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Accepted Manuscript

Corrosion behavior of Alloy 690 in aerated supercritical water

Xiangyu Zhong, En-Hou Han, Xinqiang Wu

PII: S0010-938X(12)00484-2

DOI: http://dx.doi.org/10.1016/j.corsci.2012.10.001

Reference: CS 5108

To appear in: Corrosion Science

Received Date: 3 August 2012

Accepted Date: 1 October 2012

Please cite this article as: X. Zhong, E-H. Han, X. Wu, Corrosion behavior of Alloy 690 in aerated supercritical

water, Corrosion Science(2012), doi: http://dx.doi.org/10.1016/j.corsci.2012.10.001

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Corrosion behavior of Alloy 690 in aerated supercritical water

Xiangyu Zhong, En-Hou Han*, Xinqiang Wu*

State key Laboratory for Corrosion and Protection, Liaoning Key Laboratory for

Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research,

Chinese Academy of Sciences, Shenyang 110016, P. R. China

*Corresponding author: En-Hou Han, Xinqiang Wu

State Key Laboratory for Corrosion and Protection

Institute of Metal Research,

Chinese Academy of Sciences

62 Wencui Road, Shenyang 110016, P.R. China

Tel: +86-24-2384-1883

Fax: +86-24-2389-4149

E-mail: [email protected] (E-H. Han), [email protected] (X. Wu)

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Abstract

The oxidation behavior of Alloy 690 exposed to aerated supercritical water at

different temperatures was investigated using gravimetry, scanning electron

microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray

photoelectron spectroscopy, Raman spectroscopy and electron probe micro-analyzer.

The oxide films showed a duplex layer structure with Ni and Fe rich in the outer layer,

and Cr rich in the inner layer. Some pits and nodules observed in the oxide films

could be related to TiN inclusions in the alloy matrix. The oxidation mechanism and

effects of TiN inclusions are discussed.

Key words: A: Alloy, B: SEM, B: XPS, C: High Temperature Corrosion, C:

Oxidation.

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1. Introduction

Supercritical water cooled reactor (SCWR) is one of the most promising advanced

reactor concepts for Generation IV nuclear reactors because of its high thermal

efficiency and plant simplification [1, 2]. For the safety of the nuclear power plant,

the corrosion behavior of candidate materials used in such an aggressive environment

needs to be understood. Nickel-based alloys have been considered as one kind of

candidate materials for SCWR due to their combined good corrosion resistance and

mechanical properties in high temperature and high pressure water [3-5]. Many

nickel-based alloys such as Alloy 625, Alloy 617, Alloy 718 and Hastelloy C-276

have been investigated as candidate materials for SCW systems [5-9]. The corrosion

behavior of Inconel 625 exposed to a deaerated SCW in a temperature range of

400-550 oC showed fluctuations in weight change measurements and pitting as the

predominant corrosion mode, and it was assumed that the fluctuations of weight

change were related to the pitting and spalling [7, 10]. While other authors found that

a weight gain appeared for Alloy 625 exposed to SCW contains H2O2 and C-276

exposed to deareated SCW [8, 9]. Microstructure often plays a leading role in

corrosion resistance [10]. The -phase such as Ni3(Al, Ti) is a common intermetallic

precipitate in Nickel-based alloys, the carbides M23C6 in the grain boundary have

strong influence on the resistance to intergranular corrosion and stress corrosion

cracking [11-13], and the TiN inclusions act as the initiation sites of pitting [14, 15],

corrosion fatigue [16] and stress corrosion cracking [17]. The TiN inclusions oxidized

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much faster than metal matrix and resulted in high shear strain in the oxide scales,

leading to the shrinking and cracking of the oxide films [18]. Therefore, these

inclusions have a strong influence on the corrosion behavior of nickel-based alloys in

SCW, and the effects of the inclusions need to be further investigated.

The present work is to investigate the corrosion behavior of Alloy 690 exposed to

aerated SCW with different temperature and time, and to characterize the phase

composition, morphologies and chemical composition of the oxide films formed on

the surface of Alloy 690. The related corrosion mechanism and the effects of TiN

inclusions are also discussed.

2. Experimental

2.1ApparatusExposure tests were performed with a continuous flowing SCW system consisting of

an HPLC pump (Eldex Inc., AA-100-S), a preheater, a nickel-based Alloy 625

autoclave with a volume of 850 ml, a heat exchanger and a back-pressure regulator

(BPR) (Fig. 1). The internal pressure was measured by a pressure sensor placed at the

inlet of the reactor and controlled by an HPLC pump and a BPR. A computer with

Labview 6.0 software was used to control the internal pressure and the temperature in

autoclave and preheater was monitored by two thermocouples. The test solution was

aerated pure water with 8 ppm (by weight) O2, and the flow rate was maintained at 5

ml/min.

