Effects of Electrochemical Parameters on SCC of Stainless Steels in Simulated BWR Environments
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Zhanpeng Lu, Tetsuo Shoji, Kazuhiko Sakaguchi, Fanjiang Meng, Yubing QiuFracture and Reliability Research Institute, Tohoku University
1. Introduction
Stress corrosion cracking (SCC) has been found in boiling water reactor components fabricated from various grades of stainless steels such as 304 stainless steel (SS), low-carbon stainless steels, N-containing stainless steels, and stabilized stainless steel [1-5]. Environmental parameters such as dissolved oxygen (DO) concentration, dissolved hydrogen (DH) concentration and the resultant electrochemical potential (ECP) have been found to have a strong effect on SCC of sensitized stainless steel in high temperature pure water. Decreasing potential by using hydrogen water chemistry (HWC) and later noble metal chemical addition has been found to be effective in mitigating SCC of sensitized 304SS [6]. Recently SCC has been found in BWR components such as reactor core shroud and primary loop-recirculation piping made of low-carbon grade stainless steels. Cracking of stabilized stainless steel in a BWR plant was also reported. Investigating the effects of environmental parameters on SCC of various stainless steels would provide information for understanding the elementary cracking process and for evaluating the mitigation effect of modifying water chemistry. Experiments were designed and performed and the obtained data are compared with reported data.Address: Fracture and Reliability Research Institute, Tohoku University, Aa 980-8579 Aoba 6-6-1, Aramaki, Aoba-ku, Sendai, Japan. E-mail: zhanpeng@mainte.ac.jp
2. Experimental procedures
A three-directionally (3D) 10% cold-rolled 316NG SS was used for SCC growth rate tests. The solution-annealed 316NG was cold-rolled at three directions at RT. A 2.5% reduction in thickness was achieved by the first-pass rolling. The second-pass rolling was performed perpendicular to the first rolling direction on the same plane to achieve another 2.5% reduction in thickness. The third-pass rolling to obtain a 5% reduction in thickness was performed parallel to the second rolling direction on the plane perpendicular to that used for the first and the second rolling processes. Each pass were accomplished by multiple steps to get relatively uniform deformation along the plate thickness direction. Measurements with Fischer FERITSCOPE Ferritescope indicate a low (<1%) martensite content in the cold-rolled 316NG SS. The yield strength of the 3D cold-rolled 316NG SS is 484 MPa at RT and 354 MPa at 300A°C. Contoured double cantilever beam (CDCB) specimens were used for SCC growth rate tests. The notch direction of the specimen is parallel to the final (the third) rolling direction, which is called specimen T-L orientation. The procedures for the precracking in air, in situ precracking, and SCC tests were similar to those described previously [7]. SCC tests were performed in 288A°C pure water with different dissolved oxygen (DO) and dissolved hydrogen (DH) concentrations: 1) DO=2 ppm, DH_0; 2) DO<5 ppb, DHa?“0; 3) DO<5 ppb, DH-1.4 ppm. During the SCC tests, the electrochemical potentials (ECP) of specimens were measured against a pressure-balanced external Ag-AgCl (0.1 mol/L KCl) reference electrode. The SCC test conditions were only changed after the preceding steady-state crack growth rate established from the time evolution of the ACPD data. The oxide films formed on sensitized 304SS and 316L SS after immersion in 2ppm DO pure water at 288A°C were measured by XPS.
3. Results and DiscussionObtained CGR data and literature CGR data for various stainless steels in simulated BWR water as functions of electrochemical potential are summarized. ECP is adjusted by controlling DO or sometimes H2O2 and DH contents.3.1 The effect of ECP on sensitized 304SSSCC growth rates vs. ECP for sensitized 304SS in 288A°C pure water [1, 4, 8] are shown in Fig. 1. A parameter called mitigation factor, Fm, is used to quantify the effect of changing ECP from ECP1 to ECP2 on CGR, Eq. (1).Fm(ECP1/ECP2)=CGR(ECP1)/CGR(ECP2)
Decreasing electrochemical potential has a strong effect on lowering crack growth. For example, FM(0.15VSHE/-0.5V SHE) is several tenths and can reach about 100. CGR at ECP less than -0.23VSHE is significantly lower than that at high potentials of 0.15 to 0.2 VSHE [1]. The threshold ECP for mitigating SCC of sensitized 304SS in simulated BWR environments has been reported. Similar ECP threshold of about -0.23 Vshe for corrosion fatigue of low alloy steel in simulated BWR environments was also reported [9].
