Corrosion of Stainless Steel in High Temperature Water Containing H2O2

7/27/2019 Corrosion of Stainless Steel in High Temperature Water Containing H2O2

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Corrosion of stainless steel in high temperature water containing H2O2

Tomonori Satoh*1, Takashi Tsukada*1, Shunnsuke Uchida*1

*1 Japan Atomic Energy Agency, 2-4, Shirane, Shirakata, Tokai, Ibaraki, 319-1122, JapanAuthors Email: [email protected]

Hydrogen peroxide (H2O2) plays an important role corroding the structural material of boiling water

reactors (BWRs). In this study, in order to determine how H2O2 can affect the corrosion of stainless

steel, the analyses of the oxide film on the surface of specimens after being exposed to the high

temperature water containing H2O2. And the modified double oxide layers model was developed based

on the results obtianed. The major results including the following;

1) A double oxide layer with a tight inner oxide layer and a porous outer oxide layer were confirmed. The

inner layer was formed through direct oxidation. The outer layer consisted of the oxide particles. The

oxide particles were formed by the precipitation of dissolved ferrous ions released from the inner layer.

2) Fe3O4 particles were formed from the oxidation of dissolved ferrous ions and precipitation. When theconcentration of the H2O2 was high the particles consisted of a mixture of Fe3O4 and -Fe2O3 particles

caused by the oxidation of Fe3O4 to -Fe2O3.

3) A modified double oxide layer model was developed. The calculated oxide film thickness, average

diameter and number density of outer oxide particles, hematite ratio and weight change were

qualitatively agreed with the measured data.

Introduction

The corrosive condition in boiling water

reactors (BWR) primary coolant is determined by

hydrogen peroxide (H2O2), oxygen (O2) and othercorrosive radiolytic species. H2O2 in particular

plays an important role in the corrosion of the

structural materials of reactor components [1]. One

of major indexes used to evaluate the corrosive

condition in BWR primary coolant is the

electrochemical corrosion potential (ECP) of the

structural materials used [2-4]. The ECP of

stainless steel exposed to high temperature water

containing H2O2 is higher than that exposed to O2

containing water at the same concentration [5]. In

this study, to determine how H2O2 can affect the

corrosion of stainless steel, an oxide film was

characterized using surface analysis of the test

specimens exposed to high temperature water

containing H2O2 of different concentrations and for

different exposure times. A modified double oxidelayer model was developed to confirm the

suitability of an oxide film growth mechanism

under consideration using the experimental results.

Experimental

High Temperature High Pressure H2O2 Water Loop

A schematic diagram of high temperature high

pressure H2O2 water loop with high H2O2 remaining

is given in Figure 1 [6,7]. H2O2 decomposes to O2

in high temperature water [8,9]. To avoid the

Figure 1: Schematic diagram of high temperature, high pressure hydrogen peroxide water loop

N2 gas

(bubbling)

coolersampling lineDO

k

H2O2 storage

tank

coolerion exchange

resin column

regenerating

heat exchanger

autoclave with

PTFE inner liner

main

heatermain pump

make-up

water

tank

recirculation pump

pH

DO

pH: pH

DO: dissolved O2k: conductivity

flow rate:10 l/h)

air

PTFE inner liner autoclave

(inner diameter: 30mm)

H2O2 detection

reference electrode

(Ag/AgCl)

liquid junction

lower SUS

electrode

PTFE

inner liner

PTFE inner liner

inletH2O2injection system

upper SUS

electrode

heater

outlet

N2 gas

(bubbling)

coolersampling lineDO

k

H2O2 storage

tank

coolerion exchange

resin column

regenerating

heat exchanger

autoclave with

PTFE inner liner

main

heatermain pump

make-up

water

tank

recirculation pump

pH

DO

pH: pH


pH: pH


flow rate:10 l/h)

air

PTFE inner liner autoclave

(inner diameter: 30mm)

H2O2 detection

reference electrode

(Ag/AgCl)

liquid junction

lower SUS

electrode

PTFE

inner liner

PTFE inner liner

inletH2O2injection system

upper SUS

electrode

heater

outlet

1


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decomposition of H2O2, PTFE inner liners were

installed in the injection line, sampling line and

autoclave. The injected H2O2 remained at more

than 94 % in the autoclave [6]. The loop was

designed to clean all the circulating water using an

ion exchanger resin column.

