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Effects of lead on oxidation behavior of Alloy 690TT within a high temperature aqueous environment
Applied Surface Science 426 (2017) 514–526
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Full Length Article
Effects of lead on oxidation behavior of Alloy 690TT within a high temperature aqueous environment Qiang Hou a , Zhiyong Liu a,∗ , Chengtao Li b , Xiaogang Li a,c a b c
Corrosion Protection Center, University of Science Technology Beijing, Beijing 100083, China Life Management Technology Center, Suzhou Nuclear Power Research Institute, Suzhou 215004, China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, China
a r t i c l e
i n f o
Article history: Received 21 December 2016 Received in revised form 20 July 2017 Accepted 20 July 2017 Available online 24 July 2017 Keywords: A. Alloy 690TT B. Surface analysis B. TEM C. High temperature corrosion C. Oxidation C. Duplex film model
a b s t r a c t The chemical compositions, phases and structures of two oxide films on Alloy 690TT following exposure for 4400 h in pure water with and without lead at 320 ◦ C were studied by surface analysis techniques. The analysis of a lead-doped oxide film prepared by a focused ion beam (FIB) demonstrated that both Cr-rich and Ni-rich oxides were alternatively distributed within the outer layer, whereas the inner layer was porous and poorly protected, causing severe corrosion of the alloy and a thicker film was formed. A duplex film model was proposed for the effects discussion of lead on the oxidation mechanism. © 2017 Published by Elsevier B.V.
1. Introduction Thermally treated (TT) nickel-base Alloy 690, as an alternative tubing material of Alloy 600, has been extensively utilized in steam generator (SG) systems within pressurized water reactors (PWRs), due to its outstanding corrosion and stress corrosion cracking (SCC) resistance [1]. Besides, numerous laboratory studies have reported that the existence of lead contamination could significantly increase the SCC susceptibility of Alloy 690 in an alkaline aqueous solution at an elevated temperature [2–7]. Also, the lead-induced SCC (PbSCC) is strongly related to the degradation and breakdown of oxide films based on aforementioned studies [8–13]. Until recently, considerable efforts were made in the oxidation behavior studies of austenite alloys under high temperature steam/water conditions [14–22]. Moreover, several models were proposed for the growth mechanism of oxide films formed on tubing material surfaces at the high temperature and high pressure solutions [15–18,23,24] to be explained. Sami et al. [16] found that the growth of the outer layer was governed by the transport of cations through the inner layer via an interstitialcy mechanism. The higher defects concentration seemed to modify
∗ Corresponding author. E-mail address: [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.apsusc.2017.07.201 0169-4332/© 2017 Published by Elsevier B.V.
the oxygen diffusion in the oxide scale [17]. Wu et al. [23,24] reported that the characteristics of passive films on Alloy 690 under high-temperature conditions were closely related to the pH and temperature values during testing. The films which might contain non-stoichiometric spinels (Fex Cry O4 ) were significantly complicated in near-neutral and alkaline environments [24]. Furthermore, a duplex film structured model was proposed in which the inner layer of fine-grained Cr or spinel oxide was believed to nucleate directly from the alloy matrix and the outer layer of Ni-Fe spinel oxide and Ni hydroxide was formed by a dissolution and precipitation in high-temperature alkaline solutions [23]. The microstructural characteristics of the oxide films depend on water chemistry. Within oxygenated pure water, a duplex oxide film comprising an outer layer of the granular Fe-rich spinel, NiO and a porous inner layer consisting mainly of nickel oxides was found forming on Alloy 690TT surface [19]. Ziemniak et al. [25,26] have studied the oxide films formed on nickel-based alloys in a hightemperature hydrogenated aqueous solution and found that the films consisted of an outer layer of Fe-rich spinel and an inner layer of Cr-rich spinel. It is also commonly known that impurities, especially lead contaminants which could be effectively detected in tube-sheet samples, crevice deposits and surface scales removed from SGs [27], could affect the chemical composition, structure and properties of oxides [3,11–14]. The addition of lead into oxide degrades
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Fig. 1. Microstructural characterization of Alloy 690TT. (a) Metallurgical structure and (b) high-magnification image of the framed zone in (a); (c) TEM image of Cr-rich carbides and (d) corresponding diffraction pattern.
Table 1 Chemical composition of Alloy 690TT provided by Baosteel (wt.%).
Table 2 EDX results of framed zone in Fig. 2b, e (at.%).
C
Si
Mn
P
S
N
Cr
Fe
Ni
Elements
C
O
Pb
Cr
Fe
Ni
0.020
0.21
0.14
0.007
0.001
0.027
30.47
9.97
balance
lead-free lead-containing
11.07 12.22
23.85 18.74
– 1.69
25.35 21.99
5.39 6.66
34.32 38.70
the oxide passivity and increases the SCC susceptibility [10], where the critical concentration against SCC might be as low as 0.2 ppm [2]. The effects study of lead contamination on the anodic film fracture ductility has demonstrated that the existence of lead impurities reduced the film fracture ductility [28]. The amount of lead impurities in the anodic films was strongly pH-dependent, reaching a maximum in the near-neutral environment, and either the increase or decrease of pH has caused the amount of lead added to be reduced [9]. Moreover, due to the accelerated dissolution of chromium in a lead-contaminated caustic aqueous solution [5,10], non-protective and thick oxide films could be formed on Alloy 690TT surfaces [29,30]. This could increase the detrimental effects of lead contamination on SCC resistance because alloy plasticity displayed a negative correlation with the oxide film thickness. Therefore, the mechanism of oxide film formation on Alloy 690 TT in a high-temperature solution containing lead is required to be understood for PbSCC to be controlled and mitigated. In this paper, the effects of lead on the properties of oxide films formed on Alloy 690TT were focused and clarified for the aforementioned solution. Moreover, a duplex film model was proposed for discussing the effects of lead on the oxidation mechanism.
2. Experimental procedure 2.1. Material preparation and corrosion testing The materials utilized for this work were the thermally treated (TT) Alloy 690 tubes, with an outer diameter (OD) of 19.05 mm and 1.09 mm in thickness, whereas the corresponding chemical composition is presented in Table 1. The heat treatment consisted of solution treatment at 1100 ◦ C for 5 min, followed by water quenching, tempering at 750 ◦ C for 10 h and then water quenching for continuous and semi-continuous intergranular carbides to be obtained, as presented in Fig. 1. The alloy, with a grain size of approximately 20–60 m and a low amount of typical TiN inclusions randomly distributed within the austenitic matrix, demonstrated a high density of annealing twins. The microstructure of an un-oxidized Alloy 690TT sample was also analyzed with a JEM-2010 TEM (Fig. 1c and d). The higher magnification observation of the samples with TEM imaging modes demonstrated that the carbides were surrounded by dislocations, subsequently identified by electron diffraction/STEM–EDX analysis as the Cr-rich M23 C6 carbides [31].
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Fig. 2. Low- and high-magnification SEM images and EDX spectra of oxide films formed on Alloy 690TT following exposure for 4400 h within pure water without (a, b, c) and with (d, e, f) lead at 320 ◦ C.
Fig. 3. XRD spectra from corrosion film surfaces of Alloy 690TT exposed to pure water for 4400 h with and without lead.
After Alloy 690TT tubes were cut down into 10 mm in length samples, and the faces of cut were mechanically grinded with SiC papers up to 2000 grit and the surfaces were polished. The exposure tests were executed within a static autoclave fabricated from a titanium alloy. The prepared specimens were exposed to pure water without and with lead (1000 ppm as PbO) at 320 ◦ C and 10 MPa for a period of 4400 h for the effects of lead on the formation of the corrosion films study. In order for a low oxygen content to be achieved within the autoclave, a pure nitrogen gas was continuously injected into the autoclave up to 5 MPa, and consequently the system was stabilized for 5 min and evacuated from gases. This procedure was repeated 3 times. When the pressure was reduced to 1 MPa during the third evacuation, the valve was closed and heating was initiated. The bleeder valve was consequently opened and the
Fig. 4. Raman spectroscopy of oxide films formed on Alloy 690TT immersed in high temperature water for 4400 h with and without lead.
system was evacuated at 104 ◦ C in order for the oxygen content to be controlled less than 0.2 mg/kg. Subsequently the samples were removed from the autoclave, cleaned in acetone by an ultrasonic machine, rinsed with deionized water and blow-dried for further analysis. 2.2. Analytical study The corrosion films were observed and analyzed by a Quanta 250 SEM equipped with an energy dispersive X-ray analysis (EDX) system. In order to identify the oxide phases, XRD and Raman measurements were performed by a SmartLab X-ray diffractometer with Cu radiation ( = 1.5406 Å) and an inVia-Reflex Raman microscope with a powerful laser at 532 nm, respectively. The AES
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Fig. 5. AES spectra from specimen top-surfaces exposed for 4400 h in water with and without lead at 320 ◦ C.
experiments were executed on the structures of oxide films by a PHI 700 equipped with a field emission electron gun. The further study of the oxide state was conducted by ESCSLAB 250Xi X-ray Photoelectron Spectroscopy (XPS). The photoelectron emission was generated by a monochromatic Al K␣ source operated at 150 W. Elements distribution along the depth of films was studied by Ar ion sputtering, and the rate of Ar ion sputtering is about 0.1 nm/s (relative to Ta2 O5 ). The C1s peak from the adventitious carbon at 284.8 eV was utilized as a reference for charging shifts correction. The film component peak decompositions were implemented into commercial peak fitting software (XPSpeak4.1) following Shirley background subtraction. The cross-section TEM samples of the oxide films formed on Alloy 690TT in pure water without and with lead were prepared with a dual-beam FIB within the LYRA3 TESAN FIB-SEM. The sample preparation for atomic-resolution STEM measurements at low voltages by FIB was described in details [32]. The TEM observations of the cross-sectional samples were performed by a JEM-2100F equipped with a field emission gun, an EDX device and scanning transmission electron microscopy (STEM). The STEM was combined with the high-angle annular dark-field (HAADF) technique for the chemical contrast images formed by electrons diffracted at high angle (>40 mrad) to be produced, with an intensity proportional to the Z atomic number of elements constituting the samples. The High-resolution transmission electron microscopy (HRTEM) images were obtained by a CCD camera and analyzed with fast Fourier transformations (FFT) for the crystallographic details of the corrosion products to be studied.