Pure water at 288““CSens. 3045S, Andreson et al., JNM 2008, K=27.5 MPa ms O Sens. 304SS, Kikuchi et al., Corrosion, 1997, K(Initial) -31MPa mas
Crack growth rate, CGR(m/s)
Electrochemical potential, E(V...SHEFig. 1 Dependence of CGR on ECP for sensitized 304SS in pure water at 288A°C.3.2. The effect of ECP on non-sensitized SS 3.2.1 As-solution annealed 316L(NG) SS SCC growth rates vs. ECP for solution-annealed low-carbon stainless steels in 288A°C pure water [10] are shown in Fig. 2. Crack growth rates of SA 316L and SA 316NG SS decrease drastically with decreasing potential, which are lower than 3x10-12 m/s at ECP lower than -0.230V. At medium potential of -0.1 VSHe, crack growth rates of SA low-C SS are low, close or lower than 2x10-12 m/s. The threshold ECP of about -0.23VSHE for mitigating SCC seems also applicable to low-C SS.
SHE
Pure water at 288““C, K-30 MPa m316NG SA, Itow et al., PVP2004 316L SA, Itow et al., PVP 2004
Crack growth rate, CGR(m/s)
0.0 Electrochemical potential, E(V.SHEFig. 2 Dependence of CGR on ECP for SA low-C stainless steels and 316L HAZ in pure water at 288A°C [10].3.2.2 Strain-hardened low-C SS The ACPD results for 10% 3D CW 316NG SS (L-T orientation) under constant loading in 288A°C pure water were calibrated with the measured average crack length on the fracture along the main crack path. The results at different DO and DH concentrations are shown in Fig. 3.
3D 10% CW 316NG SS, (L-T), 288A°C pure water AS22A04, 2ppm DO, K=20MPa.moS22A05, <5ppb DO, K=20MPa.mos S22A06, H-saturated, K=20MPa.m.DO<5 ppb, H, -saturatedDO<5 ppbCrack length, a(mm)2ppm DO
3500 4000 Test time, t(h)Fig. 3 Average crack length vs. test time for 10% 3D CW 316NG SS (L-T orientation) under constant loading in 288A°C pure water.Well-behaved crack growth was observed in 2ppm DO water, in deoxygenated water and in hydrogen-saturated water. CGR decreases significantly with decreasing ECP as the results of removing DO or adding DH. CGRs are calculated and plotted in Fig. 4 along with other reported data for strain-hardened low-C SS. The mitigation factor FM(0.2 Vshe/-0.57 VSHE) for 3D CW316NG is about 10, which is close to the results reported by Andresen [] for 20% cool-worked 316L SS under similar applied loading level but less than that for sensitized 304SS.