Exposure system to form oxide film

An exposure system with an in-situ ECP and

FDCI measurement system to prepare test

specimens with oxide films is given in Figure 2 [10,

11]. Four test specimens were installed in the

autoclave in series to form oxide films. The size of

each specimen was 10 mm x 10 mm x 1 mm. The

surfaces of the specimens were mechanical polished

using #2000 emery paper before exposure. ECP

and FDCI were measured to confirm the exposure

conditions. An external type Ag/AgCl reference

electrode was used to measure the ECP. With the

FDCI measurements, both the Working Electrode

(WE) and the Counter Electrode (CE) remained at

the natural potential of a working electrode, and a

minimal sine wave pattern was applied to the

working electrode so as to avoid any change in the

properties of the oxide film caused by the suppliedvoltage. The distance between the WE and the CE

was fixed at 3 mm. The frequency of the applied

sine wave pattern was changed from 1 mHz to 100

kHz for this study.

The experimental conditions are given in Table

1 and were used to simulate the conditions of the

BWR primary coolant. For use as parameters the

H2O2 and O2 concentrations were changed. In this

study, type 304 stainless steel specimens were used.

The chemical composition of the specimen is given

in Table 2. The exposure time and concentration ofH2O2 used in this study are given in Table 3.

Table 1: Major parameters

for the experimental loop

Item Parameter Parameter range

Autoclave Temperature 553 K

Pressure 8.0 MPa

Flow rate 2.8 ml s-1

Flow velocity 5.5 cm s-1

Conductivity < 0.2 S/cm

[O2] 0-8,000 ppb

[H2O2] 0-100 ppb

Make-up Temperature 280-300 K

water tank Pressure 0.1 MPa


[O2] 0-8,000 ppb[H2O2] 0 ppb

Table 1: Major parameters

for the experimental loop

Item Parameter Parameter range

Autoclave Temperature 553 K

Pressure 8.0 MPa

Flow rate 2.8 ml s-1

Flow velocity 5.5 cm s-1


[O2] 0-8,000 ppb

[H2O2] 0-100 ppb

Make-up Temperature 280-300 K

water tank Pressure 0.1 MPa


[O2] 0-8,000 ppb[H2O2] 0 ppb

Table 2: Chemical composition(mass %) of the specimens.

C Si Mn P

0.06 0.42 0.83 0.028

S Ni Cr Fe

0.005 8.41 18.31 Bal.

C Si Mn P

0.06 0.42 0.83 0.028

S Ni Cr Fe

0.005 8.41 18.31 Bal.

Table 3: Exposure condition for surfaceanalyses specimens

oxidant concentration exposure time

(ppb) (h)

H2O2 100 5, 15, 30, 50,

100, 200

20 20010 5, 15, 30, 50,

100, 200

5 200

oxidant concentration exposure time

(ppb) (h)

H2O2 100 5, 15, 30, 50,

100, 200

20 20010 5, 15, 30, 50,

100, 200

5 200

Figure 2: Measuring system

potentiostat

frequency

response

analyzeror

constant

voltage

power

supplier

PC (LabVIEW)

VECP

I

external reference

electrode (Ag/AgCl)

working electrode (SS)

PTFE lined

autoclaveupper SS electrode

counter electrode (SS) or

temporary reference electrode (Pt)

test specimen No.1 for surface analysis

No.2No.3

No.4

work

electrode

PTFE

cylinder

counter electrode or

temporary reference

electrode

potentiostat

frequency

response

analyzeror

constant

voltage

power

supplier

PC (LabVIEW)