Fig. 6. Auger depth profiles of oxide films formed on Alloy 690TT exposed to water for 4400 h (a) without and (b) with lead at 320 ◦ C.
corrosion oxide thicknesses. In Table 2 the EDX results obtained for the oxide films from two environmental corrosion specimens are presented. Lead traces could be observed on the surfaces of samples exposed to leaded water.
3. Results
3.2. XRD analysis
3.1. Surface morphology
The XRD patterns of two specimens exposed to high temperature water with and without lead presented in Fig. 3 indicated that the major oxide phases were spinels, NiO and Cr2 O3 phases; therefore the lead traces had no remarkable effect on the oxide film phase compositions. Since the XRD patterns for spinel concerning the three alloying elements (Ni, Cr, Fe) were quite similar, the spinel types could not be determined at that time [15]. The PbO phase could be detected in the oxide films formed in leaded water, whereas it was not yet determined if the PbO phases originated from the solution or corrosion products. In addition, the main characteristic peaks for the specimen exposures to leaded water were slightly weaker than the corresponding peaks for the specimens immersed in lead-free water.
The effects of lead on the surface morphologies of Alloy 690TT exposed to high temperature water are presented in Fig. 2. It can be observed that a duplex oxide film had formed on the surfaces of the specimens whether they were or were not immersed in the leaded solution. Certain faceted oxide particles nucleated on the outer layers in pure water and the inner layers were compact (Fig. 2b). As PbO was added, certain plate-like oxides were developed on the outer layers and the inner layers became porous (Fig. 2e). Moreover, the oxides were developed along the polishing scratches for specimens immersed in pure water whereas the scratches on the surfaces were covered by the films formed within leaded water, as the result of
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Fig. 7. XPS Cr 2p3/2 core level spectra (and decomposition) of oxide films on Alloy 690TT exposed to high temperature water for 4400 h (a) without and (b) with lead.
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Fig. 8. Ni 2p3/2 core level spectra (and decomposition) of oxide films formed on Alloy 690TT following exposure for 4400 h. (a) Lead-free; (b) lead-containing.
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3.3. Raman spectra In Fig. 4 the Raman spectra of the oxide films on Alloy 690TT following exposure tests within lead-free and lead-containing water at 320 ◦ C are presented. The two strongest peaks at approximately 700 and 687 cm−1 were identified as the NiFe2 O4 and NiCr2 O4 phases, respectively [33]. These results indicated that external surfaces of the oxide films formed within lead-free water was primarily NiFe2 O4 and the FeCr2 O4 phase was the main compound of the film top-surfaces formed within lead-containing water. 3.4. Oxide film structures The typical AES spectra obtained from the oxide film surfaces formed on Alloy 690TT following exposure for 4400 h in lead-free and lead-containing water are presented in Fig. 5. Both spectra demonstrated the presence of the main alloying components, such as C, Cr, O, Fe and Ni. Moreover, the spectrum from the specimens immersed within leaded water displayed the existence of Pb, most likely representing the addition of lead into the oxides. Through comparison of the two spectra, it was discovered that the Cr and O peaks from the lead-free oxide films were slightly weaker than the peaks from the lead-contaminated oxide film. It was noted that the strong C signal in Fig. 5 was still visible, possibly resulted from contamination of the specimen oxide film surfaces by the atmosphere. The typical profiles of elemental composition versus depth are presented in Fig. 6 for specimens immersed in lead-free and leadcontaining water, respectively. From Fig. 6, it can be discovered that the duplex films were formed on both surfaces of the specimens, whether lead was added or not. Nevertheless, the thickness of the films formed in leaded water was significantly thicker than the corresponding thickness formed in lead-free water. These results were consistent with SEM results. Regarding the oxide films formed in lead-free water, the profile demonstrated an important enrichment in Ni and Fe and depletion of Cr from the external layer and depletion of Ni and enrichment in Cr within the internal layer (Fig. 6a). Following corrosion in lead-contaminated water at 320 ◦ C, the profile of elemental composition was absolutely opposite to the profile of the sample immersed in pure water, as presented in Fig. 6. In addition, the content of lead was almost constant throughout the entire film, suggesting that lead was evenly distributed within the oxide films. 3.5. XPS analysis
Fig. 9. Detailed XPS spectra of the Pb 4f within the oxide film formed in leadcontaminated water at 320 ◦ C after exposure for 4400 h.
Figs. 7–9 shows the detailed XPS spectra of the Cr 2p3/2 , Ni 2p3/2 and Pb 4f peaks collected from the outer surfaces of the oxide films formed on Alloy 690TT in high temperature water without and with lead. The quantitative evaluations of all traces within the oxide films were performed on well-characterized standards from the NIST XPS database [34]. In Fig. 7 the XPS spectra and the decomposition of the Cr 2p3/2 orbital recorded following various sputtering time snapshots are presented. At the beginning of sputtering (on the top-surfaces of both specimens), the decomposition results demonstrated the presence of two components, the Cr2 O3 /NiCr2 O4 at 576.2 ± 0.3 eV and Cr(OH)3 at 577.4 ± 0.2 eV in both testing solutions [34]. The intensity of the peak at 576.2 ± 0.3 eV was slightly amplified in relation to the peak intensity at 577.4 ± 0.2 eV for the oxide films formed in pure water, whereas the opposite was observed for the lead-contaminated film. After 30 s duration of sputtering, the signal of Cr0 was detected at 574.3 eV, and simultaneously, the intensity of the peak at 576.2 ± 0.3 eV became almost similar to peak intensity at 577.4 ± 0.2 eV for the specimens immersed in lead-free water. The intensity of Cr0 increased along with the sputtering duration, whereas the relative intensity of the other two peaks was retained
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Fig. 10. Cross-sectional images of oxide films on Alloy 690TT following exposure for 4400 h in pure water (a) without and (b) with lead at 320 ◦ C.
almost constant. Regarding the oxide films formed in pure water, the intensity of the peaks at 576.2 ± 0.3 eV was almost similar to the peak intensity at 577.4 ± 0.2 eV following a sputtering of 30 s, whereas the latter was always significantly amplified than the former for the lead-contaminated oxide films. In Fig. 8 the XPS spectra and the decomposition of the Ni 2p3/2 orbital recorded following various sputtering time snapshots. On the samples’ top-surfaces (0 s sputtering time), the Ni 2p3/2 core level spectra displayed the existence of two components in both oxide films, which were related to NiO/NiCr2 O4 at 854.4 eV and Ni(OH)2 /NiFe2 O4 at 855.7 ± 0.2 eV [34]. The metallic Ni0 was also detected at 852.7 eV on the external surface of the leadcontaminated films [34]. After 30 s sputtering, the peak of metallic Ni0 was apparent and increased along with the sputtering duration for the lead-free films. Besides, after 30 s sputtering, the peak at 854.4 eV disappeared and the intensity of the peak at 855.7 ± 0.2 eV decreased along with sputtering duration and also disappeared after 1000 s sputtering. Regarding the oxide films formed in leaded water, the results of the decomposition of the Ni 2p3/2 orbital displayed the presence of only one component, being the metallic Ni0 following 30 s of sputtering. In addition, lead-contamination could be detected in the oxide film formed on Alloy 690TT exposed to leaded water, as presented in Fig. 9. At the specimen surface, only two peaks were detected at approximately 138.6 ± 0.1 and 143.5 ± 0.2 eV attributed to Pb(OH)2 [34]. Consequently, the signals of the Pb(OH)2 phase decreased, whereas simultaneously the peaks of the metallic Pb0 at 136.7 ± 0.1 and 141.6 ± 0.1 eV appeared and increased along with the sputtering duration.
3.6. TEM observations The oxide layer structures formed on Alloy 690TT exposed to high temperature water with and without lead are presented in Fig. 10. It can be observed that the two oxide films were both presented as a double layer structure, although the lead-doped film was significantly thicker. The higher sized pores in both oxide films were caused by the original pores enhancement during the sample preparation. Oxides within the outer layers of the films on Alloy 690TT immersed in leaded water displayed a banded structure and were alternatively distributed. Compared to films formed in pure water, the inner layers of the lead-doped films contained a higher amount of pores and provided a poor protection of the alloy surfaces. It was noted that the measurement of thickness by TEM figures was a demonstration of the sample volumes [35]. It involved the superposition of the microstructural characteristics within the projected figure. This effect resulted in being quite a problem as the specimens were thick and the microstructure was altered along the viewing direction. In this paper, the results measured by the AES standard were utilized.
In Fig. 11a an EDX elemental map obtained from the interface between both layers formed on Alloy 690TT following exposure for 4400 h in pure water at 320 ◦ C is presented. It demonstrates an important enrichment in Ni within the inner layers, whereas Cr was evenly distributed throughout the entire oxide films. The EDX elemental map of the outer oxide layers formed in leaded water presented an interesting phenomenon, i.e. the structure of bands of both Cr-rich oxides and Ni-rich oxides was alternatively distributed inner the external layer, whereas the O content in Crrich oxides was significantly increased than the O content in Ni-rich oxides (Fig. 11b). From Fig. 11c, it can be observed that the distribution of the main elements in the oxide film formed in leaded water, such as Cr, Ni and O, was similar to the main element distribution formed into lead-free water. Furthermore, lead-contaminants could be observed being evenly distributed throughout the entire oxide scaled image, as presented in Figs. 11b, c. It was noted that the elemental distribution results from both TEM-EDX and AES were not mutually exclusive, both suggesting the higher concentration of Ni and Cr existence within the inner layer of both oxide films. Combined with chemical composition, the crystal structures of the oxides were also determined by the corresponding lattice fringes in the HRTEM images and corresponding FFT diffractograms. In Fig. 12 and 13 the HRTEM images of the oxide films and corresponding FFT patterns of the marked areas are presented. Regarding the oxide films formed in pure water at 320 ◦ C, the oxides exhibited good crystallinity in either the outer or inner layer (Fig. 12). Also, the diffraction rings obtained from oxides in the outer layers, with certain Cr2 O3 spots, suggested the spinel-type crystal structure. Similarly to the inner layer case, the indexed diffraction pattern of the corresponding FFT diffractogram (observed from inset) also indicated the presence of spinel oxides. Besides, the HRTEM images and FFT diffractograms obtained from the oxide film framed zones on Alloy 690TT immersed in leaded water, indicated that the oxidation consisted in the formation of amorphous oxides interweaving with the crystalline-structured oxides, as presented in Fig. 13. The crystalline structure of the oxides within the outer layers (Fig. 13a) and in the inner layers (Fig. 13b) were spinel- and NiO-types, respectively.
