Electrochemical response of scratched Alloy 600 in simulated primary water and its influence with hydrogen entry
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Fracture and Reliability Research Institute, Tohoku University State Key Laboratory for Corrosion and Protection, Institute of Metal Research, China
Fanjiang Meng Fracture and Reliability Research Institute, Tohoku University
Zhanpeng Lu Fracture and Reliability Research Institute, Tohoku University
Tetsuo Shoji State Key Laboratory for Corrosion and Protection, Institute of Metal Research, China
Jianqiu Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, China
En-Hou Han
ntroduction Introduction Nickel-based Alloy 600, as a material of steam Lerator tubing, has been studied for nearly 40 years. This by is known for its susceptibility to intergranular stress rosion cracking (SCC) in high-temperature PWR water. cently, field SCC cases indicate that the dominant ure mechanism has been IGA and SCC of free-span faces, especially at abnormal surface conditions such as face dents or scratches which were produced during erting tubes into the steam generator tube support sheet. matome ANP reported that SCC developed in the face marks/scores of Alloy 600, introduced into the ernal surfaces of split tube sections prior to bending [1].ress: Fracture and Reliability Research Institute, Tohoku University, B0-8579 Aoba 6-6-1, Aramaki, Aoba-ku, Sendai, Japan, ail: fjmeng@mech.rift.tohoku.ac.jperal tubes in Unit-1 and Unit-2 of Oconee Nuclear tion were removed after eddy current detection and idual compressive stress existed on the scratch. Burst ting indicated that cracking related to surface scratch.C initiation which associated with a surface scratch at espan cold leg arose at McGuire-1 and McGuire-2 in Loy 600MA tube after only several years' service [244]. previous work, the author found scratch-induced SCC tiated in caustic lead containing solution at 330A°C [5].These cases mentioned above implied that local chanical marks/scratches could be latent sites to initiate C. It was reported that the typical scratch surface and cer surface were exposed in the same environment and emical analyses showed no difference [6]. As with many terials, cold work, similar effect of scratch, increases the e of SCC and cold worked nickel alloys was supposed Crap more hydrogen in the stress field of dislocations [7]. drogen ingress can occur via corrosion processes and/or solved hydrogen in environment, can produce various ects on their properties after entering into matrix.The presence of scratches is inevitable. In PWR mary water, high concentration of dissolved hydrogen,
which maintains a lower H2O/H2, and high temperature make hydrogen permeation easier. In order to find a guidance to explain the corrosion behavior of scratched Alloy 600, the combined effect of scratches and hydrogen was studied by performing electrochemical tests.2. Experimental process 2.1 MaterialNickel base Alloy 600 was used for this study. This Alloy was heat treated at 606-616A°C for 10h 5min and then air cooled. The chemical composition of this Alloy 600 is listed in Table 1.The specimens were cut from bulk Alloy 600 by 12x12x3 mm““. The larger faces of sheet specimens were ground with emery paper up to 2400 grit and mechanically polished using lum silica paste to get a mirror finish. Some of the specimens were chose to make some scratches (about 120um in width, 20um) by using a cone hard alloy head. And then, all the specimens were soldered with high purity Ni wires with diameter of 0.5mm, which covered with two layer shrinkable PTFE tube with aim to increase insulation.lement wt.%C 0.059Table 1 Compositions of Alloy 600 Si Mn P S Ni Cu CrFe Nb 0.13 0.21 0.006 0.001 74.19 0.12 15.57 9.41 0.05The microstructure of this heat after electro-polish was characterized using electron backscatter diffraction (EBSD) as shown in Fig. 1. EBSD measurement was conducted on Hitachi S-4300 FE-SEM, which equipped with solutions camera control system made by TSL, Co. Ltd. The data was collected and analyzed by OIMTM Software. The grains were about 80um.FUYUMFig. 1. EBSD observation of Alloy 6002. Water chemistry controlAll the test were performed in the facility of autoclave bricated by Toshin Kogyo Co., LTD, Japan, with a -circulating water loop to maintain the desired primary ater chemistry conditions. The simulated primary water as prepared with 1200ppm B as H3B04 and 2ppm Li as IOH. The inlet dissolved hydrogen (DH) was controlled = 30cc/kg and test temperature was 290A°C.