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Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water
Journal Pre-proof Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water Fangqiang Ning, Jibo Tan, Xinqiang Wu
PII:
S1005-0302(20)30131-6
DOI:
https://doi.org/10.1016/j.jmst.2020.02.004
Reference:
JMST 1980
To appear in:
Journal of Materials Science & Technology
Received Date:
16 July 2019
Revised Date:
19 November 2019
Accepted Date:
25 November 2019
Please cite this article as: Ning F, Tan J, Wu X, Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2020.02.004
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Research Article
Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water Fangqiang Ning 1, 2, Jibo Tan 1, Xinqiang Wu 1,*
1
CAS Key Laboratory of Nuclear Materials and Safety Assessment, Liaoning Key
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Laboratory for Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2
School of Materials Science and Engineering, University of Science and Technology
*
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of China, Hefei 230026, China
Corresponding author.
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E-mail address: [email protected] (X. Wu).
25 November 2019]
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[Received 16 July 2019; Received in revised form 19 November 2019; Accepted
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Effects of 405 stainless steel (405 SS) on crevice corrosion behavior of Alloy 690 in high-temperature pure water were investigated. Results revealed that the corrosion
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rate of Alloy 690 was low within the crevice. It was likely attributed to the fact that a Cr-rich inner oxide film and a Ni-rich layer beneath this oxide film formed upon
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Alloy 690, inhibiting the diffusion of oxygen towards the oxide/matrix interface. Moreover, the Fe2+ ions dissolved from 405 SS consumed most of oxygen, leading to less oxygen participating in the oxidation of Alloy 690. In addition, it was found that Fe concentration continuously decreased from the surface of the inner oxide film to the oxide/matrix interface of Alloy 690 within the crevice, which was probably due to the diffusion of Fe2+ ions dissolved from 405 SS into the inner oxide film.
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Keywords: Alloy; High-temperature corrosion; Crevice corrosion; Oxidation 1. Introduction Nickel-based alloy 690 has been extensively used in steam generator (SG) tube of pressurized water reactor (PWR) nuclear power plants (NPPs) owing to its excellent corrosion resistance [1-4]. However, many key parts of SGs in secondary circuit in a high-temperature (284-305 ºC) water environment, such as tube/tube sheets, and tube/support plate crevices, are vulnerable to crevice corrosion during
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long-term service [5-10]. The support plate is generally made of stainless steel (such as 405 SS). Crevice corrosion usually occurs in occluded regions, which is difficult to be detected and often has a long incubation period before attack takes place. Once it
initiates, it may lead to unexpected and catastrophic damage to the structural materials
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within the crevices [11-14]. Therefore, crevice corrosion has received much attention in operating PWR NPPs.
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Almost all kind of metals that rely on passive film for corrosion resistance are prone to crevice corrosion. The main reasons of crevice corrosion of structural
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materials in PWR NPPs are the presence of geometric crevices, which result in restricted mass transport between the crevice and bulk solution [11, 15, 16]. Therefore, the depletion of oxygen and abnormal ionic concentration exist within the
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crevice. Many researchers have investigated the crevice corrosion behavior of metallic materials and two mechanisms have been proposed to account for crevice corrosion,
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namely, critical crevice solution theory (CCST) and ohmic potential drop theory (IRDT) [11, 13, 17]. According to the CCST, enrichment of hydrogen ions and
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invasive ions increases the corrosivity of the crevice solution, which can destroy the passive film and cause crevice corrosion. According to the IRDT, when the potential drop (IR drop) from outside to inside of the crevice is greater than a critical potential (ΔΦ*), the state of metal within the crevice transforms from the passivity to activity, which causes crevice corrosion. Both theories have same viewpoint that oxygen is depleted within the crevice. Therefore, the dissolved oxygen (DO) plays an important role in the crevice corrosion of alloys in high-temperature water. 2
Many researchers have studied the effects of different factors (containing crevice width, crevice depth, temperature and alloy elements etc.) on the crevice corrosion behavior of alloys [9, 10, 14, 18-22]. Chen et al. [9, 10] studied the effects of crevice geometry on the crevice corrosion behavior of 304 SS in high-temperature water. They found that different crevice widths resulted in different distributions of DO and the crevice depth mainly influenced the pH value within the crevice solution, which eventually affected the development of oxide films within the crevice. Some work [19-22] focused on the effects of temperature on the crevice corrosion resistance of
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alloys. It was found that the passive film breakdown/repassivation transient had a temperature threshold, above which the crevice corrosion possibly occurred. Actually, the temperature mainly affected the critical breakthrough potential of passive film.
The critical breakthrough potential gradually decreased with increasing temperature.
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Saito et al. [18] studied the effects of alloy elements on the oxide films within the
crevice of austenitic alloys exposed to 280 oC water, they found that a protective Cr-
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rich oxide film was formed within the crevice of Alloy 600 containing higher Cr content, whereas a non-protective oxide film consisting of NiO and NiFe2O4 was
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formed within the crevice of nickel-based alloys containing lower Cr content. Abella et al. [8] studied the electrochemical nature of SG crevice corrosion in 200 oC water
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containing 10 ppb NaCl. They found that significant coupling currents flowed between the different components in a model SG tube (Alloy 600) and support plate (AISI 4140 steel) crevice. However, the effects of different metallic materials on the
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development of oxide films within the crevice of alloys in high-temperature water have not been fully clarified and reported. The investigation on this aspect is of
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significance for understanding the mechanism of crevice corrosion of different metallic materials during long-term service of PWR NPPs and finding ways to prevent crevice corrosion. The objective of the present work is to investigate the effects of 405 SS on the development of oxide films within the crevice of Alloy 690 in high-temperature water. The oxide films within the crevice of Alloy 690 and 405 SS after exposure tests in high-temperature water were analyzed by area-selected X-ray diffraction (XRD), 3
Raman spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The electrochemical impedance spectrum (EIS) was performed to investigate the effects of DO concentration on the resistance of oxide films on 405 SS and Alloy 690 in hightemperature water. 2. Experimental 2.1. Crevice specimen and apparatus of testing loop Fig. 1(a) shows the schematic diagram of the crevice corrosion test device in
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high-temperature water used in the present work. The device was composed of a zirconia bolt, top sample made of 405 SS, bottom sample made of Alloy 690 and a nut made of 405 SS. The bottom sample was immovable. The width of the crevice was
adjusted accurately by rotating angle of the top sample. The length of the crevice was
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adjusted accurately by changing the radius of the top and bottom samples. Fig. 1(b) shows the sizes of the crevice test specimens used in the present work. The crevice
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width and length were controlled constantly as 125 μm and 4 mm, respectively. Exposure tests of crevice specimens in high-temperature pressurized water were
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implemented in a 316 SS autoclave with a capacity of 2 L. More detailed information of the crevice device and testing system was described in the previous work [9, 23].
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2.2. Materials and exposure tests
Alloy 690 (used as SG tube) and 405 SS (used as support plate) were investigated in the present work. Their compositions are listed in Table 1. The
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specimens were mechanically abraded using silicon-carbide paper up to 2000 grit and degreased with ethanol in ultrasonic washer before the exposure tests.
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Table 2 shows the testing conditions in the present work. In actual secondary
water, in order to decrease corrosion rate of structure materials, the pH adjusters and corrosion inhibitors are usually added, and the DO < 5 ppb. In order to better understand the effects of 405 SS on the development of oxide films within the crevice of Alloy 690 and clarify the oxidation mechanism during the crevice corrosion in high-temperature water, a high concentration of DO (3 ppm, by weight) and pure water were used in the present work. It was believed that a high concentration of DO 4
could enhance crevice corrosion in high-temperature water, which was helpful to observe the oxide films within the crevice and to understand the effects of 405 SS. 2.3. Electrochemical measurement The electrochemical tests were conducted using a typical three electrode system. All work electrodes (405 SS and Alloy 690) with 10 mm 10 mm area were ground using silicon-carbide paper up to 2000 grit. The work electrodes of 405 SS were welded to 304 SS wires, whereas the work electrodes of Alloy 690 were welded to nickel wires. The counter electrode (Pt) was welded to a Pt wire. All wires were
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covered with heat-shrink PTFE tubes. The Ag-AgCl (0.1 M KCl) pressure-balanced external electrode was used. The reference electrode was maintained at ambient temperature with a water-cooling system.
The EIS was measured in a frequency range from 100 kHz to 10 mHz at an
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amplitude of 10 mV (rms) at open circuit potential. All the electrochemical tests were performed after 72 h exposure in high-temperature water to form relatively stable
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oxide films on the surface of specimens. The electrochemical experiments were performed with an EG&G Model 273A and a Model 52101 lock-in amplifier
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controlled by PowerSuite software. The EIS results were analyzed using Zsimpwin software.