4. Discussion 4.1. Oxide film structures The results of AES demonstrated that the external layers of the lead-free films were enriched in Ni and Fe, whereas the lead-doped films were enriched in Cr (Fig. 6). Regarding the oxide films formed in pure water, it could be inferred that the XPS peaks at 576.2 ± 0.3 and 854.4 eV were caused mainly by the NiCr2 O4 and NiO phases, respectively (Figs 7a and 8a). Also, regarding the lead-doped film, it could be determined that the peak at 854.4 eV was primarily
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Fig. 11. STEM–HAADF images and corresponding EDX spectrum images of oxide films formed on Alloy 690TT within testing solutions at 320 ◦ C after exposure for 4400 h. (a) The images of an interface between the inner and outer layers of a sample exposed to lead-free environment; (b) The images of an outer layer and (c) an interface between the inner and outer layers of a sample exposed to lead-containing environment.
increased by NiCr2 O4 (Fig. 8b). In addition, the Raman spectra suggested that the external layer comprised primarily NiFe2 O4 and NiCr2 O4 spinel oxides in the lead-free and lead-doped films, respectively (Fig. 4). Moreover, NiO and Cr2 O3 could be detected by XRD for both oxide films (Fig. 3). As aforementioned, the compounds of the outer layers formed in pure water were mainly NiFe2 O4 spinel oxides with certain Ni-rich oxides and/or hydroxides, whereas the
chemical phases of lead-doped outer layers were primarily NiCr2 O4 spinel oxides with Cr-rich oxides and/or hydroxides. Regarding the inner layer, Cr-enrichment could be observed in the lead-free oxide films, whereas the lead-doped films were enriched in Ni and depleted of Cr (Fig. 6). The FFT diffractogram (Fig. 12b) was identified as a NiCr2 O4 spinel-type structure, when the chemical composition was considered (Fig. 11a). Unlike the
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Fig. 12. HRTEM images of (a) the outer layer and (b) the inner layer of oxide films developed on Alloy 690TT exposed to high temperature pure water for 4400 h. FFT diffractograms of the framed zone (inset images).
lead-free oxide films, the lead-doped inner layers consisted mainly of NiO and amorphous structure oxides, as presented in Fig. 13b. It was worth being noted that the metallic Ni appeared on the external surface of the lead-doped films, whereas no metallic Ni was detected on the surface of the lead-free films (Fig. 8). It was suggested that a portion of Ni was not oxidized [36], as presented in Fig. 11b. According to the Pourbaix diagram of Pb at 300 ◦ C [37], this might have occurred because the corrosion potential of Ni was similar to the transition potential of the Ni/NiO phases, consequently the Ni became thermodynamically stable within the lead-containing solution [36].
4.2. Lead contamination effects The general process of passivation [38]: the gel-like or amorphous structure hydroxides formation onto the metal surface, then dehydration, finally, formation of a duplex layer film composed of spinel oxides. Kim et al. [10] estimated that the pH value of highpure water at 315 ◦ C was 5.8 and the solution was mild caustic (pH up to 7.9) when PbO (10000 ppm) was added. It was therefore inferred that the test solutions wee mild caustic when lead traces (1000 ppm) were added. According to the literature [37], the PbO added within mild caustic water would hydrate to form Pb(OH)2 or HPbO2 − . Dissolved lead traces were quite easy to be adsorbed
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Fig. 13. HRTEM images of (a) the outer layer and (b) the inner layer of oxide films developed on Alloy 690TT exposed to lead-containing water at 320 ◦ C for 4400 h. FFT diffractograms of the framed zone (inset images).
on the metal surface [9], whereas might be added into oxides by the following reactions to form lead-doped mixtures [8,14], as presented in Fig. 11b, c:
(OH)2 M − O − Pb − O − M(OH)2 → M2 O3 PbO + 2H2 O, (M = Cr, Fe) (4)
Pb(OH)2 · 2M(OH)2 → (OH)2 M − O − Pb − O − M (OH) + 2H2 O, (M = Ni, Fe)
(1)
Pb(OH)2 · 2M(OH)3 → (OH)2 M − O − Pb − O − M(OH)2 + 2H2 O, (M = Cr, Fe)
(2)
These reactions also indicated that lead contamination could inhibit the dehydration of amorphous structured hydroxides and consequently prevent the formation of spinel oxides (Fig. 13). Therefore lead traces could subsequently be detected as Pb(OH)2 within the films (Fig. 9). Moreover, a portion of lead-doped oxyhydroxides further formed the lead-doped complex oxides [8,9,14]. (OH) M − O − Pb − O − M (OH) → 2MOPbO + H2 O, (M = Ni, Fe) (3)
The addition of lead traces could result in the lattice of the oxide to be mismatched and cause a higher amount of defects within films [9,13,14]. The passivity was degraded and a significantly porous and poor protective film was formed (Fig. 10b). Moreover, in the literature [10,30], it was reported that lead traces could be reduced to the metallic Pb by the following electrodeposition reactions: Ni + Pb2+ → Ni2+ + Pb
(5)
2Cr + 3Pb2+ → 2Cr 3+ + 3Pb
(6)
2+
Fe + Pb
→ Fe
2+
+ Pb
(7)
Also, the electrochemical investigations [5,39] indicated that a lead-promoted anodic dissolution of Alloy 690 would occur, during the oxidation of metallic Pb electrodeposited on the alloy surfaces.
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Fig. 14. Proposed sketch of oxidation mechanisms of Alloy 690TT in high temperature water with lead.
In Fig. 9, the metallic Pb might had originated from the reduction of lead oxides and/or hydroxides during sputtering and electrodeposition [11]. 4.3. Growth mechanism The studies of electrochemical behaviors [40] displayed that the corrosion potential of Alloy 600 within de-aerated mild alkaline water at 315 ◦ C with Pb (1000 ppm) was approximately −0.86 V (SHE). It could therefore be considered that the corresponding potential of Alloy 690TT within leaded (1000 ppm) mild caustic (pH approximately 8) water at 320 ◦ C should have been approximately −0.86 V (SHE) [10]. According to the Pourbaix diagrams of Ni, Cr and Ni-Cr-Fe at 300 ◦ C [23,41], the possible stable forms might have been the Cr2 O3 and (Ni, Fe)Cr2 O4 phases, whereas the potential of Ni appeared to be similar to the transition potential of the Ni/NiO phases. This was well consistent with the analysis results of the tested lead-doped films. A duplex film model was proposed for discussing the effects of lead on the oxidation mechanism, as presented in Fig. 14. Due to the preferential dissolution of Ni and Fe from the matrix [15,23], the Cr(OH)3 prefered to nucleate and increase onto the metal surfaces. Consequently the Cr(OH)3 reacted with the adsorbed Pb(OH)2 to produce the lead-doped oxyhydroxides (Reaction (2)). Further, a portion of oxyhydroxides was converted to the lead-doped oxides (Reaction 4), containing a great deal of defects and became poor in protection (Step 1). The coalescence of the Cr2 O3 islands resulted in the formation of continuous Cr2 O3 layers. Also, the thermodynamically stable Ni diffused outwards and remained within the oxide film as metallic Ni (Fig. 8b). The originally dissolved Ni precipitated on the surfaces to form lead-doped Ni(OH)2 and NiO (Reaction (1) and (3)) as presented in Step 2. Due to lead-promoted selective dissolution of Cr from the corrosion films [5], a portion of metallic Ni was oxidized to form lead-doped NiO inner layers (Fig. 13b),
which were porous and could not act as protective barrier layers. Furthermore, the precipitated NiO has reacted with the dissolved Cr to produce lead-doped NiCr2 O4 (Step 3). Simultaneously, OH− ions continuously diffused inwards through the non-protective film and corrosion continued. As a result, the lead-doped oxide films were constructed by a Ni-rich inner layer (mainly NiO and amorphous structure) and a Cr-rich outer layer (primarily NiCr2 O4 and amorphous structure).
5. Conclusions Lead contamination could easily be adsorbed on the metal surfaces and be incorporated into films, which caused a great deal of defects within the oxides. Also, lead traces could inhibit the dehydration of hydroxides and the formation of stable spinel oxides. Severe alloy corrosion is a result of the porous and poor protective oxide film nucleated on the surface of Alloy 690TT exposed to leaded water. A mechanism was proposed to explain the evolution of the leaddoped films on Alloy 690TT in high-temperature water. Lead traces participated in the oxidation. The inner layers of both Ni-rich oxides and amorphous structures were considered to be produced by oxidation of a later Ni diffusion from the matrices, whereas the outer layers of Cr-rich oxides and amorphous structures were formed by reactions of dissolved Cr from the inner layers with the dissolved Ni.
Acknowledgements This work is supported by the National Basic Research Program of China (973 Program project, No. 2014CB643300), the Chinese National Science Foundation (Nos. U1260201 and 51471034).
