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Accepted Manuscript

Title: Influence of Conductivity on Corrosion Behavior of 304 Stainless Steel

in High Temperature Aqueous Environment

Author: Jiazhen Wang, Jianqiu Wang, En-Hou Han

PII: S1005-0302(15)00223-6

DOI: http://dx.doi.org/doi: 10.1016/j.jmst.2015.12.008

Reference: JMST 617

To appear in: Journal of Materials Science & Technology

Received date: 19-1-2015

Revised date: 16-2-2015

Accepted date: 4-3-2015

Please cite this article as: Jiazhen Wang, Jianqiu Wang, En-Hou Han, Influence of Conductivity

on Corrosion Behavior of 304 Stainless Steel in High Temperature Aqueous Environment,

Journal of Materials Science & Technology (2015), http://dx.doi.org/doi:

10.1016/j.jmst.2015.12.008.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service

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1

Influence of Conductivity on Corrosion Behavior of 304 Stainless Steel

in High Temperature Aqueous Environment

Jiazhen Wang, Jianqiu Wang*, En-Hou Han

Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese

Academy of Sciences, Shenyang 110016, China

[Received in 19 January 2015; Received in revised form 16 February 2015; Accepted 4 March 2015]

*Corresponding author. Ph.D., Prof.; Tel.: +86 24 23893723; Fax: +86 24 23894149.

E-mail address: [email protected] (Jianqiu Wang).

The influence of conductivity on corrosion behavior of 304 stainless steel (SS) in high temperature

water was investigated by using in-situ potentiodynamic polarization curves, electrochemical

impedance spectra (EIS) at 300 °C and ex-situ scanning electron microscopy (SEM), X-ray

diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The structures of oxide films

formed on 304 SS change with different conductivities at 300 °C. With the increase in conductivity,

the passive current density increases while the resistances of oxide films decrease. But, the

resistances do not decrease lineally with the increase in conductivity. A modified double-layer model

for oxide structure was proposed to explain the influence mechanism of conductivity on the oxide

films on 304 SS in high temperature water. Improving the 10B enrichment level can reduce the

conductivity of primary water and increase the corrosion resistance of 304 SS.

Key words: Stainless steel; Polarization; Electrochemical impedance spectra; High temperature

corrosion

1. Introduction

In all pressurized water reactors (PWRs), boric acid is used as a chemical shim to control excess

reactivity in the fuel assemblies during a fuel cycle [1,2]. Although natural boric acid is normally used,

only the 10B isotope has a high capacity for thermal neutron capture by the following reaction[3,4]:

B Li H e10 1 7 4

5 0 3 2

n (1)

The injection of B in the typical ingredients of primary PWR coolant is 900 ‒ 1800 ppm (by weight)

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as burnable neutron absorbers to control nuclear reactions. Besides, 2 ‒ 5 ppm lithium hydroxide

(LiOH) is also injected to achieve a desired pH range between 6.9 and 7.4, which minimizes the

corrosion of structural materials, as well as the solubility and transport of activated corrosion

products[1,5].

In support of advanced fuel management trends, a higher 10B content is required at the beginning

of the fuel cycle. About 80% of total B in nature exists in the form of 11B isotope, which is

essentially useless as a neutron absorber [1,3,6]. Moreover, 11B isotope increases the total concentration

of required boron acid (H3BO3) to obtain a target10B isotope content[7,8]. However, increasing boron

concentration brings many difficulties associated with the need to increase the concentration of

LiOH in primary PWR to maintain the relative stability of pH values. The increased levels of H3BO3

and LiOH concentration, along with the rise of subsequent conductivity aggravate the boron

deposition on fuel and accelerate the corrosion rates of fuel cladding and Alloy 600[1,2,4,6].