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2.4. Analysis of the oxide films
After exposure tests, the surfaces of specimens were cleaned using cotton that absorbed deionized water and dried carefully using absorbent paper. The surface
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macro-morphologies of specimens were examined using a Leica S6D and an OLS4000 3D stereomicroscope. The micro-morphologies of specimens were
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examined using a scanning electron microscopy (SEM) (FEI XL30) equipped with an energy-dispersive spectrometer (EDS). The structure of oxide film was analyzed by a BWS905 custom Raman system and a Bruker D8 Discover area-selected X-ray diffraction (XRD) analyzer with Co K alpha radiation. The detection range of the area-selected XRD analysis was a circle with a diameter of 0.5 mm, which could accurately analyze the oxides in each region. An ESCALAB250 X-ray photoelectron spectrometer (XPS) was used to analyze the thickness and composition of oxide film. 5
The accurate depth profile and crystallographic details on the cross-section of the oxide film were analyzed using a JEM-2100 TEM operating at 200 kV. To investigate crystallographic details of the inner layer of oxide film, the high-resolution TEM images were captured by a Gatan 4k charge coupled device camera and analyzed using Fast Fourier Transform (FFT) technique. The TEM samples of the oxide films were prepared by a focused ion beam with a Ga ion sputtering (FEI Quanta 200 3D). 3. Results 3.1. Surface macro-morphologies
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Fig. 2 shows the surface morphology of Alloy 690 (Fig. 2(a, c)) and 405 SS (Fig. 2(b, d)) after 500 h exposure tests in 290 ºC water containing 3 ppm DO. The
variations in color and corrosion morphology of different regions indicate that crevice corrosion occurred and the oxide films were different in different regions after the
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exposure tests. The difference in composition and structure of the oxide films formed in different regions was discussed in the following sections. Based on the differences
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in corrosion morphology, composition and structure of the oxide films, surface of the Alloy 690 could be divided into four regions, namely, free surface (region A), crevice
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mouth (region B), within the crevice (region C) and deeper site within the crevice (region D). In contrast, the surface of 405 SS could be divided into three regions
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except for region A, a free surface part. From crevice mouth to deeper site within the crevice, the widths of regions B, C and D were approximately 0.7, 1.8 respectively.
and 1.5 mm,
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3.2. XRD analysis
Fig. 3 shows the XRD patterns of the oxide films formed on Alloy 690 (Fig. 3(a))
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and 405 SS (Fig. 3(b)) respectively. The characteristic peaks of spinel oxide, NiO and Ni(OH)2 were detected in the region A of Alloy 690. The previous work [24, 25] has proven that the spinel oxide was (Ni, Fe)Fe2O4. In the other regions, the characteristic peaks of oxide films formed on Alloy 690 were the same as those on 405 SS. The characteristic peaks of spinel and hematite oxides were detected in the region B. The characteristic peaks of hematite oxide were detected in the region C and the characteristic peaks of spinel oxide were detected in the region D. In addition, in the 6
region C, the intensity of characteristic peaks of hematite oxide on Alloy 690 was stronger than that on 405 SS, suggesting that the content of hematite oxide on Alloy 690 was more than that on 405 SS. In the region D, the intensity of characteristic peaks of spinel oxide on Alloy 690 was weaker than that on 405 SS, suggesting that the content of spinel oxide on Alloy 690 was less than that on 405 SS. For Alloy 690, the intensity of characteristic peaks of hematite oxide in the region C was stronger than that of spinel oxide in the region D, suggesting that the content of hematite oxide in the region C was more than that of spinel oxide in the region D, and the reverse was
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true for 405 SS. 3.3. Raman spectra
Fig. 4 shows the Raman spectra of oxide films formed on Alloy 690 (Fig. 4(b-e)) and 405 SS (Fig. 4(g-i)), respectively. The Raman peaks of NiO (478 cm-1) [26, 27]
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and NiFe2O4 (693 cm-1) [27, 28] were detected in the region A. The Raman peaks of Fe2O3 (222, 292, 402, 478, 1304 cm-1) [29, 30] and NiFe2O4 were detected in the
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region B. Only the Raman peaks of Fe2O3 and Fe3O4 (660 cm-1) [29, 30] were detected in the regions C and D, respectively. This was consistent with the previous
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XRD results.
3.4. Surface morphologies of oxide films
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Fig. 5 shows the SEM morphology and EDS results of the oxide films formed on Alloy 690 and 405 SS. The oxide films in different regions or on different materials showed different characteristics. Sparsely dispersed oxide clusters with some straight
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sidesteps grew on the top of a porous inner layer in the region A (Fig. 5(a)) of Alloy 690. The previous work [24, 25] has proven that the oxide clusters were NiFe2O4 and
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the porous inner oxides were NiO. Densely dispersed typical spinel oxides and big equiaxed oxide particles were observed in the region B (Fig. 5(c, d and g)). The spinel oxides were NiFe2O4 and the equiaxed oxide particles were Fe2O3 by combining the EDS results with the XRD and Raman results. In the region C, the big oxide particles were observed on Alloy 690 (Fig. 5(e)) and the small oxide particles were observed on 405 SS (Fig. 5(h)). In the region D, sparsely dispersed typical spinel oxides were observed on Alloy 690 (Fig. 5(f)) and densely dispersed typical spinel oxides were 7
observed on 405 SS (Fig. 5(i)). Because the Fe content in Alloy 690 was approximately 10 at.% and the previous work [31] has proven that the Cr-rich oxides were formed within the crevice made of the same type of Alloy 690, it could be speculated that the Fe element in the Fe-rich outer oxide particles in the regions B, C and D of Alloy 690 mainly came from 405 SS. 3.5. XPS results Fig. 6 shows the XPS depth profiles of the oxide films formed in different regions of Alloy 690. Because the absorbed oxygen and carbon were removed during
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the initial sputtering time (0-60 s), the elements (Fe, Cr, Ni, O) content after 60 s sputtering was taken as the initial value. The thickness of the oxide films is defined
from the depth profile where the intensity of oxygen level reaches 50% of its initial value [24, 32-37]. The thickness of oxide films from region B to region D was
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gradually decreased (Fig. 6(d)). The Fe content in all regions was higher than that in matrix, and it was highest in the regions B and C (Fig. 6(a)). Except that the Ni
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content in the region A was slightly higher than that in matrix, it in the other regions was lower than that in matrix (Fig. 6(b)). The Cr content in all regions was lower than
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that in matrix (Fig. 6(c)). The XPS results also revealed that the oxides formed in the region B were mainly Ni-Fe oxides and those in the region C and D were mainly Fe-
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rich oxides. It was noticeable that the Fe content in the inner layer of oxide films within the crevice (regions C and D) was much higher than that in matrix. Therefore, the effects of 405 SS on the inner layer of oxide films formed in the region B, C and
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D of Alloy 690 can be expected.
Fig. 7 shows the XPS depth profiles of O 1s in the oxide films formed in the
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region D of Alloy 690 and 405 SS. The thickness of oxide films formed on 405 SS was more than that on Alloy 690. The oxide film formed on Alloy 690 for the crevice made of the same type of Alloy 690 [31] was thicker than that on Alloy 690 for the crevice made of different metals (Alloy 690/405 SS). It could be speculated that the presence of 405 SS decreased the thickness of oxide film formed on Alloy 690. 3.6. Cross-section observation and analysis by TEM Figs. 8 to 10 show the TEM observation and analysis of the oxide films formed 8
in the regions B, C and D of Alloy 690, respectively. The oxide film was a double layer structure, and a clear interface was observed between the outer and inner layers. However, the oxide films in different regions showed different thickness, composition and structure. The inner layer of oxide film in the region B was thicker than that in the regions C and D, and it is porous and not protective, suggesting that the corrosion of Alloy 690 in the region B was more serious than that in the regions C and D. According to the XPS (Fig. 6(d)) results, the oxide films in the region C were thicker than that in the region D. However, the TEM observation (Figs. 9 and 10) suggested
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that the thickness of the inner layer of oxide films in these two regions was almost the same, with about 20 nm. It could be concluded that the outer layer of oxide film
consisting of Fe2O3 in the region C were thicker than that consisting of Fe3O4 in the region D.
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In the region B (Fig. 8), the EDX and SAED analyses reveal that two kinds of oxide particles in the outer layer of oxide film were Ni-Fe spinel and Fe2O3,
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respectively. The crystallographic analysis of the inner layer of oxide film by FFT (equivalent to electron diffraction patterns) indicates that the inner layer of oxide film
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was mainly composed of NiO with a normalized composition of 87.6 at.% Ni, 7.5 at.% Fe and 4.9 at.% Cr. The average contents of Fe, Cr and Ni in the inner layer of
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oxide film were obtained from a number of EDX spot analyses conducted in the middle of the inner layer of oxide film. The NiO layer was porous and could not prevent oxygen from diffusing inward, which resulted in more serious corrosion of
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Alloy 690 in the region B. The thickness of the inner layer of oxide film was approximately 370 nm.
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In the region C (Fig. 9), EDX and SAED analyses reveal that the outer layer of
oxide film was mainly Fe2O3 with a normalized composition of 97.2 at.% Fe, 1.8 at.% Cr and 1.0 at.% Ni. The inner layer of oxide film was mainly composed of Fe(Fe,Cr)2O4 and Cr2O3. The composition of the inner layer of oxide film was 35.7 at.% Fe, 58.5 at.% Cr and 5.8 at.% Ni. EDX mapping (Fig. 9(b)) and line-scans (Fig. 9(c)) show enrichments of Fe and Cr in the inner layer of oxide film. An enrichment of Ni and a depletion of Cr in the matrix beneath the oxide film were observed, while 9
no such a segregation of Fe was observed, suggesting that the Fe element in the innerlayer film was mainly from the solution in crevice and the Cr element in the innerlayer film was from the matrix. In the region D (Fig. 10), EDX and SAED analyses indicate that the outer layer of oxide film was mainly Fe3O4 with a normalized composition of 95.3 at.% Fe, 4.5 at.% Cr and 0.2 at.% Ni. The inner layer of oxide film with a normalized composition of 26.2 at.% Fe, 56.2 at.% Cr and 17.6 at.% Ni was consistent with that in the region C. An enrichment of Ni and a depletion of Cr in the matrix beneath the oxide film
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were also observed. 3.7. Electrochemical measurements
Chen et al. [10] have studied the distribution of DO within the crevice of 304 SS with the crevice width of 125 m and crevice length of 4 mm in 290 oC water
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containing 3 ppm DO, which is consistent with the experimental conditions of the
present work. They found that the DO concentration was almost equal to 3 ppm in the
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region B, approximately 200 ppb in the region C, and less than 5 ppb in the region D. Therefore, the electrochemical measurements were conducted at DO = 5 ppb and DO
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= 200 ppb in the present work to compare the film resistance of Alloy 690 and 405 SS.