Pure water at 288A°C Toloczko et al, 13th ED, 20073 Cold-work 316SS (Hat #1). K: 27.2 MPa mosO Cool-work 316LSS (heat #2), K: 28.5 MPa mos Andresen et al, J Nucl. Mater., 2008A Cool-work 316LSS (heat 2).. K: 32.5 MPa mos FRI data3D cold-work 316NG (L-T orientation), K: 20 MPa mo
Crack growth rate, CGR(m/s)
-0.4 -0.2 0.0 0.2 Electrochemical potential, ECP(VSHE)
Fig. 4 Dependence of CGR on ECP for strain-hardened low-C stainless steels in pure water at 288A°C [4, 11].As shown in Fig. 4, Toloczko et al [11] has reported that the mitigation factor FM is strongly dependent on the grain boundary properties, yield strength as the results of cold work and applied load level. FM (2ppm DO/63ppb DH) for 20% cold worked heat #1 316SS (with 0.06 wt% C, little or no significant Mo-segregation at grain boundaries) is about 100, which is much higher than that for 21% cool-worked (at 140A°C) heat #2 316L SS (with 0.014 wt% C, >10% Mo segregation at grain boundaries). FM for the later is about 3 or 4 at stress intensity factor between 25 and 30 MPa mo., which becomes only 1.4 if K is in the range of 35 to 42 MPa mo:). The decreasing of Fm with increasing K also shows the strong mechanochemical interaction in the crack tip oxidation kinetics3.2.3 Ni-base alloy and weld metal Several typical sets of CGR vs. ECP data for Ni-base alloy 600 and Alloy 182 weld metal in 288A°C pure water are plotted in Fig. 5. Similar to the results of low-C stainless steel, decreasing ECP decreases CGR. FM is high for Ni-base alloys without cold work. Cold worked Alloy 600 still exhibit a high CGR of 3.2x10'' m/s at a low ECP of about -0.54 VSHE
Pure water at 288A°C Andresen et al., NACE 2002, J. Nuclear. Mater., 2008SA+20% CW Alloy 600, K: 30 MPa mosSens. + 20pctCW Alloy 600, K: 30 MPa mos A As-welded Alloy 182, K: 28.4 MPa mosSensitized Alloy 182, K:30 MPa mos itow et al., 8th Env. Ded., 1997PWHT Alloy 182, K: 36.5-41.5 MPa mos O PWHT Alloy 182, K: 34.9-38.0 MPa mos
Crack growth rate, CGR(m/s)
-0.4 -0.2 0.0 0.2 Electrochemical potential, ECP(VsHe)
Fig. 5 Dependence of CGR on ECP for Alloy 600 and Alloy 182 in pure water at 288A°C [4, 10].
3.4. Analysis of the effect of environmental parameters Results in Figs. 1-5 imply that the effects of environmental parameters such as ECP become less significant if alloys are heavily cold worked or highly loaded. The crack growth can be viewed as a mechanochemical oxidation process. Surface oxide film plays an important role in SCC processes. The XPS results of oxide films formed on sensitized 304SS and 316L SS in 288A°C pure water with 2ppm DO are shown in Figs. 6a and 6b. There is no significant difference in the values of thickness of oxide films formed on sensitized 304SS and SA 316L SS in oxygenated high temperature pure water. There is a trend that the outer oxide layer of SA 316L SS contains more nickel while has less iron. Since the data are obtained on bulk surface, more systematic analysis of local oxidation behavior especially at grain boundaries would give more insight for the environmental parameters on SCC.
Sensitized 304 SS288A°C pure water, 2ppm DO [ 300 h immmersionz u oooAtomic Concentration (%)
a) Sensitized 3045SSA 316L SS 288A°C pure water, 2ppm DO 300 h immersionAtomic Concentration (%)
150 200 Sputter Time, t (min)b) SA 316L SS Fig. 6 XPS results of oxide films on a) sensitized 304SS, and b) SA 316L SS after 300h immersion 288A°C pure water with 2ppm DO.The schematic of a stress corrosion cracking system is shown in Fig. 7.
Bulk solution Bks) -----------Bulk solution(BKS) Voks Crack mouth(CKM) SCEN /Bulk oxide(BOX)Bulk metal(BM) -Wall metal(WM) Wall oxide(WOX) o.