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References [1] A.J. Sedriks, J.W. Schultz, M.A. Cordovi, Inconel Alloy 690-A New Corrosion Resistant Material, Zairyo-to-Kankyo 28 (1979) 82–95. [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 2, Corrosion 60 (2) (2003) 115–180, http:// dx.doi.org/10.5006/1.3299232. [3] B. Lu, L. Tian, R. Zhu, J. Luo, Y. Lu, Effects of dissolved calcium and magnesium ions on lead-induced stress corrosion cracking susceptibility of nuclear steam generator tubing alloy in high temperature crevice solutions, Electrochim. Acta 56 (4) (2011) 1848–1855, http://dx.doi.org/10.1016/j.electacta.2010.09. 104. [4] Y. Hu, J. Wang, W. Ke, E.-H. Han, Intergranular attack and intergranular stress corrosion cracking of Alloy 690 TT in high temperature lead-containing caustic solution, Mater. Corros. 63 (3) (2012) 238–246, http://dx.doi.org/10. 1002/maco.201005725. [5] S.S. Hwang, Y.S. Lim, H.P. Kim, J.S. Kim, L. Thomas, Electrochemical interpretation of a stress corrosion cracking of thermally treated Ni base alloys in a lead contaminated water, Solid State Phenom. 124–126 (2007) 1545–1548, http://dx.doi.org/10.4028/www.scientific.net/ssp.124-126.1545. [6] D.J. Kim, P.K. Hong, S.S. Hwang, Susceptibility of alloy 690 to stress corrosion cracking in caustic aqueous solutions, Nucl. Eng. Des. 45 (45) (2013) 67–72, http://dx.doi.org/10.5516/NET.07.2012.021. [7] Z. Zhang, J. Wang, E.-H. Han, W. Ke, Trans-twins stress corrosion cracking behaviors of Alloy 690TT in lead-containing caustic solution at 330◦ C, Nucl. Eng. Des. 241 (12) (2011) 4944–4952, http://dx.doi.org/10.1016/j.nucengdes. 2011.09.025. [8] B. Peng, B. Lu, J. Luo, Y. Lu, H. Ma, Investigation of passive films on nickel Alloy 690 in lead-containing environments, J. Nucl. Mater. 378 (3) (2008) 333–340, http://dx.doi.org/10.1016/j.jnucmat.2008.06.037. [9] B. Lu, J. Luo, Y. Lu, Effects of pH on lead-induced passivity degradation of nuclear steam generator tubing alloy in high temperature crevice chemistries, Electrochim. Acta 87 (1) (2013) 824–838, http://dx.doi.org/10.1016/j. electacta.2012.10.006. [10] D.J. Kim, H.C. Kwon, H.W. Kim, S.S. Hwang, P.K. Hong, Oxide properties and stress corrosion cracking behaviour for Alloy 600 in leaded caustic solutions at high temperature, Corros. Sci. 53 (4) (2011) 1247–1253, http://dx.doi.org/ 10.1016/j.corsci.2010.12.016. [11] B. Lu, Y. Lu, J. Luo, A mechanistic study on lead-Induced passivity-degradation of nickel-Based alloy, J. Electrochem. Soc. 54 (8) (2007) C379–C389, http://dx. doi.org/10.1149/1.2741204. [12] Z. Zhang, J. Wang, E.-H. Han, W. Ke, Effects of surface state and applied stress on stress corrosion cracking of alloy 690TT in lead-containing caustic solution, J. Mater. Sci. Technol. 28 (9) (2012) 785–792, http://dx.doi.org/10. 1016/S1005-0302(12)60131-5. [13] A. Palani, B. Lu, L. Tian, J. Luo, Y. Lu, Effect of magnesium on the lead induced corrosion and SCC of alloy 800 in neutral crevice solution at high temperature, J. Nucl. Mater. 396 (2–3) (2010) 189–196, http://dx.doi.org/10. 1016/j.jnucmat.2009.11.004. [14] B. Lu, J. Luo, Y. Lu, Passivity degradation of nuclear steam generator tubing alloy induced by Pb contamination at high temperature, J. Nucl. Mater. 429 (1–3) (2012) 305–314, http://dx.doi.org/10.1016/j.jnucmat.2012.06.021. [15] W. Kuang, X. Wu, E.-H. Han, J. Rao, The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water, Corros. Sci. 53 (11) (2011) 3853–3860, http://dx.doi.org/10.1016/j.corsci.2011.07.038. [16] S. Penttilä, I. Betova, M. Bojinov, P. Kinnunen, A. Toivonen, Oxidation model for construction materials in supercritical water—Estimation of kinetic and transport parameters, Corros. Sci. 100 (9) (2015) 3454–3463, http://dx.doi. org/10.1016/j.corsci.2015.06.033. [17] H. Lefaix-Jeuland, L. Marchetti, S. Perrin, M. Pijolat, M. Sennour, R. Molins, Oxidation kinetics and mechanisms of Ni-base alloys in pressurised water reactor primary conditions: influence of subsurface defects, Corros. Sci. 53 (12) (2011) 3914–3922, http://dx.doi.org/10.1016/j.corsci.2011.07.024. [18] K.I. Choudhry, D.A. Guzonas, D.T. Kallikragas, I.M. Svishchev, On-line monitoring of oxide formation and dissolution on alloy 800H in supercritical water, Corros. Sci. 111 (2016) 574–582, http://dx.doi.org/10.1016/j.corsci. 2016.05.042. [19] F. Huang, J. Wang, E.-H. Han, W. Ke, Microstructural characteristics of the oxide films formed on Alloy 690 TT in pure and primary water at 325◦ C, Corros. Sci. 76 (10) (2013) 52–59, http://dx.doi.org/10.1016/j.corsci.2013.06. 023.
[20] S.Y. Persaud, A. Korinek, J. Huang, G.A. Botton, R.C. Newman, Internal oxidation of Alloy 600 exposed to hydrogenated steam and the beneficial effects of thermal treatment, Corros. Sci. 86 (9) (2014) 108–122, http://dx.doi. org/10.1016/j.corsci.2014.04.041. [21] S.L. Yun, S.W. Kim, S.S. Hwang, P.K. Hong, C. Jang, Intergranular oxidation of Ni-based Alloy 600 in a simulated PWR primary water environment, Corros. Sci. 108 (2016) 125–133, http://dx.doi.org/10.1016/j.corsci.2016.02.040. [22] L. Marchetti, S. Perrin, F. Jambon, M. Pijolat, Corrosion of nickel-base alloys in primary medium of pressurized water reactors: new insights on the oxide growth mechanisms and kinetic modelling, Corros. Sci. 102 (2015) 24–35, http://dx.doi.org/10.1016/j.corsci.2015.09.001. [23] J. Huang, X. Wu, E.-H. Han, Electrochemical properties and growth mechanism of passive films on Alloy 690 in high-temperature alkaline environments, Corros. Sci. 52 (10) (2010) 3444–3452, http://dx.doi.org/10. 1016/j.corsci.2010.06.016. [24] J. Huang, X. Wu, E.-H. Han, Influence of pH on electrochemical properties of passive films formed on Alloy 690 in high temperature aqueous environments, Corros. Sci. 51 (12) (2009) 2976–2982, http://dx.doi.org/10. 1016/j.corsci.2009.08.002. [25] S.E. Ziemniak, M. Hanson, Corrosion behavior of NiCrMo Alloy 625 in high temperature, hydrogenated water, Corros. Sci. 45 (7) (2003) 1595–1618, http://dx.doi.org/10.1016/S0010-938X(02)00230-5. [26] S.E. Ziemniak, M. Hanson, Corrosion behavior of NiCrFe Alloy 600 in high temperature, hydrogenated water, Corros. Sci. 48 (2) (2006) 498–521, http:// dx.doi.org/10.1016/j.corsci.2005.01.006. [27] K. Fruzzetti, Workshop of Effects of Pb and S on the Performance of Secondary Side Tubing of Steam Generators in PWRs, ANL, IL May 24–27 2005. [28] B. Lu, J. Luo, Y. Lu, Correlation between film rupture ductility and PbSCC of Alloy 800, Electrochim. Acta 53 (12) (2008) 4122–4136, http://dx.doi.org/10. 1016/j.electacta.2007.12.070. [29] Y. Tan, J. Yang, W. Wang, R. Shi, K. Liang, S. Zhang, Effects of PbO on the oxide films of incoloy 800HT in simulated primary circuit of PWR, J. Nucl. Mater. 473 (2016) 119–124, http://dx.doi.org/10.1016/j.jnucmat.2016.02.011. [30] H.P. Kim, D.J. Kim, S.W. Kim, Y.S. Lim, S.S. Hwang, Ex situ and in situ characterization of stress corrosion cracking of nickel-base alloys at high temperature, J. Solid State Chem. 18 (2) (2014) 309–323, http://dx.doi.org/10. 1007/s10008-013-2248-3. [31] G. Bertali, F. Scenini, M.G. Burke, Advanced microstructural characterization of the intergranular oxidation of Alloy 600, Corros. Sci. 100 (2) (2015) 169–173, http://dx.doi.org/10.1016/j.corsci.2015.08.010. [32] M. Schaffer, B. Schaffer, Q. Ramasse, Sample preparation for atomic-resolution STEM at low voltages by FIB, Ultramicroscopy 114 (2) (2012) 62–71, http://dx. doi.org/10.1016/j.ultramic.2012.01.005. [33] J.H. Kim, I.S. Hwang, Development of an in situ Raman spectroscopic system for surface oxide films on metals and alloys in high temperature water, J. Nucl. Eng. Des. 235 (9) (2005) 1029–1040, http://dx.doi.org/10.1016/j. nucengdes.2004.12.002. [34] NIST XPS database. https://srdata.nist.gov/xps/selEnergyType.aspx. [35] M. Sennour, L. Marchetti, F. Martin, S. Perrin, R. Molins, M. Pijolat, A detailed TEM and SEM study of Ni-base alloys oxide scales formed in primary conditions of pressurized water reactor, Surf. Interface Anal. 47 (5) (2015) 632–642, http://dx.doi.org/10.1016/j.jnucmat.2010.05.010. [36] J.H. Liu, R. Mendonc¸a, Characterization of oxide films formed on alloy 182 in simulated PWR primary water, J. Nucl. Mater. 393 (2) (2009) 242–248, http:// dx.doi.org/10.1016/j.jnucmat.2009.06.012. [37] E. Protopopoff, P. Marcus, Potential-pH diagrams for Pb adsorption on Cu and Ni surfaces at 25◦ C and 300◦ C, J. Electrochem. Soc. 160 (3) (2013), http://dx. doi.org/10.1149/2.017303jes. [38] N. Sato, 1989 whitney award lecture: toward a more fundamental understanding of corrosion processes, Corrosion–Houston 45 (5) (1989) 354–368, http://dx.doi.org/10.5006/1.3582030. [39] B.T. Lu, J.L. Luo, B. Peng, A. Palani, Y.C. Lu, Condition for lead-induced corrosion of alloy 690 in an alkaline steam generator crevice solution, Corrosion −Houston Tx-. 65 (9) (2009) 601–610, http://dx.doi.org/10.5006/1.3319163. [40] D.J. Kim, B.H. Mun, H.P. Kim, S.S. Hwang, Investigation of oxide property on Alloy 600 with the immersion time in a high-temperature leaded alkaline solution, Met. Mater. Int. 20 (6) (2014) 1059–1065, http://dx.doi.org/10.1007/ s12540-014-6009-3. [41] W.G. Cook, R.P. Olive, Pourbaix diagrams for the nickel-water system extended to high-subcritical and low-supercritical conditions, Corros. Sci. 58 (2012) 284–290, http://dx.doi.org/10.1016/j.corsci.2012.02.007.