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Fig. 11 shows the EIS results for Alloy 690 and 405 SS after 72 h exposure tests in 290 oC pure water with different DO levels. Two unfinished semicircles could be distinguished in all Nyquist diagrams (Fig. 11(a)). The semicircle radius of Alloy 690
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was larger than that of 405 SS at DO = 5 ppb, and the reverse was true at DO = 200 ppb. A Warburg impedance should be considered if one assumes that the diffusion
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process plays an important role. An equivalent circuit [34, 35, 38] as shown in Fig. 11(b) was used to fit the EIS
results. A constant phase element (CPE) was introduced to represent a non-ideal capacitance. A CPE has an impedance equal to 1/Y0(j)n, where Y0 is the CPE coefficient and n is the CPE exponent. The meaning of each element is described below. Rsol is the resistance of solution, CPE1 is the non-ideal capacitance of the double layer, Rct is the resistance of charge transfer, W is the Warburg impedance, 10
CPE2 is the non-ideal capacitance of the oxide film, and Rfilm is the resistance of the inner layer of oxide film. The corrosion resistance of a material can be qualitatively assessed by the Rfilm [34, 35, 38-40]. Fig. 11(a) shows the fitting results of Rfilm. The Rfilm of Alloy 690 was larger than that of 405 SS at DO = 5 ppb, suggesting that the inner layer of oxide film formed on Alloy 690 was more protective than that on 405 SS. However, the inner layer of oxide film formed on 405 SS showed better protectivity than that on Alloy 690 at DO = 200 ppb. 4. Discussion
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The present work indicates that the oxide films formed on the free surface (region A) and within the crevice (regions B, C and D) of Alloy 690 after 500 h
exposure tests exhibit different morphologies (Fig. 5), structures (Figs. 3 and 4) and thicknesses (Figs. 6-10). In addition, it is found that 405 SS can significantly affect
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the formation and characteristic of oxide films within the crevice of Alloy 690. Fig. 12 shows the crevice corrosion process and the effects of 405 SS on the development of
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oxide films within the crevice of Alloy 690 during the crevice corrosion. During the crevice corrosion, the presence of crevice geometry can restrict the
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transport of the ions and oxygen between the bulk and the crevice solutions [11, 13, 17], which eventually influence the ions and DO concentration distributions within
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the crevice. The consumption rate of DO within the crevice is faster than the diffusion rate of DO from the bulk solution into crevice solution [15], which results in the depletion of DO within the crevice as shown in Fig. 12(a). In addition, the
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conductivity of water used in the present work is 0.06-0.075 S/cm, which is close to the limit of pure water, suggesting that the aggressive anions concentration in water is
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extremely low. Although the aggressive anions can be enriched in solution within the crevice with the development of crevice corrosion, it is believed that their concentrations are not too high. The acidification degree of the solution within the crevice is not serious due to the electroneutrality principle. Chen et al. [10] have found that the pH value decreased to about 5 within the crevice of 304 SS under the condition of 125 m crevice width and 4 mm crevice length in 290 oC pure water, which is close to neutral solution with a pH value of 5.6 at this temperature [41]. It 11
can be speculated that the pH within the crevice is close to neutral in the present work. Therefore, the DO concentration distribution plays an important role on the formation of oxide films within the crevice in the present work. The main difference between the oxide films formed on Alloy 690 in deaeration and oxygenated high-temperature water is that the oxide film formed in deaeration water is Cr-rich, and the oxide film formed in oxygenated water is Ni-Fe-rich [24, 25, 42, 43]. For the general corrosion of Alloy 690 in high-temperature pure water containing 3 ppm DO, the oxide film was composed of an outer layer with oxide particles of Ni-
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Fe spinel and an inner porous layer of NiO [24, 25]. In the present work, the oxide film formed in the region A (Fig. 5(a)) has similar structure and composition as the
general corrosion. However, the change in the hydrochemistry (including metal ions, DO) within the crevice leads to the formation of different oxide films in different
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regions.
The corrosion resistance of 405 SS is inferior to that of Alloy 690 in the region D
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(Fig. 11), indicating that it can release a large amount of Fe2+ ions into the crevice solution. Because the Fe2+ ions concentration near the surface of 405 SS is higher than
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that of Alloy 690, the Fe2+ ions can diffuse from 405 SS to Alloy 690. Abella et al. [8] have also found that significant coupling current flowed from AISI 4140 steel to Alloy
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600 within the crevice in high-temperature deaerated water. It can be speculated that the coupling current is produced by the diffusion of Fe2+ ions dissolved from AISI 4140 steel, which is consistent with the present work. Meanwhile, the crevice
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corrosion current flows from the region D to region A [11, 13, 17], resulting in the Fe2+ ions to diffuse in the same direction. Eventually, Fe2+ ions diffuse simultaneously
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to the Alloy 690 and outside of the crevice as shown in Fig. 12(b), resulting in a distribution of Fe2+ ions concentration near the surface of Alloy 690 within the crevice as shown in Fig. 12(c), which is well confirmed by the depth profile of Fe (Fig. 6(a)). In the region C, the good film resistance of 405 SS and small amount of oxides on 405 SS (Fig. 5(h)) also suggest that most of Fe elements in oxide film formed on Alloy 690 may be from the region D. In the region D, the DO concentration of less than 5 ppb is in the potential-pH 12
region of stable Fe3O4 in high-temperature water, which has been justified by the previous work [9, 10]. A large amount of Fe3O4 oxide particles formed on 405 SS by dissolution-precipitation mechanism [36-38, 44, 45] (Fig. 5(i)), which can consume most of oxygen. To form Fe3O4, the oxygen needs continually to diffuse to 405 SS (Fig. 12(d)), resulting in depletion of oxygen near Alloy 690 (Fig. 12(d)). Therefore, there is only a small amount of oxygen to oxidize Alloy 690, resulting in a thin oxide film formed on Alloy 690, as evidenced by the depth profiles of oxygen (Figs. 7 and 10).
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In the region C, because the DO concentration of 200 ppb is in the potential-pH region of stable Fe2O3 in high-temperature water, the Fe2+ ions migrated from the
region D are oxidized to Fe2O3 on Alloy 690. Due to the high concentration of Fe2+ ions near Alloy 690 (Fig. 12(c)), most of oxygen is used to oxidize Fe2+ ions (Fig.
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12(d)), eventually forming a thicker layer of Fe2O3 (Fig. 12(e)). The EIS results (Fig. 11) suggested that the corrosion resistance of Alloy 690 is not good in high-
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temperature water containing 200 ppb DO. Wang et al. [46] also found that the corrosion resistance of Alloy 800 (43 at.% Fe) is better than Alloy 690 (10 at.% Fe)
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when the DO concentration was out of the potential-pH region of stable Cr2O3. Some previous work [25, 35, 46] has found that the corrosion resistance of materials can
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benefit from higher Fe concentration at high DO concentration (DO 100 ppb) in high-temperature water. In the present work, the presence of Fe2O3 also proves that the DO in high-temperature water is more than 100 ppb [47], suggesting that the
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corrosion resistance of Alloy 690 is not good. However, the TEM results (Figs. 9 and 10) showed that the inner layer of oxide film in the region C was as thin as that in the
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region D, suggesting that Alloy 690 is not severely corroded in the region C. Furthermore, a thicker outer-layer oxide film consisting of Fe2O3 was observed. It can be speculated that the Fe2+ ions consume a large amount of oxygen to form Fe2O3, thus only a small amount of oxygen is available to oxidize Alloy 690, which can mitigate corrosion of Alloy 690. In the regions C and D, the Fe2+ ions dissolved from 405 SS can diffuse into the inner layer of oxide film formed on Alloy 690. Along the thickness direction of inner 13
layer of oxide film, a decrease of Fe concentration was observed from the surface of the inner layer to the oxide/matrix interface (Figs. 9 and 10), also suggesting the diffusion of Fe2+ ions into inner layer of oxide film. In the middle region of inner layer of oxide film, the Cr concentration was highest, indicating Cr-rich oxides formed in the middle region of inner layer of oxide film. The previous work [31] has found that the inner layer of oxide film formed within the crevice made of the same type of Alloy 690 was Cr-rich oxides. Therefore, according to the point-defect mechanism for Ni-Cr-Fe alloys [44, 48-51], the formation of Fe-Cr spinel is
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determined by the diffusion of Fe2+ ions into the Cr-rich inner layer of oxide film by the following reactions. Fe(aq) Fei(chromia)
(1)
Fei(chromia) + 2CrCr(chromia) + 3OO(chromia) FeFe(spinel) + 2CrCr(spinel)
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+ 3OO(spinel) + VO(spinel)
(2)
where Fe(aq) represents Fe2+ ions in solution, Fei(chromia) represents interstitial cation
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in chromia, MM(chromia) and (spinel) represent normal metal cation lattice sites in oxide (M = Fe and Cr), OO(chromia) and (spinel) represent oxygen ion lattice sites in oxide, and
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VO(spinel) represents oxygen vacancy in oxide. According to the Gibbs free energytemperature (G0-T) map of Fe-, Cr- and Ni-oxides, the critical oxygen partial
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pressure required for formation of the oxide is Cr < Fe < Ni, indicating that the oxidation of Cr takes precedence over Ni and Fe [37, 52, 53]. When oxygen diffuses inward, the Cr is preferentially oxidized to form Cr-rich oxide at oxide/matrix
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interface. The local oxygen concentration at the oxide/matrix interface is not high enough to oxidize Ni. Due to a lower diffusion rate of Ni in Cr-rich oxide, the
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unoxidized Ni is retained in oxide films. With increasing exposure time, the formation of a large amount of Cr-rich oxides and the diffusion of Fe2+ into Cr-rich oxides can push the unoxidized Ni into the base metal to occupy the lattice sites of Cr and Fe in Alloy 690 according to reaction (3) [54, 55]. Nii(oxide) + VM(metal) NiNi,M(metal)
(3)
where Nii(oxide) represents Ni in oxide, VM(metal) represents metal vacancy in the alloy (M = Cr and Fe), Ni Ni,M (metal) represents that Ni occupy the lattice sites of Fe and Cr 14
in the alloy. This leads to the enrichment of Ni in the matrix beneath the inner layer of oxide film as shown in Fig. 12(e). Since Ni is not easily oxidized at low oxygen concentration, the oxidation process, namely, reaction (4) [44, 49, 50] is inhibited at Ni-rich layer/oxide interface. M MM(oxide)+(/2)VO(oxide) + e
(4)
where M represents metal atom in the alloy, MM(oxide) represents metal cation lattice sites in oxide (M = Ni, Fe and Cr), VO(oxide) represents oxygen vacancy in oxide. This leads to a decrease of oxygen vacancy in oxides, which can further inhibit the
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progress of reaction (5) [44, 49,50] at oxide/solution interface. H2O + VO(oxide) OO + 2H+
(5)
where OO represent oxygen ion at anion sites in oxide. The inhibition of the reaction
(5) at oxide/solution interface indicates that the diffusion of oxygen through the inner
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layer of oxide film towards oxide/matrix interface is inhibited. The above process is shown in Fig. 13. Therefore, the Ni-rich layer can act as a barrier for oxide growth
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and oxygen diffusion through oxide film [54, 55], which is one of the reasons for the formation of a thin inner layer of oxide film within the crevice of Alloy 690. In
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addition, another main reason for the thinning of the oxide film on Alloy 690 is that the continuous Cr-rich inner layer can also act as a barrier layer to resist the corrosion
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of alloy [31, 56].