-3. Hydrogen charging and electrochemical testThe specimens for electrochemical tests were listed in able 2.Table 2 Test detail of Alloy 600 SpecimennChargingTest contentPlainNoEIS, AP,ScratchedNoEIS, AP,PlainYes, 86.5hEIS, AP,ScratchedYes, 86.5hEIS, AP,* AP: Anodic polarization.A three-electrode system was used, which concluded he external pressure balanced calomel electrode SCE 0.1N) acted as a reference electrode, a platinum wire as a counter electrode and work electrode. All the measured potentials were transferred to potentials referred to tandard hydrogen electrode. The potentials of Alloy 600 vere monitored with POTENTIOSTAT/GALVANOSTAT HA-151A. EIS was measured by using Solartron mpedance/gain-phase analyzer SI 1260 every two days until the corrosion potential of all the specimens became relatively stable. Some of the specimens were anodic polarized using Solartron electrochemical interface SI 1287.After this, some specimens were chosen to charge hydrogen cathodically in the simulated primary water. The Charged current density was controlled at a small constant of 0.35mA/cm' at 290A°C, which was in order to minimize Ehe damage of cathodic hydrogen charging to polished and scratched Alloy 600 surface. The total hydrogen charging time was 86.5h for each specimen. Anodic polarization was performed for the charged Alloy 600 after for 4h later.Time dependent potential of Alloy 600 in simulated primary water containing 1200ppm B and 2ppm Li with DH of 30cc/kg at 290A°CPlain specimen Specimen with scratchesPotential(SCE0 50 100 150 200 250 300 350Immersion time (h) Fig.2. Time dependent potential of Alloy 600 in simulated primary water at room temperature, during heating up and at 290A°C3. Results and discussion 3.1. The effect of scratches on corrosion of Alloy 600 3.1.1. Oxidation of scratched Alloy 600 by EIS monitoringThe time dependent potentials for plain and scratched Alloy 600 were shown in Fig.2. For these two specimens, the potentials all dropped negatively during the whole monitoring process. At the start of the test, the potentials315shifted negative dramatically. It took about 7 days to reach a stable value in the simulated primary water with a DH 30cc/kg, which means that oxide formation and matrix dissolution needed longer time to get a balance. Measurement of anodic polarization in less than 7 days' immersion may not reflect the electrochemical behavior of long-term service of Alloy 600. However, there seems to be no difference for the corrosion potential between scratched and plain specimens.During potential monitoring, EISs both for plain (Fig.3a) and scratched (Fig.3b) Alloy 600 were traced every two days. For the low frequencies impedance arc in Fig.3 which stands for the electric double layer, there were no obvious differences between plain and scratched Alloy 600 with increasing immersion time.-2500EIS evolution of plain Alloy 600 in 290A°C simulated primary water with DH content of 30cc/kg-2000-15009 dayZimgrohm-cmi5““day-5001““ day5001000150020002500300035004000Zre(ohm cm') (a) Plain Alloy 600-2500EIS evolution of scratched Alloy 600 in 290A°C simulated primary water with DH content of 30cc/kg-2000davlohm.cm3 day““dayOL50010001500250030003500400045002000Zre(ohm-cm')(b) Scratched Alloy 600 Fig.3. Evolution of Nyquist diagrams for plain and scratched Alloy 600 in simulated primary waterHowever, significant difference can be found in the high and intermediate frequencies impedance arc as shown in Figs.3a and 3b. This part of impedance arc can reflect characters of oxide film. It is noteworthy that there is no persistent increase of imaginary part during the evolution of oxide film in the simulated primary water. The imaginary part in high and intermediate frequencies dropped after 5 days' immersion. Oxides became more stable from 7th day and Nyquist diagram did not change a lot from that time on. This is consistent with the result of potential monitoring.3.1.2. Anodic behavior of scratched Alloy 600The polarization curves for plain and scratched Alloy 600 were shown in Fig.4. Even though Alloy 600 was scratched, there was no apparent effect on anodic behavior com- pared to plain Alloy 600. The identical character may be caused by dissolved hydrogen. The dissolved hydrogen can easily enter into the alloy 600 matrix [7]. Because of the uniform hydrogen in the aqueous environment, surface distribution of hydrogen permeated into the oxide film and matrix was supposed to be homogeneous in both kinds of specimens. At corrosion potential, dissolved hydrogen can partly reduce the surface film and the film toward corrosion for both scratched and plain Alloy 600. The high anodic current also suggests that matrix dissolution was not restrained even though there covered oxide film on the surface. Scratching brought about a