17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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SCC AND CREVICE CORROSION RESISTANCES OF TANTALUM-MODIFIED STAINLESS STEEL

J. Kaneda1, Y. Wang2, Y. Maruno2, M. Iwanami1, S.Ishioka1, N.Shigenaka1, and A. Hasegawa3

1 Hitachi-GE Nuclear Energy, Ltd., 3-1-1 Saiwai-cho, Hitachi, Ibaraki, 317-0073, Japan 2 Hitachi Research Laboratory, Hitachi, Ltd., 3-1-1 Saiwai-cho, Hitachi, Ibaraki, 317-0073, Japan

3 Tohoku University, 6-6-01-2 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, 980-8579, Japan

ABSTRACT

Irradiation assisted stress corrosion cracking (IASCC) resistant austenitic stainless steel is being developed for reactor internal components. It is also needed to improve resistances to stress corrosion cracking (SCC) and crevice corrosion. It was reported that Ta improves crevice corrosion resistance in Ni-based alloys and traps vacancies to reduce radiation damage as an over-size element in austenitic stainless steel.

In this paper, several Ta-modified stainless steels (SSs) with different contents of Ta and C were produced to investigate their crevice corrosion resistance, SCC susceptibility and crack growth rate (CGR). In the crevice corrosion test under γ-ray irradiation, two out of four specimens had no crack in the Ta-modified SSs whereas all four specimens had cracks in type 316L SS. The maximum crack depth in the Ta-modified SSs was shallower than that in type 316L SS. From the crevice corrosion test, it was judged that the Ta-modified SSs had better crevice corrosion resistance compared to type 316L SS. The creviced bent beam (CBB) test was performed to evaluate SCC susceptibility. There were no SCCs in any of the Ta-modified SS specimens. For type 316L SS, cracks were found in four out of seven specimens. The CGR test was also conducted using 0.5T-CT specimens. Crack growth was recognized only in type 316L SS; there was almost no crack growth in the Ta-modified SSs under constant load. From the CBB and CGR test results, it was concluded that the Ta-modified SSs had better SCC resistance compared to type 316L SS.

Keywords: crevice corrosion resistance, stress corrosion cracking, crack growth rate, Ta-modified stainless steels, type 316L stainless steel

1. INTRODUCTION

Austenitic stainless steel has been used for reactor internal components in light water reactors. When the operating period is increased, it is known that irradiation assisted stress corrosion cracking (IASCC) susceptibility is raised due to the increasing neutron fluence. It is believed that IASCC is caused by mechanical property changes associated with microstructure changes and radiation induced segregation (RIS) [1-3]. These phenomena result from formation of point defect clusters such as dislocation loops and Cr depletion due to diffusion of Cr atoms toward grain interior areas while vacancies diffuse in the opposite direction toward grain boundaries. It is thought that the above phenomena causing IASCC can be suppressed by trapping point defects to increase their recombination probability. In austenitic stainless steel, it is well known that addition of oversized solutes such as Zr, Hf and Ta can lower RIS or inhibit Cr depletion [4, 5] and subsequently, suppress IASCC.

It was reported that Ta addition to Ni-based alloys increased crevice corrosion resistance in 25% NaCl solution at 107 ºC and increased corrosion resistance in H2SO4 solution at 120 ºC [6, 7]. It was also reported that Ta addition could improve pitting corrosion resistance and crevice corrosion resistance in duplex stainless steels [8, 9].Therefore, Ta addition would be expected to improve crevice corrosion resistance and stress corrosion cracking (SCC) resistance in austenitic stainless steels as well. Nb is added to austenitic stainless steel to get the stabilised stainless steel, type 347 stainless steel, and Nb

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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would possibly be a candidate element for addition to the austenitic stainless steel of reactor internals since it is in Group VB of the periodic table, the same as Ta. But it was reported that Nb addition to austenitic stainless steel was not effective or did not have a clear effect to lower IASCC susceptibility [10, 11]. Then, the development of Ta-modified austenitic stainless steel was considered as the way to improve SCC resistance and IASCC resistance for reactor internals. It was also reported that reduction of the amounts of Si, P and S in the stainless steel improved IASCC resistance [10, 11]. Then Si, P and S amounts were reduced in the finally developed material.

