CORROSION BEHAVIOR OF ALLOY 690 IN SIMULATED PWR …envdeg2015.org/final-proceedings/ENVDEG/papers/ENVDEG055.pdf · Corrosion behavior of alloy TT690 tubing at high flow velocity

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

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CORROSION BEHAVIOR OF ALLOY 690 IN SIMULATED PWR PRIMARY WATER AT HIGH FLOW VELOCITY

Yasuhiro Masaki1, Yumi Momozono1, Manabu Kanzaki1, Shunsuke Taniguchi1, Takafumi Amino1, and Osamu Miyahara2

1 NIPPON STEEL & SUMITOMO METAL CORPORATION, R & D Laboratories, 1-8 Fuso-Cho, Amagasaki, Hyogo 660-0891, Japan

2 NIPPON STEEL & SUMITOMO METAL CORPORATION, Amagasaki Works, 1 Nishino-Cho, Higashi-Mukojima, Amagasaki, Hyogo 660-0856, Japan

ABSTRACT

Corrosion behavior of alloy TT690 tubing at high flow velocity (1.7 m/s) in the simulated PWR primary water has been characterized as a basic study of Ni release from Ni-based alloy. The physical analysis (SEM, TEM, AES, and STEM) of the tubing tested for 860 hours has revealed that the oxide film formed on the alloy consists of three layers: an external layer consisting of lath-shaped crystallites, an inner one consisting of NiCr2O4 crystallites, and an innermost one consisting of Cr2O3 crystallites. Each layer of Cr2O3 and NiCr2O4 shows continuity in structure. Lath-shaped crystallites are rich in Cr and Ni and are supposed to be formed by promoted corrosion by high flow velocity, followed by precipitation. It has been demonstrated that Cr2O3 in the oxide film is formed by oxidation due to inner growth of oxide scale, by assignment of an intrinsic bandgap value (2.9 eV) of Cr2O3 found in photoelectrochemical analysis. It is presumed that high flow velocity promotes the internal oxidative formation of Cr2O3 beneath the surface of the alloy to organize the continuous Cr2O3 layer.

Keywords: alloy TT690, SG, Cr2O3, PWR, high flow velocity, corrosion, oxidation, continuous layer

1. INTRODUCTION

Suppressing Ni release from Ni-based alloys as materials for steam generator tubing is very important, because 58Ni released in the primary water is activated into 58Co in the nuclear core, increasing the global radioactivity of the primary circuit of PWR. In the PWR primary water, passive oxide films formed on the alloys play key roles in the corrosion phenomena including Ni release from the alloy, in addition to stress corrosion cracking (SCC) of Ni alloy. Many basic studies have been addressed to the nature and growth of such protective oxide films in simulated PWR primary water [1-14].

It has been apparent that such oxide films formed on Ni-based alloys take a duplex structure divided into two layers: an external layer formed by precipitation phenomena and an internal one formed by oxidation. It is often the case that NiFe2O4 crystallites shape an external layer sometimes with Ni(OH)2, and Cr-rich oxides such as (FeNi)Cr2O4 constitute the continuous internal layer [1-5]. Recent attention has been paid to whether Cr2O3 crystallites organize a continuous layer or discontinuous one in the case of internal layers including Cr2O3 [6-10,13] because of the barrier property of the continuous Cr2O3 layer for corrosion.

The structure and growth of the oxide films are influenced by various environmental factors in simulated PWR primary water, e.g., temperature [12], dissolved H2 concentration [13,14], dissolved O2

concentration [11], and metal cation concentration [7]. Information obtained in such studies is useful for understanding of corrosion behavior of Ni-based alloy as well as water chemistry management. High flow velocity is one of the most important features of PWR primary water conditions. In primary actual plants, water management requests high flow rate in recirculated systems, where flow velocity in SG tubing reaches several meters per second [15], in order to raise the efficiency of heat exchange between a primary circuit and the secondary one. It can be inferred that high flow velocity affects corrosion behavior in the PWR primary water because corrosion proceeds by mass transport or diffusion process in


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water. However, little attention has been paid to actually exploring corrosion behavior in simulated PWR primary water at high flow velocity.