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Full Length Article
Effects of lead on oxidation behavior of Alloy 690TT within a high temperature aqueous environment Qiang Hou a , Zhiyong Liu a,∗ , Chengtao Li b , Xiaogang Li a,c a b c
Corrosion Protection Center, University of Science Technology Beijing, Beijing 100083, China Life Management Technology Center, Suzhou Nuclear Power Research Institute, Suzhou 215004, China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, China
a r t i c l e
i n f o
Article history: Received 21 December 2016 Received in revised form 20 July 2017 Accepted 20 July 2017 Available online 24 July 2017 Keywords: A. Alloy 690TT B. Surface analysis B. TEM C. High temperature corrosion C. Oxidation C. Duplex film model
a b s t r a c t The chemical compositions, phases and structures of two oxide films on Alloy 690TT following exposure for 4400 h in pure water with and without lead at 320 ◦ C were studied by surface analysis techniques. The analysis of a lead-doped oxide film prepared by a focused ion beam (FIB) demonstrated that both Cr-rich and Ni-rich oxides were alternatively distributed within the outer layer, whereas the inner layer was porous and poorly protected, causing severe corrosion of the alloy and a thicker film was formed. A duplex film model was proposed for the effects discussion of lead on the oxidation mechanism. © 2017 Published by Elsevier B.V.
1. Introduction Thermally treated (TT) nickel-base Alloy 690, as an alternative tubing material of Alloy 600, has been extensively utilized in steam generator (SG) systems within pressurized water reactors (PWRs), due to its outstanding corrosion and stress corrosion cracking (SCC) resistance [1]. Besides, numerous laboratory studies have reported that the existence of lead contamination could significantly increase the SCC susceptibility of Alloy 690 in an alkaline aqueous solution at an elevated temperature [2–7]. Also, the lead-induced SCC (PbSCC) is strongly related to the degradation and breakdown of oxide films based on aforementioned studies [8–13]. Until recently, considerable efforts were made in the oxidation behavior studies of austenite alloys under high temperature steam/water conditions [14–22]. Moreover, several models were proposed for the growth mechanism of oxide films formed on tubing material surfaces at the high temperature and high pressure solutions [15–18,23,24] to be explained. Sami et al. [16] found that the growth of the outer layer was governed by the transport of cations through the inner layer via an interstitialcy mechanism. The higher defects concentration seemed to modify
∗ Corresponding author. E-mail address: [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.apsusc.2017.07.201 0169-4332/© 2017 Published by Elsevier B.V.
the oxygen diffusion in the oxide scale [17]. Wu et al. [23,24] reported that the characteristics of passive films on Alloy 690 under high-temperature conditions were closely related to the pH and temperature values during testing. The films which might contain non-stoichiometric spinels (Fex Cry O4 ) were significantly complicated in near-neutral and alkaline environments [24]. Furthermore, a duplex film structured model was proposed in which the inner layer of fine-grained Cr or spinel oxide was believed to nucleate directly from the alloy matrix and the outer layer of Ni-Fe spinel oxide and Ni hydroxide was formed by a dissolution and precipitation in high-temperature alkaline solutions [23]. The microstructural characteristics of the oxide films depend on water chemistry. Within oxygenated pure water, a duplex oxide film comprising an outer layer of the granular Fe-rich spinel, NiO and a porous inner layer consisting mainly of nickel oxides was found forming on Alloy 690TT surface [19]. Ziemniak et al. [25,26] have studied the oxide films formed on nickel-based alloys in a hightemperature hydrogenated aqueous solution and found that the films consisted of an outer layer of Fe-rich spinel and an inner layer of Cr-rich spinel. It is also commonly known that impurities, especially lead contaminants which could be effectively detected in tube-sheet samples, crevice deposits and surface scales removed from SGs [27], could affect the chemical composition, structure and properties of oxides [3,11–14]. The addition of lead into oxide degrades
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Fig. 1. Microstructural characterization of Alloy 690TT. (a) Metallurgical structure and (b) high-magnification image of the framed zone in (a); (c) TEM image of Cr-rich carbides and (d) corresponding diffraction pattern.
Table 1 Chemical composition of Alloy 690TT provided by Baosteel (wt.%).
Table 2 EDX results of framed zone in Fig. 2b, e (at.%).
C
Si
Mn
P
S
N
Cr
Fe
Ni
Elements
C
O
Pb
Cr
Fe
Ni
0.020
0.21
0.14
0.007
0.001
0.027
30.47
9.97
balance
lead-free lead-containing
11.07 12.22
23.85 18.74
– 1.69
25.35 21.99
5.39 6.66
34.32 38.70
the oxide passivity and increases the SCC susceptibility [10], where the critical concentration against SCC might be as low as 0.2 ppm [2]. The effects study of lead contamination on the anodic film fracture ductility has demonstrated that the existence of lead impurities reduced the film fracture ductility [28]. The amount of lead impurities in the anodic films was strongly pH-dependent, reaching a maximum in the near-neutral environment, and either the increase or decrease of pH has caused the amount of lead added to be reduced [9]. Moreover, due to the accelerated dissolution of chromium in a lead-contaminated caustic aqueous solution [5,10], non-protective and thick oxide films could be formed on Alloy 690TT surfaces [29,30]. This could increase the detrimental effects of lead contamination on SCC resistance because alloy plasticity displayed a negative correlation with the oxide film thickness. Therefore, the mechanism of oxide film formation on Alloy 690 TT in a high-temperature solution containing lead is required to be understood for PbSCC to be controlled and mitigated. In this paper, the effects of lead on the properties of oxide films formed on Alloy 690TT were focused and clarified for the aforementioned solution. Moreover, a duplex film model was proposed for discussing the effects of lead on the oxidation mechanism.
2. Experimental procedure 2.1. Material preparation and corrosion testing The materials utilized for this work were the thermally treated (TT) Alloy 690 tubes, with an outer diameter (OD) of 19.05 mm and 1.09 mm in thickness, whereas the corresponding chemical composition is presented in Table 1. The heat treatment consisted of solution treatment at 1100 ◦ C for 5 min, followed by water quenching, tempering at 750 ◦ C for 10 h and then water quenching for continuous and semi-continuous intergranular carbides to be obtained, as presented in Fig. 1. The alloy, with a grain size of approximately 20–60 m and a low amount of typical TiN inclusions randomly distributed within the austenitic matrix, demonstrated a high density of annealing twins. The microstructure of an un-oxidized Alloy 690TT sample was also analyzed with a JEM-2010 TEM (Fig. 1c and d). The higher magnification observation of the samples with TEM imaging modes demonstrated that the carbides were surrounded by dislocations, subsequently identified by electron diffraction/STEM–EDX analysis as the Cr-rich M23 C6 carbides [31].
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Fig. 2. Low- and high-magnification SEM images and EDX spectra of oxide films formed on Alloy 690TT following exposure for 4400 h within pure water without (a, b, c) and with (d, e, f) lead at 320 ◦ C.
Fig. 3. XRD spectra from corrosion film surfaces of Alloy 690TT exposed to pure water for 4400 h with and without lead.
After Alloy 690TT tubes were cut down into 10 mm in length samples, and the faces of cut were mechanically grinded with SiC papers up to 2000 grit and the surfaces were polished. The exposure tests were executed within a static autoclave fabricated from a titanium alloy. The prepared specimens were exposed to pure water without and with lead (1000 ppm as PbO) at 320 ◦ C and 10 MPa for a period of 4400 h for the effects of lead on the formation of the corrosion films study. In order for a low oxygen content to be achieved within the autoclave, a pure nitrogen gas was continuously injected into the autoclave up to 5 MPa, and consequently the system was stabilized for 5 min and evacuated from gases. This procedure was repeated 3 times. When the pressure was reduced to 1 MPa during the third evacuation, the valve was closed and heating was initiated. The bleeder valve was consequently opened and the
Fig. 4. Raman spectroscopy of oxide films formed on Alloy 690TT immersed in high temperature water for 4400 h with and without lead.
system was evacuated at 104 ◦ C in order for the oxygen content to be controlled less than 0.2 mg/kg. Subsequently the samples were removed from the autoclave, cleaned in acetone by an ultrasonic machine, rinsed with deionized water and blow-dried for further analysis. 2.2. Analytical study The corrosion films were observed and analyzed by a Quanta 250 SEM equipped with an energy dispersive X-ray analysis (EDX) system. In order to identify the oxide phases, XRD and Raman measurements were performed by a SmartLab X-ray diffractometer with Cu radiation ( = 1.5406 Å) and an inVia-Reflex Raman microscope with a powerful laser at 532 nm, respectively. The AES
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Fig. 5. AES spectra from specimen top-surfaces exposed for 4400 h in water with and without lead at 320 ◦ C.
experiments were executed on the structures of oxide films by a PHI 700 equipped with a field emission electron gun. The further study of the oxide state was conducted by ESCSLAB 250Xi X-ray Photoelectron Spectroscopy (XPS). The photoelectron emission was generated by a monochromatic Al K␣ source operated at 150 W. Elements distribution along the depth of films was studied by Ar ion sputtering, and the rate of Ar ion sputtering is about 0.1 nm/s (relative to Ta2 O5 ). The C1s peak from the adventitious carbon at 284.8 eV was utilized as a reference for charging shifts correction. The film component peak decompositions were implemented into commercial peak fitting software (XPSpeak4.1) following Shirley background subtraction. The cross-section TEM samples of the oxide films formed on Alloy 690TT in pure water without and with lead were prepared with a dual-beam FIB within the LYRA3 TESAN FIB-SEM. The sample preparation for atomic-resolution STEM measurements at low voltages by FIB was described in details [32]. The TEM observations of the cross-sectional samples were performed by a JEM-2100F equipped with a field emission gun, an EDX device and scanning transmission electron microscopy (STEM). The STEM was combined with the high-angle annular dark-field (HAADF) technique for the chemical contrast images formed by electrons diffracted at high angle (>40 mrad) to be produced, with an intensity proportional to the Z atomic number of elements constituting the samples. The High-resolution transmission electron microscopy (HRTEM) images were obtained by a CCD camera and analyzed with fast Fourier transformations (FFT) for the crystallographic details of the corrosion products to be studied.