heavy plastic deformation to the surface, but dissolved hydrogen as the leading role, suppressed the effect of scratches on the surface.Anodic polarization curves for scratched and plain Alloy 600 in simulated primary water at 290A°CPlain specimen Specimen with scratches210fPotential(VSHP)0.0000001E-6 5 E- 1 E-4Current density. A cm? Fig. 4. Anodic polarization curves for plain and scratched Alloy 600 in simulated water containing 1200 ppm B and 2 ppm Li at 290A°C.A decrease of current density from about +0.2V/SHE to +0.6V/SHE was observed, which is not a real passivation but a pseudo-passivation. This decrease of current density indicates that there was pre-passivation process during anodic scanning below this potential range. The competition probably occurs between matrix dissolution and film formation at anodic potentials. When the electrochemical reaction of film formation is more rapid than matrix dissolution, pre-passivation below +0.2V/SHE and pseudo-passivation processes take place as presented in Fig. 4.The film formation can be contributed to some possible reasons. At corrosion potential, Ni is stable under a DH of 30cc/kg at 290A°C [8] and the oxide film formed on Alloy 600 was rich in Fe and Cr [9]. According to potential-pH diagram of Ni, Cr, Fe in high temperature [10], at anodic potentials in studied environment (pH=7), only Ni can be oxidized as Nio and Cr and Fe are stable oxides as Cr2O3 and Fe2O3, respectively. Oxides such as NiCr204 and NiFe2O4 may form on the surface during anodic scanning. With increasing anodic potential, the protective of film become more compact, which induced a pseudopassivation zone. Over +0.6V/SHE, oxides dissolution or316oxygen evolution may start.3.2. The electrochemical response of Alloy 600 to charged hydrogenAfter hydrogen charged in simulated primary water, Fig.5 shows the anodic polarization curves obtained from plain Alloy 600. To some extent, the current density at pseudo-passivation zone for the charged specimen increased, i.e., the formation of compact oxide film, which has been discussed in Section 3.1.2, was retarded.Anodic polarization curves for charged and uncharged Alloy 600 in simulated primary water at 290-C--Plain specimen, uncharged -- Plain specimen, precharged for 86.5hPotential(VSME)1E- 7 1E-6 E-5 1E-4 1E-3Current density, A/cm? Fig.5. Anodic polarization curves for plain Alloy 600 after charging in simulated water containing 1200 ppm B and 2 ppm Li at 290A°C.Corrosion pit15.0KV 15.2mm X250 SE(M)200 umFig.6. SEM micrograph revealing the surface corrosion pit of 86.5h hydrogen charged plain Alloy 600 after anodic polarizationThis phenomenon can be explained by the permeated hydrogen during charging process in the oxide and Alloy 600 matrix. The hydrogen entered into the matrix is divided into two parts: the reversible hydrogen and irreversible hydrogen. According to M.Uhlemann [7], carbide/carbonitride precipitates were principal irreversible traps and hydrogen reversibly trapped in the interstitial atoms could release at a temperature higher than 250A°C. There are chromium carbides or titanium nitrides existed in nickel alloys. Hydrogen can be trapped in these precipitates even at 290A°C, which results in local higher hydrogen concentration than that of uncharged Alloy 600.Fig.6 depicts the optical micrograph of hydrogen charged Alloy 600 after anodic polarization in simulated high temperature primary water. It was found that a corrosion pit appeared on the surface. This extrairreversible hydrogen could slow down the formation of oxide film and accelerate matrix dissolution.3.3. The combined effect of scratches and charged hydrogenAnodic polarization curve for scratched Alloy 600 (Fig. 7) was measured after hydrogen charging for 86.5h in simulated primary water, too. The pseudo-passivation zone over +0.3V/SHE after hydrogen charging, compared to the uncharged but scratched one, disappeared.2Anodic polarization curves for seratched Alloy 600 charged and | uncharged in simulated primary water at 290 C1.5 HScratched and uncharged Scratched and precharged 86.5hPotentialVSHE)0.0000011E- 40.001E- 5Current density, A/cm2 Fig. 7. Anodic polarization curves for scratched Alloy 600 after hydrogen charging in simulated water containing 1200 ppm B and 2 ppm Li at 290A°C.orrosiony pitsCorrosios pits15.OKV 15.1mm x300 SEcum(a)Top view of a scratchCorrosion pit!!!1900/01/1415,1mm' 42.00K SEM20.00m(b) Marked area in (a) with a high magnification Fig. 8. Surface observation of 86.5h hydrogen charged Alloy 600 with scratches after anodic polarizationIn the test condition, charged hydrogen in the interstitial atoms or the stress field of dislocation already