We have introduced the loop test equipment, which makes it possible to recirculate high temperature water through metal tubing at high velocity (1.7 m/s) comparable to those in the actual plants. The present study has been conducted in order to explore the effect of high flow velocity on the structure and growth of oxide film on alloy TT690 in simulated PWR primary water. The structure and growth of such an oxide film is revealed by physical analysis (SEM, TEM, STEM, and AES) and photoelectrochemical analysis. The oxide film prepared under a static condition is treated as a reference to discuss the effect of flow velocity more clearly.

2. EXPERIMENTAL

2.1 Specimens

Alloy TT690 tubing was used in this study. Chemical composition of the alloy is given in Table 1. The composition of the tubing was determined by the standard test methods in accordance with ASTM E1473. The composition of the tubing was determined by the standard test methods in accordance with ASTM E1473. Alloy 690 TT tubing was prepared by annealing at 1100°C in H2 and thermally-treating at 725 °C for 10 hours in vacuum atmosphere and used as a specimen as it was after these heat treatments (without surface treatment). The tested tubing had an OD of 19 mm and a wall thickness of 1.0 mm.

2.2 Corrosion tests

Corrosion tests were conducted in the simulated PWR primary water condition using two types of testers: the loop tester and the static one. The test was performed at temperature of 325°C. The solution was deaerated water with the addition of 1000 ppm boron and 2 ppm lithium. After the corrosion test for a given duration (860 h), the test tubing was cooled down in the tester, taken out, and then provided for analysis.

In the loop test, pressured water by a high pressure pump was introduced into the preheating unit and heated up to 325°C. High pressured and high temperature water was flown with 5.0 L/min into Alloy TT690 tubing of 1.1 m length installed in the heating unit. In the tubing, an inner bar as a tang was inserted with a clearance of 1.5 mm between the inner surface of tubing and the inner bar in order to raise flow velocity in the tubing to 1.7 m/s. The hot water passed through the tubing was cooled down in a cooling unit, and stocked in the tank once (200L). The adjustment of concentration oxygen and hydrogen were performed in the tank by bubbling pure argon gas followed by pure hydrogen at room temperature. The concentration of dissolved H2 and O2 was maintained at 2.6 ppm and less than 10 ppb, respectively. Without purification by ion-exchange resin, water was recirculated in such a way. All the component materials, e.g., tubing, tanks, valves, and inner parts in some pumps and heaters, which have contact with fluid, are made of titanium for high temperature section and Teflon for low temperature section.

The static test was conducted in the alloy TT690 tubing of 17 cm length, where water was put inside by plugging on both ends using stopcocks made of titanium. All preparations were made in a glove box. After argon gas bubbling for a half day, hydrogen and argon mixed gas (Ar-based 10 vol% H2) was bubbled into water (18 ml) for 10 min, and the other end was then sealed by plugging. Introduction of the gas mixture to water permits hydrogen to dissolve in water at 30 cc/kg (STP). The test tubing prepared in this way was soaked in a stainless autoclave, and the corrosion test was performed at 325°C.

2.3 Physical analysis

A field emission scanning electron microscope (FESEM) JEOL JSM-7001F was used to investigate the surface morphology of oxide films formed on the inner surface of the 690TT tubing after the corrosion test. Characterizations by transmission electron microscopy (TEM) were made on the cross-section of


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specimens. Thin-foil specimens were prepared using a focused ion beam (FIB) Hitachi FB2000A with Ga ion sputtering after a protective resin was covered on the oxide film. TEM and field emission TEM (FETEM) investigations were conducted using JEOL JEM-200CX and JEOL JEM2100-F, respectively. Concentration profiles of Ni, Cr, Fe, and O were measured using an EDS system attached to a scanning TEM (STEM; FEI Titan3 60-300) operating at 300 kV. The diameter of the incident electron beam for EDS analysis was less than 0.1 nm. A part of the interface between the matrix and the oxide layer contiguous with the matrix set to be the incident electron beam for the EDS analysis.