VECP

I

external reference

electrode (Ag/AgCl)

working electrode (SS)

PTFE lined

autoclaveupper SS electrode

counter electrode (SS) or

temporary reference electrode (Pt)

test specimen No.1 for surface analysis

No.2No.3

No.4

work

electrode

PTFE

cylinder

counter electrode or

temporary reference

electrode

2


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Characterization of oxide film

The surface of the test specimens were

characterized using the surface analysis after being

exposed. The analytical methods for the surface

analysis used in this study are given in Table 4.

Surface images were captured using a Scanning

Electron Microscope (SEM) [14] while the

chemical composition of the oxide film was

observed with a Laser Raman Spectroscope (LRS)

[15]. The depth profile of the oxide film was

measured with a Secondary Ion Mass Spectroscope

(SIMS) [15].

Results

ECP and FDCI measurements

The time dependent ECP and Cole-Cole plots of

measured FDCI after 200 hours exposure at 100ppb, 10 ppb and 5 ppb of H2O2 are given in Figure

3. The ECP became saturated condition over

exposure time. The time at which the ECP became

saturated increased as the concentration of H2O2

decreased. The saturated ECP with 10 ppb of H2O2

was almost the same as that with 100 ppb of H2O2.

Two half circles were obtained in the Cole-Coleplots of the measured FDCIs. One of the half

circles was from 100 kHz to 10 Hz, which is

referred to as the high frequency semi-circle in this

paper, and the other was from 10 Hz to 1 mHz,

which is referred to as the low frequency semi-

circle in this paper. The low frequency semi-circle

increased as the concentration of H2O2 decreased,

while the saturated ECP remained at the same level.

Surface observation

SEM images of the surfaces of the test

specimens exposed to 100, 20, 10 and 5 ppb of

H2O2 conditions for 200 hours are given in Figure 4.

Oxide particles covered the surface with the size of

Table 4: Instruments for multilateral surface analyses

Instruments Incident Beam Incident Detected Obtained information

beam size condition particle

Scanning electron electrons energy: electrons surface image

microscope (SEM) 20keV

Laser Raman visible rays 1m wavelength: scattered chemical form of oxide

spectroscopy (LRS) 632.8nm laser light (thin layers of surface)

Secondary ion mass Cs+ 500x500m2 energy: Cs clusters isotope distribution

spectroscopy (SIMS) 5keV (through depth)

Table 4: Instruments for multilateral surface analyses

Instruments Incident Beam Incident Detected Obtained information

beam size condition particle

Scanning electron electrons energy: electrons surface image

microscope (SEM) 20keV

Laser Raman visible rays 1m wavelength: scattered chemical form of oxide

spectroscopy (LRS) 632.8nm laser light (thin layers of surface)

Secondary ion mass Cs+ 500x500m2 energy: Cs clusters isotope distribution

spectroscopy (SIMS) 5keV (through depth)

0 50 100 150 200 250 300-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

corrosionpotentialV-SHE)

time (h)

100 ppb10 ppb5 ppb

x 105

-Im

[Z](ohmcm2)

x 105Re [Z] (ohm cm2)

1.0

0.5

0

1.5

0 0.5 1.0 1.5 2.0 2.5 3.0

a) Time dependent ECP

Figure 3: Measured ECP and FDCI for 100ppb, 10ppb and 5ppb H2O2 conditions

b) Cole-Cole plot of measure FDCI at 200 hours

100 ppb

10 ppb5 ppb

0 50 100 150 200 250 3000 50 100 150 200 250 300-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

corrosionpotentialV-SHE)

time (h)

100 ppb10 ppb5 ppb

100 ppb10 ppb5 ppb

x 105

-Im

[Z](ohmcm2)

x 105Re [Z] (ohm cm2)