In the region B, because the DO concentration of approximate 3 ppm is out of the potential-pH region of stable Cr2O3 and Fe2+ ions migrated from the regions C and
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D can not consume so much oxygen, the Cr in the inner layer of oxide film enters into solution as HCrO4- or CrO42- [24, 25]. The inner layer of oxide film becomes porous
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and less protective NiO layer. So, oxygen can continuously diffuse inward and the oxidation processes keep going on, resulting in a thicker inner layer of oxide film (Fig. 8). Meanwhile, Ni and Fe dissolve selectively from the matrix into solution because the diffusion rate of elements in oxide is Fe > Ni > Cr [25, 57, 58]. The previous work [31] has found that when the crevice was made of the same type of Alloy 690, a large amount of NiO was observed in the region B, suggesting that the amount of Ni2+ ions dissolved from the matrix of Alloy 690 are much more than Fe2+ 15
ions because the Ni content in Alloy 690 is much higher than Fe. However, in the present work, there is a large amount of Fe2+ ions in the region B, most of which is migrated from the regions C and D, and a small part is released from Alloy 690 and 405 SS in the region B. Ni2+ ions and O2 react with Fe2+ ions to produce NiFe2O4, and the remaining Fe2+ ions react with O2 to form Fe2O3, which results in a increase of thickness of the outer layer of oxide film (Fig. 12(e)). As a result, the thick oxide film in the region B can enhance the occlusion effect of the crevice, restricting the mass transport between the bulk and the crevice solutions, which further exacerbate the
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crevice corrosion and affect the development of the oxide films within the crevice. 5. Conclusions
Effects of 405 SS on crevice corrosion behavior of Alloy 690 in 290 oC pure
water containing 3 ppm DO were investigated. Some conclusions could be drawn as
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follows.
(1) A thin inner layer of oxide film was formed within the crevice of Alloy 690,
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suggesting a low corrosion rate. It was attributed to the formation of continuous Cr-rich protective inner layer and the formation of Ni-rich layer beneath this inner
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layer formed on Alloy 690 as well as the consumption of most of oxygen by Fe2+ ions dissolved from 405 SS.
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(2) The outer layer of oxide film on Alloy 690 within the crevice and at crevice mouth became thick due to the precipitation of Fe2+ ions dissolved from 405 SS. (3) Along the thickness of the inner layer of oxide film on Alloy 690 within the
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crevice, there was a decrease of Fe content from the surface of the inner layer to the oxide/matrix interface due to the diffusion of Fe2+ ions into the inner layer,
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while the Cr concentration was highest in the middle region of the inner layer.
(4) The outer layer of oxide film mainly consisted of Ni-Fe spinel and Fe2O3 at the crevice mouth, Fe2O3 within the crevice, and Fe3O4 at the deeper site within the crevice. The inner layer of oxide film mainly consisted of Fe-Cr spinel and Cr2O3 within the crevice, and porous NiO at the crevice mouth. Acknowledgements This study was jointly supported by the National Natural Science Foundation of 16
China (No. 51671201); the National Science and Technology Major Project (No. 2017ZX06002003-004-002); the Key Programs of the Chinese Academy of Sciences (Research on the Development of Nuclear Power Materials and Service Security Technology, No. ZDRW-CN-2017-1); and the Innovation Fund of Institute of Metal Research, Chinese Academy of Sciences (No. SCJJ-2013-ZD-02). The authors are grateful to Xue Liang, Yifeng Li and Xiaodong Lin for their support to perform the
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TEM analyses.
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[49] D.D. Macdonald, J. Electrochem. Soc. 139 (1992) 3432-3449. [50] D.D. Macdonald, Pure Appl. Chem. 71 (1999) 951-978. [51] B. Stellwag, Corros. Sci. 40 (1998) 337-370. [52] P. Deng, Q. Peng, E.H. Han, W. Ke, Corrosion 73 (10) 1237-1249. [53] R.J. Lemire, G.A. McRae, J. Nucl. Mater. 294 (2001) 141-147. [54] K. Kruska, S. Lozano-Perez, D.W. Saxey, T. Terachi, T. Yamada, T. Smith, D.W. George, Corros. Sci. 63 (2012) 225-233. [55] N.K. Das, K. Suzuki, K. Ogawa, T. Shoji, Corros. Sci. 51 (2009) 908-913.
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Figure list
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Fig. 1. Schematic diagrams of crevice corrosion device (a) and corresponding crevice
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specimen geometry (b) used in high-temperature water.
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Fig. 2. Surface morphologies of crevice specimens after 500 h exposure test in 290 oC
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water containing 3 ppm DO: (a) Alloy 690; (b) 405 SS; (c) magnified image of
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rectangular area in (a); (d) magnified image of rectangular area in (b).
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Fig. 3. XRD patterns of oxide films formed on crevice specimens after 500 h
exposure test in 290 oC water containing 3 ppm DO: (a) Alloy 690, (b) 405 SS, (c, d)
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magnified images of rectangular areas in (a); (e, f, and g) magnified images of
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rectangular areas in (b).
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Fig. 4. Raman spectra of oxide films formed on crevice specimens after 500 h
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exposure test in 290 oC water containing 3 ppm DO: (a-e) Alloy 690; (f-i) 405 SS.
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Fig. 5. SEM morphologies of oxide films formed on crevice specimens after 500 h
exposure test in 290 oC water containing 3 ppm DO: (a) free surface of Alloy 690; (b,
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c, and d) crevice mouth of Alloy 690; (e) site within the crevice of Alloy 690; (f) deeper site within the crevice of Alloy 690; (g) crevice mouth of 405 SS; (h) site
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within the crevice of 405 SS; (i) deeper site within the crevice of 405 SS.
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Fig. 6. XPS depth profiles of Fe (a), Ni (b), Cr (c), and O (d) in the oxide films
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formed on Alloy 690 after 500 h exposure test in 290 oC water containing 3 ppm DO.
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Fig. 7. XPS depth profiles of O in oxide films formed on deeper sites within the
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crevice of Alloy 690 or 405 SS after 500 h exposure tests in 290 oC water containing 3
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ppm DO.
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Fig. 8. TEM observation and analysis of cross-section of oxide film formed at crevice
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mouth of Alloy 690 after 500 h exposure test in 290 oC water containing 3 ppm DO: (a) TEM observation of the cross-section of the oxide film; (b) mappings for Ni, Cr, Fe and O; (c) EDX point-scan collected along line shown in (a); (d) selected area
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electron diffraction patterns and high resolution TEM observation of oxide film.
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Fig. 9. TEM observation and analysis of the cross-section of oxide film formed within the crevice of Alloy 690 after 500 h exposure tests in 290 oC water containing 3 ppm DO: (a) TEM observation of the cross-section of the oxide film; (b) mappings for Ni,
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Cr, Fe and O; (c) EDX point-scan collected along line shown in (a); (d) selected area
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electron diffraction patterns and high resolution TEM observation of oxide film.
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Fig. 10. TEM observation and analysis of cross-section of oxide film formed at deeper
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site within the crevice of Alloy 690 after 500 h exposure tests in 290 oC water containing 3 ppm DO: (a) TEM observation of the cross-section of the oxide film; (b) mappings for Ni, Cr, Fe and O; (c) EDX point-scan collected along line shown in (a);
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oxide film.
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(d) selected area electron diffraction patterns and high resolution TEM observation of
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Fig. 11. EIS results of Alloy 690 and 405 SS in 290 oC water with different DO levels:
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(a) Nyquist plots; (b) equivalent circuit used to simulate the EIS results.
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Fig. 12. Schematics showing the influencing mechanism of 405 SS on oxidation
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behavior of Alloy 690 during crevice corrosion in 290 oC water containing 3 ppm DO.
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Fig. 13. Schematics showing formation mechanism of Ni-rich layer and mechanism of
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metal corrosion mitigated by Ni-rich layer.