Because the corrosion of alloys in high temperature water is an electrochemical process in nature,

a large number of studies have investigated the influences of temperature [9 ‒ 12], pH value[11,13,14],

dissolved oxygen (DO)[15] and dissolved hydrogen (DH)[16] on electrochemical characters of the

alloys by in-situ electrochemical measurement techniques in high temperature water. However, few

studies focus on how conductivity influences the corrosion behavior of stainless steels (SS) and

nickel-based alloys in high temperature water. Liu et al. [14] investigated the influence of pH on the

corrosion behavior of 316L SS at 300 °C, and found that the corrosion rate of 316L SS reduced with

the increase of pH value. However, the pH and conductivity of the experimental solutions changed at

the same time and the influence of conductivity on corrosion behavior of 316L SS was not discussed.

Thus, the main purpose of the present work is to study the effect of conductivity on corrosion

behavior of 304 SS in high temperature aqueous environments by means of in-situ potentiodynamic

polarization, electrochemical impedance spectra (EIS), ex-situ scanning electron microscopy (SEM),

X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis. One duplex-layer

model is proposed to understand the evolution process of oxides on 304 SS in high temperature

aqueous solutions with different conductivities.

2.

Experimental Procedures

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2.1. Samples and solutions

The composition (wt%) of 304 SS used for this work is 18.65 Cr, 9.27 Ni, 1.88 Mn, 0.035 C,

0.66 Si and Fe balance. The material was solution annealed at 1100±3 °C for 1 h and then water

quenched. The samples with a size of 10 mm × 10 mm × 1 mm were used for electrochemical

experiments and were carefully ground with SiC papers up to 2000 grit. After that, all the samples

were polished with 3.5 μm diamond paste. The samples were subsequently welded to 316 SS wires

shielded with heat-shrinkable polytetrafluoroethylene tubes. There are three parallel electrochemical

samples in each electrochemical experiment and another two parallel samples for SEM, XRD and

XPS analysis. The autoclave was connected with a re-circulation loop, and the water chemistry

parameters were monitored on-line, including DO, DH, pH value and electrical conductivity. The DO

and DH values were automatically controlled and adjusted using software LabView 8.5. The control

error was lower than 1%. The autoclave and re-circulation loop were pre-oxidized in the relevant

testing solution before the formal experiment. The testing electrolytes were four kinds of lithiated

and borated solutions in the high- purity deionized water (conductivity: 18.25 MΩ cm). The

concentrations of B and Li, the pH values and conductivities of the solutions at 300 °C are listed in

Table 1. All the data in the table have been calculated using commercial software pHSC4 from Duke

Power Company. The testing solution was firstly deaerated by continuously bubbling with N 2 gas

(99.99%) until the DO was less than 0.01 ppm, and then inflated with H2 gas (99.999%) until the DH

reached 2.3 ppm. The samples were immersed in 300±1 °C and 10 MPa high temperature water in a

2 L Ni-lined stainless steel autoclave for 72 h.

2.2. Electrochemical measurements

A Gamry Reference 600 Potentiostat and corresponding softwares were used for all

electrochemical measurements. The three-electrode system including working, counter and external

Ag/AgCl reference electrodes were used. The counter electrode was a platinum foil with a size of 20

mm 20 mm, and the concentration of KCl in the Ag/AgCl reference electrode was 0.1 mol/L. Prior

to the heating process of autoclave, the samples were reduced potentiostatically at −1.0 V (vs SHE—

standard hydrogen electrode) for 10 min to remove the surface oxides on SS and then kept in the

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solution at 300±1 °C for 72 h. According to American Society for Testing and Materials F2129

Standard and Ref.[17,18], 72 h of pre-oxidation could allow the extent of oxide films achieved, and the

open circuit potential (OCP) could reach the quasi-steady condition at the same time.

The OCPs of 304 SS in testing solutions at 300 °C were measured at intervals. The EIS

measurements were performed at OCP. The alternating current signal amplitude was ±10 mV rms

and the frequency ranged from 100 kHz to 10 mHz. The potentiodynamic polarization experiments

were performed at a scanning rate of 0.5 mV/s starting from −0.1 V vs OCP. A Gamry Echem

Analyst Software was used for calculating the fitting results of EIS.

All the potentials have been converted into SHE by the following equation[19]:

3r

92r

7r obsSHE )(1003.3)(10745.1)(001.02866.0 T T T T T T E E

(2)

where E SHE is the electrode potential vs SHE, E obs is the measured electrode potential, T r is room

temperature (25 °C) , and T is the measured experimental temperature (300 °C).