Auger electron spectroscopy (AES) measurements were conducted for investigating compositions and thickness of oxide films using ULVAC-PHI model-680. Depth-profiling analysis was conducted on the area of about 2 mm square with Argon ion sputtering. The sputtering rate as a reference of SiO2 layer was about 0.75 nm/s. The observed area by AES was about 2 m square.

2.4 Photoelectrochemical method and analysis

The photoelectrochemical measurements were conducted in a three-electrode electrochemical cell at 30°C. The test tubing obtained by corrosion test was cut down and processed to a working electrode in such a way that the inner side of the tubing was exposed to the solution and irradiated. A platinum plate was used as a counter electrode and a KCl-saturated (SCE) electrode was used as a reference. The deaerated aqueous solution containing 0.5 mol/L Na2SO4 was used as an electrolyte solution. The working electrode was irradiated through a quartz window of the electrochemical cell with a 300 W Xenon lamp equipped with a monochrometer. The photocurrent was detected with a lock-in amplifier with the light chopped at a frequency of 20Hz. This lock-in technique removed any interference from the sample dark current. A Si-photodiode connected to a digital ammeter was used to record the light intensity in function of the incident wavelength. The photocurrent was measured at a fixed potential (0 V vs. SCE) by scanning the light wavelength from 200 nm (6.2 eV) to 800 nm (1.55 eV).

There is a relationship between photocurrent (Iph) and the bandgap (Eg) of the many semiconductors, as written in the form (1) [16]:

(|Iph| h )1/n = A(h Eg) (1)

where |Iph| is the photocurrent divided by incident photon flux at wavelength of irradiation, h is the energy of the photon, and A is a constant; the value of n depends on the nature of the optical transition, where n = 2 for the indirect transition and n = 1/2 for the direct transition of semiconductors [17,18]. For the analysis of photocurrent, bandgap energy of oxides contained in films was drawn by this relationship, where n = 2 was adopted by considering well-known indirect transition of Cr2O3 and previous reports [9,17,18] showing that n = 2 is appropriate for photoelectrochemical behaviors of passive films of chromium oxides.

3. RESULTS

3.1 FESEM and TEM analysis

Figure 1 shows the surface morphology of oxide films formed on alloy 690TT exposed in simulated PWR primary water. The surface of the oxide film obtained in the loop test (Figure 1a) is covered with numerous lath-shaped crystallites of 100–200 nm in the long side and differs from one in the static test (Figure 1b).

The cross-section views of these specimens are shown in Figure 2. Each oxide film in the loop test and static one reveals a duplex structure consisting of an internal layer and external one. As to the oxide film for the loop test, the internal layer is 30–40 nm in thickness. The external layer consists of stacked lath- shaped crystallites, whose view is in agreement with FESEM observation (Figure 1a). Comparing two oxide films, the oxide film for the loop test seems denser than that for the static test.


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3.2 AES analysis

Depth profiles of elements of TT690 after exposure to simulated PWR primary water are shown in Figure 3. Because both the surface and the interface between oxide films and alloys are uneven at least within 2 m square (Figure 2) which corresponds to the AES observed area, these AES results provide average depth information. For specimens prepared in the loop test (Figure 3a), there are inflection points of 30–40 nm in depth commonly on the profiles of O, Cr, and Ni. It is thought that the boundary between the internal layer and the external one lies around here. At the shallow position, the O concentration is considerably high accompanying the very low concentration of Cr and Ni; this is possibly due to adhesive contamination on the surface. The external layer consists of oxide rich in Cr and Ni with very few of Fe. The internal layer seems to mainly consist of Cr-enriched oxide because the ratio of Cr to the total of Cr, Fe, and Ni within the internal oxide layer is higher than that in Alloy 690 (approximately 30 %). The total thickness of the oxide film obtained in the loop test is 60–70 nm by the depth profile of O. On the other hand, as shown in Figure 3b, the oxide film for the static test also has the duplex structure where the boundary between the internal layer and the external one is likely to lie on approximately 30 nm depth. The total thickness of the oxide film seems to be approximately 50 nm by the depth profile of O. Comparing two oxide films, the total thickness is higher for the loop test than for the static one. Furthermore, it is noted that the both Cr contents in the external oxide layer and the internal one are higher for the loop test than for the static one.