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2.2Materials and SpecimensAlloy 690 used in the present work was solution annealed at 1333 K for 0.5 h and

then water quenched. Table 1 is the chemical composition of the alloy. Fig. 2a and 2b

are the metallographic images of the as received alloy. The microstructure of the alloy

is typical austenite with many annealing twins (Fig. 2a) and TiN inclusions were

clearly observed in the alloy (Fig. 2b). Specimens (10 mm 12.5 mm 2 mm) for

expose test in SCW were mechanically ground progressively with fine grit

silicon-carbide paper up to 2000# grit, and final mechanical polished with 2.5 m

diamond paste. Prior to each test, the specimens were cleaned with ethanol and

ultrasonically rinsed with deionized water for 30 min.

2.3 MethodologyThe specimens were mounted on a rack and put into the autoclave. The testing

conditions were maintained at 450 oC /25 MPa and 550 oC/25 MPa and the exposure

time was up to 500 h. After exposure test, the specimens were cleaned with deionized

water and dried. Then they were characterized by weight gain measurement, surface

analysis and cross-section analysis. The weight of all specimens before and after

exposure was measured using Sartorius BP211D microbalance with a resolution of

10-5 g. Corrosion products analysis were performed by D/Max 2400 X-ray

diffractometer (XRD) using copper radiation (=1.542o

A ) and a BWS905 custom

Raman system. The Raman system contains a powerful laser at 532 nm with a

maximum power of 1.5 W. The Raman shift range is 175-2875 cm-1 and the spectral

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resolution is 12 cm-1. The integration time used was 15 or 20 s depending on the

signal intensity of the specimens. The surface and cross-section morphologies and

compositions of oxide films were performed using scanning electron microscopy

(SEM) (FEI INSPECT F50) equipped with an energy-dispersive X-ray spectroscopy

(EDS) system. To investigate the cross-sections of the oxide films, some specimens

were coated in Ni-P solution to protect the films. The coated specimens were mounted

with epoxy resin and then polished. The corresponding cross-sections of the oxide

films were examined using SEM and EDS. An electron probe micro-analyzer

(EPMA-6010) was used to detect the distribution of O, Ni, Cr and Fe elements on

cross-section under the operating condition of U = 15 kV and I = 100 nA. X-ray

photoelectron spectroscopy (XPS) measurements were performed with

ESCALAB250 X-ray photoelectron spectrometer. Photoelectron emission was excited

by monochromatic Al Ka source operated at 150 W with initial photo energy 1486.6

eV. The C1s peak from adventitious carbon at 285 eV was used as a reference to

correct the charging shifts. Depth profile information was performed over an area of 2

2 mm2 under 2 keV Ar-ion sputtering and spectra were obtained over a 0.5 mm spot

using a focusing X-ray monochromator. Sputtering rate was determined to be about

0.2 nm/s (vs. Ta2O5). The peak decomposition of the species in the oxide films was

carried out with a commercial peak fitting software (XPSpeak4.1) using

Gaussian-Lorentzian peak fitting after Savitzky-Golay smoothing and Shirley

background subtraction, and the peak areas were converted to atomic concentrations

as depending on sputtering depth.

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3. Results

3.1GravimetryFig. 3 shows the weight gain of Alloy 690 exposed to SCW at 450 oC/25 MPa and 550

oC/25 MPa for different times. The weight gain of Alloy 690 at 550 oC is higher than

that at 450 oC, and the trend of the weight gain at 550 oC is different from that at 450

oC. At 550 oC, the weight gain increases first, and reaches a peak after 100 h exposure,

and then decreases with further increasing exposure time. At 450 oC, some

fluctuations in weight gain are observed. The different trend of the weight gain may

be caused by the competing processes of weight gain due to oxidation and weight loss

due to pitting or oxide dissolution [7, 10, 19]. At 450 oC, the competing processes of

oxidation and dissolution keep balance. Therefore, the weight gain of Alloy 690

fluctuated with the increasing of exposure time. At 550 oC, the oxidation rate

increased, and it is easy to form a compact Cr2O3 layer, this layer can hinder the

inward diffusion of oxygen and the oxidation rate can be decreased. On the other hand,

the dissolution rate of the oxide film increased quickly. As a result, the weight gain

exhibited decreasing phenomenon.