2.3. Analysis of surface film

To observe the microstructure of corrosion product on the surface of 304 SS, an FEI XL30 field

emission environmental scanning electron microscope (ESEM) was used. The phases of oxide films

on 304 SS after immersion were identified using a PANalytical X’Pert Powder X-ray diffraction

(XRD), with Al K α radiation. The generator voltage and tube current were 40 kV and 40 mA,

respectively. The scanning range of the detector was from 10° to 90°, the step size was 0.05°, and the

time per step was 0.8 s. XPS analysis was performed using a VG ESCALAB 250 X-ray

photoelectron spectrometer. Composition-depth profiles were obtained with 2.0 keV Ar ions at a

target current of 2.0 μA/cm2. The sputtering rate was estimated to be about 0.1 nm/s (vs Ta2O5). To

characterize the thickness of oxide film qualitatively, the equivalent oxide thickness is defined from

the XPS depth profiles where the atomic percent of O reaches 10%.

3. Results

3.1.

Change of OCP and Potentiodynamic polarization curves

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Fig. 1 shows the evolution of OCPs for 304 SS in different solutions at 300 °C. The OCPs of

304 SS reduce with the increase of B and Li injection. The OCP values of 304 SS in 1#, 2#, 3# and

4# solutions are −0.594, −0.625, −0.673 and −0.685 V, respectively.

Fig. 2 shows the potentiodynamic polarization curves for 304 SS after 72 h in different

concentrations of B and Li solutions at 300 °C. All the passive potentials of 304 SS are nearly

located around −0.4 V, which can be due to the transformation of Fe ‒ Cr oxides into Ni ‒ Fe oxides

based on the E – pH diagram[20 ‒ 23]. The anodic current density, the cathodic current density and the

passive current density all increase with the rise of conductivity. The passive current density of 304

SS in 1#, 2#, 3# and 4# solutions are 26, 62, 96 and 200 μA/cm2, respectively.

3.2.Electrochemical impedance spectroscopy

Fig. 3 shows the EIS results for the oxide films on 304 SS after 72 h exposure in different

concentrations of B and Li solutions at 300 °C. The impedance magnitude |Z| reduces with the

increase of conductivity and two time constants could be observed according to the bode diagram in

Fig. 3(a). The approximate frequency ranges, where two time constants arise, are shown in Fig. 3(a)

with red arrows. The Nyquist diagram in Fig. 3(b) shows a large semi-circle at high frequency region

and a small semi-circle at low frequency region. With the increase of conductivity, the radiuses of

both the large semi-circle and the small semi-circle decrease.

3.3. Surface film observation and analysis

3.3.1.

SEM results

Fig. 4 shows morphologies of oxide films on 304 SS in different concentrations of B and Li

injection at 300 °C. With the rise of conductivity, the sizes of spinel oxide particles on the outer layer

gradually become larger and the number of the particles gradually increases, as shown in Fig. 4(a, b).

Besides, oxide particles with a size of smaller than 200 nm start to appear on the outer layer. But

most interestingly, with the continuing increase of conductivity, while the maximum size of the

outer-layer oxide particles increases and the number of these particles decreases, the number of oxide

particles with relatively smaller size grows obviously (Fig. 4(c)). This suggests that the increase of B

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6

and Li injection can apparently alter the morphologies of the oxide films on 304 SS.

3.3.2. XRD results

Fig. 5 shows XRD patterns of the oxide films on 304 SS in different concentrations of B and Li

solutions at 300 °C. The diffraction peaks mainly correspond to Fe3O4 (JCPDS 19-0629), FeCr 2O4

(JCPDS 34-0140), with the exception of substrate. All the positions of diffraction peaks match well

with those from Fe3O4 and FeCr 2O4 according to the published data[21,24,25]. For SSs, Fe3O4 and

FeCr 2O4 were considered as the main corrosion products immersed in high temperature water [26 ‒ 28].

3.3.3.

