue the section of test specimens were fractured and the maximum depth of SCC cracking measured by an optical microscope and an electron microscope. Figs. 6 and 7 show examples of obtained data and summarized data are shown in Table 2. In the accelerated test, SCC propagation rates da/dt of Types 304 and 304L steel were about 1.0E 10 to 3.5E 9 m/s at the Stress Intensity Factor K in the range of MPa m 1/2.On Table 1 Lifetime of canister materials for SCC initiation Material Loaded stress Lifetime of canister for SCC initiation (day) Relative ratio Natural exposure Accelerated test B A1/B A2/B Direct exposure A1 Under glass exposure A2 Base metal 1.0 PS Over Over PS Weld PS Over L Base metal 1.0 PS Over Over PS 5 Max Over198 Over 198 Weld Over PS Over Base metal 1.0 PS Over 686 Over PS Weld Over 581 Over PS Notes: (1) PS: 0.2% proof stress. (2) For example, Over 860 means that SCC had not yet occurred after 860 days exposure. (3) For example, means that SCC had not yet occurred after 364 days exposure, but was found after 573 days.

5 A. Kosaki / Nuclear Engineering and Design 238 (2008) Fig. 6. SCC propagation rate of (natural exposure test). (a) Penetrate pre-crack; (b) half elliptical surface pre-crack. Fig. 7. SCC propagation rate of (accelerated test). (a) Penetrate pre-crack; (b) half elliptical surface pre-crack. Table 2 Propagation rate of SCC in canister materials Material SCC propagation rate/natural exposure (m/s) SCC propagation Relative ratio rate/accelerated Direct exposure A1 Under glass exposure A2 B/A1 B/A2 test B (m/s) Base metal 6.4E 12 (no SCC) 9.3E 10 (1.2E 10 to 2.7E 9) 145 Weld 6.6E 12 (2.1E 12 to 1.8E 11) 3.2E 12 (1.2E 12 to 6.4E 12) 1.2E 09 (1.9E 10 to 3.1E 9) L Base metal 1.5E 11 (no SCC) 1.0E 09 (2.9E 10 to 2.7E 9) 67 Weld 7.5E 12 (1.2E 12 to 1.7E 11) 4.2E 12 (4.1E 12, 4.3E 12) 9.7E 10 (1.0E 10 to 3.5E 9) Base metal (no SCC) (no SCC) 1.4E 10 Weld 9.8E 12 (2.6E 12 to 2.1E 11) 6.4E E Note: Propagation rates of SCC show average values and values in ( ) show scattering range of data.

6 1238 A. Kosaki / Nuclear Engineering and Design 238 (2008) Fig. 8. The 1/5 small scale models after losing sealability by accelerated test. (a) (Joint A); (b) L (Joint A). the other hand, in the natural exposure test, SCC propagation rates da/dt were about 1.2E 12 to 1.8E 11 m/s at K in the range of MPa m 1/2, that is about two orders lower than in the accelerated test. The SCC propagation rates under both the natural and the accelerated conditions were independent of the Stress Intensity Factor K in the above range, but SCC can be propagated in a small K value below 1 MPa m 1/2. The da/dt values obtained of 1.2E 12 to 1.8E 11 m/s under natural exposure conditions were mm/year, which would take from about years to penetrate through a thickness of 15 mm. lifetime of sealability under natural conditions are shown in Table 4. Using the Eq. (2) with the lifetime of SCC initiation, SCC propagation rate and time dependent temperature of the canister, the lifetime of sealability can be obtained. These results may be more conservative than the lifetime of sealability obtained by 1/5 models. To estimate the lifetime at the actual storage site, the same method as in this study should be applied, taking account of the actual temperature profile of the canister after the storage site is determined Lifetime of sealability in a structure using 1/5 scale model (Kosaki, 2004) The lifetime of sealability in a structure was measured using 1/5 scale models under the same accelerated test conditions as shown in Fig. 8. Fig. 9 shows the configuration of two types of test model. Each model has a welding Joint A or B. Table 3 shows a list of the test models and test results. Lifetime to penetrate the wall thickness of 13mm (for the 1/5 scale model) using the accelerated test data in Tables 1 and 2, and lifetime obtained by the sealability test in the 1/5 scale model under the same accelerated test are compared in Table 3. Lifetime of sealability in the 1/5 small scale model is greater than the lifetime of sealability estimated by small test bending specimens. Thus, the scale model has a safety margin as a structure. This could be considered due to the effect of residual stress distribution through the thickness direction in the structure Evaluation of lifetime of sealability under natural condition As shown in Tables 1 and 2, both lifetime of SCC initiation and SCC propagation rate obtained under natural conditions were similar to the accelerated conditions, allowing estimation of the lifetime of sealability. From the natural exposure test results at Miyakojima Island, the estimated results of Fig. 9. Configuration of 1/5 scale models. (a) Test model with Joint A (mm); (b) test model with Joint B (mm).