3.2XRD analysisFig. 4 shows the XRD spectra of Alloy 690 after exposure tests at different

temperatures (Fig. 4a) and for different time (Fig. 4b). The major phases in the oxide

films are spinel (PDF card NO. 23-1271 and 10-0325), Cr2O3 (PDF card NO.

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38-1479), NiO (PDF card NO. 44-1159) and Ni(OH)2 (PDF card NO. 03-0177). Since

XRD patterns for spinels concerning the three alloying constituents (Fe, Cr, Ni) are

very similar, the spinel type cannot be clarified right now. A further analysis is needed

to identify the composition of the oxide film. The intensity of the Cr2O3peak increase

with increasing exposure temperature and time. The intensity of the peak relates to the

content of phases in the oxide films, indicating that the Cr2O3 in the oxide film

increase with increasing temperature and time.

3.3Raman spectraFig. 5 shows the Raman spectra of the oxide films on Alloy 690 after exposure tests in

SCW at 450 oC and 550 oC. The peaks on the spectra show no obvious change with

increasing exposure temperature and time. Spinel structure is responsible for the

Raman peaks at 693 cm-1 [20, 21]. Due to the limited detecting depth of Raman

spectroscopy, only the outmost surface of the oxide films can be detected. The present

results indicate that the outmost layer of the oxide film is spinel.

3.4Surface morphologyFig. 6 and Fig. 7 show the effects of exposure temperature and time on the surface

morphologies of the Alloy 690 exposed to 450 oC and 550 oC SCW. It can be seen that

the oxide films present as layered oxide under all test conditions. The outer layer

consists of large sparsely distributed, discrete polyhedral crystallites and some

slide-shaped oxide particles, and the inner layer contains fine and uniform oxide

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particles (Fig. 6 and Fig. 7). The size of polyhedral crystallites increases with the

increasing exposure temperature and time (Fig. 6 and Fig. 7).After exposing to theSCW for 50h, there are some large polyhedral crystallites sparsely distributed on the

surface (Fig. 7a). With increasing exposure time, the surface cover rate of the large

polyhedral crystallites increased (Fig. 7b), corresponding to the increasing of the

weight gain. After exposure in the SCW for 500 h, the size of the large polyhedral

crystallites increased, but the cover rate decreased (Fig. 7c), and the pitting was also

observed (Fig. 7d). The pitting and the decrease of the polyhedral crystallites may be

the reasons for the decrease of the weight gain. The growth process of the oxide films

during exposure tests can be described as an initial oxide nucleation on selected sites

followed by a uniform growth of oxide particles until they connect with each other

and result in a compact layer.

On the oxide surface, the grain boundaries were outlined by topographically elevated

oxides (Fig. 8b), similar to the work of Ren et al. [7]. They found that O and Cr were

enriched at grain boundaries and Ni was depleted. This indicates that Cr was

segregated at the grain boundaries and the chromium carbide formed in the grain

boundaries of the alloy was oxidized faster than the grains [22]. The oxidation

morphologies at the grain boundaries are different from that in the bulk grains. In the

grains, there are some large polyhedral particles (as shown label 1 in Fig. 8b) and

slide-shape oxide particles (as shown label 3 in Fig. 8b). But at the grain boundaries,

the polyhedral oxides are much smaller (particle 2 in Fig. 8b). Some quadrate-like

holes and cauliflower-like oxides are observed on the surface of oxide films (Fig. 8c

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and Fig. 8d). EDS analysis indicate that Ti content is enriched in the hole and the

center of the cauliflower, while Ni and Cr content are depleted (Table 2). Same results

are also obtained for the specimens exposed to 450 oC SCW (Fig. 6c and Fig. 6d). As

shown Fig. 6c and Fig. 6d, there are many large oxides in the TiN inclusions, while

few oxides are observed around the inclusions. The above results indicate that the

selective oxidation of the TiN inclusions has occurred, the holes and nodules in the

oxide films originate from local attack at the TiN inclusions.

3.5Structure of the oxide filmFig. 9 shows the cross-sectional morphologies of oxide films and composition depth

profiles for the Alloy 690 specimens exposed to 450 oC and 550 oC SCW for 500 h. A

typical duplex oxide structure is detected, consisting of a Ni-rich outer layer and a

Cr-rich inner layer.The number 1, 2, 3, 4 present the metal matrix, the inner layer, the

outer layer and the Ni coating layer respectively. Since the oxide layer is plated with

Ni, the Ni composition of the oxide layer measured by EDS is not reliable, especially

considering the very thin oxidation layer (0.25 to 1m), but the evolution trend of the

concentration of Ni and Cr can be observed from the EDS result. Some polyhedral

crystallites in the outer oxide layer are rich in Ni and Fe (Fig. 9d). Combined with the

results of XRD and Raman spectroscopy, the polyhedral crystallites may be NiFe2O4

spinel.