In this study, SCC and crevice corrosion properties of Ta-modified austenitic stainless steel (SS) with reduced amounts of Si, P and S are investigated in comparison to type 316L SS.

2. EXPERIMENTAL

2.1 Test Materials

Four kinds of Ta-modified SS test materials (TA, TB, TC and TD) with different contents of Ta and C were produced by vacuum melting. Their chemical compositions are shown in Table 1. Ta and C contents increased in the order of TA< TB < TC. TB and TD had similar Ta and C contents but Ni content of TB was about 1.5wt% larger than that of TD. Ingot size of TA and TC was 10 kg and that of TB and TD was 150 kg. The ingots were hot forged and hot rolled then given solution heat treatment (SHT). SHT conditions are shown in Table 2. Ta should be in a solid solution in the material to prevent RIS but stabilizing heat treatment is preferable to stabilize C as Ta carbide in the grain interior areas to prevent sensitization. In this study, the stabilizing heat treatment was done at 900 ºC for 1 h followed by air cooling (AC) for the Ta-modified SS materials after SHT [12]. Type 316L SS with SHT was used as a comparison material. Test specimens were machined from the test materials for the tensile test, crevice corrosion test, creviced bent beam (CBB) test and crack growth rate test.

2.2 Tensile Test

Tensile test specimens were cut from the test materials of TA, TB, TC and TD. The tensile tests were performed at room temperature (RT) and 302 ºC in accordance with Japanese Industrial Standards (JIS) Z 2241 and G 0567 corresponding to American Society for Testing and Materials (ASTM) E8 and E21, respectively. Tensile tests were performed for both the SHT and the stabilized specimens of TA, TB and TC but only for the stabilized specimens of TD. Yield strength and tensile strength were evaluated in the tensile tests.

2.3 Crevice Corrosion Test Under γ-ray Irradiation

Stabilized heat treated materials of TA, TB and TC and type 316L SS were used for the crevice corrosion test. Coupon specimens were cut from these materials. Dimensions of coupon specimens for the crevice corrosion test were 50 × 10 × 1.5 mm. Four coupon specimens were machined from each test material for the crevice corrosion test. The surface of each specimen was emery-polished down to #600. Two coupon specimens of the same material were spot-welded to make a 35 mm long central crevice with metal touching as shown in Figure 1; this was one set of test specimens. Thus, two set of spot-welded crevice corrosion test specimens were made for each stabilized test material and type 316L SS. The test was carried out in an autoclave.

The crevice corrosion test conditions are shown in Table 3. The crevice corrosion test temperature and pressure simulated the water conditions in a boiling water reactor (BWR), and the spot-welded specimens were irradiated with γ-rays during the test. γ-ray irradiation is to simulate radiolysis in reactor water and to accelerate crevice corrosion.

After the test, the spot-welded specimen was taken out of the autoclave and cut along the center line in a longitudinal direction. The cross section was polished up to mirror finish. The cross-sectional

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investigation of crevice corrosion was done in a whole crevice area between the welding spots with an optical microscope.

2.4 Creviced Bent Beam (CBB) Test

Creviced bent beam (CBB) specimens were applied to investigate SCC initiation susceptibility of test materials. Stabilized materials of TA, TB and TC and type 316L SS were subjected to sensitizing heat treatment at 700°C for 30 min with water quench (WQ) followed by 40% cold rolling to enhance SCC susceptibility. Seven CBB specimens with the dimensions of 50 × 10 × 2 mm were machined from each cold rolled plate of test material. Both sides of the CBB specimen surface were emery-polished down to #600.

The CBB specimen and a graphite fiber wool sheet of the same dimensions were tightened together using a holder with a curvature R of 100 mm. Then, the holder was placed in the autoclave and immersed in high temperature water. DO and conductivity in the test water were set to accelerate SCC initiation. The CBB test conditions are shown in Table 4.