1.0

0.5

0

1.5

0 0.5 1.0 1.5 2.0 2.5 3.0

a) Time dependent ECP

Figure 3: Measured ECP and FDCI for 100ppb, 10ppb and 5ppb H2O2 conditions

b) Cole-Cole plot of measure FDCI at 200 hours

100 ppb

10 ppb5 ppb

100 ppb

10 ppb5 ppb

5mH2O2 20ppb H2O2 5ppbH2O2 10ppbH2O2 100ppb 5m5mH2O2 20ppb H2O2 5ppbH2O2 10ppbH2O2 100ppb H2O2 20ppb H2O2 5ppbH2O2 10ppbH2O2 100ppb

Figure 4: SEM images of the surface of test specimens after 200 hours exposure

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them increasing as the concentration of H2O2

decreased. The particle size was determined by a

balance of particle growth and the dissolution of

oxide from the particle surface. With the lower

concentration of H2O2, the oxide growth rate should

have been smaller. The dependency of particle sizeon the concentration of H2O2 might be caused by

the higher oxide dissolution rate with the higher

concentration of H2O2. 100 ppb of H2O2 resulted in

a mix of small and large particles. The large

particles might be -Fe2O3, which has a lower

dissolution rate than Fe3O4. The average diameters

of the oxide particles captured in SEM images are

given in Figure 5. The diameter of the oxide

particles decreased as the concentration of H2O2

increased because of the change in balance between

oxide growth and oxide dissolution.

100 101 102

small particle

large particles

0

0.5

1.5

1.0

oxidant concentration (ppb)

averagedi

ameter(m)

100 101 102

small particle

large particles

0

0.5

1.5

1.0

0

0.5

1.5

1.0


averagedi

ameter(m)

Figure 5: Relationship between averagediameter of outer oxide particles andoxidant concentration

In order to confirm the change in surface

conditions over exposure time, SEM observations

were made of the surfaces of test specimens

exposed to 100 ppb and 10 ppb of H2O2 with

different exposure times. The resulting SEM

images are given in Figure 6. With 100 ppb of

H2O2, the surface of the specimen was already

covered in small oxide particles after 5 hours of

exposure, while the specimens exposed to 10 ppb

were covered after 30 hours. The relationship

between the number density of oxide particles and

exposure time is given in Figure 7. The large

particles were observed after 100 hours of exposureat 100 ppb and indicated that -Fe2O3 was

generated between 50 and 100 hours at 100 ppb of

H2O2.

Chemical composition of the oxide

LRS was used in order to determine the

chemical form of the oxide formed in H2O2

conditions. As an example of the results, the

Raman spectrum of a test specimen exposed to 100

ppb of H2O2 is given in Figure 8. Fe3O4, -Fe2O3

and NiFe2O4 peaks were observed for all the

5 h 30 h15 h 50 h 200 h100 h

5 m(a) 10 ppb H2O2

5 h 50 h 200 h100 h30 h15 h

5 m(b) 100 ppb H2O2

5 h 30 h15 h 50 h 200 h100 h

5 m(a) 10 ppb H2O2

5 h 50 h 200 h100 h30 h15 h

5 m(b) 100 ppb H2O2Figure 6: SEM images of the surface of test specimens with changing exposure time

Figure 7: Relationship between numberdensities of outer oxide particlesand exposure time

100 103101 102

exposure time (h)

100ppb H2O2small particles

10ppb H2O2

1011

1014

1013

1012

numberdensity(m-2)

100ppb H2O2large particles

100 103101 102

exposure time (h)


10ppb H2O2

1011

1014

1013

1012

numberdensity(m-2)


100 103101 102

exposure time (h)


10ppb H2O2

1011

1014

1013

1012

numberdensity(m-2)


Intensity

[-]

1

0

NiFe2O

4

Fe3O

4

Fe2O3

200 400 600 800

wave number [cm-1]