XPS results

Fig. 6 shows the normalized composition depth profiles of Cr, Ni, Fe in the oxide films on 304

SS with different injection of B and Li at 300 °C. Considering the fact that the sputtering depth is not

linear with the sputtering time for the XPS analysis [21,29], the original sputtering time is used to show

the concentration distributions of elements in the oxide films on 304 SS. For the oxide films in 1#

solution, Fe, Ni and Cr are homogeneously distributed within the whole sputtering range of the oxide

films depth. And the inner layer of the oxide films is slightly Cr-rich. For the oxide films in 3#

solution, the outer layer of the oxide films is obviously Fe-rich while the inner layer is Cr-rich. By

comparing Fig. 6(a) with 6(b), it can be seen that the increase of conductivity can enlarge the Fe-rich

extent in the outer layer and Cr-rich extent in the inner layer of the oxide films. Besides, the

thickness of oxide films increases with the rise of conductivity by comparing Fig. 6(a) with 6(b).

4. Discussion

4.1. Effect of conductivity on the electrochemical behavior of 304 SS

To qualitatively analyze how the B and Li injection affects the electrochemical characteristics in

this study, the Evans diagram is used as shown in Fig. 7. According to Fig. 2, the cathodic current

density, the anodic current density, and the passivity current density for 304 SS increase as the B and

Li injection goes up. It indicates that the increase of conductivity can promote both the cathodic

reaction and the anodic reaction at the same time. The reason could be that the increase of

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conductivity weakens the compactness of the oxide films and speeds up the diffusion process of

metal atoms, which will be further discussed in 4.2. The increase in the current paths of the cathodic

and anodic reactions can lead to the consumption of the electrical charges generated by anodic

dissolution on the metal surface through oxidation-reduction reaction away from the metal surface,

which increases the area of the cathodic reaction, and thus accelerate the corrosion rate [30,31]. Besides,

the corrosion potentials of 304 SS decrease, indicating that the increase of conductivity has a more

obvious effect on the anodic corrosion promotion of 304 SS as shown in Fig. 7. Thus, the increase of

conductivity can remarkably reduce the corrosion resistance of 304 SS at 300 °C.

According to Ref.[32], the relationship between X i and I p has been developed to explain the

surface reactions between ith ions X i (e.g. Li+

, Zn2+

) and the passive film. And the passive current

density I p could be expressed as Eq. (3)[33].

+ +2 2

OH

p H O H OH H

OH

1 /

( )1 /

i i

i

i i

i

c e

i X

s u i

c e

i X

i

I a a

I k a k aK a a

(3)

where I i is a composite parameter, which contains the quasi equilibrium constant K i and the forward

dissolution rate constant k j; the subscript i denotes the ith active ion; s, u, ci and ei are the reaction

orders as to the activity of j, respectively; a j represents the activity of some species j (e.g. H+, X i and

OH−). The other parameters can be deduced based on the dependence of I p on the concentrations of

ions (e.g. B+, Li+) in Eq. (3). According to Table 1, the four testing solutions in our experiment have

the same pH value at 300 °C, so aH+ in the solutions has the same value. Because of the absence of

active ions (e.g. Cl−) in solutions, the change of parameters in the bracket of Eq. (3) can be

ignored[21,23,34]. Due to the less protective oxide films on 304 SS by increasing B and Li

concentration, the value of K i becomes smaller while I i becomes larger [20,21,23,35]

. Thus, the increase

in B and Li concentrations increases I p and accelerates the dissolution of oxides.

For the materials in high temperature water, the slower oxygen (O) diffusion process across the

oxide films leads to the lower corrosion resistance[36,37]. The impedance modulus | Z | at low frequency

is supposed to reflect how difficult O diffuses through the oxide layer into the substrate [23]. Based on

Fig. 3(a), with the increase of conductivity, the impedance modulus | Z | at low frequency tends to

decrease due to the less protective oxide films. Besides, it can be inferred that the increase of

conductivity promotes the O diffusion process across the barrier layer into the substrate, and thus

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increases the speed of the cathodic reaction speed. This is consistent with the potentiodynamic

polarization shown in Fig. 2. There are two capacitive arcs obtained from the oxide films on 304 SS

in Fig. 3(b). The larger arc is associated with the relatively difficult electronic transmission of barrier

layer while the smaller one is assigned to the relatively easy transportation of substances at the

barrier layer/solution boundary.