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Table list: Table 1: Chemical compositions of Alloy 690 and 405 SS (wt.%). Materials C N S P Mn Ti Al Si Cu Fe Cr Ni Alloy 0.03 0.013 0.001 0.007 0.29 0.2 0.2 0.29 0.01 10.5 29.73 57.7 690 405 SS 0.05 0.001 0.015 0.45 0.01 0.02 0.33 86.66 12.37 0.094
Table 2: Experimental conditions in the present work. Deionized water
Inlet water conductivity
0.06-0.075 S cm-1
Flow rate
9-10 L h-1
Pressure
8 MPa
Temperature in autoclave
290 ± 2 °C
Dissolved oxygen
3 ppm (by weight)
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Solution
500 h
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Exposure time
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PII:
S1005-0302(20)30131-6
DOI:
https://doi.org/10.1016/j.jmst.2020.02.004
Reference:
JMST 1980
To appear in:
Journal of Materials Science & Technology
Received Date:
16 July 2019
Revised Date:
19 November 2019
Accepted Date:
25 November 2019
Please cite this article as: Ning F, Tan J, Wu X, Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2020.02.004
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Research Article
Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water Fangqiang Ning 1, 2, Jibo Tan 1, Xinqiang Wu 1,*
1
CAS Key Laboratory of Nuclear Materials and Safety Assessment, Liaoning Key
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Laboratory for Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2
School of Materials Science and Engineering, University of Science and Technology
*
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of China, Hefei 230026, China
Corresponding author.
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E-mail address: [email protected] (X. Wu).
25 November 2019]
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[Received 16 July 2019; Received in revised form 19 November 2019; Accepted
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Effects of 405 stainless steel (405 SS) on crevice corrosion behavior of Alloy 690 in high-temperature pure water were investigated. Results revealed that the corrosion
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rate of Alloy 690 was low within the crevice. It was likely attributed to the fact that a Cr-rich inner oxide film and a Ni-rich layer beneath this oxide film formed upon
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Alloy 690, inhibiting the diffusion of oxygen towards the oxide/matrix interface. Moreover, the Fe2+ ions dissolved from 405 SS consumed most of oxygen, leading to less oxygen participating in the oxidation of Alloy 690. In addition, it was found that Fe concentration continuously decreased from the surface of the inner oxide film to the oxide/matrix interface of Alloy 690 within the crevice, which was probably due to the diffusion of Fe2+ ions dissolved from 405 SS into the inner oxide film.
1
Keywords: Alloy; High-temperature corrosion; Crevice corrosion; Oxidation 1. Introduction Nickel-based alloy 690 has been extensively used in steam generator (SG) tube of pressurized water reactor (PWR) nuclear power plants (NPPs) owing to its excellent corrosion resistance [1-4]. However, many key parts of SGs in secondary circuit in a high-temperature (284-305 ºC) water environment, such as tube/tube sheets, and tube/support plate crevices, are vulnerable to crevice corrosion during
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long-term service [5-10]. The support plate is generally made of stainless steel (such as 405 SS). Crevice corrosion usually occurs in occluded regions, which is difficult to be detected and often has a long incubation period before attack takes place. Once it
initiates, it may lead to unexpected and catastrophic damage to the structural materials
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within the crevices [11-14]. Therefore, crevice corrosion has received much attention in operating PWR NPPs.
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Almost all kind of metals that rely on passive film for corrosion resistance are prone to crevice corrosion. The main reasons of crevice corrosion of structural
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materials in PWR NPPs are the presence of geometric crevices, which result in restricted mass transport between the crevice and bulk solution [11, 15, 16]. Therefore, the depletion of oxygen and abnormal ionic concentration exist within the
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crevice. Many researchers have investigated the crevice corrosion behavior of metallic materials and two mechanisms have been proposed to account for crevice corrosion,
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namely, critical crevice solution theory (CCST) and ohmic potential drop theory (IRDT) [11, 13, 17]. According to the CCST, enrichment of hydrogen ions and
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invasive ions increases the corrosivity of the crevice solution, which can destroy the passive film and cause crevice corrosion. According to the IRDT, when the potential drop (IR drop) from outside to inside of the crevice is greater than a critical potential (ΔΦ*), the state of metal within the crevice transforms from the passivity to activity, which causes crevice corrosion. Both theories have same viewpoint that oxygen is depleted within the crevice. Therefore, the dissolved oxygen (DO) plays an important role in the crevice corrosion of alloys in high-temperature water. 2
Many researchers have studied the effects of different factors (containing crevice width, crevice depth, temperature and alloy elements etc.) on the crevice corrosion behavior of alloys [9, 10, 14, 18-22]. Chen et al. [9, 10] studied the effects of crevice geometry on the crevice corrosion behavior of 304 SS in high-temperature water. They found that different crevice widths resulted in different distributions of DO and the crevice depth mainly influenced the pH value within the crevice solution, which eventually affected the development of oxide films within the crevice. Some work [19-22] focused on the effects of temperature on the crevice corrosion resistance of
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alloys. It was found that the passive film breakdown/repassivation transient had a temperature threshold, above which the crevice corrosion possibly occurred. Actually, the temperature mainly affected the critical breakthrough potential of passive film.
The critical breakthrough potential gradually decreased with increasing temperature.
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Saito et al. [18] studied the effects of alloy elements on the oxide films within the
crevice of austenitic alloys exposed to 280 oC water, they found that a protective Cr-
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rich oxide film was formed within the crevice of Alloy 600 containing higher Cr content, whereas a non-protective oxide film consisting of NiO and NiFe2O4 was
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formed within the crevice of nickel-based alloys containing lower Cr content. Abella et al. [8] studied the electrochemical nature of SG crevice corrosion in 200 oC water
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containing 10 ppb NaCl. They found that significant coupling currents flowed between the different components in a model SG tube (Alloy 600) and support plate (AISI 4140 steel) crevice. However, the effects of different metallic materials on the
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development of oxide films within the crevice of alloys in high-temperature water have not been fully clarified and reported. The investigation on this aspect is of
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significance for understanding the mechanism of crevice corrosion of different metallic materials during long-term service of PWR NPPs and finding ways to prevent crevice corrosion. The objective of the present work is to investigate the effects of 405 SS on the development of oxide films within the crevice of Alloy 690 in high-temperature water. The oxide films within the crevice of Alloy 690 and 405 SS after exposure tests in high-temperature water were analyzed by area-selected X-ray diffraction (XRD), 3
Raman spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The electrochemical impedance spectrum (EIS) was performed to investigate the effects of DO concentration on the resistance of oxide films on 405 SS and Alloy 690 in hightemperature water. 2. Experimental 2.1. Crevice specimen and apparatus of testing loop Fig. 1(a) shows the schematic diagram of the crevice corrosion test device in
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high-temperature water used in the present work. The device was composed of a zirconia bolt, top sample made of 405 SS, bottom sample made of Alloy 690 and a nut made of 405 SS. The bottom sample was immovable. The width of the crevice was
adjusted accurately by rotating angle of the top sample. The length of the crevice was
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adjusted accurately by changing the radius of the top and bottom samples. Fig. 1(b) shows the sizes of the crevice test specimens used in the present work. The crevice
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width and length were controlled constantly as 125 μm and 4 mm, respectively. Exposure tests of crevice specimens in high-temperature pressurized water were
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implemented in a 316 SS autoclave with a capacity of 2 L. More detailed information of the crevice device and testing system was described in the previous work [9, 23].
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2.2. Materials and exposure tests
Alloy 690 (used as SG tube) and 405 SS (used as support plate) were investigated in the present work. Their compositions are listed in Table 1. The
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specimens were mechanically abraded using silicon-carbide paper up to 2000 grit and degreased with ethanol in ultrasonic washer before the exposure tests.
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Table 2 shows the testing conditions in the present work. In actual secondary
water, in order to decrease corrosion rate of structure materials, the pH adjusters and corrosion inhibitors are usually added, and the DO < 5 ppb. In order to better understand the effects of 405 SS on the development of oxide films within the crevice of Alloy 690 and clarify the oxidation mechanism during the crevice corrosion in high-temperature water, a high concentration of DO (3 ppm, by weight) and pure water were used in the present work. It was believed that a high concentration of DO 4
could enhance crevice corrosion in high-temperature water, which was helpful to observe the oxide films within the crevice and to understand the effects of 405 SS. 2.3. Electrochemical measurement The electrochemical tests were conducted using a typical three electrode system. All work electrodes (405 SS and Alloy 690) with 10 mm 10 mm area were ground using silicon-carbide paper up to 2000 grit. The work electrodes of 405 SS were welded to 304 SS wires, whereas the work electrodes of Alloy 690 were welded to nickel wires. The counter electrode (Pt) was welded to a Pt wire. All wires were
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covered with heat-shrink PTFE tubes. The Ag-AgCl (0.1 M KCl) pressure-balanced external electrode was used. The reference electrode was maintained at ambient temperature with a water-cooling system.
The EIS was measured in a frequency range from 100 kHz to 10 mHz at an
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amplitude of 10 mV (rms) at open circuit potential. All the electrochemical tests were performed after 72 h exposure in high-temperature water to form relatively stable
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oxide films on the surface of specimens. The electrochemical experiments were performed with an EG&G Model 273A and a Model 52101 lock-in amplifier
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controlled by PowerSuite software. The EIS results were analyzed using Zsimpwin software.