Figure 8: Examples of Ramanspectra

Intensity

[-]

1

0

NiFe2O

4

Fe3O

4

Fe2O3

200 400 600 800

wave number [cm-1]

Figure 8: Examples of Ramanspectra

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specimens. In order to determine the existence

ratio of each oxide in the formed oxide film, fitting

spectra were calculated using the Raman spectra of

the standard oxides, which were -Fe2O3, Fe3O4,

FeCr2O4 and NiFe2O4 [15]. The hematite ratio,

which is the existence ratio of

-Fe2O3, is plottedagainst the concentration of H2O2 in Figure 9. The

-Fe2O3 content increased as the concentration of

H2O2 increased, thus indicating that the -Fe2O3

rich oxide film forms in higher concentrations of

H2O2. In order to confirm the -Fe2O3 formation in

the oxide film the hematite ratios of the oxide film

formed at the surface of test specimens exposed to

100 ppb and 10 ppb of H2O2 with different

exposure times were calculated. The results

obtained are given in Figure 10. The -Fe2O3

content increased after 30 hours with both the 10

ppb and 100 ppb of H2O2 exposure. This indicatedthat the Fe3O4 particles were precipitated by the

oxidation of dissolved ferrous ions and that the -

Fe2O3 then formed after 30 hours of exposure

through the Fe3O4 oxidizing. The hematite particles

grew into large particles with the higher

concentration of H2O2 because of the higher

hematite growth rate. Conversely the hematite

particles that formed might not have grown very

much because of the lower growth rate in lower

concentrations of H2O2.

Depth profile of oxide film

In order to determine the depth profiles of the

oxygen, chromium and nickel in the oxide film thatformed its depth profile was measured using SIMS.

The obtained depth profile of the oxide film formed

after 200 hours of exposure to 100 ppb of H2O2 is

given in Figure 11. The chromium enriched layer

was not observed in all the specimens exposed to

high temperature water containing H2O2.

The thickness of the oxide film was determined

using the depth profile of the oxygen, and the

results given in Figure 12. The thickness decreased

as the concentration of H2O2 increased. The

thickness of the oxide film was determined by the

balance between the growth and dissolution of the

oxide film. This result indicated that the

dissolution rate of the inner oxide layer increased

along with as oxide growth rate as the concentration

of H2O2 increased.

0.00

0.05

0.10

0.15

0.20


100 101 102

hematiteratio

(-)

Figure 9: Relationship between H2O2

concentration and hematiteratio

0.00

0.05

0.10

0.15

0.20


100 101 102

hematiteratio

(-)

Figure 9: Relationship between H2O2

concentration and hematiteratio

100 103101 102

0.05

0. 10

0.15

0.00

100ppb H2O

2

10ppb

H2O

2

exposure time (h)

hematiteratio(-)

Figure 10: Relationship hematite ratioand exposure time

0.20

100 103101 102

0.05

0. 10

0.15

0.00

100ppb H2O

2

10ppb

H2O

2

exposure time (h)

hematiteratio(-)

Figure 10: Relationship hematite ratioand exposure time

0.20

1.0

0.0

0.2

0.4

0.6

0.8

0.0 0.1 0.2 0.3 0.4 0.5

depth (m)

100 ppb H2O

2

Cr

O

Ni

RatioofeachelecmenttoFe(-) 1.0

0.0

0.2

0.4

0.6

0.8

0.0 0.1 0.2 0.3 0.4 0.5

depth (m)

100 ppb H2O

2

Cr

O

Ni

RatioofeachelecmenttoFe(-)

Figure 11: Depth profiles of O, Cr and Niin the oxide film obtained bySIMS

Figure 12: Relationship between [H2O

2],

and oxide film thickness.