Interestingly, both the bode diagram and Nyquist diagram in Fig. 3 are different from those in

the previous work [14,22,23]. The impedance magnitudes shown in Fig. 3(a) are obviously smaller than

those in the previous work for nickel-based alloys and stainless steels[12,14,15,22,23]. The reason is that

the dissolved hydrogen in high temperature water promotes the cathodic process and reduces the

corrosion resistance of materials[16]

.

An equivalent circuit R ‒ (QR) ‒ (QR) is adopted as shown in Fig. 8[9,23,35], where Rs is the solution

resistance, R1 is the charge transfer resistance of the Faraday process, CPE1 is the constant phase

element for the duplex layer, R2 is the oxide resistance, and CPE2 is the constant phase element for

oxide. For the applied equivalent circuit in Fig. 8, the transfer function is given as follows:

s

1 21 1

1 1 1 2 2 2[ 1 ( ) ] [ 1 ( ) ]

n n Z R R j R Q R j R Q

(4)

where j is the imaginary unit, ω is the angular frequency, Q denotes the CPE coefficient, n is the CPE

exponent indicating the deviation of pure capacitor, and n1, n2 represent the fitting parameters at the

high frequency and low frequency, respectively. The fitted values of parameters of the above

equivalent circuit are listed in Table 2. Although the change of Rs is irregular with the increase of

conductivity, R1 and R2 reduce with the increase of conductivity. It is interesting that R1 and R2 do not

decrease linearly with the increase in conductivity as shown in Fig. 9. When the conductivity

changed from 113.9 to 164.3 μS/cm, a quick reduction of R1 and R2 occurred. However, the

decreasing rates of R1 and R2 become lower at larger conductivity value (>164.3 μS/cm). When R1 is

ignored, the value of R2 can denote the protectiveness of the oxide films in high temperature water.

Based on Fig. 9, it can be inferred that a porous oxide film is formed on surface of 304 SS in the

present water chemistry environment with high conductivity.

4.2. Effect of conductivity on the composition of oxide film on 304 SS

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Fig. 10 shows the potential ‒ pH diagrams for Ni, Cr, and Fe at 300 °C, which were summarized

by Staehle and Gorman[38]. According to Table 1 and Fig. 1, the calculated pH for the test solutions at

300 °C during the electrochemical measurements is 7.29, and the measured OCP range in different

solutions is between −0.596 V and −0.684 V. In Fig. 10, pH of 7.29 is shown using red vertical lines,

while the measured potentials at four different solutions are located within the range of the horizontal

bars. Therefore, based on the measured corrosion potentials in the primary water, the stable states of

Cr, Fe, and Ni are Cr 2O3, Fe3O4, Ni, respectively. Fe3O4 can be understood as the mixture of Fe2+ and

Fe3+ according to the following reaction[15]:

2 3 3 4F e O F e O F e O (5)

In high-temperature pressurized water, interactions among oxides (e.g. Cr 2O3, FeO, Fe2O3, NiO)

are inevitable[15], which are mainly:

2 3 2 4F e O C r O F e C r O (6)

2 3 2 4N iO C r O N iC r O (7)

2 3 2 4N iO F e O N iF e O (8)

2 3 2 3 4N iO C r O F e O N iC r F e O2 2 (9)

The concentration of Ni (< 10 wt%) in 304 SS is limited and the stable form of Ni under the

experimental conditions is elemental Ni, so NiCr 2O4, NiFe2O4 and NiCrFeO4 in Reactions (7), (8)

and (9) could not be formed. This is consistent with the XRD result shown in Fig. 5. Therefore, the

change of conductivity does not alter the phases of oxide films on the surface of 304 SS.