Fig. 6. Auger depth profiles of oxide films formed on Alloy 690TT exposed to water for 4400 h (a) without and (b) with lead at 320 ◦ C.
corrosion oxide thicknesses. In Table 2 the EDX results obtained for the oxide films from two environmental corrosion specimens are presented. Lead traces could be observed on the surfaces of samples exposed to leaded water.
3. Results
3.2. XRD analysis
3.1. Surface morphology
The XRD patterns of two specimens exposed to high temperature water with and without lead presented in Fig. 3 indicated that the major oxide phases were spinels, NiO and Cr2 O3 phases; therefore the lead traces had no remarkable effect on the oxide film phase compositions. Since the XRD patterns for spinel concerning the three alloying elements (Ni, Cr, Fe) were quite similar, the spinel types could not be determined at that time [15]. The PbO phase could be detected in the oxide films formed in leaded water, whereas it was not yet determined if the PbO phases originated from the solution or corrosion products. In addition, the main characteristic peaks for the specimen exposures to leaded water were slightly weaker than the corresponding peaks for the specimens immersed in lead-free water.
The effects of lead on the surface morphologies of Alloy 690TT exposed to high temperature water are presented in Fig. 2. It can be observed that a duplex oxide film had formed on the surfaces of the specimens whether they were or were not immersed in the leaded solution. Certain faceted oxide particles nucleated on the outer layers in pure water and the inner layers were compact (Fig. 2b). As PbO was added, certain plate-like oxides were developed on the outer layers and the inner layers became porous (Fig. 2e). Moreover, the oxides were developed along the polishing scratches for specimens immersed in pure water whereas the scratches on the surfaces were covered by the films formed within leaded water, as the result of
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Fig. 7. XPS Cr 2p3/2 core level spectra (and decomposition) of oxide films on Alloy 690TT exposed to high temperature water for 4400 h (a) without and (b) with lead.
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Fig. 8. Ni 2p3/2 core level spectra (and decomposition) of oxide films formed on Alloy 690TT following exposure for 4400 h. (a) Lead-free; (b) lead-containing.
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3.3. Raman spectra In Fig. 4 the Raman spectra of the oxide films on Alloy 690TT following exposure tests within lead-free and lead-containing water at 320 ◦ C are presented. The two strongest peaks at approximately 700 and 687 cm−1 were identified as the NiFe2 O4 and NiCr2 O4 phases, respectively [33]. These results indicated that external surfaces of the oxide films formed within lead-free water was primarily NiFe2 O4 and the FeCr2 O4 phase was the main compound of the film top-surfaces formed within lead-containing water. 3.4. Oxide film structures The typical AES spectra obtained from the oxide film surfaces formed on Alloy 690TT following exposure for 4400 h in lead-free and lead-containing water are presented in Fig. 5. Both spectra demonstrated the presence of the main alloying components, such as C, Cr, O, Fe and Ni. Moreover, the spectrum from the specimens immersed within leaded water displayed the existence of Pb, most likely representing the addition of lead into the oxides. Through comparison of the two spectra, it was discovered that the Cr and O peaks from the lead-free oxide films were slightly weaker than the peaks from the lead-contaminated oxide film. It was noted that the strong C signal in Fig. 5 was still visible, possibly resulted from contamination of the specimen oxide film surfaces by the atmosphere. The typical profiles of elemental composition versus depth are presented in Fig. 6 for specimens immersed in lead-free and leadcontaining water, respectively. From Fig. 6, it can be discovered that the duplex films were formed on both surfaces of the specimens, whether lead was added or not. Nevertheless, the thickness of the films formed in leaded water was significantly thicker than the corresponding thickness formed in lead-free water. These results were consistent with SEM results. Regarding the oxide films formed in lead-free water, the profile demonstrated an important enrichment in Ni and Fe and depletion of Cr from the external layer and depletion of Ni and enrichment in Cr within the internal layer (Fig. 6a). Following corrosion in lead-contaminated water at 320 ◦ C, the profile of elemental composition was absolutely opposite to the profile of the sample immersed in pure water, as presented in Fig. 6. In addition, the content of lead was almost constant throughout the entire film, suggesting that lead was evenly distributed within the oxide films. 3.5. XPS analysis
Fig. 9. Detailed XPS spectra of the Pb 4f within the oxide film formed in leadcontaminated water at 320 ◦ C after exposure for 4400 h.
Figs. 7–9 shows the detailed XPS spectra of the Cr 2p3/2 , Ni 2p3/2 and Pb 4f peaks collected from the outer surfaces of the oxide films formed on Alloy 690TT in high temperature water without and with lead. The quantitative evaluations of all traces within the oxide films were performed on well-characterized standards from the NIST XPS database [34]. In Fig. 7 the XPS spectra and the decomposition of the Cr 2p3/2 orbital recorded following various sputtering time snapshots are presented. At the beginning of sputtering (on the top-surfaces of both specimens), the decomposition results demonstrated the presence of two components, the Cr2 O3 /NiCr2 O4 at 576.2 ± 0.3 eV and Cr(OH)3 at 577.4 ± 0.2 eV in both testing solutions [34]. The intensity of the peak at 576.2 ± 0.3 eV was slightly amplified in relation to the peak intensity at 577.4 ± 0.2 eV for the oxide films formed in pure water, whereas the opposite was observed for the lead-contaminated film. After 30 s duration of sputtering, the signal of Cr0 was detected at 574.3 eV, and simultaneously, the intensity of the peak at 576.2 ± 0.3 eV became almost similar to peak intensity at 577.4 ± 0.2 eV for the specimens immersed in lead-free water. The intensity of Cr0 increased along with the sputtering duration, whereas the relative intensity of the other two peaks was retained
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Fig. 10. Cross-sectional images of oxide films on Alloy 690TT following exposure for 4400 h in pure water (a) without and (b) with lead at 320 ◦ C.
almost constant. Regarding the oxide films formed in pure water, the intensity of the peaks at 576.2 ± 0.3 eV was almost similar to the peak intensity at 577.4 ± 0.2 eV following a sputtering of 30 s, whereas the latter was always significantly amplified than the former for the lead-contaminated oxide films. In Fig. 8 the XPS spectra and the decomposition of the Ni 2p3/2 orbital recorded following various sputtering time snapshots. On the samples’ top-surfaces (0 s sputtering time), the Ni 2p3/2 core level spectra displayed the existence of two components in both oxide films, which were related to NiO/NiCr2 O4 at 854.4 eV and Ni(OH)2 /NiFe2 O4 at 855.7 ± 0.2 eV [34]. The metallic Ni0 was also detected at 852.7 eV on the external surface of the leadcontaminated films [34]. After 30 s sputtering, the peak of metallic Ni0 was apparent and increased along with the sputtering duration for the lead-free films. Besides, after 30 s sputtering, the peak at 854.4 eV disappeared and the intensity of the peak at 855.7 ± 0.2 eV decreased along with sputtering duration and also disappeared after 1000 s sputtering. Regarding the oxide films formed in leaded water, the results of the decomposition of the Ni 2p3/2 orbital displayed the presence of only one component, being the metallic Ni0 following 30 s of sputtering. In addition, lead-contamination could be detected in the oxide film formed on Alloy 690TT exposed to leaded water, as presented in Fig. 9. At the specimen surface, only two peaks were detected at approximately 138.6 ± 0.1 and 143.5 ± 0.2 eV attributed to Pb(OH)2 [34]. Consequently, the signals of the Pb(OH)2 phase decreased, whereas simultaneously the peaks of the metallic Pb0 at 136.7 ± 0.1 and 141.6 ± 0.1 eV appeared and increased along with the sputtering duration.
3.6. TEM observations The oxide layer structures formed on Alloy 690TT exposed to high temperature water with and without lead are presented in Fig. 10. It can be observed that the two oxide films were both presented as a double layer structure, although the lead-doped film was significantly thicker. The higher sized pores in both oxide films were caused by the original pores enhancement during the sample preparation. Oxides within the outer layers of the films on Alloy 690TT immersed in leaded water displayed a banded structure and were alternatively distributed. Compared to films formed in pure water, the inner layers of the lead-doped films contained a higher amount of pores and provided a poor protection of the alloy surfaces. It was noted that the measurement of thickness by TEM figures was a demonstration of the sample volumes [35]. It involved the superposition of the microstructural characteristics within the projected figure. This effect resulted in being quite a problem as the specimens were thick and the microstructure was altered along the viewing direction. In this paper, the results measured by the AES standard were utilized.
In Fig. 11a an EDX elemental map obtained from the interface between both layers formed on Alloy 690TT following exposure for 4400 h in pure water at 320 ◦ C is presented. It demonstrates an important enrichment in Ni within the inner layers, whereas Cr was evenly distributed throughout the entire oxide films. The EDX elemental map of the outer oxide layers formed in leaded water presented an interesting phenomenon, i.e. the structure of bands of both Cr-rich oxides and Ni-rich oxides was alternatively distributed inner the external layer, whereas the O content in Crrich oxides was significantly increased than the O content in Ni-rich oxides (Fig. 11b). From Fig. 11c, it can be observed that the distribution of the main elements in the oxide film formed in leaded water, such as Cr, Ni and O, was similar to the main element distribution formed into lead-free water. Furthermore, lead-contaminants could be observed being evenly distributed throughout the entire oxide scaled image, as presented in Figs. 11b, c. It was noted that the elemental distribution results from both TEM-EDX and AES were not mutually exclusive, both suggesting the higher concentration of Ni and Cr existence within the inner layer of both oxide films. Combined with chemical composition, the crystal structures of the oxides were also determined by the corresponding lattice fringes in the HRTEM images and corresponding FFT diffractograms. In Fig. 12 and 13 the HRTEM images of the oxide films and corresponding FFT patterns of the marked areas are presented. Regarding the oxide films formed in pure water at 320 ◦ C, the oxides exhibited good crystallinity in either the outer or inner layer (Fig. 12). Also, the diffraction rings obtained from oxides in the outer layers, with certain Cr2 O3 spots, suggested the spinel-type crystal structure. Similarly to the inner layer case, the indexed diffraction pattern of the corresponding FFT diffractogram (observed from inset) also indicated the presence of spinel oxides. Besides, the HRTEM images and FFT diffractograms obtained from the oxide film framed zones on Alloy 690TT immersed in leaded water, indicated that the oxidation consisted in the formation of amorphous oxides interweaving with the crystalline-structured oxides, as presented in Fig. 13. The crystalline structure of the oxides within the outer layers (Fig. 13a) and in the inner layers (Fig. 13b) were spinel- and NiO-types, respectively.