10

0

10

1

10

2


0.0

0.1

0.2

0.3

0.4

0.5

thickness(m)

10

0

10

1

10

2


0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.5

thickness(m)

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Discussion

Mechanism of oxide film formation and growth

The oxide film formed on the surface of

stainless steel in high temperature water consisted

of a tight inner oxide layer and precipitated outeroxide particles. The tight inner oxide layer was

formed through direct oxidation of the base metal.

The formed inner oxide layer dissolved in the water

as ferrous ions with higher concentration of H2O2

and dissolved ferrous ions precipitated as Fe3O4

particles at the surface of the inner oxide layer

because of the oxidation of ferrous ions. Pert of the

precipitated Fe3O4 particles oxidized into -Fe2O3.

And as a result the outer oxide layer consisted of a

mixture of Fe3O4 and -Fe2O3 particles. The -

Fe2O3 particles increased in size due to the lower

dissolution of -Fe2O3. Conversely the Fe3O4

particles did not increase in size due to the high

dissolution of Fe3O4. And as a result, the large

hematite particles and small magnetite particles

became mixed with higher concentration of H2O2.

Modified double oxide layers model

Usually, to calculate the oxide film growth on

the surface of stainless steel, an empirical

calculation model, a double oxide layers model, has

been used [17]. However, with a double oxide

layers model the outer oxide was calculated as a

porous oxide layer rather than particles. Aschematic view of the double oxide layers model is

given in Figure 13 a). However, the double oxide

layer model could not be used to calculate the

properties of the oxide particles. Hence a modified

double oxide layer model, in which the outer layer

was treated as particles, was developed to evaluate

the growth and dissolution of the inner oxide layer

and outer oxide particles [18]. A schematic view of

the modified double oxide layers model is given in

Figure 13 b). The inner oxide layer is formed

through direct oxidation. Part of the inner oxide

layer that formed dissolved in the boundary layer as

ferrous ions, which then precipitated to the surface

as oxide particles. On the surface of each oxide

particle the growth and dissolution of oxide

occurred, with part of the oxide particles then

oxidizing. In this study it was assumed that theinner oxide layer and outer oxide particles just after

precipitation would be Fe3O4, and that part of the

precipitated Fe3O4 would oxidize into -Fe2O3.

A modified double oxide layer model was

developed to evaluate the suitability of the oxide

film formation and growth mechanism. Basic

equations for use with the modified double oxide

layer model are given in Equations (1) - (6).

Inner oxide layer

dM/dt = /M - inM/Tm (1)

The inner oxide layer was formed by direct

oxidation. is a corrosion rate coefficient of thedirect oxidation. That was suppressed by aresistance of the inner oxide layer against the

diffusion of the oxidant through the inner oxide

layer. in is a release rate of ferrous ions from innerlayer. In Equation (1), the resistance of theprecipitated outer oxide particles against thedissolution of inner oxide was introduced.

Ferrous ion concentration in boundary layer

dC/dt = inM/Tm/b - mCTm2/3Cm1/3fpb

- hCTh2/3Ch

1/3fhb - kgCfb(C)

- k(C - Cb) + mTm / b + hTh / b (2)

The dissolved ferrous ions were releasedthrough the dissolution of the inner layer and the

outer oxide particles. C was a dissolved ferrous ion

concentration. m and h were release rates offerrous ions from Fe3O4 and -Fe2O3 respectively.

The precipitation and the transfer of ions fromboundary layer to bulk water decreased the ferrousion concentration. kg was a generation rate

coefficient of Fe3O4 particles and k was a mass

base metal

boundarylayer

bulkmass transfer

outer layer

dissolution precipitation,absorption

oxidation

flow

oxide particle(hematite)

oxide particle(magnetite)

oxide particle(hematite)

oxide particle(magnetite)

directoxidation

inner layer

Figure 13: Schematic view of the double oxide layer model andmodified double oxide layer model

outer

inner

(bottom)base metal

inner

coolant water

direct oxidation

dissolution

precipitation

outer

inner

(bottom)base metal

inner

coolant water

direct oxidation

dissolution

precipitation

a) Double oxidelayer model

b) Modified double oxide layer model developed in this study

6


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transfer rate of ferrous ions. A part of dissolvedions were consumed in the growth of Fe3O4 and -

Fe2O3 particles. m and h were growth ratecoefficients of Fe3O4 and -Fe2O3 respectively.