After the CBB test, the specimen was taken out of the autoclave and cut along the center line in a longitudinal direction. The cross section was polished up to mirror finish and then it was bent with a curvature R of 50 mm to be able to easily observe cracks. The cross-sectional investigation of SCC initiation was done with an optical microscope.

2.5 Crack Growth Rate Test

The objective of this crack growth rate (CGR) test is to confirm the CGRs of CT specimens made of different test materials with similar Ta contents, compared to those of type 316L CT specimens. Stabilized materials of TB, TC and TD and type 316L SS were used for the CGR test. A 0.5T-compact tension (CT) specimen for TC and TD and two 0.5T-CT specimens for TB and type 316L SS were provided for the CGR test. The CT specimens with pre-crack about 2 mm in length were set in autoclaves. Test conditions are shown in Table 5. Loading conditions were changed from STEP 1 to STEP 3 as shown in Table 6 to transition to the intergranular crack and to get sufficient engagement. According to the previous CGR test data, the fracture surface under the STEP 3 condition was almost an intergranular crack and it was a fully intergranular crack in the constant load. Crack length was monitored by the direct current potential drop (DCPD) system. After the test, the crack was advanced by fatigue loading to open the fracture surface, which was observed to determine the actual crack length. The DCPD data were corrected by the actual crack length and then the CGR was obtained from the relationship between the crack length and test time.

3. RESULTS AND DISCUSSION

3.1 Tensile Properties

Figure 2 shows yield strength and tensile strength at RT and 302°C of Ta-modified SS materials with SHT and with SHT + stabilizing heat treatment. The C contents of the Ta-modified materials were 0.01 wt% for TA, 0.029 wt% for TB, 0.046 wt% for TC and 0.029 wt% for TD. According to tensile test results, yield strength and tensile strength tended to increase with C content and such behavior is well known in metallurgy. If high yield strength and tensile strength are needed for a certain component, high C content is preferable. But stainless steel with high C content gets easily sensitized and then high Ta content is required to stabilize C in the matrix. It is necessary to optimize C and Ta contents in consideration of the requirements of material properties for the components.

3.2 Crevice Corrosion

Typical cross-sectional optical microscope observation results of the crevice corrosion test specimens are shown in Figure 3 at the location with the maximum depth of corrosion or cracking. Corrosion or cracks at the grain boundaries were observed in the four spot-welded specimens of Ta-modified materials. The

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maximum depths of corrosion or cracking were 33μm, 27μm and 14μm in TA, TB and TC, respectively. On the other hand, grain boundary cracks were observed in all four spot-welded specimens for type 316L SS and the maximum crack depth was 103μm. In some locations, grain detachment was also observed. Maximum depth and numbers of grain boundary (GB) corrosion sites or cracks are shown in Figure 4 for each specimen. In some TB and TC specimens, number of corrosion sites or cracks at grain boundaries was comparable to that in type 316L SS, but the maximum crack depth of Ta-modified SS specimens was significantly smaller than that of type 316L SS specimens. As a result of the test, crevice corrosion resistance of the Ta-modified SS seemed to be better than that of type 316L SS. In terms of the effects of Ta contents, the maximum depth tended to decrease with increasing Ta content among test specimens, although the number of corrosion sites or cracks showed a different trend. The effect of Ta content on crevice corrosion resistance was not clarified in this study.

3.3 SCC Initiation Susceptibility

Typical cross-sectional observations of the CBB test specimens are shown in Figure 5 at the location with the maximum crack depth. The presence and absence of SCC is shown in Table 7, which included the maximum crack depth when SCC occurred. SCC was observed in four specimens of type 316L SS out of seven specimens. The maximum crack depth was 62 μm. On the other hand, there was no SCC in the Ta-modified SS materials, TA, TB and TC. As a result of the CBB test, every type of Ta-modified SS had lower SCC initiation susceptibility and better SCC resistance than type 316L SS.