Fig. 10 shows the EPMA results of O, Ni, Cr and Fe elements distribution in the oxide

film on Alloy 690 after 500 h exposure test in 550 oC SCW. Coinciding with the

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previous results of EDS line scan, Ni and Fe are rich in the outer layer and Cr is rich

in the inner layer. Ni and Cr maps complement each other since Cr-rich regions are

Ni-depleted and vice-versa. The O map shows strong enhancement at the location of

Cr-rich zones.

Fig. 11 shows the cross-sectional morphologies of inclusion-involved oxide films.

Corresponding to the surface morphologies of inclusion-involved oxide films (Fig. 6

to Fig. 8), the TiN inclusions which are located at or close to the metal surface are

selectively oxidized, and the oxide shrinks due to the high shear strain at the interface

between the oxide and TiN inclusions when the oxidation proceed (Fig. 11a). In Fig.

11b, there are two TiN inclusions on the cross section close to the metal/oxide

interface. The inclusion 2 is closer to the metal/oxide interface than the inclusion 3,

EDS results (Table 3) show that the element N is detected in the inclusion 3, while no

N is detected in the inclusion 2. The above results seem to indicate that the inclusion 2

oxidizes faster than the inclusion 3 during exposure tests in SCW.

3.6XPS analysisFig. 12 shows the XPS depth profiles of the oxide film formed on Alloy 690 exposed

to 450 oC SCW for 100 h. It is obvious that Cr content first increases from the

outermost surface and then decreases gradually, while Ni content first decreases from

the outermost surface and then increases gradually, O content decreases from the

outermost surface to the inner layer, and Fe content keeps at a low level. Before

sputtering, the atomic ratio O/Ni=2:1, and after 180 s sputtering, the atomic ratio

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Ni/Cr=1:2, indicating that the outermost layer contains Ni(OH)2 and the inner layer

contains spinel NiCr2O4 [23].

Fig. 13 shows the Ni 2p3/2 core level spectra recorded after different sputtering time.

At the specimen surface (0 s sputtering time), Ni 2p3/2 spectrum consists of the intense

peak at the binding energy (BE) of 856.3 0.5 eV, and its satellite peak at about 860.5

0.3 eV. These two features originate from Ni(OH)2 [8, 23-25]. And the intensity of

the peak decreases with increasing sputtering time, and disappears finally after 60 s

sputtering. After 30 s sputtering, the peak at 852.4 0.2 eV appears and become

dominant. And the peak at 853.2 0.3 eV springs out and increases first and then

decreases after 390 s sputtering. The BE of 853.2 0.3 eV, corresponding to NiO or

NiCr2O4, and the other one located at BEs of 852.4 0.2 eV corresponding to metallic

Ni0 [8, 23, 24]. As XPS identifies the chemical composition on surface within only

several tens of nanometers, this information indicates that a Ni(OH)2 layer was

formed in the outermost layer in contact with SCW. The above results suggest that the

out layer of the oxide film mainly contains Ni(OH)2, and the inner layer consists of

NiCr2O4. Metallic Ni0 can be detected after 30 s sputtering. The metallic Ni0 becomes

the dominant peak after 180 s sputtering (Fig. 12). Machet et al. [23, 26] thought that

the detected Ni0 most probably resulted from chemical reduction during ion sputtering

whileLiu et al. [27] believed Ni was not oxidized during exposure tests. In the present

work, considering the significant intensity ratio of Ni0, it is believed that the signal of

metallic Ni0 detected after 30 s and 180s sputtering result from the reduction during

sputtering, and the metallic Ni0 detected after 360 s and 690 s sputtering come from

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the metal matrix.

Fig. 14 shows the Cr 2p3/2 core level spectra recorded after different sputtering time.

The Cr 2p3/2 peaks have been systematically decomposed into up to two components:

one located at a BE of 573.8 0.2 eV, corresponding to metallic Cr, and the other one

located at BE of 576.2 0.3 eV, corresponding to Cr2O3 or NiCr2O4 [23, 26]. With

increasing sputtering time, the intensity of the signal at BE of 576.2 0.3 eV

increases first and then decreases. After 360 s sputtering, the peak of metallic Cr0

springs out and becomes dominant, indicating the oxide/matrix interface.