Fig. 11 shows the atomic percent of Fe oxide to total metal oxides (ηFe/(Cr + ηFe), η = 0.29)

after 72 h immersion in 1# and 3# solutions at 300 °C. The ratio of 0.29Fe/(Cr + 0.29Fe) remains

about 50% (blue line) for the oxide films of 304 SS in 1# solution. This indicates that the proportions

of FeCr 2O4 and Fe3O4 oxides on 304 SS are about 33 to 67 in 1# solution. However, for the oxide

films on 304 SS immersed in 3# solution, the 0.29Fe/(Cr + 0.29Fe) value of the outer film is much

bigger than 50%, indicating that the main phase of the outer oxide films formed in 3# solution is

Fe3O4. At the meantime, the 0.29Fe/(Cr + 0.29Fe) value of the inner oxide films formed in 3#

solution is about 40%, which proves that the phases of the inner oxide films are FeCr 2O4 and Fe3O4

(the proportions are 50 to 50). Based on Figs. 6 and 11, it can be inferred that the diffusion process of

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Fe toward outer oxide layer is promoted by the increasing conductivity. Besides, for the oxides of Fe,

Cr, and Ni, the diffusion rates of metals in their relevant oxides are shown as follows[37]:

Fe2+ > Ni2+ >> Cr 3+ (10)

According to Robertson[37], the metal ions are the faster moving species and determine the

oxidation rates. So, the promoted diffusion rate of Fe ions due to the increase of conductivity

accelerates the corrosion rate of 304 SS. This is consistent with the potentiodynamic results and EIS

results. The metal ions in oxides diffuse toward oxide/solution interface, and the vacancies will be

formed in the inner layer. The rapid diffusion of Fe ions in the oxide films will form a large number

of vacancies in the inner layer, and then these vacancies will gather together and form pores if the

vacancies cannot be supplemented by the diffusion of metal ions from the substrate. These pores

reduce the compactness and protectiveness of the inner oxide layer. This also explains why resistance

R2 of oxide layer in high conductivity solution is smaller than that in low conductivity solution,

although an obvious Cr-rich layer is formed in the inner layer of oxide films in high conductivity

solution (Figs. 6 and 11).

After the protective oxide films are formed on SSs, the oxidation process is virtually a slow

oxidation process along with the simultaneous pseudo steady-state actions of the direct oxidation,

precipitation and dissolution of oxides. Compared with the outer layer of the films in high

temperature water, the inner layer is relatively compact, which is probably formed by solid-state

reaction processes[39,40]. The oxides dissolute and grow at the same time on the surface of each oxide

particle in high temperature water. The dissolved ions are precipitated as the outer particles due to

their oxidation[21]. Based on Fig. 4, a schematic drawing of the double-layer oxide films on 304 SS in

different conductivity solutions at 300 °C is shown in Fig. 12. When the conductivity is relatively low,

the high resistance of oxide inhibits the diffusion process of metal ions toward oxide/solution

interface. Thus only oxide particles with a small size are formed through the precipitation of

dissolved ions at the outer layer (Fig. 4(a)). As the conductivity increases, the protectiveness of inner

oxides becomes poor and the diffusion process is promoted, and the size of precipitated oxide

particles at the outer layer of oxide films becomes bigger (Fig. 4(b)). According to Fig. 11, which

shows that Fe3O4 is the main phase of the outer oxide films formed in 3# solution, Fe3O4 is possibly

formed by the following equilibrium equation

[14]

:

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11

+

2 3 4 2F e H O F e O H H

2

3 4 6

(11)

As the conductivity continues to increase, the increased diffusion channels due to pores in the oxide

layer further promote the diffusion of metal ions (e.g. Fe2+)[41]. Therefore, it can be inferred that the

tendency of Fe3O4 formation increases, leading to more nucleation sites on the surface of oxides.

And more nucleation sites can make the size of oxide particles become smaller, which is also

consistent with the SEM image in Fig. 4(c).

The increase of conductivity has a bad effect on corrosion resistance of 304 SS and other

structural materials. In order to obtain a target 10B isotope content, a proper method is to improve the

10B enrichment level and decrease the total concentrations of H3BO3 and LiOH in primary water at

the same time. This method will contribute to reducing the conductivity of the primary water and

increasing the corrosion resistance of 304 SS and other structural materials based on the above

results.