3.4 SCC Crack Growth Rate

Figure 6 shows the fracture surface of a CT specimen of type 316L SS after the CGR test. Notch area of the CT specimen, the fracture surface of a pre-crack formed in air, crack growth in the environment and coercive fracture were identified in the fracture surface. Average crack length of both the pre-crack formed in air and crack growth in the environment was calculated according to the crack surface area divided by the specimen width. Initial crack length was the sum of the notch depth and average crack length of the pre-crack formed in air. DCPD data were corrected by the initial crack length and the average crack length. The corrected DCPD data are shown in Figure 7. Cracks largely propagated in STEP 1 and crack propagation decreased with decreasing cyclic frequency in every specimen. In this CGR test, it was thought that the fracture surface was almost an intergranular crack in STEP 3 and it was a fully intergranular crack in the constant load. Then CGRs, which were calculated from crack propagation divided by time, are shown in Figure 8. Under the constant load, CGR of type 316L SS specimens was about 1x10-11 m/s, whereas that of TB specimens was about 1x10-13 m/s, which was two order of magnitude less than type 316L SS. There was no crack growth in TC and TD specimens. In STEP 3, CGR of type 316L SS was about 4x10-11 m/s, whereas that of Ta-modified SS specimens was about 7x10-12 m/s, which was one-fifth compared to type 316L SS.

Ta-modified SS materials had smaller CGR in the simulated BWR environment and better SCC resistance than type 316L SS. Ta contents of the Ta- modified SS materials used for the CGR test were 0.55 wt% for TB, 0.53 wt% for TC and 0.61% for TD. There was no effect of Ta content on CGR.

3.5 Discussion of Ta Effects on Corrosion Resistance

Ta contents of the Ta-modified SS materials in this study were from 0.13 wt% to 0.61 wt%. The test material with 0.13 wt% Ta (TA) had better crevice corrosion resistance and SCC resistance as well as the materials with 0.53 wt% Ta (TB) and 0.61 wt% Ta (TC) than type 316L SS. On the other hand, it has been reported that the repassivation potentials for crevice corrosion increased with Ta contents in Ni-based alloys, although the data were for Ta contents larger than 1 wt% [6]. Improvement of corrosion resistance of Ni-based alloys in hot sulfuric acids appeared for Ta content larger than 1.5 wt% but there was no improvement at 0.7 wt% [7]. It was assumed that Ta was introduced into the oxide film, which was stabilized to increase corrosion resistance [6, 7]. In this study, improvement of crevice corrosion resistance and SCC resistance were observed at lower Ta contents than the above references. But

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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dependence of Ta contents on corrosion resistance was not observed and a threshold Ta content could not be identified. Further investigation is needed to clarify the Ta effects and mechanisms of enhancing corrosion resistance.

In the Ta-modified SS, Si, P and S were lowered to improve IASCC resistance. Effects from lowering those element amounts on corrosion resistance are expected. It was reported that higher Si contents than 1 wt% elevated CGRs [13]. However, the effects of Si on corrosion resistance are not clear in the range of less than 0.5 wt%. S contents of the Ta-modified SS were almost equivalent to that of type 316L SS and there seemed to be no effects of S content on the results in this study. Although lowering P contents increased IASCC resistance in the range of less than 0.02 wt% [11], the effects of P contents on crevice corrosion resistance and SCC resistance under non-irradiated conditions are not clear. Therefore, the effects of decreasing Si, P and S contents on corrosion resistance are also not clear.

CONCLUSIONS

The crevice corrosion test, CBB test and CGR test were performed to investigate crevice corrosion and SCC resistances of Ta-modified SS in high temperature water in comparison with type 316L SS. The following results were obtained.

1) Yield strength and tensile strength tended to increase with C contents as is well known in metallurgy. It is necessary to optimize C and Ta contents in consideration of the requirements of material properties for the components.

2) Maximum crevice corrosion depths of Ta-modified SS material specimens were shallower than that of type 316L SS specimens. Numbers of crevice corrosion sites or cracks of Ta-modified SS specimens were also smaller than that of type 316L SS specimens.

3) There was no SCC in Ta-modified SS material specimens whereas type 316L SS specimens exhibited SCC during the CBB test. In the CGR test, CGRs of Ta-modified SS material specimens were less than one-fifth of that of type 316L SS specimens.