Fig. 15 shows the Fe 2p3/2 core lever spectra after different sputtering time. The peak

located at BE of 711.0 0.1 eV corresponding to Fe3+ and 709.6 0.1 eV

corresponding to Fe2+ [26]. After 360 s sputtering, the peak at BE of 711.0 0.1 eV

and 709.6 0.1 eV disappears, and the peak at the BE of 707 0.2 eV and its satellite

peak at the BE of 711.6 0.2 eV corresponding to metallic Fe0 become dominant. The

Fe0 peak appears after sputtering 360 s, the signal of Fe0 comes from the metal matrix.

Fig. 16 shows the O 1s core lever spectra recorded after different sputtering time. The

O 1s peaks have been systematically decomposed into up to two components: one

located at a BE of 530.3 0.3 eV, corresponding to O2- [23, 28] and the other one

located at BE of 531.7 0.5 eV, corresponding to OH- [23, 28]. With increasing

sputtering time, the intensity of the signal at BE of 531.7 0.5 eV decreases and that

at BE of 530.3 0.3 eV increases, indicating that the outmost layer of the oxide film

contains hydroxide.

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4. Discussion4.1The fluctuation of weight changeIn the present work, it is found that the weight change of Alloy 690 is fluctuated (Fig.

3), coinciding with the corrosion behavior of Inconel 625 and Hastelloy C-276

exposed to SCW reported by Allen et al. [10], Was et al. [19] and Cook et al. [29].

They assumed that the fluctuation of weight change is due to the competition between

the oxidation and pitting or spalling of the oxide films. But Sun et al. [8] and Zhang et

al. [9] reported that the Alloy 625 and C-276 exhibited weight gain with increasing

exposure time without fluctuations. Chang et al. [30] found that the weight change of

Alloy 625 were slight and fluctuated in 400 oC and 500 oC SCW with 8.3 ppm (by

weight) O2, and the weight gain followed a parabolic law in 600oC SCW. Kuang et al.

[31] investigated the corrosion behavior of Alloy 690 in 290 oC oxygenated high

temperature water, they found that the oxide film dissolved with increasing exposure

time due to the release of Cr in the oxide into the solution as CrO 42-. In the present

work, the dissolution of the oxide particles and some pits were also observed at the

surface of the oxide film (Fig. 7 and Fig. 8). Therefore, the fluctuation of the weight

change may be caused by the competing processes of weight gain due to oxidation

and weight loss due to pitting or oxide dissolution during exposure tests [7, 10, 19].

At 450 oC, the competing processes of oxidation and dissolution keep balance.

Therefore, the weight gain of Alloy 690 fluctuated with the increasing of exposure

time. But at 550 oC, after exposure for 100 h, the surface cover rate of the large

polyhedral crystallites increased than that for 50 h (Fig. 7a and b), the oxidation rate

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increased.And it is easy to form a compact Cr2O3 layer at initial stage, this layer can

hinder the inward diffusion of oxygen and the oxidation rate can be decreased in the

later oxidation stage. On the other hand, the pitting caused by the selective oxidation

of TiN inclusions and the dissolution rate of the oxide film are more severe than that

at 450 oC (Fig. 6 to Fig. 8). As a result, the weight gain showed a decrease

phenomenon.

4.2Effects of inclusionsIn addition to the general oxidation, some selective oxidation of the inclusions (Fig.

6c and Fig. 6d) and some pits and nodules are also observed on the surface of Alloy

690 (Fig. 8c and Fig. 8d). The EDS analysis results indicated that these pits and

nodules are related to the TiN inclusions in the alloy (Table 2). Ren et al. [7] and Tan

et al. [10] have observed several pits formed on Alloy 625 and Alloy 718 exposed to

500 oC SCW with 25 ppb O2 (by weight). They believed that the pits maybe

associated with the Nb- and/or Ti-rich precipitates (likely -phase) leading to

galvanic corrosion due to the difference of electrochemical potential between the

matrix and -phase. Zhang et al. [32] also found some nodules and cauliflower-like

oxides originated from the local attack of the -phase precipitates and scattered on

the uniform scale formed on Alloy 625, C-276 and X-750 in 500 oC SCW. Lim et al.