5. Conclusions

(1)

The structures of oxide films formed on 304 SS change with different conductivities at 300 °C.

The oxide films are composed of a mixture Fe3O4 and FeCr 2O4 spinel oxides. As the conductivity

increases, the passive current density increases and the resistances of oxide films decrease.

(2)

The resistances of oxide films do not decrease lineally with the increase of conductivity in high

temperature water; when the conductivity is lower than 164.3 μS/cm, a slight increase in

conductivity causes an obvious change of the resistances of oxide films while further increase in

conductivity shows little effect on reducing the resistances of films.

(3) A modified model with double-layer oxide structure is proposed to interpret the influence

mechanism of conductivity in high temperature water. The increase of conductivity directly

influences the diffusion process and the oxide structure. And the protectiveness of oxide films

decreases with the increase of conductivity in high temperature water.

(4) Improving the 10B enrichment level can reduce the conductivity of primary water in PWR and

enhance the corrosion resistance of 304 SS.

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Acknowledgements

This work was jointly supported by the National Key Science and Technology Project of China

(No. 2011ZX06004-017) and the National Natural Science Foundation of China (No. 51025104).

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Table Captions

Table 1 Compositions of testing solutions and the corresponding pH values and conductivities at

300 °C

Table 2 Fitting results for 304 SS in different solutions at 300 °C

Figure captions

Fig. 1. Evolution of OCP for 304 SS in different solutions at 300 °C.

Fig. 2. Potentiodynamic polarization curves for 304 SS in different solutions at 300 °C.

Fig. 3. EIS data for 304 SS in different solutions at 300 °C: (a) Bode diagram, (b) Nyquist diagram.

Fig. 4. SEM photographs of oxide films on 304 SS in different solutions at 300 °C: (a) 1#, (b) 2#, (c)

3#.

Fig. 5. XRD patterns of oxide films on 304 SS in different solutions at 300 °C.

Fig. 6. XPS depth profiles of the oxide films on 304 SS in different solutions at 300 °C: (a) 1#, (b)

3#.

Fig. 7. Evans’ diagram used to interpret the influence of conductivity on change of OCP.

Fig. 8. Equivalent circuit used in the present study.

Fig. 9. Change in R1 and R2 as a function of conductivity for the oxide films on 304 SS at 300 °C.

Fig. 10. Potential-pH diagrams for Ni, Cr, and Fe at 300 °C[38].

Fig. 11. Atomic percent of Fe oxide to total metal oxides (ηFe/(ηFe + Cr), η = 0.29) in the oxide

films on 304 SS in different solutions at 300 °C.

Fig. 12. Schematic drawing of the double-layer oxide model on 304 SS in different solutions at

300 °C.

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Table 1 Contents of different solutions and the corresponding pH values and conductivities at 300 °C

Solution B (ppm) Li (ppm) pH300℃a

Conductivity300℃ b

(μS/cm)

1#

2#

3#

4#

0

50

200

480

0.56

0.72

1.24

2.3

7.29

7.29

7.29

7.29

113.88

125.31

164.34

242.64

Notes: a,b pH300℃ and Conductivity300℃ of testing solutions are calculated by a commercial

software pHSC4.

Table 2 Fitting results for 304 SS in different solutions at 300 °C

Rs (Ω cm2) R1 (Ω cm

2)CPE1 (mS

s-n)n1

R2 (Ω

cm2)

CPE2 (nS

s-n)n2

1# 1436.7 1154.4 5.35 0.60 4768.6 30.6 0.89

2# 1882.2497.4

7.09 0.61 2336.7 21.4 0.88

3# 848.7328.9

9.61 0.65 1478.4 34.9 0.91

4# 597.9 307.1 6.83 0.53 972.2 16.2 0.95

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Figure List

Fig. 1

Fig. 2

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Fig. 3

Fig. 4

Fig. 5

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Fig. 6

Fig. 7

Fig. 8

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Fig. 9

Fig. 10

Fig. 11

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Fig. 12

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