4) This study made it clear that Ta-modified SS had better crevice corrosion resistance and SCC resistance than type 316L SS. However, effects of Ta contents on crevice corrosion and SCC resistances were not clear. It is necessary to determine the mechanisms of effects of Ta addition on corrosion properties in a future study.

REFERENCES

[1] Scott, P. “A review of irradiation assisted stress corrosion cracking,” J. Nucl. Mater., Vol.211, Iss.2, 1994, pp.101-122.

[2] Bruemmer, S. M., Simonen, E. P., Scott, P. M., Andresen, P. L., Was G. S. and Nelson, J. L. “Radiation induced material changes and susceptibility to intergranular failure of light-water-reactor core internals,” J. Nucl. Mater., Vol.274, Iss.3, 1999, pp.299-314.

[3] Wass G. S and Andresen, P. L. “Stress corrosion cracking behavior of alloys in aggressive nuclear reactor core environments,” Corrosion, Vol.63, No.1, 2007, pp.19-45.

[4] Kato, T., Takahashi, H. and Izumiya, M. “Grain boundary segregation under electron irradiation in austenitic stainless steels modified with oversized elements,” J. Nucl. Mater., Vol.189, Iss.2, 1992, pp.167-174.

[5] Shigeneka, N., Ono, S., Isobe, Y., Hashimoto, T., Fujimoris H. and Uchida, S. “Effect of zirconium addition to austenitic stainless steels on suppression of radiation induced chromium segregation at grain boundaries under ion irradiation,” J. Nucl. Sci. Tech., Vol.33, Iss.7, 1996, pp.577-581.

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[6] Sakai, H., Sugawara K. and Saito, H. “Effect of Ta addition on crevice corrosion resistance for Ni based corrosion resistant alloys,” Zairyo-to-Kankyo, Vol.59, No.6, 2010, pp.222-227.

[7] Sugawaras K. and Takizawa, Y. “The corrosion behavior of Ni-Cr-Mo ternary alloys in hot concentrated sulfuric acids with active carbon (Part 3) -The roles of small addition of tantalum to Ni-Cr-Mo alloys-,” Zairyo-to-Kankyo, Vol.47, No.4, 1998, pp.239-245.

[8] Kawamori, M., Kinugawa, J., Yonenaga, Y., Fukuda, Y., Shimamoto, M., Sugimura, T., Sato, T., Nishizawa, N. and Nagao, M. “Effect of tantalum addition on inclusion formation and pitting corrosion resistance of super duplex stainless steels,” 227th ECS Meeting, May 24-28, 2015, Chicago, IL.

[9] Kawamori, M., Kinugawa, J., Yonenaga, Y. Fukuda, Y., Shimamoto, M., Sugimura, T., Sato, T., Nishizawa, N. and Nagao, M. Zairyo-to-Kankyo, 2015, B-310

[10] Fukuya, K., Shima, S., Nakata, N., Kasahara, S., Jacobs, A.J., Wozadlo, G.P., Suzuki, S. and Kitamura, M. “Mechanical properties and IASCC susceptibility in irradiated stainless steels,” Proceedings of the Sixth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, August 1-5, San Diego, CA, The Minerals, Metals & Materials Society, 1993, p. 565-572.

[11] Kasahara, S., Nakata, K., Jacobs, A.J., Wozadlo, G.P., Fukuya, K., Shima, S. and Suzuki, S. “The effects of minor elements on IASCC susceptibility in austenitic stainless steels irradiated with neutrons,” Proceedings of the Sixth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, August 1-5, San Diego, CA, The Minerals, Metals & Materials Society, 1993, p. 615-622.

[12] Kowaka, M., Kudo, T. and Ota, K. “Protection of polythionic acidic stress corrosion cracking on stainless steel,” The Sumitomo Search, No.18, November 1977.

[13] Andresen, P. L. and Morra, M.M. “Effects of Si on SCC of irradiated and unirradiated stainless steels and nickel alloys,” Proceedings of the 12th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, August 14-18, Salt Lake City, UT, The Minerals, Metals & Materials Society, 2005, pp.87-106.