[15] also found that some pits initiated at TiN inclusions in Alloy 600. (Mg, Ca)S and

angular TiN particles acted as a preferential site for the film breakdown for pitting

corrosion, and the micro-crevice formed at the angular TiN/matrix interface facilitated

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the formation of an occluded cell for pitting corrosion. Chang et al. [30] investigated

the corrosion behavior of Alloy 625 in 400-600 oC aerated SCW. They found that

some pits initiated from the (Nb, Ti)C inclusions in the substrate. The TiN or TiC

inclusions were selectively oxidized if they were located at or close to the metal

surface. Moreover, the inclusions oxidized faster than the metal matrix [18], the

volume increased when the oxidation proceeded (the ratio of molar volume

TiO2/TiC=1.28 [18]), resulting in high shear strain at the interface between the oxide

film and TiN inclusion, and the oxide shrank with development of the oxidation, as

shown in Fig. 10a. As a result, some cracks could be generated in the shrunk oxide

films with continuous oxidation. These cracks may promote the short circuit diffusion

of aggressive components such as oxygen penetrating into the metal matrix, thereby

causing internal oxidation processes around the TiN inclusions [18].

4.3Multi-layer oxide structureIn the present work, the oxide films formed on Alloy 690 in aerated SCW is a duplex

structure, consisting of a loose outer layer with large polyhedral crystallites and a

dense inner layer with fine oxide particles (Fig. 6 and Fig. 7). The outer layer is

Ni-rich and the inner layer consists of Cr-rich oxides (Fig. 9, Fig. 10 and Fig. 12). The

outer layer is composed of polyhedral nickel ferrite crystallites with sizes of

micrometric scale (Fig. 6 and Fig. 7). Besides the outer polyhedral oxide particles,

some plate-like oxides (particle 3 in Fig. 8b) and faceted oxide particles (particle 2 in

Fig. 8b) are growing up on the surface layer. The plate-like oxides are also observed

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around the TiN inclusions (Fig. 6c, Fig. 6d and Fig. 8c). According to the XPS

analyses in the present work (Fig. 12), the Ni2+ signal at BE of 856.3 0.5 eV and O

1s signal at BE of 531.7 0.5 eV can be assigned to Ni(OH)2 [23, 25]. The results of

XRD (Fig. 4) also reveal that the oxide films consist of Ni(OH)2. Angeliu and Was [25]

have also found plate-like oxide formed on Nickel-based alloy had a hexagonal

crystal structure corresponding to Ni(OH)2. The presence of nickel hydroxide in the

external layer of oxide film was identified using XPS by several other authors [8, 23,

24]. In the present work, it is believed that the plate-like oxides are Ni(OH)2. Carrete

and Machet [26] found that some iron hydroxide also existed as intermediate

precipitates in the external layer formed on Alloy 690. However, the iron hydroxide is

not stable, it would react with nickel hydroxide, and then the spinel NiFe 2O4 formed

in the external layer according to reaction (1) [33]. This mechanism results from the

precipitation phenomenon due to the local saturation of Ni and Fe.

2 2 2 4 2 2Ni(OH) +2Fe(OH) NiFe O +2H O+H (1)

The Ni2+ signal at BE of 853.2 0.3 eV and O 1s signal at BE of 530.3 0.3 eV are

assigned to NiO [23], the Cr signal at BE of 576.2 0.3 eV and O 1s single at BE of

530.3 0.3 eV correspond to Cr2O3 [23, 26]. The E-pH diagrams of Ni and Cr in

SCW at 400 oC proposed by Cook et al. [34-38] reveal that, NiO and Cr2O3 are

chemical stable phases in such an environment. However, they just consider the pure

metal, if taking the elements into account, the oxides maybe more complicate. The

E-pH diagrams of Alloy 690 in PWR primary condition proposed by Huang et al. [39]

revealed that NiCr2O4 and NiFe2O4 were the thermodynamic stable oxides. Sennour et

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al. [33] calculated the thermodynamic stability of some oxides in PWR primary

conditions and found that the stability of oxides as a decreasing rank

Cr2O3>FeCr2O4>NiCr2O4>NiFe2O4>NiO. These results mean that the NiCr2O4 is

more stable than NiO in high temperature water. The XRD results in the present work

(Fig. 4) indicate that the intensity of NiO is weaker than that of spinel, suggesting that

the content of NiO in the oxide film is less than spinel. This could be explained by

considering the solid-state reaction between NiO and Cr2O3. With the development of

oxidation, a part of NiO react with Fe3+ and OH- to form NiFe2O4 according reaction

(2) [39], and a part of NiO react with Cr2O3 to form spinel NiCr2O4 by the reaction (3)

[40]. As a result, the outer layer formed on Alloy 690 consists of Ni(OH)2, NiO and

NiFe2O4. And the inner layer of the oxide film formed on Alloy 690 mainly contains

Cr2O3 and NiCr2O4 after long-term exposure test in SCW.