7 A. Kosaki / Nuclear Engineering and Design 238 (2008) Table 3 List of test models and comparison of lifetime of sealability in 1/5 scale models (accelerated test) Materials and type of weld joint Lifetime of sealability in 1/5 scale model A (day) Lifetime of sealability estimated by small test bending specimens SCC initiation B1 (day) SCC propagation B2 (day) Lifetime to penetrate thickness B3 (=B1 + B2) (day) Remarks Joint B 1201 Joint A 980 L Joint B 719 Joint A Max Average 162 (49 792) (Note 1) Average 155 ( ) (Note 2) Average (59 807) Average 160 ( ) Joint A Over 638 (test is continued) (Note 3) B3<A Generally B3 < A Notes: This table shows the example of accelerated test results of welds under 1.0 PS (PS: 0.2% proof stress) tensile stress. Procedure for obtaining B2 value: B2 = (13 mm [minimum thickness of 1/5 scaled model])/(scc propagation rate [da/dt] obtained by small test bending specimens in accelerated test). (1) Average: thickness 13 mm/(9.3e 10 m/s = 29 mm/year) = 162 days; scattering range of data: thickness 13 mm/(1.9e 10 to 3.1E 9 m/s = 6 98 mm/year) = days. (2) Average: thickness 13 mm/(9.7e 10 m/s = 31 mm/year) = 155 days; scattering range of data: thickness 13 mm/(1.0e 10 to 3.5E 9 m/s = mm/year) = days. (3) Thickness 13 mm/(5.0e 11 m/s = 1.6 mm/year) = 3009 days; SCC propagation rate (da/dt = 5.0E 11 m/s) of is for only one piece of data. Table 4 Lifetime of sealability of canister in natural atmosphere Lifetime of SCC initiation A (year) Lifetime of propagation of SCC Propagation rate of SCC B1 (mm/year) average (scattering range) Lifetime to penetrate the thickness B2 (thickness 13 mm/b1) (year) average (scattering range) Lifetime of sealability (year) C (=A + B2) Direct exposure Base metal Weld Over ( ) 65 (22 185) L Base metal Over Weld Over ( ) 65 (26 325) 68 Base metal Over 1.9 (no SCC) Weld Over ( ) 43 (19 163) 45 Under glass exposure Base metal Over 2.7 (no SCC) Weld ( ) 130 (65 325) 131 L Base metal Over (no SCC) Weld (0.1, 0.1) Base metal Over Weld Over Estimated from the results of exposure tests using small bending test specimens on Miyakojima Island Conclusion Natural exposure tests and accelerated corrosion tests of candidate materials for canisters of conventional Types 304, 304L, and 316(LN) stainless steel, for concrete casks were conducted and the lifetime of canister sealability estimated. The results are as follows: (1) The maximum pitting rate of stainless steel was around 20 m/year (0.8 mm/40 years) and the maximum crevice corrosion rate about 30 m/year (1.2 mm/40 years). These rates were too small to penetrate the thickness of the canister during the storage period of about 40 years. (2) Many SCC in the small bending test specimens can be found to initiate from the bottom of the corrosion area

8 1240 A. Kosaki / Nuclear Engineering and Design 238 (2008) by pitting and crevice corrosion under the corrosion products. (3) The residual stress at the welding Joint A between the upper lid and the shell body, and at Joint B between the shell body and the lower lid of the canister, shows tensile stress of a value near the 0.2% PS. The residual stress value of the welded surface measured by X-ray is greater than of the average value through thickness measured by the strain gauge method with incremental cutting and relaxation. (4) The SCC propagation rate in Types 304 and 304L stainless steel under natural conditions was about 1.2E 12 to 1.8E 11 m/s in a K range of MPa m 1/2, and that of the accelerated test was about 1.0E 10 to 3.5E 9 m/s in a K range of MPa m 1/2. The former is smaller than the latter by two orders of magnitudes. (5) The SCC propagation rates under both natural and accelerated conditions were independent of Stress Intensity Factor K in the above range. (6) The lifetime of sealability estimated from the 1/5 scale models was longer than that from the small bending test specimens. The 1/5 scale models have safety margins as a structure. The reason could be the effect of residual stress distribution through the thickness direction in the structure. Acknowledgment This work has been being executed under contract with Ministry of Economy, Trade and Industry of the Japanese government. References Kosaki, A., An example of corrosion estimation of metal cask. In: The 2002 meeting for Weathering Technology, Japan Weathering Test Center. Kosaki, A., The evaluation of corrosion lifetime of canister weld (no. 2) the lifetime of sealability against stress corrosion cracking under accelerated condition. In: Abstract of 2004 Fall Meeting of the Atomic Energy Society of Japan, B4, The Atomic Energy Society of Japan, p Kosaki, A., Inohara, Y., Evaluation of Crevice Corrosion Initiation of Corrosion Resisting Alloys-Crevice Corrosion Diagram in the Natural Water Environment. CRIEPI Report U Kosaki, A., Inohara, Y., et al., Advanced R&D on spent fuel storage-spent high burn-up fuel and MOX (mixed-oxide: Pu and U) fuel. In: Proceedings of the 14th INMM Spent Fuel Management Seminar, INMM (Institute of Nuclear Materials Management), Washington, DC, pp Kosaki, A., Saegusa, T., Urabe, N., Fujiwara, H., Verification study for concrete cask (4)-measured results of residual stress of canister weld. In: Abstract of 2002 Fall Meeting of the Atomic Energy Society of Japan, I48, The Atomic Energy Society of Japan, p Shinohara, T., Tsujikawa, S., Hisamatsu, T., Boshoku-Gijutsu 34, 283. Tsujikawa, S., Tamaki, K., Hisamatsu, T., Tetsu-to-Hagane 66, Tsujikawa, S., Akashi, M., et al., Corrosion Q&A/Corrosion 110. Japan Society of Corrosion Engineering, Maruzen.