released at 290A°C. Cold worked Alloy 600 can trap more hydrogen in the matrix since the existence of microvoids317as irreversible traps [7, 11]. Scratches have a similar effect. For nickel based alloy, scratching the surface introduced many micro-voids and caused a heavy deformational layer at the scratch groove which has been investigated by F. Meng et al. [5]. Amounts of irreversible hydrogen were trapped in the deformed layer around the scratch groove.The anodic current over +0.2V/SHE for scratched Alloy 600 was not related to increased potential, which implies that anodic dissolution was controlled by diffusion. The irreversibly trapped hydrogen in cold worked Alloy 600 can be as high as 6ppm at this test temperature [7], which was higher than the dissolved hydrogen (about 2.67ppm) in the simulated primary water. The irreversible hydrogen was sufficient to inhibit formation of oxide film such as NiCr204 and NiFe204 and quickened anodic dissolution by oxidizing (Fe,Cr)203 and Cr2O3. As displayed in Fig.8, a lot of corrosion pits appeared at two banks of scratch and the scratch groove. The pits illuminate that the protective chromium oxide was also dissolved.According to B.T. Lu et al. [12], the synergistic effect due to the interaction of charged hydrogen and local stress field is negligible. In present test, the superimposition of dissolved hydrogen in the primary water and scratch-induced local stress field can be ignored as indicated in Fig.4. However, the charged irreversible hydrogen caused by scratching have a strongly interaction with active dissolution of Alloy 600.4. ConclusionThe effect of scratches on electrochemical behavior of Alloy 600 and its influence with hydrogen entry was investigated in simulated primary water at 290A°D!. Some conclusions could be drawn as following:I. Oxide film reached a stable condition until 7 days' immersion according evolution of corrosion potentials and EIS. The scratched Alloy 600 possessed almost the same evolution of oxide film with plain specimens.II. Scratched Alloy 600 did not change the anodic behavior at all. Uniform dissolved hydrogen plays a leading role in matrix dissolution in simulated primary water and may suppress effect of scratches.III. The anodic current changes of scratched Alloy 600 after hydrogen charging were not the simple superimposition between that of scratched and charged plain Alloy 600. The irreversibly trapped hydrogen could accelerate the oxidation by enhancing dissolution of chromium oxides at more noble potentials.However, additional studies are needed for a deeper and more quantitative analysis of the structure of corrosion scale for charged and uncharged Alloy 600.Acknowledgements A part of this work has been performed by the support of PEACE-E program. Authors thank Prof. Yubing Qiu, Tohoku University, Japan, for the helpful discussion.References [1] Materials Reliability Program: Resistance to Primary Water Stress Corrosion Cracking of Alloys 690, 52, and 152 in Pressurized Water Reactors(MRP-111), EPRI, Palo Alto, CA, U.S. Department of Energy, Washington, DC: 2004. 1009801. [2] R.W. Staehle, J.A. Gorman, Quantitative assessment of submodes of stress corrosion cracking on the secondary side of steam generator tubing in pressurized water reactors: Part I, Corrosion 59(11) (2003) 931-994. [3] R.W. Staehle, A statistically based model for the initiation of SCC of alloy 600MA the stat-phys model of SCC. [4] P.E. MacDonald, V.N. Shah, L.W. Ward, P.G. Ellison, Steam generator tube failures[r], NUREG/CR-6365. [5] F. Meng, J. Wang, E.-H. Han, W. Ke, Effects of scratching on corrosion and stress corrosion cracking of alloy 690TT at 58A°C and 330A°C, Corrosion Science, 51(2009): 2761a?“2769. [6] R.W. Staehle, J.A. Gorman, Quantitative assessment of submodes of stress corrosion cracking on the secondary side of steam generator tubing in pressurized water reactors: Part II, Corrosion, 60(1 )(2004): 5-63. [7] M. Uhlemann, B. Pound, Diffusivity, solubility and trapping behavior of hydrogen in alloys 600, 690tt and 800, Corrosion Science 40 (1998) 645-662. [8] T. NAKAGAWA, N. TOTSUKA, T. TERACHI, N. NAKAJIMA, Influence of Dis- solved Hydrogen on Oxide Film and PWSCC of Alloy 600 in PWR Primary Water, Journal of Nuclear Science and Technology 40(1) (2003): 39-43. [9] T. Terachi, N. Totsuka, T. Yamada, T. Nakagawa, Influence of dissolved hydrogen on structure of oxide film on alloy 600 formed in primary water of pressurized water reactors, Journal of Nuclear Science and Technology, 40 (2009): 509a?“516. [10] C. Chen, K. Aral, G. Theus, Computer-calculated potential pH diagrams to 300A°C. Volume 2: Handbook of diagrams, EPRI NP 3137, Vol. 2. Electric Power Research Institute, Palo Alto, CA, 1983. [11] G. Young, J. Scully, Evidence that carbide precipitation produces hydrogen traps in Ni-17Cr-8Fe alloys, Scripta Materialia 36(6)(1997): 713a?“719. [12] B. Lu, J. Luo, P. Norton, H. Ma, Effects of dissolved hydrogen and elastic and plastic deformation on active dissolution of pipeline steel in an aerobic groundwater of near- neutral pH. Acta Materialia, 57(1) (2009): 41a?“49.318“ “Electrochemical response of scratched Alloy 600 in simulated primary waterand its influence with hydrogen entry“ “Fanjiang Meng,Zhanpeng Lu,Tetsuo Shoji,Jianqiu Wang ,En-Hou Han