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2.4. Analysis of the oxide films
After exposure tests, the surfaces of specimens were cleaned using cotton that absorbed deionized water and dried carefully using absorbent paper. The surface
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macro-morphologies of specimens were examined using a Leica S6D and an OLS4000 3D stereomicroscope. The micro-morphologies of specimens were
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examined using a scanning electron microscopy (SEM) (FEI XL30) equipped with an energy-dispersive spectrometer (EDS). The structure of oxide film was analyzed by a BWS905 custom Raman system and a Bruker D8 Discover area-selected X-ray diffraction (XRD) analyzer with Co K alpha radiation. The detection range of the area-selected XRD analysis was a circle with a diameter of 0.5 mm, which could accurately analyze the oxides in each region. An ESCALAB250 X-ray photoelectron spectrometer (XPS) was used to analyze the thickness and composition of oxide film. 5
The accurate depth profile and crystallographic details on the cross-section of the oxide film were analyzed using a JEM-2100 TEM operating at 200 kV. To investigate crystallographic details of the inner layer of oxide film, the high-resolution TEM images were captured by a Gatan 4k charge coupled device camera and analyzed using Fast Fourier Transform (FFT) technique. The TEM samples of the oxide films were prepared by a focused ion beam with a Ga ion sputtering (FEI Quanta 200 3D). 3. Results 3.1. Surface macro-morphologies
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Fig. 2 shows the surface morphology of Alloy 690 (Fig. 2(a, c)) and 405 SS (Fig. 2(b, d)) after 500 h exposure tests in 290 ºC water containing 3 ppm DO. The
variations in color and corrosion morphology of different regions indicate that crevice corrosion occurred and the oxide films were different in different regions after the
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exposure tests. The difference in composition and structure of the oxide films formed in different regions was discussed in the following sections. Based on the differences
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in corrosion morphology, composition and structure of the oxide films, surface of the Alloy 690 could be divided into four regions, namely, free surface (region A), crevice
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mouth (region B), within the crevice (region C) and deeper site within the crevice (region D). In contrast, the surface of 405 SS could be divided into three regions
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except for region A, a free surface part. From crevice mouth to deeper site within the crevice, the widths of regions B, C and D were approximately 0.7, 1.8 respectively.
and 1.5 mm,
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3.2. XRD analysis
Fig. 3 shows the XRD patterns of the oxide films formed on Alloy 690 (Fig. 3(a))
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and 405 SS (Fig. 3(b)) respectively. The characteristic peaks of spinel oxide, NiO and Ni(OH)2 were detected in the region A of Alloy 690. The previous work [24, 25] has proven that the spinel oxide was (Ni, Fe)Fe2O4. In the other regions, the characteristic peaks of oxide films formed on Alloy 690 were the same as those on 405 SS. The characteristic peaks of spinel and hematite oxides were detected in the region B. The characteristic peaks of hematite oxide were detected in the region C and the characteristic peaks of spinel oxide were detected in the region D. In addition, in the 6
region C, the intensity of characteristic peaks of hematite oxide on Alloy 690 was stronger than that on 405 SS, suggesting that the content of hematite oxide on Alloy 690 was more than that on 405 SS. In the region D, the intensity of characteristic peaks of spinel oxide on Alloy 690 was weaker than that on 405 SS, suggesting that the content of spinel oxide on Alloy 690 was less than that on 405 SS. For Alloy 690, the intensity of characteristic peaks of hematite oxide in the region C was stronger than that of spinel oxide in the region D, suggesting that the content of hematite oxide in the region C was more than that of spinel oxide in the region D, and the reverse was
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true for 405 SS. 3.3. Raman spectra
Fig. 4 shows the Raman spectra of oxide films formed on Alloy 690 (Fig. 4(b-e)) and 405 SS (Fig. 4(g-i)), respectively. The Raman peaks of NiO (478 cm-1) [26, 27]
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and NiFe2O4 (693 cm-1) [27, 28] were detected in the region A. The Raman peaks of Fe2O3 (222, 292, 402, 478, 1304 cm-1) [29, 30] and NiFe2O4 were detected in the
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region B. Only the Raman peaks of Fe2O3 and Fe3O4 (660 cm-1) [29, 30] were detected in the regions C and D, respectively. This was consistent with the previous
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XRD results.
3.4. Surface morphologies of oxide films
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Fig. 5 shows the SEM morphology and EDS results of the oxide films formed on Alloy 690 and 405 SS. The oxide films in different regions or on different materials showed different characteristics. Sparsely dispersed oxide clusters with some straight
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sidesteps grew on the top of a porous inner layer in the region A (Fig. 5(a)) of Alloy 690. The previous work [24, 25] has proven that the oxide clusters were NiFe2O4 and
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the porous inner oxides were NiO. Densely dispersed typical spinel oxides and big equiaxed oxide particles were observed in the region B (Fig. 5(c, d and g)). The spinel oxides were NiFe2O4 and the equiaxed oxide particles were Fe2O3 by combining the EDS results with the XRD and Raman results. In the region C, the big oxide particles were observed on Alloy 690 (Fig. 5(e)) and the small oxide particles were observed on 405 SS (Fig. 5(h)). In the region D, sparsely dispersed typical spinel oxides were observed on Alloy 690 (Fig. 5(f)) and densely dispersed typical spinel oxides were 7
observed on 405 SS (Fig. 5(i)). Because the Fe content in Alloy 690 was approximately 10 at.% and the previous work [31] has proven that the Cr-rich oxides were formed within the crevice made of the same type of Alloy 690, it could be speculated that the Fe element in the Fe-rich outer oxide particles in the regions B, C and D of Alloy 690 mainly came from 405 SS. 3.5. XPS results Fig. 6 shows the XPS depth profiles of the oxide films formed in different regions of Alloy 690. Because the absorbed oxygen and carbon were removed during
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the initial sputtering time (0-60 s), the elements (Fe, Cr, Ni, O) content after 60 s sputtering was taken as the initial value. The thickness of the oxide films is defined
from the depth profile where the intensity of oxygen level reaches 50% of its initial value [24, 32-37]. The thickness of oxide films from region B to region D was
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gradually decreased (Fig. 6(d)). The Fe content in all regions was higher than that in matrix, and it was highest in the regions B and C (Fig. 6(a)). Except that the Ni
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content in the region A was slightly higher than that in matrix, it in the other regions was lower than that in matrix (Fig. 6(b)). The Cr content in all regions was lower than
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that in matrix (Fig. 6(c)). The XPS results also revealed that the oxides formed in the region B were mainly Ni-Fe oxides and those in the region C and D were mainly Fe-
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rich oxides. It was noticeable that the Fe content in the inner layer of oxide films within the crevice (regions C and D) was much higher than that in matrix. Therefore, the effects of 405 SS on the inner layer of oxide films formed in the region B, C and
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D of Alloy 690 can be expected.
Fig. 7 shows the XPS depth profiles of O 1s in the oxide films formed in the
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region D of Alloy 690 and 405 SS. The thickness of oxide films formed on 405 SS was more than that on Alloy 690. The oxide film formed on Alloy 690 for the crevice made of the same type of Alloy 690 [31] was thicker than that on Alloy 690 for the crevice made of different metals (Alloy 690/405 SS). It could be speculated that the presence of 405 SS decreased the thickness of oxide film formed on Alloy 690. 3.6. Cross-section observation and analysis by TEM Figs. 8 to 10 show the TEM observation and analysis of the oxide films formed 8
in the regions B, C and D of Alloy 690, respectively. The oxide film was a double layer structure, and a clear interface was observed between the outer and inner layers. However, the oxide films in different regions showed different thickness, composition and structure. The inner layer of oxide film in the region B was thicker than that in the regions C and D, and it is porous and not protective, suggesting that the corrosion of Alloy 690 in the region B was more serious than that in the regions C and D. According to the XPS (Fig. 6(d)) results, the oxide films in the region C were thicker than that in the region D. However, the TEM observation (Figs. 9 and 10) suggested
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that the thickness of the inner layer of oxide films in these two regions was almost the same, with about 20 nm. It could be concluded that the outer layer of oxide film
consisting of Fe2O3 in the region C were thicker than that consisting of Fe3O4 in the region D.
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In the region B (Fig. 8), the EDX and SAED analyses reveal that two kinds of oxide particles in the outer layer of oxide film were Ni-Fe spinel and Fe2O3,
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respectively. The crystallographic analysis of the inner layer of oxide film by FFT (equivalent to electron diffraction patterns) indicates that the inner layer of oxide film
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was mainly composed of NiO with a normalized composition of 87.6 at.% Ni, 7.5 at.% Fe and 4.9 at.% Cr. The average contents of Fe, Cr and Ni in the inner layer of
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oxide film were obtained from a number of EDX spot analyses conducted in the middle of the inner layer of oxide film. The NiO layer was porous and could not prevent oxygen from diffusing inward, which resulted in more serious corrosion of
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Alloy 690 in the region B. The thickness of the inner layer of oxide film was approximately 370 nm.
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In the region C (Fig. 9), EDX and SAED analyses reveal that the outer layer of
oxide film was mainly Fe2O3 with a normalized composition of 97.2 at.% Fe, 1.8 at.% Cr and 1.0 at.% Ni. The inner layer of oxide film was mainly composed of Fe(Fe,Cr)2O4 and Cr2O3. The composition of the inner layer of oxide film was 35.7 at.% Fe, 58.5 at.% Cr and 5.8 at.% Ni. EDX mapping (Fig. 9(b)) and line-scans (Fig. 9(c)) show enrichments of Fe and Cr in the inner layer of oxide film. An enrichment of Ni and a depletion of Cr in the matrix beneath the oxide film were observed, while 9
no such a segregation of Fe was observed, suggesting that the Fe element in the innerlayer film was mainly from the solution in crevice and the Cr element in the innerlayer film was from the matrix. In the region D (Fig. 10), EDX and SAED analyses indicate that the outer layer of oxide film was mainly Fe3O4 with a normalized composition of 95.3 at.% Fe, 4.5 at.% Cr and 0.2 at.% Ni. The inner layer of oxide film with a normalized composition of 26.2 at.% Fe, 56.2 at.% Cr and 17.6 at.% Ni was consistent with that in the region C. An enrichment of Ni and a depletion of Cr in the matrix beneath the oxide film
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were also observed. 3.7. Electrochemical measurements
Chen et al. [10] have studied the distribution of DO within the crevice of 304 SS with the crevice width of 125 m and crevice length of 4 mm in 290 oC water
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containing 3 ppm DO, which is consistent with the experimental conditions of the
present work. They found that the DO concentration was almost equal to 3 ppm in the
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region B, approximately 200 ppb in the region C, and less than 5 ppb in the region D. Therefore, the electrochemical measurements were conducted at DO = 5 ppb and DO
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= 200 ppb in the present work to compare the film resistance of Alloy 690 and 405 SS.