3+ -

2 4 2NiO+2Fe +6OH NiFe O +3H O (2)

2 3 2 4NiO+Cr O NiCr O (3)

Machet et al. [23] and Mclntyre et al. [41] proposed that the oxide films formed on

Alloy 600 consist of an outer layer of Ni(OH)2/NiO and an inner of Cr2O3. However,

both of them did not consider the transport of Fe ions in the oxide during long-term

oxidation. The transport of Fe through Cr oxide at high temperatures has been

observed in dry oxidation experiments [42]. If considering the transport of Fe through

the oxide film, a more complicate spinel (Ni, Fe)(Cr, Fe)2O4 may be formed in the

oxide films after long-term oxidation. Combined with the results of XRD, Raman

spectra and XPS, the outer layer consist of NiO, Ni(OH)2 and NiFe 2O4, and the inner

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layer consists of Cr2O3 and NiCr2O4. This type of oxide film has been observed by

many other authors in nickel-based alloys in sub-critical water and SCW [7-10, 24, 25,

33, 43].

4.4Oxidation mechanismSeveral mechanisms have been proposed to explain the duplex oxide layer formed on

metals in SCW [8, 9, 33, 43-45], in which the solid-stated growth mechanism [44] and

metal dissolution/oxide precipitation mechanism [45] are generally accepted. In the

present work, the experimental results and related analyses indicate that the oxides

grew via a mixed mechanism. The porous outer layer grows via metal dissolution and

oxide precipitation mechanism, and the compact inner layer grows via solid-state

growth mechanism. Fig. 17 shows a schematic of the oxidation process of Alloy 690

in aerated SCW. At the beginning of exposure test, Cr oxidized preferentially by

reacting with dissolved oxygen to form Cr2O3. Simultaneously, Fe and Ni can

selectively dissolve into SCW at the active sites. With an increase in dissolved metal

ions concentrations, the metal cations may combine with anions to form oxides or

hydroxides and precipitate on the surface of the specimen. Ni(OH)2 and Fe(OH)2

precipitated on the surface (Fig. 17a), and then the outer layer of NiFe2O4 spinel is

formed according to the reaction (1). Cr diffuses more slowly than Ni and Fe in the

oxide, the lattice diffusion coefficient of the Cr ions in the Cr2O3 layer were two

orders of magnitude lower than that of Ni ions [46], so Cr is retained and enriched in

the inner layer. The anion (O2-) diffuses inward to the oxide/metal interface through

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the short circuit paths such as micro-pores and grain boundaries [40, 47], and reacts

with the enriched Cr in the inner layer. As a result, a continuous compact inner layer is

formed (Fig. 17b). Oxidation can continue by outward diffusion of cation (Ni2+) to the

oxide/water interface and reaction with oxygen or high temperature water, resulting in

a NiO layer. With the development of oxidation, some NiO react with Fe3+ and OH- to

form NiFe2O4 in the outer layer according reaction (2). And some NiO react with

Cr2O3 to form spinel NiCr2O4 according to the reaction (3), as shown in Fig. 17c.

With the outwards diffusion of Fe and Ni through the oxide layers, the process of

oxide precipitation and solid-state reactions continue. As a result, a thicker oxide film

was formed.

5. Conclusions

The weight gain, phase compositions, surface and cross-section morphologies and

elements distribution of the oxide films formed on Alloy 690 exposed to 450 oC and

550 oC aerated SCW has been investigated. The growth mechanism of the oxide films

is also discussed. The following conclusions can be drawn.

(1)General corrosion, pitting corrosion and nodule corrosion are observed on Alloy690 exposed to aerated SCW. The weight change is fluctuated and a weight loss is

observed with increasing exposure time, which could be attributed to the pitting.

(2)The corrosion of TiN inclusions is faster than that of alloy matrix, resulting inhigh shear strain in the oxide films, followed by scale shrinking and cracking.

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These inclusions play an important role in the pitting and nodule corrosion.

(3)The oxide films formed on Alloy 690 in SCW show a duplex layer structure witha Cr-rich compact inner layer grown via solid-state growth mechanism and Ni-rich

porous outer layer grown via metal dissolution/oxide precipitation mechanism.

The outer layer consists of NiFe2O4 spinel, NiO and Ni(OH)2, and the inner layer

contains Cr2O3 and NiCr2O4 spinel.

Acknowledgments

This study was jointly supported by National Basic Research Program of China

(2011CB610501), National Science and Technology Major Project (2011ZX06004009)

and Innovation Fund of Institute of Metal Research (IMR), Chinese Academy of

Sciences (CAS).