Fanjiang Meng Fracture and Reliability Research Institute, Tohoku University
Zhanpeng Lu Fracture and Reliability Research Institute, Tohoku University
Tetsuo Shoji State Key Laboratory for Corrosion and Protection, Institute of Metal Research, China
Jianqiu Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, China
En-Hou Han
ntroduction Introduction Nickel-based Alloy 600, as a material of steam Lerator tubing, has been studied for nearly 40 years. This by is known for its susceptibility to intergranular stress rosion cracking (SCC) in high-temperature PWR water. cently, field SCC cases indicate that the dominant ure mechanism has been IGA and SCC of free-span faces, especially at abnormal surface conditions such as face dents or scratches which were produced during erting tubes into the steam generator tube support sheet. matome ANP reported that SCC developed in the face marks/scores of Alloy 600, introduced into the ernal surfaces of split tube sections prior to bending [1].ress: Fracture and Reliability Research Institute, Tohoku University, B0-8579 Aoba 6-6-1, Aramaki, Aoba-ku, Sendai, Japan, ail: fjmeng@mech.rift.tohoku.ac.jperal tubes in Unit-1 and Unit-2 of Oconee Nuclear tion were removed after eddy current detection and idual compressive stress existed on the scratch. Burst ting indicated that cracking related to surface scratch.C initiation which associated with a surface scratch at espan cold leg arose at McGuire-1 and McGuire-2 in Loy 600MA tube after only several years' service [244]. previous work, the author found scratch-induced SCC tiated in caustic lead containing solution at 330A°C [5].These cases mentioned above implied that local chanical marks/scratches could be latent sites to initiate C. It was reported that the typical scratch surface and cer surface were exposed in the same environment and emical analyses showed no difference [6]. As with many terials, cold work, similar effect of scratch, increases the e of SCC and cold worked nickel alloys was supposed Crap more hydrogen in the stress field of dislocations [7]. drogen ingress can occur via corrosion processes and/or solved hydrogen in environment, can produce various ects on their properties after entering into matrix.The presence of scratches is inevitable. In PWR mary water, high concentration of dissolved hydrogen,
which maintains a lower H2O/H2, and high temperature make hydrogen permeation easier. In order to find a guidance to explain the corrosion behavior of scratched Alloy 600, the combined effect of scratches and hydrogen was studied by performing electrochemical tests.2. Experimental process 2.1 MaterialNickel base Alloy 600 was used for this study. This Alloy was heat treated at 606-616A°C for 10h 5min and then air cooled. The chemical composition of this Alloy 600 is listed in Table 1.The specimens were cut from bulk Alloy 600 by 12x12x3 mm““. The larger faces of sheet specimens were ground with emery paper up to 2400 grit and mechanically polished using lum silica paste to get a mirror finish. Some of the specimens were chose to make some scratches (about 120um in width, 20um) by using a cone hard alloy head. And then, all the specimens were soldered with high purity Ni wires with diameter of 0.5mm, which covered with two layer shrinkable PTFE tube with aim to increase insulation.lement wt.%C 0.059Table 1 Compositions of Alloy 600 Si Mn P S Ni Cu CrFe Nb 0.13 0.21 0.006 0.001 74.19 0.12 15.57 9.41 0.05The microstructure of this heat after electro-polish was characterized using electron backscatter diffraction (EBSD) as shown in Fig. 1. EBSD measurement was conducted on Hitachi S-4300 FE-SEM, which equipped with solutions camera control system made by TSL, Co. Ltd. The data was collected and analyzed by OIMTM Software. The grains were about 80um.FUYUMFig. 1. EBSD observation of Alloy 6002. Water chemistry controlAll the test were performed in the facility of autoclave bricated by Toshin Kogyo Co., LTD, Japan, with a -circulating water loop to maintain the desired primary ater chemistry conditions. The simulated primary water as prepared with 1200ppm B as H3B04 and 2ppm Li as IOH. The inlet dissolved hydrogen (DH) was controlled = 30cc/kg and test temperature was 290A°C.