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Fig. 11 shows the EIS results for Alloy 690 and 405 SS after 72 h exposure tests in 290 oC pure water with different DO levels. Two unfinished semicircles could be distinguished in all Nyquist diagrams (Fig. 11(a)). The semicircle radius of Alloy 690
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was larger than that of 405 SS at DO = 5 ppb, and the reverse was true at DO = 200 ppb. A Warburg impedance should be considered if one assumes that the diffusion
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process plays an important role. An equivalent circuit [34, 35, 38] as shown in Fig. 11(b) was used to fit the EIS
results. A constant phase element (CPE) was introduced to represent a non-ideal capacitance. A CPE has an impedance equal to 1/Y0(j)n, where Y0 is the CPE coefficient and n is the CPE exponent. The meaning of each element is described below. Rsol is the resistance of solution, CPE1 is the non-ideal capacitance of the double layer, Rct is the resistance of charge transfer, W is the Warburg impedance, 10
CPE2 is the non-ideal capacitance of the oxide film, and Rfilm is the resistance of the inner layer of oxide film. The corrosion resistance of a material can be qualitatively assessed by the Rfilm [34, 35, 38-40]. Fig. 11(a) shows the fitting results of Rfilm. The Rfilm of Alloy 690 was larger than that of 405 SS at DO = 5 ppb, suggesting that the inner layer of oxide film formed on Alloy 690 was more protective than that on 405 SS. However, the inner layer of oxide film formed on 405 SS showed better protectivity than that on Alloy 690 at DO = 200 ppb. 4. Discussion
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The present work indicates that the oxide films formed on the free surface (region A) and within the crevice (regions B, C and D) of Alloy 690 after 500 h
exposure tests exhibit different morphologies (Fig. 5), structures (Figs. 3 and 4) and thicknesses (Figs. 6-10). In addition, it is found that 405 SS can significantly affect
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the formation and characteristic of oxide films within the crevice of Alloy 690. Fig. 12 shows the crevice corrosion process and the effects of 405 SS on the development of
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oxide films within the crevice of Alloy 690 during the crevice corrosion. During the crevice corrosion, the presence of crevice geometry can restrict the
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transport of the ions and oxygen between the bulk and the crevice solutions [11, 13, 17], which eventually influence the ions and DO concentration distributions within
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the crevice. The consumption rate of DO within the crevice is faster than the diffusion rate of DO from the bulk solution into crevice solution [15], which results in the depletion of DO within the crevice as shown in Fig. 12(a). In addition, the
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conductivity of water used in the present work is 0.06-0.075 S/cm, which is close to the limit of pure water, suggesting that the aggressive anions concentration in water is
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extremely low. Although the aggressive anions can be enriched in solution within the crevice with the development of crevice corrosion, it is believed that their concentrations are not too high. The acidification degree of the solution within the crevice is not serious due to the electroneutrality principle. Chen et al. [10] have found that the pH value decreased to about 5 within the crevice of 304 SS under the condition of 125 m crevice width and 4 mm crevice length in 290 oC pure water, which is close to neutral solution with a pH value of 5.6 at this temperature [41]. It 11
can be speculated that the pH within the crevice is close to neutral in the present work. Therefore, the DO concentration distribution plays an important role on the formation of oxide films within the crevice in the present work. The main difference between the oxide films formed on Alloy 690 in deaeration and oxygenated high-temperature water is that the oxide film formed in deaeration water is Cr-rich, and the oxide film formed in oxygenated water is Ni-Fe-rich [24, 25, 42, 43]. For the general corrosion of Alloy 690 in high-temperature pure water containing 3 ppm DO, the oxide film was composed of an outer layer with oxide particles of Ni-
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Fe spinel and an inner porous layer of NiO [24, 25]. In the present work, the oxide film formed in the region A (Fig. 5(a)) has similar structure and composition as the
general corrosion. However, the change in the hydrochemistry (including metal ions, DO) within the crevice leads to the formation of different oxide films in different
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regions.
The corrosion resistance of 405 SS is inferior to that of Alloy 690 in the region D
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(Fig. 11), indicating that it can release a large amount of Fe2+ ions into the crevice solution. Because the Fe2+ ions concentration near the surface of 405 SS is higher than
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that of Alloy 690, the Fe2+ ions can diffuse from 405 SS to Alloy 690. Abella et al. [8] have also found that significant coupling current flowed from AISI 4140 steel to Alloy
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600 within the crevice in high-temperature deaerated water. It can be speculated that the coupling current is produced by the diffusion of Fe2+ ions dissolved from AISI 4140 steel, which is consistent with the present work. Meanwhile, the crevice
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corrosion current flows from the region D to region A [11, 13, 17], resulting in the Fe2+ ions to diffuse in the same direction. Eventually, Fe2+ ions diffuse simultaneously
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to the Alloy 690 and outside of the crevice as shown in Fig. 12(b), resulting in a distribution of Fe2+ ions concentration near the surface of Alloy 690 within the crevice as shown in Fig. 12(c), which is well confirmed by the depth profile of Fe (Fig. 6(a)). In the region C, the good film resistance of 405 SS and small amount of oxides on 405 SS (Fig. 5(h)) also suggest that most of Fe elements in oxide film formed on Alloy 690 may be from the region D. In the region D, the DO concentration of less than 5 ppb is in the potential-pH 12
region of stable Fe3O4 in high-temperature water, which has been justified by the previous work [9, 10]. A large amount of Fe3O4 oxide particles formed on 405 SS by dissolution-precipitation mechanism [36-38, 44, 45] (Fig. 5(i)), which can consume most of oxygen. To form Fe3O4, the oxygen needs continually to diffuse to 405 SS (Fig. 12(d)), resulting in depletion of oxygen near Alloy 690 (Fig. 12(d)). Therefore, there is only a small amount of oxygen to oxidize Alloy 690, resulting in a thin oxide film formed on Alloy 690, as evidenced by the depth profiles of oxygen (Figs. 7 and 10).
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In the region C, because the DO concentration of 200 ppb is in the potential-pH region of stable Fe2O3 in high-temperature water, the Fe2+ ions migrated from the
region D are oxidized to Fe2O3 on Alloy 690. Due to the high concentration of Fe2+ ions near Alloy 690 (Fig. 12(c)), most of oxygen is used to oxidize Fe2+ ions (Fig.
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12(d)), eventually forming a thicker layer of Fe2O3 (Fig. 12(e)). The EIS results (Fig. 11) suggested that the corrosion resistance of Alloy 690 is not good in high-
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temperature water containing 200 ppb DO. Wang et al. [46] also found that the corrosion resistance of Alloy 800 (43 at.% Fe) is better than Alloy 690 (10 at.% Fe)
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when the DO concentration was out of the potential-pH region of stable Cr2O3. Some previous work [25, 35, 46] has found that the corrosion resistance of materials can
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benefit from higher Fe concentration at high DO concentration (DO 100 ppb) in high-temperature water. In the present work, the presence of Fe2O3 also proves that the DO in high-temperature water is more than 100 ppb [47], suggesting that the
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corrosion resistance of Alloy 690 is not good. However, the TEM results (Figs. 9 and 10) showed that the inner layer of oxide film in the region C was as thin as that in the
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region D, suggesting that Alloy 690 is not severely corroded in the region C. Furthermore, a thicker outer-layer oxide film consisting of Fe2O3 was observed. It can be speculated that the Fe2+ ions consume a large amount of oxygen to form Fe2O3, thus only a small amount of oxygen is available to oxidize Alloy 690, which can mitigate corrosion of Alloy 690. In the regions C and D, the Fe2+ ions dissolved from 405 SS can diffuse into the inner layer of oxide film formed on Alloy 690. Along the thickness direction of inner 13
layer of oxide film, a decrease of Fe concentration was observed from the surface of the inner layer to the oxide/matrix interface (Figs. 9 and 10), also suggesting the diffusion of Fe2+ ions into inner layer of oxide film. In the middle region of inner layer of oxide film, the Cr concentration was highest, indicating Cr-rich oxides formed in the middle region of inner layer of oxide film. The previous work [31] has found that the inner layer of oxide film formed within the crevice made of the same type of Alloy 690 was Cr-rich oxides. Therefore, according to the point-defect mechanism for Ni-Cr-Fe alloys [44, 48-51], the formation of Fe-Cr spinel is
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determined by the diffusion of Fe2+ ions into the Cr-rich inner layer of oxide film by the following reactions. Fe(aq) Fei(chromia)
(1)
Fei(chromia) + 2CrCr(chromia) + 3OO(chromia) FeFe(spinel) + 2CrCr(spinel)
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+ 3OO(spinel) + VO(spinel)
(2)
where Fe(aq) represents Fe2+ ions in solution, Fei(chromia) represents interstitial cation
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in chromia, MM(chromia) and (spinel) represent normal metal cation lattice sites in oxide (M = Fe and Cr), OO(chromia) and (spinel) represent oxygen ion lattice sites in oxide, and
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VO(spinel) represents oxygen vacancy in oxide. According to the Gibbs free energytemperature (G0-T) map of Fe-, Cr- and Ni-oxides, the critical oxygen partial
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pressure required for formation of the oxide is Cr < Fe < Ni, indicating that the oxidation of Cr takes precedence over Ni and Fe [37, 52, 53]. When oxygen diffuses inward, the Cr is preferentially oxidized to form Cr-rich oxide at oxide/matrix
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interface. The local oxygen concentration at the oxide/matrix interface is not high enough to oxidize Ni. Due to a lower diffusion rate of Ni in Cr-rich oxide, the
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unoxidized Ni is retained in oxide films. With increasing exposure time, the formation of a large amount of Cr-rich oxides and the diffusion of Fe2+ into Cr-rich oxides can push the unoxidized Ni into the base metal to occupy the lattice sites of Cr and Fe in Alloy 690 according to reaction (3) [54, 55]. Nii(oxide) + VM(metal) NiNi,M(metal)
(3)
where Nii(oxide) represents Ni in oxide, VM(metal) represents metal vacancy in the alloy (M = Cr and Fe), Ni Ni,M (metal) represents that Ni occupy the lattice sites of Fe and Cr 14
in the alloy. This leads to the enrichment of Ni in the matrix beneath the inner layer of oxide film as shown in Fig. 12(e). Since Ni is not easily oxidized at low oxygen concentration, the oxidation process, namely, reaction (4) [44, 49, 50] is inhibited at Ni-rich layer/oxide interface. M MM(oxide)+(/2)VO(oxide) + e
(4)
where M represents metal atom in the alloy, MM(oxide) represents metal cation lattice sites in oxide (M = Ni, Fe and Cr), VO(oxide) represents oxygen vacancy in oxide. This leads to a decrease of oxygen vacancy in oxides, which can further inhibit the
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progress of reaction (5) [44, 49,50] at oxide/solution interface. H2O + VO(oxide) OO + 2H+
(5)
where OO represent oxygen ion at anion sites in oxide. The inhibition of the reaction
(5) at oxide/solution interface indicates that the diffusion of oxygen through the inner
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layer of oxide film towards oxide/matrix interface is inhibited. The above process is shown in Fig. 13. Therefore, the Ni-rich layer can act as a barrier for oxide growth
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and oxygen diffusion through oxide film [54, 55], which is one of the reasons for the formation of a thin inner layer of oxide film within the crevice of Alloy 690. In
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addition, another main reason for the thinning of the oxide film on Alloy 690 is that the continuous Cr-rich inner layer can also act as a barrier layer to resist the corrosion
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of alloy [31, 56].