4. Discussion 4.1. Oxide film structures The results of AES demonstrated that the external layers of the lead-free films were enriched in Ni and Fe, whereas the lead-doped films were enriched in Cr (Fig. 6). Regarding the oxide films formed in pure water, it could be inferred that the XPS peaks at 576.2 ± 0.3 and 854.4 eV were caused mainly by the NiCr2 O4 and NiO phases, respectively (Figs 7a and 8a). Also, regarding the lead-doped film, it could be determined that the peak at 854.4 eV was primarily
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Fig. 11. STEM–HAADF images and corresponding EDX spectrum images of oxide films formed on Alloy 690TT within testing solutions at 320 ◦ C after exposure for 4400 h. (a) The images of an interface between the inner and outer layers of a sample exposed to lead-free environment; (b) The images of an outer layer and (c) an interface between the inner and outer layers of a sample exposed to lead-containing environment.
increased by NiCr2 O4 (Fig. 8b). In addition, the Raman spectra suggested that the external layer comprised primarily NiFe2 O4 and NiCr2 O4 spinel oxides in the lead-free and lead-doped films, respectively (Fig. 4). Moreover, NiO and Cr2 O3 could be detected by XRD for both oxide films (Fig. 3). As aforementioned, the compounds of the outer layers formed in pure water were mainly NiFe2 O4 spinel oxides with certain Ni-rich oxides and/or hydroxides, whereas the
chemical phases of lead-doped outer layers were primarily NiCr2 O4 spinel oxides with Cr-rich oxides and/or hydroxides. Regarding the inner layer, Cr-enrichment could be observed in the lead-free oxide films, whereas the lead-doped films were enriched in Ni and depleted of Cr (Fig. 6). The FFT diffractogram (Fig. 12b) was identified as a NiCr2 O4 spinel-type structure, when the chemical composition was considered (Fig. 11a). Unlike the
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Fig. 12. HRTEM images of (a) the outer layer and (b) the inner layer of oxide films developed on Alloy 690TT exposed to high temperature pure water for 4400 h. FFT diffractograms of the framed zone (inset images).
lead-free oxide films, the lead-doped inner layers consisted mainly of NiO and amorphous structure oxides, as presented in Fig. 13b. It was worth being noted that the metallic Ni appeared on the external surface of the lead-doped films, whereas no metallic Ni was detected on the surface of the lead-free films (Fig. 8). It was suggested that a portion of Ni was not oxidized [36], as presented in Fig. 11b. According to the Pourbaix diagram of Pb at 300 ◦ C [37], this might have occurred because the corrosion potential of Ni was similar to the transition potential of the Ni/NiO phases, consequently the Ni became thermodynamically stable within the lead-containing solution [36].
4.2. Lead contamination effects The general process of passivation [38]: the gel-like or amorphous structure hydroxides formation onto the metal surface, then dehydration, finally, formation of a duplex layer film composed of spinel oxides. Kim et al. [10] estimated that the pH value of highpure water at 315 ◦ C was 5.8 and the solution was mild caustic (pH up to 7.9) when PbO (10000 ppm) was added. It was therefore inferred that the test solutions wee mild caustic when lead traces (1000 ppm) were added. According to the literature [37], the PbO added within mild caustic water would hydrate to form Pb(OH)2 or HPbO2 − . Dissolved lead traces were quite easy to be adsorbed
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Fig. 13. HRTEM images of (a) the outer layer and (b) the inner layer of oxide films developed on Alloy 690TT exposed to lead-containing water at 320 ◦ C for 4400 h. FFT diffractograms of the framed zone (inset images).
on the metal surface [9], whereas might be added into oxides by the following reactions to form lead-doped mixtures [8,14], as presented in Fig. 11b, c:
(OH)2 M − O − Pb − O − M(OH)2 → M2 O3 PbO + 2H2 O, (M = Cr, Fe) (4)
Pb(OH)2 · 2M(OH)2 → (OH)2 M − O − Pb − O − M (OH) + 2H2 O, (M = Ni, Fe)
(1)
Pb(OH)2 · 2M(OH)3 → (OH)2 M − O − Pb − O − M(OH)2 + 2H2 O, (M = Cr, Fe)
(2)
These reactions also indicated that lead contamination could inhibit the dehydration of amorphous structured hydroxides and consequently prevent the formation of spinel oxides (Fig. 13). Therefore lead traces could subsequently be detected as Pb(OH)2 within the films (Fig. 9). Moreover, a portion of lead-doped oxyhydroxides further formed the lead-doped complex oxides [8,9,14]. (OH) M − O − Pb − O − M (OH) → 2MOPbO + H2 O, (M = Ni, Fe) (3)
The addition of lead traces could result in the lattice of the oxide to be mismatched and cause a higher amount of defects within films [9,13,14]. The passivity was degraded and a significantly porous and poor protective film was formed (Fig. 10b). Moreover, in the literature [10,30], it was reported that lead traces could be reduced to the metallic Pb by the following electrodeposition reactions: Ni + Pb2+ → Ni2+ + Pb
(5)
2Cr + 3Pb2+ → 2Cr 3+ + 3Pb
(6)
2+
Fe + Pb
→ Fe
2+
+ Pb
(7)
Also, the electrochemical investigations [5,39] indicated that a lead-promoted anodic dissolution of Alloy 690 would occur, during the oxidation of metallic Pb electrodeposited on the alloy surfaces.
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Fig. 14. Proposed sketch of oxidation mechanisms of Alloy 690TT in high temperature water with lead.
In Fig. 9, the metallic Pb might had originated from the reduction of lead oxides and/or hydroxides during sputtering and electrodeposition [11]. 4.3. Growth mechanism The studies of electrochemical behaviors [40] displayed that the corrosion potential of Alloy 600 within de-aerated mild alkaline water at 315 ◦ C with Pb (1000 ppm) was approximately −0.86 V (SHE). It could therefore be considered that the corresponding potential of Alloy 690TT within leaded (1000 ppm) mild caustic (pH approximately 8) water at 320 ◦ C should have been approximately −0.86 V (SHE) [10]. According to the Pourbaix diagrams of Ni, Cr and Ni-Cr-Fe at 300 ◦ C [23,41], the possible stable forms might have been the Cr2 O3 and (Ni, Fe)Cr2 O4 phases, whereas the potential of Ni appeared to be similar to the transition potential of the Ni/NiO phases. This was well consistent with the analysis results of the tested lead-doped films. A duplex film model was proposed for discussing the effects of lead on the oxidation mechanism, as presented in Fig. 14. Due to the preferential dissolution of Ni and Fe from the matrix [15,23], the Cr(OH)3 prefered to nucleate and increase onto the metal surfaces. Consequently the Cr(OH)3 reacted with the adsorbed Pb(OH)2 to produce the lead-doped oxyhydroxides (Reaction (2)). Further, a portion of oxyhydroxides was converted to the lead-doped oxides (Reaction 4), containing a great deal of defects and became poor in protection (Step 1). The coalescence of the Cr2 O3 islands resulted in the formation of continuous Cr2 O3 layers. Also, the thermodynamically stable Ni diffused outwards and remained within the oxide film as metallic Ni (Fig. 8b). The originally dissolved Ni precipitated on the surfaces to form lead-doped Ni(OH)2 and NiO (Reaction (1) and (3)) as presented in Step 2. Due to lead-promoted selective dissolution of Cr from the corrosion films [5], a portion of metallic Ni was oxidized to form lead-doped NiO inner layers (Fig. 13b),
which were porous and could not act as protective barrier layers. Furthermore, the precipitated NiO has reacted with the dissolved Cr to produce lead-doped NiCr2 O4 (Step 3). Simultaneously, OH− ions continuously diffused inwards through the non-protective film and corrosion continued. As a result, the lead-doped oxide films were constructed by a Ni-rich inner layer (mainly NiO and amorphous structure) and a Cr-rich outer layer (primarily NiCr2 O4 and amorphous structure).
5. Conclusions Lead contamination could easily be adsorbed on the metal surfaces and be incorporated into films, which caused a great deal of defects within the oxides. Also, lead traces could inhibit the dehydration of hydroxides and the formation of stable spinel oxides. Severe alloy corrosion is a result of the porous and poor protective oxide film nucleated on the surface of Alloy 690TT exposed to leaded water. A mechanism was proposed to explain the evolution of the leaddoped films on Alloy 690TT in high-temperature water. Lead traces participated in the oxidation. The inner layers of both Ni-rich oxides and amorphous structures were considered to be produced by oxidation of a later Ni diffusion from the matrices, whereas the outer layers of Cr-rich oxides and amorphous structures were formed by reactions of dissolved Cr from the inner layers with the dissolved Ni.
Acknowledgements This work is supported by the National Basic Research Program of China (973 Program project, No. 2014CB643300), the Chinese National Science Foundation (Nos. U1260201 and 51471034).