3.3 STEM analysis

STEM analysis was performed on the oxide film formed on alloy TT690 exposed on simulated PWR primary water in the loop test to characterize the film structure, oxide composition, and its distribution in the film. Figure 4 depicts the STEM image obtained in high angle annular dark field (HAADF) mode on the oxide film. The oxide film seems to be compact and continues. Also it was confirmed that the oxide film consisted of polycrystalline structure by bright field TEM observation.

In order to investigate the chemical composition of the oxides and their distribution in the film, EDS line mapping was conducted on three lines related to the interface between the film and the alloy: one line (line X) is orthogonal to the interface line and two lines (line Y and Z) are parallel to one, as shown in Figure 4. The reason why this interface is selected for analysis is that the high contrast between the oxide film and the alloy (Figure 4) means the configuration of the interface parallel to incidence of electronic beam for EDS, which permits line analysis to demonstrate the inherent elemental concentration in the film without the interference of alloy matrix. Furthermore, the diameter of incident electron beam for EDS analysis is so small (less than 0.1 nm) that one can conduct the line analysis at the spatial resolution of at the most 2 nm. As shown in Figure 5a, the concentration of Cr and Ni on line X was changing from the outermost surface to the alloy in such a way that similar gradual peaks appear for Ni and Cr within approximately 40 nm at position, and two sharp peaks appear around 50 and 60 nm at position for Ni and Cr, respectively. This result shows that the oxide film consists of an external layer rich in Cr and Ni and an internal layer sub-divided into two layers: an inner layer richer in Ni and an innermost one enriched most in Cr. One of the key questions is whether each of the two internal layers is continuous or discontinuous from the standpoint of corrosion resistance of oxide films. To confirm the continuity, analyses of line Y and line Z were performed along the innermost layer and inner one, respectively (Figure 5b, 5c). Analysis of line Y shows that the innermost layer mainly includes Cr and O in concentration with an almost constant relative ratio (average O/Cr ratio of 1.78) and very few amounts of Ni and Fe. Figure 6a shows one example of electron diffraction patterns on the area including line Y. These patterns were taken by nano-beam diffraction of FETEM (the beam diameter of less than 10 nm) and obtained by tilting the specimen by appropriate angles. The analysis for the diffraction pattern revealed crystallographic distance and angles in agreement with the “corundum” crystallographic structure. Such analyses performed on different interfaces, whose area selection was obeyed in a foregoing manner, permitted a similar interpretation. These results draw the rationale that close set Cr2O3 crystallites constitutes the innermost layer as a continuous one. The thickness of the Cr2O3 layer is


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estimated to be approximately 15 nm by the Cr peak width in the Cr profile in Figure 5a. On line Z (Figure 5c) for the inner layer, Cr, Ni, and O were contained in almost constant relative ratios at least within 30nm (average ratio approximately 2.0 and approximately 0.47 for O/Cr and Ni/Cr, respectively) by EDS line analysis. Furthermore, as shown in Figure 6b, the “spinel” crystallographic structure was confirmed on the area including such line Z by diffraction pattern analysis. Considering that very less Fe is included in this layer (Figure 5c), the spinel crystallites can be assigned to NiCr2O4 ones. These crystallites constitute the inner layer whose thickness is approximately 15 nm (Figure 5a). At the position over 30 nm on line Z, Ni decreases a bit with an increase of O in concentration (Figure 5c), possibly owing to penetration of the external layer having high O content. These results of STEM and FETEM analyses explain that the oxide film formed alloy TT690 exposed to simulated PWR primary water at high velocity should have a stratified structure arranged in the order of lath-shaped crystallites, NiCr2O4 ones, and Cr2O3 ones from the surface to the interface. With regard to the oxide film prepared in the static test, the existences of NixFe1-xCr2O4 and Cr2O3 were confirmed, but at least the film continuity of Cr2O3 could not be revealed.