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Figure Captions

Fig. 1 Schematic diagram of supercritical water experimental apparatus.

Fig.2 (a) The metallographic image of as received Alloy 690, (b) Light optical

morphologies of the TiN on Alloy 690 surface.

Fig. 3 The weight change of Alloy 690 exposed to SCW at 450 oC and 550 oC.

Fig. 4 (a) XRD patterns of the oxide films formed on Alloy 690 exposed to SCW at

450 oC and 550 oC. (b) XRD patterns of the oxide films formed on Alloy 690 exposed

to SCW at 550 oC for different time.

Fig. 5 (a) Raman spectroscopy of the oxide films formed on Alloy 690 exposed to

SCW at 450 oC and 550 oC. (b) Raman spectroscopy of the oxide films formed on

Alloy 690 exposed to SCW at 550 oC for different time.

Fig. 6 SEM morphologies of the oxide film formed on Alloy 690 surface exposed to

SCW at 450 oC for different time. (a, c) 450 oC for 100 h, (b, d) 450 oC for 500 h,

Fig. 7 SEM morphologies of the oxide film formed on Alloy 690 surface exposed to

SCW at 550 oC for different time. (a) 50 h, (b) 100 h, (c, d) 500 h.

Fig. 8 Details of SEM morphologies of the oxide films formed at 550 oC for 100 h. (a)

low amplification image, (b) high amplification image of area A in Fig. 8a, (c)

cauliflower-like oxide on the oxide surface area B in Fig. 8a, (d) quadrate-like hole on

the oxide surface.

Fig. 9 SEM morphologies and EDS line scan of cross-section of the oxide films

formed on Alloy 690 surface exposed to SCW at 450 oC and 550 oC for about 500 h.

(a, b) 450 oC, (c, d) 550 oC.

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Fig. 10 The distribution of O, Fe, Ni, Cr in the oxide film formed on Alloy 690

surface exposed to SCW at 550 oC for 500 h.

Fig. 11 The cross-section morphologies of oxidized TiN beneath the oxide film

formed on Alloy 690 exposed to SCW at 550 oC for different time. (a) 100 h and (b)

500 h.

Fig. 12 XPS depth profiles of the oxide films formed on Alloy 690 exposed to 450oC

SCW for 100 h.

Fig. 13 XPS Ni 2p3/2 core level spectra (and their decomposition) of the oxide films

formed on Alloy 690 exposed to 450 oC SCW for 100 h.

Fig. 14 Cr 2p3/2 core level spectra (and their decomposition) of the oxide films formed

on Alloy 690 exposed to 450 oC SCW for 100 h.

Fig. 15 Fe 2p3/2 core level spectra (and their decomposition) of the oxide films formed

on Alloy 690 exposed to 450 oC SCW for 100 h.

Fig. 16 O 1s core level spectra (and their decomposition) of the oxide films formed on

Alloy 690 exposed to 450 oC SCW for 100 h.

Fig. 17 Schematics of oxidation process of Alloy 690 exposed to aerated SCW.

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Table Captions

Table 1 Chemical compositions of Alloy 690 (wt.%).

Table 2 EDS results of site 1 to 5 in Fig. 8c and Fig. 8d (at.%).

Table 3 EDS results of site 1 to 3 in Fig. 11 (at.%).

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Table 1 Chemical compositions of Alloy 690 (wt%).

C N Cr Fe Mn P Si Al Ti Cu Nb Co Ni

0.013 0.01 29.15 9.19 0.21 0.01 0.02 0.26 0.305 0.01 0.01 0.01 Bal

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Table 2 EDS results of site 1 to 5 in Fig. 8c and Fig. 8d (at%).

Elements 1 2 3 4 5

O 75.27 48.20 51.90 60.77 32.82

Ti 15.77 1.62 0.65 19.82 0.80

Cr 7.60 21.23 19.18 7.30 21.47

Fe 0.40 4.28 4.81 1.72 5.75

Ni 0.97 24.68 23.45 10.38 39.17

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Table 3 EDS results of site 1 to 3 in Fig. 11 (at%).

Elements 1 2 3

O 63.33 68.27 -

Ti 20.48 23.51 45.89

Cr 10.67 3.21 2.22

Fe 0.85 - -

Ni 4.67 5.01 1.85

N - - 50.04

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Highlights:

Weight gain of Alloy 690 fluctuated in aerated SCW. Ni and Fe are rich in outer layer of oxide scale, and Cr is rich in inner layer.

Pits and nodules observed in oxide scales could be related to TiN inclusions.