* Crack tip(CT)Tip solution(TS) Tip oxide(TOX) Tip metal (TM)Fig. 7 Schematic of a stress corrosion cracking system with different interfaces.da - K. (Ed)““LCrack growth rates of austenitic alloys in high temperature water can be expressed by the following equation.(2) dt Where a is the crack length, da/dt is the crack growth rate, ka is crack tip oxidation rate constant, act is crack tip strain rate, m is the slope of the oxidation rate decay curve. For slip-dissolution/oxidation mechanism, there is M ,1, Irto)““(3) ka 7.p.F.(1a?“ m) Es For solid state oxidation mechanism, there is ka = [(,)(1-m). (g)(-m ]Results in Figs. 1-5 indicate that the effect of ECP on oxidation rate constant ka is also related to the material property such as the degree of cold work. According to slip-dissolution/oxidation mechanism, m is also function of ECP. In solid-state oxidation mechanism, it is expected that ECP would affect more the absolute value of oxidation rate than the oxidation rate law, i.e., ECP would have more significant effect on k1 than on m. Oxidation rate constant k. Oxidation kinetics at the SCC tip can be viewed as a quasi-solid state oxidation kinetics where the mass transport in the solid oxide film, the mass transport inside the crack enclave and surface electrochemistry would play important roles in determining the reaction rate. A. Turnbull [12] proposed the general equation for the mass transport in a SCC crack, Eq. (5). OC - OE≫c;, z;D;F O OOO i ax? RT ox““: Ox'(C; 2) + A? R1,,homo +? or (5)1, j, heteroWhere Rij,hom is homogeneous equilibrium reaction process that can occur in the solution in the crack, Rij.bet are heterogeneous reactions processes that can occur on the walls of the crack (including redox reactions and interfacial precipitation reactions), & is a half of the crack width at a distance x from the crack mouth. The interaction between crack tip mechanics and crack tip oxidation kinetics is also revealed by the different responses of CGR of cold-worked and non-cold workedto changing ECP by adjust DO or DHstainless steels to changing ECP by adjust DO or DH concentrations.4. Conclusions The effects of electrochemical potential on SCC growth rates of various austenitic SS are analyzed based on obtained experimental results and reported data. The effects of electrochemical conditions would become less significant if alloys are heavily cold-worked. The results provide crucial information for understanding mechanochemical interactions involved in SCC of materials in high temperature water and for selecting mitigation methods.Acknowledgements This work has been performed as a part of the PEACE-E program jointly supported EDF, EPRI, SSM, TEPCO, KEPCO, TohokuEPCO, ChubuEPCO, JAPCO, HITACHI Ltd., MHI, TOSHIBA Co., and IHI. This work has been also performed under the support of Grant-in-Aid for Scientific Research (S) 17106002 and (C) 20560063, Japan Society for the Promotion of Science.References [1] P.L. Andresen, F.P. Ford, Mater. Sci. Eng. A 103(1988) 167. [2] S. Suzuki, K. Kumagayi, C. Shitara, et al.,Maintenology 3 (2004) 65. [3] T. Shoji, Proc. 11th Int. Symp. Environ. Degradationof Materials in Nuclear Power Systems-Water Reactors, held August 10-14, 2003, Skamania,Stevenson, Washington, ANS, 2003, pp. 588a?“598. [4] P.L. Andresen, M.M. Morra, J. Nucl. Mater. 383(2008) 97a?“111. [5] U. Ehrnsten, H. Hanninen, P. Aaltonen, et al., Proc.10th Int. Symp. Environmental Degradation of Materials in Nuclear Power Systems-WaterReactors, 2001, NACE, CDROM. [6] Y.J. Kim and P.L. Andresen, Corrosion 59 (2003)511. [7] Z.P. Lu, T. Shoji, Y. Takeda, et al., Corros. Sci. 50(2008) 698. [8] E. Kikuchi, M. Itow, J. Kuniya, H. Sakamoto, M.Yamamoto, A. Sudo, S. Suzuki and M. Kitamura,Corrosion 53 (1997) 306-311. [9] J. Congleton, T. Shoji, R.N. Parkins. Cor. Sci.,25(1985) 633. [10] M. Itow, M. Kikuchi, N. Tanaka, et al., Proc. 2004ASME Pressure Vessels and Piping DivisionConference, ASME, 2004, pp. 167-173. [11] M. Toloczko, M., S.M. Bruemmer, P.L. Andresen,Proc. 13th Int. Symp. Environmental Degradation of Materials in Nuclear Power Systems-WaterReactors, 2007. [12] A. Turnbull. Corrros. Sci., 39(1997)789.“ “Effects of Electrochemical Parameters on SCC of Stainless Steels inSimulated BWR Environments“ “Zhanpeng Lu, Tetsuo Shoji, Kazuhiko Sakaguchi, Fanjiang Meng, Yubing Qiu