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References [1] A.J. Sedriks, J.W. Schultz, M.A. Cordovi, Inconel Alloy 690-A New Corrosion Resistant Material, Zairyo-to-Kankyo 28 (1979) 82–95. [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 2, Corrosion 60 (2) (2003) 115–180, http:// dx.doi.org/10.5006/1.3299232. [3] B. Lu, L. Tian, R. Zhu, J. Luo, Y. Lu, Effects of dissolved calcium and magnesium ions on lead-induced stress corrosion cracking susceptibility of nuclear steam generator tubing alloy in high temperature crevice solutions, Electrochim. Acta 56 (4) (2011) 1848–1855, http://dx.doi.org/10.1016/j.electacta.2010.09. 104. [4] Y. Hu, J. Wang, W. Ke, E.-H. Han, Intergranular attack and intergranular stress corrosion cracking of Alloy 690 TT in high temperature lead-containing caustic solution, Mater. Corros. 63 (3) (2012) 238–246, http://dx.doi.org/10. 1002/maco.201005725. [5] S.S. Hwang, Y.S. Lim, H.P. Kim, J.S. Kim, L. Thomas, Electrochemical interpretation of a stress corrosion cracking of thermally treated Ni base alloys in a lead contaminated water, Solid State Phenom. 124–126 (2007) 1545–1548, http://dx.doi.org/10.4028/www.scientific.net/ssp.124-126.1545. [6] D.J. Kim, P.K. Hong, S.S. Hwang, Susceptibility of alloy 690 to stress corrosion cracking in caustic aqueous solutions, Nucl. Eng. Des. 45 (45) (2013) 67–72, http://dx.doi.org/10.5516/NET.07.2012.021. [7] Z. Zhang, J. Wang, E.-H. Han, W. Ke, Trans-twins stress corrosion cracking behaviors of Alloy 690TT in lead-containing caustic solution at 330◦ C, Nucl. Eng. Des. 241 (12) (2011) 4944–4952, http://dx.doi.org/10.1016/j.nucengdes. 2011.09.025. [8] B. Peng, B. Lu, J. Luo, Y. Lu, H. Ma, Investigation of passive films on nickel Alloy 690 in lead-containing environments, J. Nucl. Mater. 378 (3) (2008) 333–340, http://dx.doi.org/10.1016/j.jnucmat.2008.06.037. [9] B. Lu, J. Luo, Y. Lu, Effects of pH on lead-induced passivity degradation of nuclear steam generator tubing alloy in high temperature crevice chemistries, Electrochim. Acta 87 (1) (2013) 824–838, http://dx.doi.org/10.1016/j. electacta.2012.10.006. [10] D.J. Kim, H.C. Kwon, H.W. Kim, S.S. Hwang, P.K. Hong, Oxide properties and stress corrosion cracking behaviour for Alloy 600 in leaded caustic solutions at high temperature, Corros. Sci. 53 (4) (2011) 1247–1253, http://dx.doi.org/ 10.1016/j.corsci.2010.12.016. [11] B. Lu, Y. Lu, J. Luo, A mechanistic study on lead-Induced passivity-degradation of nickel-Based alloy, J. Electrochem. Soc. 54 (8) (2007) C379–C389, http://dx. doi.org/10.1149/1.2741204. [12] Z. Zhang, J. Wang, E.-H. Han, W. Ke, Effects of surface state and applied stress on stress corrosion cracking of alloy 690TT in lead-containing caustic solution, J. Mater. Sci. Technol. 28 (9) (2012) 785–792, http://dx.doi.org/10. 1016/S1005-0302(12)60131-5. [13] A. Palani, B. Lu, L. Tian, J. Luo, Y. Lu, Effect of magnesium on the lead induced corrosion and SCC of alloy 800 in neutral crevice solution at high temperature, J. Nucl. Mater. 396 (2–3) (2010) 189–196, http://dx.doi.org/10. 1016/j.jnucmat.2009.11.004. [14] B. Lu, J. Luo, Y. Lu, Passivity degradation of nuclear steam generator tubing alloy induced by Pb contamination at high temperature, J. Nucl. Mater. 429 (1–3) (2012) 305–314, http://dx.doi.org/10.1016/j.jnucmat.2012.06.021. [15] W. Kuang, X. Wu, E.-H. Han, J. Rao, The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water, Corros. Sci. 53 (11) (2011) 3853–3860, http://dx.doi.org/10.1016/j.corsci.2011.07.038. [16] S. Penttilä, I. Betova, M. Bojinov, P. Kinnunen, A. Toivonen, Oxidation model for construction materials in supercritical water—Estimation of kinetic and transport parameters, Corros. Sci. 100 (9) (2015) 3454–3463, http://dx.doi. org/10.1016/j.corsci.2015.06.033. [17] H. Lefaix-Jeuland, L. Marchetti, S. Perrin, M. Pijolat, M. Sennour, R. Molins, Oxidation kinetics and mechanisms of Ni-base alloys in pressurised water reactor primary conditions: influence of subsurface defects, Corros. Sci. 53 (12) (2011) 3914–3922, http://dx.doi.org/10.1016/j.corsci.2011.07.024. [18] K.I. Choudhry, D.A. Guzonas, D.T. Kallikragas, I.M. Svishchev, On-line monitoring of oxide formation and dissolution on alloy 800H in supercritical water, Corros. Sci. 111 (2016) 574–582, http://dx.doi.org/10.1016/j.corsci. 2016.05.042. [19] F. Huang, J. Wang, E.-H. Han, W. Ke, Microstructural characteristics of the oxide films formed on Alloy 690 TT in pure and primary water at 325◦ C, Corros. Sci. 76 (10) (2013) 52–59, http://dx.doi.org/10.1016/j.corsci.2013.06. 023.
[20] S.Y. Persaud, A. Korinek, J. Huang, G.A. Botton, R.C. Newman, Internal oxidation of Alloy 600 exposed to hydrogenated steam and the beneficial effects of thermal treatment, Corros. Sci. 86 (9) (2014) 108–122, http://dx.doi. org/10.1016/j.corsci.2014.04.041. [21] S.L. Yun, S.W. Kim, S.S. Hwang, P.K. Hong, C. Jang, Intergranular oxidation of Ni-based Alloy 600 in a simulated PWR primary water environment, Corros. Sci. 108 (2016) 125–133, http://dx.doi.org/10.1016/j.corsci.2016.02.040. [22] L. Marchetti, S. Perrin, F. Jambon, M. Pijolat, Corrosion of nickel-base alloys in primary medium of pressurized water reactors: new insights on the oxide growth mechanisms and kinetic modelling, Corros. Sci. 102 (2015) 24–35, http://dx.doi.org/10.1016/j.corsci.2015.09.001. [23] J. Huang, X. Wu, E.-H. Han, Electrochemical properties and growth mechanism of passive films on Alloy 690 in high-temperature alkaline environments, Corros. Sci. 52 (10) (2010) 3444–3452, http://dx.doi.org/10. 1016/j.corsci.2010.06.016. [24] J. Huang, X. Wu, E.-H. Han, Influence of pH on electrochemical properties of passive films formed on Alloy 690 in high temperature aqueous environments, Corros. Sci. 51 (12) (2009) 2976–2982, http://dx.doi.org/10. 1016/j.corsci.2009.08.002. [25] S.E. Ziemniak, M. Hanson, Corrosion behavior of NiCrMo Alloy 625 in high temperature, hydrogenated water, Corros. Sci. 45 (7) (2003) 1595–1618, http://dx.doi.org/10.1016/S0010-938X(02)00230-5. [26] S.E. Ziemniak, M. Hanson, Corrosion behavior of NiCrFe Alloy 600 in high temperature, hydrogenated water, Corros. Sci. 48 (2) (2006) 498–521, http:// dx.doi.org/10.1016/j.corsci.2005.01.006. [27] K. Fruzzetti, Workshop of Effects of Pb and S on the Performance of Secondary Side Tubing of Steam Generators in PWRs, ANL, IL May 24–27 2005. [28] B. Lu, J. Luo, Y. Lu, Correlation between film rupture ductility and PbSCC of Alloy 800, Electrochim. Acta 53 (12) (2008) 4122–4136, http://dx.doi.org/10. 1016/j.electacta.2007.12.070. [29] Y. Tan, J. Yang, W. Wang, R. Shi, K. Liang, S. Zhang, Effects of PbO on the oxide films of incoloy 800HT in simulated primary circuit of PWR, J. Nucl. Mater. 473 (2016) 119–124, http://dx.doi.org/10.1016/j.jnucmat.2016.02.011. [30] H.P. Kim, D.J. Kim, S.W. Kim, Y.S. Lim, S.S. Hwang, Ex situ and in situ characterization of stress corrosion cracking of nickel-base alloys at high temperature, J. Solid State Chem. 18 (2) (2014) 309–323, http://dx.doi.org/10. 1007/s10008-013-2248-3. [31] G. Bertali, F. Scenini, M.G. Burke, Advanced microstructural characterization of the intergranular oxidation of Alloy 600, Corros. Sci. 100 (2) (2015) 169–173, http://dx.doi.org/10.1016/j.corsci.2015.08.010. [32] M. Schaffer, B. Schaffer, Q. Ramasse, Sample preparation for atomic-resolution STEM at low voltages by FIB, Ultramicroscopy 114 (2) (2012) 62–71, http://dx. doi.org/10.1016/j.ultramic.2012.01.005. [33] J.H. Kim, I.S. Hwang, Development of an in situ Raman spectroscopic system for surface oxide films on metals and alloys in high temperature water, J. Nucl. Eng. Des. 235 (9) (2005) 1029–1040, http://dx.doi.org/10.1016/j. nucengdes.2004.12.002. [34] NIST XPS database. https://srdata.nist.gov/xps/selEnergyType.aspx. [35] M. Sennour, L. Marchetti, F. Martin, S. Perrin, R. Molins, M. Pijolat, A detailed TEM and SEM study of Ni-base alloys oxide scales formed in primary conditions of pressurized water reactor, Surf. Interface Anal. 47 (5) (2015) 632–642, http://dx.doi.org/10.1016/j.jnucmat.2010.05.010. [36] J.H. Liu, R. Mendonc¸a, Characterization of oxide films formed on alloy 182 in simulated PWR primary water, J. Nucl. Mater. 393 (2) (2009) 242–248, http:// dx.doi.org/10.1016/j.jnucmat.2009.06.012. [37] E. Protopopoff, P. Marcus, Potential-pH diagrams for Pb adsorption on Cu and Ni surfaces at 25◦ C and 300◦ C, J. Electrochem. Soc. 160 (3) (2013), http://dx. doi.org/10.1149/2.017303jes. [38] N. Sato, 1989 whitney award lecture: toward a more fundamental understanding of corrosion processes, Corrosion–Houston 45 (5) (1989) 354–368, http://dx.doi.org/10.5006/1.3582030. [39] B.T. Lu, J.L. Luo, B. Peng, A. Palani, Y.C. Lu, Condition for lead-induced corrosion of alloy 690 in an alkaline steam generator crevice solution, Corrosion −Houston Tx-. 65 (9) (2009) 601–610, http://dx.doi.org/10.5006/1.3319163. [40] D.J. Kim, B.H. Mun, H.P. Kim, S.S. Hwang, Investigation of oxide property on Alloy 600 with the immersion time in a high-temperature leaded alkaline solution, Met. Mater. Int. 20 (6) (2014) 1059–1065, http://dx.doi.org/10.1007/ s12540-014-6009-3. [41] W.G. Cook, R.P. Olive, Pourbaix diagrams for the nickel-water system extended to high-subcritical and low-supercritical conditions, Corros. Sci. 58 (2012) 284–290, http://dx.doi.org/10.1016/j.corsci.2012.02.007.