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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Table 1 Chemical compositions of test materials (wt%)

ID C Si Mn P S Ni Cr Mo Ta Nb N Ta/C

TA 0.01 0.01 1.8 0.005 0.003 10.68 19.0 0.02 0.13 0.008 0.008 13.00

TB 0.029 0.02 1.7 0.001 0.001 10.67 18.94 0.02 0.53 0.002 <0.01 18.28

TC 0.046 0.01 1.7 0.004 0.003 10.7 19.2 0.02 0.61 0.008 0.007 13.26

TD 0.029 0.02 1.6 0.001 0.001 9.11 19.34 0.02 0.55 0.001 <0.01 18.97

316L 0.005 0.51 0.83 0.014 0.001 12.13 17.36 2.12 - - - -

Fe: Balance

Table 2 Solution heat treatment conditions

ID SHT conditions

TA、TC 1050 ºC / 1h / WQa

TB 1050 ºC/ 30min / WQ

TD 1050 ºC/ 50min / WQ

316L 1050 ºC/ 15min / WQ a WQ: water quench

Table 3 Crevice corrosion test conditions

Items Conditions

Temperature 288 ºC

Pressure 8 MPa

Dissolved oxygen (DO) 8 ppm

Sulfate ion 50 ppb

Conductivity 1.0 - 1.2 μS/cm

γ-ray dose rate 20 kGy/h

Test time 1000 h

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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Table 4 CBB test conditions

Items Conditions

Temperature 288 ºC

Pressure 8.2 MPa

Dissolved oxygen (DO) 30 - 35 ppm

Sulfate ion 50 ppb

Conductivity 1.0 μS/cm

Test time 500 h

Table 5 CGR test conditions

Items Conditions

Temperature 288 ºC

ECP ≥ 150 mV SHE

Conductivity Inlet: ≤ 0.1 μS/cm, Outlet: ≤ 0.2 μS/cm

Ion concentration Sulfate ion: ≤ 5ppb, Muriatic acid: ≤ 3ppb, Nitrate ion: ≤ 3ppb

Target K 15 MPa√m

Table 6 Loading conditions of CGR test

Steps Conditions

STEP 1 R=0.6, 0.01Hz

STEP 2 R=0.6, 0.001Hz

STEP 3 R=0.6, 0.0001Hz

Table 7 CBB test results

Test materials No.1 No.2 No.3 No.4 No.5 No.6 No.7

TA No crack No crack No crack No crack No crack No crack No crack

TB No crack No crack No crack No crack No crack No crack No crack

TC No crack No crack No crack No crack No crack No crack No crack

316L No crack Cracked

(62μm)

Cracked

(60μm)

Cracked

(28μm)

Cracked

(46μm) No crack No crack

(Numbers in parentheses): maximum crack depth

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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Figure 1 Crevice corrosion test specimen (unit: mm)

Figure 2 Yield strength and tensile strength at RT and 302 ºC of Ta-modified SS test materials (STA: Stabilizing heat treatment)

7.5 7.5

50

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351.5

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Fig

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(a) Maximum depth (b) Number

Figure 4 Maximum depth and number of corrosion sites or cracks at the grain boundaries

Figure 5 Cross-sectional observation of CBB test specimens

(b) TB No.2 (a) TA No.2

(c) TC No.2

(d) 316L No.2 (e) 316L No.3

(f) 316L No.4 (g) 316L No.5

Observed surface

CBB specimen cut in the longitudinal direction

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Figure 6 Fracture surface of CT specimen of type 316L SS

Figure 7 Crack length versus test time of CT specimens

Notch of CT specimen

Pre-crack in air

Crack in the environment

Coercive fracture

Specimen width

12.8

13.0

13.2

13.4

13.6

13.8

14.0

0 500 1000 1500

Cra

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STEP 1 STEP 2 STEP 3 Constant Load

TB-2

TB-1

316L-2

316L-1

TC

TD

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Figure 8 CGRs under constant load and cyclic load condition of STEP 3

1.E-13

1.E-12

1.E-11

1.E-10

TB-1 TB-2 TC TD 316L-1 316L-2

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