-3. Hydrogen charging and electrochemical testThe specimens for electrochemical tests were listed in able 2.Table 2 Test detail of Alloy 600 SpecimennChargingTest contentPlainNoEIS, AP,ScratchedNoEIS, AP,PlainYes, 86.5hEIS, AP,ScratchedYes, 86.5hEIS, AP,* AP: Anodic polarization.A three-electrode system was used, which concluded he external pressure balanced calomel electrode SCE 0.1N) acted as a reference electrode, a platinum wire as a counter electrode and work electrode. All the measured potentials were transferred to potentials referred to tandard hydrogen electrode. The potentials of Alloy 600 vere monitored with POTENTIOSTAT/GALVANOSTAT HA-151A. EIS was measured by using Solartron mpedance/gain-phase analyzer SI 1260 every two days until the corrosion potential of all the specimens became relatively stable. Some of the specimens were anodic polarized using Solartron electrochemical interface SI 1287.After this, some specimens were chosen to charge hydrogen cathodically in the simulated primary water. The Charged current density was controlled at a small constant of 0.35mA/cm' at 290A°C, which was in order to minimize Ehe damage of cathodic hydrogen charging to polished and scratched Alloy 600 surface. The total hydrogen charging time was 86.5h for each specimen. Anodic polarization was performed for the charged Alloy 600 after for 4h later.Time dependent potential of Alloy 600 in simulated primary water containing 1200ppm B and 2ppm Li with DH of 30cc/kg at 290A°CPlain specimen Specimen with scratchesPotential(SCE0 50 100 150 200 250 300 350Immersion time (h) Fig.2. Time dependent potential of Alloy 600 in simulated primary water at room temperature, during heating up and at 290A°C3. Results and discussion 3.1. The effect of scratches on corrosion of Alloy 600 3.1.1. Oxidation of scratched Alloy 600 by EIS monitoringThe time dependent potentials for plain and scratched Alloy 600 were shown in Fig.2. For these two specimens, the potentials all dropped negatively during the whole monitoring process. At the start of the test, the potentials315shifted negative dramatically. It took about 7 days to reach a stable value in the simulated primary water with a DH 30cc/kg, which means that oxide formation and matrix dissolution needed longer time to get a balance. Measurement of anodic polarization in less than 7 days' immersion may not reflect the electrochemical behavior of long-term service of Alloy 600. However, there seems to be no difference for the corrosion potential between scratched and plain specimens.During potential monitoring, EISs both for plain (Fig.3a) and scratched (Fig.3b) Alloy 600 were traced every two days. For the low frequencies impedance arc in Fig.3 which stands for the electric double layer, there were no obvious differences between plain and scratched Alloy 600 with increasing immersion time.-2500EIS evolution of plain Alloy 600 in 290A°C simulated primary water with DH content of 30cc/kg-2000-15009 dayZimgrohm-cmi5““day-5001““ day5001000150020002500300035004000Zre(ohm cm') (a) Plain Alloy 600-2500EIS evolution of scratched Alloy 600 in 290A°C simulated primary water with DH content of 30cc/kg-2000davlohm.cm3 day““dayOL50010001500250030003500400045002000Zre(ohm-cm')(b) Scratched Alloy 600 Fig.3. Evolution of Nyquist diagrams for plain and scratched Alloy 600 in simulated primary waterHowever, significant difference can be found in the high and intermediate frequencies impedance arc as shown in Figs.3a and 3b. This part of impedance arc can reflect characters of oxide film. It is noteworthy that there is no persistent increase of imaginary part during the evolution of oxide film in the simulated primary water. The imaginary part in high and intermediate frequencies dropped after 5 days' immersion. Oxides became more stable from 7th day and Nyquist diagram did not change a lot from that time on. This is consistent with the result of potential monitoring.3.1.2. Anodic behavior of scratched Alloy 600The polarization curves for plain and scratched Alloy 600 were shown in Fig.4. Even though Alloy 600 was scratched, there was no apparent effect on anodic behavior com- pared to plain Alloy 600. The identical character may be caused by dissolved hydrogen. The dissolved hydrogen can easily enter into the alloy 600 matrix [7]. Because of the uniform hydrogen in the aqueous environment, surface distribution of hydrogen permeated into the oxide film and matrix was supposed to be homogeneous in both kinds of specimens. At