3.4 Photoelectrochemical characterization

Photoelectrochemical analysis was conducted in order to obtain further information for the structure and formation of oxide films formed on alloy TT690 by their bandgap which oxides possessing semiconductor properties show as intrinsic values. Figure 7 shows (|Iph| x h)0.5 vs. h plots for oxide films on alloy TT690 exposed in simulated PWR primary water in the loop test and the static test. In the oxide film formed in the loop test (Figure 7a), there appeared four oxide components whose bandgap values are approximately 2.3 eV, 2.9 eV, 4.1 eV, and 4.5 eV. According to the previous report [19], the component for 2.9 eV can be attributed to Cr2O3 located in the inner part of oxide scale, possibly due to the inward growing direction of the subscale. The bandgap value of 4.1 eV is identical to that of pure NiCr2O4 formed by oxidation of Ni-base alloy without Fe [9]. This assignment of two oxides agrees with the result acquired from those of the STEM study in the previous section. The bandgap value of approximately 2.3 eV is probably attributed to Ni(OH)2 (the reported value is approximately 2.25 eV [20]) or Cr(OH)3 (the reported value is approximately 2.4 eV [20,21]). The component for approximately 4.5 eV may be the spinel-type Ni1-xFexCr2O4, as reported by Marchetti [9,22]. Compared with the oxide film obtained by the static test (Figure 7b), there are some evident differences in the oxide component between two oxide films. Notably, in the oxide film for the loop test, only one type of Cr2O3 with a bandgap of approximately 2.9 eV attributed to Cr2O3 due to inward oxidation appeared, while two types of Cr2O3 with a bandgap of approximately 3.4 eV (attributed to Cr2O3 due to external growth of subscale [9,19]) as well as approximately 3.0 eV (attributable to Cr2O3 due to inward oxidation) appeared in the oxide film for the static test.

4. DISCUSSION

4.1 Structure of oxide film

The structure of the oxide film formed on alloy TT690 in simulated PWR primary water at high flow velocity is schematized in Figure 8. The external layer consists of the stacked lath-shaped crystallites of the oxide rich in Cr and Ni, including few amounts of Fe, whose shape makes us believe that the external layer is formed by precipitation phenomena. It is supposed that this oxide may be one of the mixed hydroxides of Cr(OH)3 and Ni(OH)2 because of high oxygen content (Figure 3a) and the appearance of hydroxide components in the photoelectrochemical analysis (Figure 7a). The internal layer is sub-divided into two continuous layers: an inner layer composed of NiCr2O4 and innermost one composed of Cr2O3. Such structure has not been observed in the oxide film for the static test in this study. This difference in structure suggests that high flow velocity would promote the formation of a continuous Cr2O3 layer with a continuous NiCr2O4 one on alloy TT690. The dual continuous layers are probably strong enough to resist the corrosion in simulated PWR primary water.