In the region B, because the DO concentration of approximate 3 ppm is out of the potential-pH region of stable Cr2O3 and Fe2+ ions migrated from the regions C and
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D can not consume so much oxygen, the Cr in the inner layer of oxide film enters into solution as HCrO4- or CrO42- [24, 25]. The inner layer of oxide film becomes porous
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and less protective NiO layer. So, oxygen can continuously diffuse inward and the oxidation processes keep going on, resulting in a thicker inner layer of oxide film (Fig. 8). Meanwhile, Ni and Fe dissolve selectively from the matrix into solution because the diffusion rate of elements in oxide is Fe > Ni > Cr [25, 57, 58]. The previous work [31] has found that when the crevice was made of the same type of Alloy 690, a large amount of NiO was observed in the region B, suggesting that the amount of Ni2+ ions dissolved from the matrix of Alloy 690 are much more than Fe2+ 15
ions because the Ni content in Alloy 690 is much higher than Fe. However, in the present work, there is a large amount of Fe2+ ions in the region B, most of which is migrated from the regions C and D, and a small part is released from Alloy 690 and 405 SS in the region B. Ni2+ ions and O2 react with Fe2+ ions to produce NiFe2O4, and the remaining Fe2+ ions react with O2 to form Fe2O3, which results in a increase of thickness of the outer layer of oxide film (Fig. 12(e)). As a result, the thick oxide film in the region B can enhance the occlusion effect of the crevice, restricting the mass transport between the bulk and the crevice solutions, which further exacerbate the
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crevice corrosion and affect the development of the oxide films within the crevice. 5. Conclusions
Effects of 405 SS on crevice corrosion behavior of Alloy 690 in 290 oC pure
water containing 3 ppm DO were investigated. Some conclusions could be drawn as
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follows.
(1) A thin inner layer of oxide film was formed within the crevice of Alloy 690,
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suggesting a low corrosion rate. It was attributed to the formation of continuous Cr-rich protective inner layer and the formation of Ni-rich layer beneath this inner
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layer formed on Alloy 690 as well as the consumption of most of oxygen by Fe2+ ions dissolved from 405 SS.
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(2) The outer layer of oxide film on Alloy 690 within the crevice and at crevice mouth became thick due to the precipitation of Fe2+ ions dissolved from 405 SS. (3) Along the thickness of the inner layer of oxide film on Alloy 690 within the
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crevice, there was a decrease of Fe content from the surface of the inner layer to the oxide/matrix interface due to the diffusion of Fe2+ ions into the inner layer,
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while the Cr concentration was highest in the middle region of the inner layer.
(4) The outer layer of oxide film mainly consisted of Ni-Fe spinel and Fe2O3 at the crevice mouth, Fe2O3 within the crevice, and Fe3O4 at the deeper site within the crevice. The inner layer of oxide film mainly consisted of Fe-Cr spinel and Cr2O3 within the crevice, and porous NiO at the crevice mouth. Acknowledgements This study was jointly supported by the National Natural Science Foundation of 16
China (No. 51671201); the National Science and Technology Major Project (No. 2017ZX06002003-004-002); the Key Programs of the Chinese Academy of Sciences (Research on the Development of Nuclear Power Materials and Service Security Technology, No. ZDRW-CN-2017-1); and the Innovation Fund of Institute of Metal Research, Chinese Academy of Sciences (No. SCJJ-2013-ZD-02). The authors are grateful to Xue Liang, Yifeng Li and Xiaodong Lin for their support to perform the
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TEM analyses.
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Figure list
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Fig. 1. Schematic diagrams of crevice corrosion device (a) and corresponding crevice
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specimen geometry (b) used in high-temperature water.
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Fig. 2. Surface morphologies of crevice specimens after 500 h exposure test in 290 oC
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water containing 3 ppm DO: (a) Alloy 690; (b) 405 SS; (c) magnified image of
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rectangular area in (a); (d) magnified image of rectangular area in (b).
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Fig. 3. XRD patterns of oxide films formed on crevice specimens after 500 h
exposure test in 290 oC water containing 3 ppm DO: (a) Alloy 690, (b) 405 SS, (c, d)
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magnified images of rectangular areas in (a); (e, f, and g) magnified images of
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rectangular areas in (b).
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Fig. 4. Raman spectra of oxide films formed on crevice specimens after 500 h
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exposure test in 290 oC water containing 3 ppm DO: (a-e) Alloy 690; (f-i) 405 SS.
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Fig. 5. SEM morphologies of oxide films formed on crevice specimens after 500 h
exposure test in 290 oC water containing 3 ppm DO: (a) free surface of Alloy 690; (b,
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c, and d) crevice mouth of Alloy 690; (e) site within the crevice of Alloy 690; (f) deeper site within the crevice of Alloy 690; (g) crevice mouth of 405 SS; (h) site
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within the crevice of 405 SS; (i) deeper site within the crevice of 405 SS.
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Fig. 6. XPS depth profiles of Fe (a), Ni (b), Cr (c), and O (d) in the oxide films
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formed on Alloy 690 after 500 h exposure test in 290 oC water containing 3 ppm DO.
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Fig. 7. XPS depth profiles of O in oxide films formed on deeper sites within the
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crevice of Alloy 690 or 405 SS after 500 h exposure tests in 290 oC water containing 3
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ppm DO.
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Fig. 8. TEM observation and analysis of cross-section of oxide film formed at crevice
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mouth of Alloy 690 after 500 h exposure test in 290 oC water containing 3 ppm DO: (a) TEM observation of the cross-section of the oxide film; (b) mappings for Ni, Cr, Fe and O; (c) EDX point-scan collected along line shown in (a); (d) selected area
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electron diffraction patterns and high resolution TEM observation of oxide film.
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Fig. 9. TEM observation and analysis of the cross-section of oxide film formed within the crevice of Alloy 690 after 500 h exposure tests in 290 oC water containing 3 ppm DO: (a) TEM observation of the cross-section of the oxide film; (b) mappings for Ni,
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Cr, Fe and O; (c) EDX point-scan collected along line shown in (a); (d) selected area
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electron diffraction patterns and high resolution TEM observation of oxide film.
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Fig. 10. TEM observation and analysis of cross-section of oxide film formed at deeper
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site within the crevice of Alloy 690 after 500 h exposure tests in 290 oC water containing 3 ppm DO: (a) TEM observation of the cross-section of the oxide film; (b) mappings for Ni, Cr, Fe and O; (c) EDX point-scan collected along line shown in (a);
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oxide film.
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(d) selected area electron diffraction patterns and high resolution TEM observation of
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Fig. 11. EIS results of Alloy 690 and 405 SS in 290 oC water with different DO levels:
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(a) Nyquist plots; (b) equivalent circuit used to simulate the EIS results.
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Fig. 12. Schematics showing the influencing mechanism of 405 SS on oxidation
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behavior of Alloy 690 during crevice corrosion in 290 oC water containing 3 ppm DO.
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Fig. 13. Schematics showing formation mechanism of Ni-rich layer and mechanism of
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metal corrosion mitigated by Ni-rich layer.
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Table list: Table 1: Chemical compositions of Alloy 690 and 405 SS (wt.%). Materials C N S P Mn Ti Al Si Cu Fe Cr Ni Alloy 0.03 0.013 0.001 0.007 0.29 0.2 0.2 0.29 0.01 10.5 29.73 57.7 690 405 SS 0.05 0.001 0.015 0.45 0.01 0.02 0.33 86.66 12.37 0.094
Table 2: Experimental conditions in the present work. Deionized water
Inlet water conductivity
0.06-0.075 S cm-1
Flow rate
9-10 L h-1
Pressure
8 MPa
Temperature in autoclave
290 ± 2 °C
Dissolved oxygen
3 ppm (by weight)
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Solution
500 h
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Exposure time
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