corrosion potential, dissolved hydrogen can partly reduce the surface film and the film toward corrosion for both scratched and plain Alloy 600. The high anodic current also suggests that matrix dissolution was not restrained even though there covered oxide film on the surface. Scratching brought about a heavy plastic deformation to the surface, but dissolved hydrogen as the leading role, suppressed the effect of scratches on the surface.Anodic polarization curves for scratched and plain Alloy 600 in simulated primary water at 290A°CPlain specimen Specimen with scratches210fPotential(VSHP)0.0000001E-6 5 E- 1 E-4Current density. A cm? Fig. 4. Anodic polarization curves for plain and scratched Alloy 600 in simulated water containing 1200 ppm B and 2 ppm Li at 290A°C.A decrease of current density from about +0.2V/SHE to +0.6V/SHE was observed, which is not a real passivation but a pseudo-passivation. This decrease of current density indicates that there was pre-passivation process during anodic scanning below this potential range. The competition probably occurs between matrix dissolution and film formation at anodic potentials. When the electrochemical reaction of film formation is more rapid than matrix dissolution, pre-passivation below +0.2V/SHE and pseudo-passivation processes take place as presented in Fig. 4.The film formation can be contributed to some possible reasons. At corrosion potential, Ni is stable under a DH of 30cc/kg at 290A°C [8] and the oxide film formed on Alloy 600 was rich in Fe and Cr [9]. According to potential-pH diagram of Ni, Cr, Fe in high temperature [10], at anodic potentials in studied environment (pH=7), only Ni can be oxidized as Nio and Cr and Fe are stable oxides as Cr2O3 and Fe2O3, respectively. Oxides such as NiCr204 and NiFe2O4 may form on the surface during anodic scanning. With increasing anodic potential, the protective of film become more compact, which induced a pseudopassivation zone. Over +0.6V/SHE, oxides dissolution or316oxygen evolution may start.3.2. The electrochemical response of Alloy 600 to charged hydrogenAfter hydrogen charged in simulated primary water, Fig.5 shows the anodic polarization curves obtained from plain Alloy 600. To some extent, the current density at pseudo-passivation zone for the charged specimen increased, i.e., the formation of compact oxide film, which has been discussed in Section 3.1.2, was retarded.Anodic polarization curves for charged and uncharged Alloy 600 in simulated primary water at 290-C--Plain specimen, uncharged -- Plain specimen, precharged for 86.5hPotential(VSME)1E- 7 1E-6 E-5 1E-4 1E-3Current density, A/cm? Fig.5. Anodic polarization curves for plain Alloy 600 after charging in simulated water containing 1200 ppm B and 2 ppm Li at 290A°C.Corrosion pit15.0KV 15.2mm X250 SE(M)200 umFig.6. SEM micrograph revealing the surface corrosion pit of 86.5h hydrogen charged plain Alloy 600 after anodic polarizationThis phenomenon can be explained by the permeated hydrogen during charging process in the oxide and Alloy 600 matrix. The hydrogen entered into the matrix is divided into two parts: the reversible hydrogen and irreversible hydrogen. According to M.Uhlemann [7], carbide/carbonitride precipitates were principal irreversible traps and hydrogen reversibly trapped in the interstitial atoms could release at a temperature higher than 250A°C. There are chromium carbides or titanium nitrides existed in nickel alloys. Hydrogen can be trapped in these precipitates even at 290A°C, which results in local higher hydrogen concentration than that of uncharged Alloy 600.Fig.6 depicts the optical micrograph of hydrogen charged Alloy 600 after anodic polarization in simulated high temperature primary water. It was found that a corrosion pit appeared on the surface. This extrairreversible hydrogen could slow down the formation of oxide film and accelerate matrix dissolution.3.3. The combined effect of scratches and charged hydrogenAnodic polarization curve for scratched Alloy 600 (Fig. 7) was measured after hydrogen charging for 86.5h in simulated primary water, too. The pseudo-passivation zone over +0.3V/SHE after hydrogen charging, compared to the uncharged but scratched one, disappeared.2Anodic polarization curves for seratched Alloy 600 charged and | uncharged in simulated primary water at 290 C1.5 HScratched and uncharged Scratched and precharged 86.5hPotentialVSHE)0.0000011E- 40.001E- 5Current density, A/cm2 Fig. 7. Anodic polarization curves for scratched Alloy 600 after hydrogen charging in simulated water containing 1200 ppm B and 2 ppm Li at 290A°C.orrosiony pitsCorrosios pits15.OKV 15.1mm x300 SEcum(a)Top view of a