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4.2 Mechanism under high flow velocity

As shown in Figure 3, the depth profile shows that Cr is most included in the external layer in the oxide film obtained by the loop test, in contrast to the case of the static test. The order of solubility of oxide for main constituting elements of Ni-base alloy is Fe > Ni > Cr at 325°C, as reported in the literature [23]. Therefore, it is suggested that there occurs remarkable corrosion followed by precipitation on alloy TT690 in the loop test. This presumption seems right, considering that high flow velocity happens to accelerate the corrosion in tubing by the increase of mass transport. The respective metal ions of Cr, Ni, and Fe are released from alloy TT690 by corrosion. Among them, the species related to Ni2+ and Cr3+ would partially precipitate to form lath-shaped crystallites, but Fe2+ would be carried away into fluid with high velocity because of high solubility of Fe hydroxides, as expected by data shown in the literature [24]. However, the occurrence of such corrosion should be limited by the time continuous protective layers of Cr2O3 and NiCr2O4 are formed. Indeed the releases of Ni from SG tubing gradually decrease and finally are saturated early or late with time [25].

In photoelectrochemical analysis (Figure 7), one bandgap (value of approximately 2.9 eV) for Cr2O3 in the film for the loop test means that Cr2O3 included there is exclusively formed through one pathway, possibly due to the inward growing direction of the subscale [19]. Two bandgap values (approximately 3.0 eV and approximately 3.4 eV in this study) for Cr2O3 in the film for the static test were drawn, and this observation is true for the previous results treating oxide films formed on Ni-base alloys in recirculation autoclaves [9,22]. The bandgap value of 3.4 eV can be attributed to bulk Cr2O3, due to outward growth of subscale [9,19]. The above results demonstrate that the flow velocity in simulated PWR primary water affects the oxidation pathway for Cr2O3 formation.

Delabrouille et al described that increasing Cr in the Ni-Cr-Fe alloy resulted in an increase in the Cr content of Cr-enriched oxides with development of the Cr depletion zone beneath the oxide layers in simulated PWR primary water [5]. Lefaix-Jeuland et al. made structural defects on the surface of alloy 690 by xenon implantation and revealed that the surface defects increased the preferential nucleation sites for Cr2O3, thereby giving numerous Cr2O3 nodules at the interface between the internal spinel layer and the alloy, accompanying the Cr depleted zone [8]. They presume that such surface defects will play a crucial role even in the formation of continuous Cr2O3 layers. In this study, the specimens were used without any surface modification and damaging, and actually the Cr depletion was not confirmed by detailed STEM analysis (Figure 5a). One of the keys for understanding the mechanism underlying high flow velocity is that Cr2O3 formation is predominantly due to oxidation by the internal growth of oxide scale.

As mentioned above, a bandgap of 2.9 eV attributed to Cr2O3 formed by oxidation due to inward growth of oxide scale is observed for the oxide films prepared in the static test in this study and in the recirculation autoclaves [9,22] as well as in the loop test. It is suggested that regardless of flow velocity, the oxidative formation of Cr2O3 by inward growth of oxide scale occurs somewhat on the Ni-based alloy in simulated PWR primary water, presumably at the initial stage of oxidation. The oxidation due to inward growth of oxide scale, that is, the internal oxidation of the alloy 690 (with Cr of 30 mass %), will give discrete Cr2O3 particles beneath the alloy surface. Therefore, the effect of high flow velocity on oxidation is to promote further internal growth of such Cr2O3 particles without shifting the external oxidation, probably leading to organization of the continuous layer. The mechanism seems complicated; however, probably, the alloy surface change to a composition acceptable to oxidation would be important in the oxidation at high flow velocity: the surface change could be caused by remarkable corrosion accompanying metal release under high flow velocity. Furthermore, it may be necessary to take the participation of hydrogen in the oxidation into account because hydrogen enhances oxygen diffusion in the alloy by solid dissolving [26] and plays a role as an oxygen carrier in the Ni-based alloy [27], thereby promoting oxidation. In order to clarify the mechanism, the metal release behavior and corresponding structure change of oxide film will be investigated.


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CONCLUSIONS

The corrosion behavior of alloy TT690 tubing at high flow velocity (1.7 m/s) in simulated PWR primary water has been investigated as a basic study of Ni release from SG tubing. The oxide film formed on alloy TT 690 has shown the unique structure and formation, which seem to be peculiar to high flow velocity.

The oxide film has a triple-layered structure arranged in the order of lath-shaped crystallite layer of Cr and Ni mixed oxide or hydroxide (approximately 40 nm in thickness), NiCr2O4 one (approximately 15 nm in thickness), and Cr2O3 one (approximately 15 nm in thickness) from the surface to the interface.

Each NiCr2O4 and Cr2O3 layer is continuous in structure.

Cr2O3 formation under high flow velocity is predominantly due to oxidation by the internal growth of oxide scale, differing from that under the static condition due to oxidation by both internal and external growth of oxides.

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(a) (b)

(b)(a)

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Table 1 Chemical composition of test specimens (mass %)

C Si Mn S Ni Cr Cu Fe

0.02 0.3 0.3 <0.001 60.3 29.6 <0.1 balance

Figure 1 Secondary electron FESEM images (acquired at 15 kV) of the surface of alloy TT690 exposed in simulated PWR primary water (325 °C) for 860 h: (a) in the loop test and (b) in the static test

Figure 2 Bright field TEM cross-section images (acquired at 200 kV) of alloy TT690 exposed in simulated PWR primary water (325 °C) for 860 h: (a) in the loop test and (b) in the static test.


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(a) (b) Figure 3 AES-Depth profiles of the elements of alloy TT690 exposed in simulated PWR primary water (325 °C) for 860 h: (a) in the loop test and (b) in the static test.

Figure 4 HAADF-STEM image for cross-section of the oxide film formed on alloy TT690 in the loop test exposed in simulated PWR primary water (325 °C) for 860 h.

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t (at

%)

Depth (nm)

Cr

Fe

ONi

Externallayer

Internallayer


11

(b)(a)

(a)

(b)

(c)

Figure 5 EDS Line profiles obtained on (a) line X, (b) line Y, and (c) line Z shown in Figure 4.

Figure 6 Crystal orientation analysis of the diffraction patterns performed on the areas including (a) line Y and (b) line Z in Figure 4 (framed by blue squares (10 nm × 10 nm); they are assigned to Cr2O3 structure and NiCr2O4 one, respectively.

0

20

40

60

80

0 100 200 300 400

Ato

mic

%

position(nm)

Ni

Cr

Fe

O

X1 X2

0

20

40

60

80

0 10 20 30 40 50

Ato

mic

%

position(nm)position(nm)

Ni

Cr

Fe

OY1 Y2

0

20

40

60

80

0 10 20 30 40 50

Ato

mic

%

position(nm)position(nm)

Ni

Cr

Fe

OZ1 Z2


12

(a) (b)

Figure 7 Photoresponse represented by (|Iph|x hv)0.5 vs energy incident light hfor the oxide films formed on alloy TT690 exposed in simulated PWR primary water (325 °C) for 860 h: (a) in the loop test and (b) in the static test.

Figure 8 Schematic diagram of the oxide film formed on alloy TT690 exposed in simulated PWR primary water (325 °C) at high flow velocity for 860 h.

Cr2O3 (Innermost layer)

Alloy TT690

NiCr2O4 (Inner layer)

Ni-Cr-oxide 30–40 nm

Internal layer

~ 15nm

~ 15nm

External layer

0

0.005

0.01

0.015

2 3 4 5 6

(|Iph

|・h

）0.5 (

a.u.

)h(eV)

~ 2.2 eV

~ 3.0 eV

~ 3.4 eV

0

0.01

0.02

0.03

0.04

0.05

2 3 4 5 6

(|Iph

|・h

）0.5 (

a.u.

)

h(eV)

~ 2.3 eV

~ 2.9 eV

~ 4.1 eV~ 4.5 eV

Documents

CORROSION BEHAVIOR OF ALLOY 690 IN SIMULATED PWR …envdeg2015.org/final-proceedings/ENVDEG/papers/ENVDEG055.pdf · Corrosion behavior of alloy TT690 tubing at high flow velocity