scratchCorrosion pit!!!1900/01/1415,1mm' 42.00K SEM20.00m(b) Marked area in (a) with a high magnification Fig. 8. Surface observation of 86.5h hydrogen charged Alloy 600 with scratches after anodic polarizationIn the test condition, charged hydrogen in the interstitial atoms or the stress field of dislocation already released at 290A°C. Cold worked Alloy 600 can trap more hydrogen in the matrix since the existence of microvoids317as irreversible traps [7, 11]. Scratches have a similar effect. For nickel based alloy, scratching the surface introduced many micro-voids and caused a heavy deformational layer at the scratch groove which has been investigated by F. Meng et al. [5]. Amounts of irreversible hydrogen were trapped in the deformed layer around the scratch groove.The anodic current over +0.2V/SHE for scratched Alloy 600 was not related to increased potential, which implies that anodic dissolution was controlled by diffusion. The irreversibly trapped hydrogen in cold worked Alloy 600 can be as high as 6ppm at this test temperature [7], which was higher than the dissolved hydrogen (about 2.67ppm) in the simulated primary water. The irreversible hydrogen was sufficient to inhibit formation of oxide film such as NiCr204 and NiFe204 and quickened anodic dissolution by oxidizing (Fe,Cr)203 and Cr2O3. As displayed in Fig.8, a lot of corrosion pits appeared at two banks of scratch and the scratch groove. The pits illuminate that the protective chromium oxide was also dissolved.According to B.T. Lu et al. [12], the synergistic effect due to the interaction of charged hydrogen and local stress field is negligible. In present test, the superimposition of dissolved hydrogen in the primary water and scratch-induced local stress field can be ignored as indicated in Fig.4. However, the charged irreversible hydrogen caused by scratching have a strongly interaction with active dissolution of Alloy 600.4. ConclusionThe effect of scratches on electrochemical behavior of Alloy 600 and its influence with hydrogen entry was investigated in simulated primary water at 290A°D!. Some conclusions could be drawn as following:I. Oxide film reached a stable condition until 7 days' immersion according evolution of corrosion potentials and EIS. The scratched Alloy 600 possessed almost the same evolution of oxide film with plain specimens.II. Scratched Alloy 600 did not change the anodic behavior at all. Uniform dissolved hydrogen plays a leading role in matrix dissolution in simulated primary water and may suppress effect of scratches.III. The anodic current changes of scratched Alloy 600 after hydrogen charging were not the simple superimposition between that of scratched and charged plain Alloy 600. The irreversibly trapped hydrogen could accelerate the oxidation by enhancing dissolution of chromium oxides at more noble potentials.However, additional studies are needed for a deeper and more quantitative analysis of the structure of corrosion scale for charged and uncharged Alloy 600.Acknowledgements A part of this work has been performed by the support of PEACE-E program. Authors thank Prof. Yubing Qiu, Tohoku University, Japan, for the helpful discussion.References [1] Materials Reliability Program: Resistance to Primary Water Stress Corrosion Cracking of Alloys 690, 52, and 152 in Pressurized Water Reactors(MRP-111), EPRI, Palo Alto, CA, U.S. Department of Energy, Washington, DC: 2004. 1009801. [2] R.W. Staehle, J.A. Gorman, Quantitative assessment of submodes of stress corrosion cracking on the secondary side of steam generator tubing in pressurized water reactors: Part I, Corrosion 59(11) (2003) 931-994. [3] R.W. 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Totsuka, T. Yamada, T. Nakagawa, Influence of dissolved hydrogen on structure of oxide film on alloy 600 formed in primary water of pressurized water reactors, Journal of Nuclear Science and Technology, 40 (2009): 509a?“516. [10] C. Chen, K. Aral, G. Theus, Computer-calculated potential pH diagrams to 300A°C. Volume 2: Handbook of diagrams, EPRI NP 3137, Vol. 2. Electric Power Research Institute, Palo Alto, CA, 1983. [11] G. Young, J. Scully, Evidence that carbide precipitation produces hydrogen traps in Ni-17Cr-8Fe alloys, Scripta Materialia 36(6)(1997): 713a?“719. [12] B. Lu, J. Luo, P. Norton, H. Ma, Effects of dissolved hydrogen and elastic and plastic deformation on active dissolution of pipeline steel in an aerobic groundwater of near- neutral pH. Acta Materialia, 57(1) (2009): 41a?“49.318“ “Electrochemical response of scratched Alloy 600 in simulated primary waterand its influence with hydrogen entry“ “Fanjiang Meng,Zhanpeng Lu,Tetsuo Shoji,Jianqiu Wang ,En-Hou Han