Outer layer (magnetite particles)

dCm/dt = kgCfb(C) / (Wm) - ( + km)Cm (3)

dTm/dt = mCTm2/3Cm

1/3fmb2 + kgC fb(C)b

- (m + + km)Tm (4)

Outer layer (hematite particles)Outer layer (hematite particles)

dCh/dt = Cm - khCh (5)dC

dTh/dt = Tm + hCTh2/3Ch

1/3fhb2dT

- (h + kh)Th (6)- (

Fe3O4 particles were generated by the

precipitation.

-Fe2O3 particles were generated bythe oxidation of Fe3O4. Cm and Ch were the number

densities of Fe3O4 and -Fe2O3 respectively. was

a generation rate coefficient of-Fe2O3. Tm and Thwere total mole concentrations of Fe3O4 and -Fe2O3 layers per square meter. The diameter of

oxide particles was determined by the number

density and total mole concentration.

Fe

In this study it was assumed that the

precipitation would be affected by the difference

between the concentration of ferrous ions in the

boundary layer and the saturated concentration offerrous ions as provided for in Equation (7).

In this study it was assumed that the

precipitation would be affected by the difference

between the concentration of ferrous ions in the

boundary layer and the saturated concentration offerrous ions as provided for in Equation (7).

fb(C)f

= exp[-{(Cs-C)2 + ((Cs-C) |Cs-C|)}] (7)= exp[-{(Cs-C)

The major parameters used in the calculations

are given in Table 5.

The major parameters used in the calculations

are given in Table 5.

Calculated results of modified double oxide layersmodelCalculated results of modified double oxide layersmodel

In order to confirm the suitability of the

modified double oxide layers model the oxide film

growth on the surface of stainless steel exposed to

H2O2 was calculated and compared with

experimental data. The calculated results of the

dependency of the thickness of the oxide film, the

number density of the outer oxide particles and the

hematite ratio on the concentration of H2O2 are

given in Figure 14 along with the experimental data.

The calculated thickness of the oxide film

successfully reproduced the dependency seen in the

measured oxide film thickness. The concentration

of H2O2 that gave the maximum thickness of the

oxide film thickness was 20 ppb in the calculation,

which was larger than with experimental data. This

indicated that the oxide dissolution in the

calculation was too large. The calculated result for

the average diameter of the magnetite particles in

lower concentration of H2O2 did not agree with the

measured result. With lower oxidation Fe3O4

particles were generated though the precipitation

reaction between the dissolved ferrous ions and thewater. With lower concentration of H2O2 the

contribution of Fe3O4 being generated through the

In order to confirm the suitability of the

modified double oxide layers model the oxide film

growth on the surface of stainless steel exposed to

Hh/dt = Cm - khCh (5)

h/dt = Tm + hCTh2/3Ch

1/3fhb2

h + kh)Th (6)

3O4 particles were generated by the

precipitation.

-Fe2O3 particles were generated bythe oxidation of Fe3O4. Cm and Ch were the number

densities of Fe3O4 and -Fe2O3 respectively. was

a generation rate coefficient of-Fe2O3. Tm and Thwere total mole concentrations of Fe3O4 and -Fe2O3 layers per square meter. The diameter of

oxide particles was determined by the number

density and total mole concentration.

b(C)2 + ((Cs-C) |Cs-C|)}] (7)

2O2 was calculated and compared with

experimental data. The calculated results of the

dependency of the thickness of the oxide film, the

number density of the outer oxide particles and the

hematite ratio on the concentration of H2O2 are

given in Figure 14 along with the experimental data.

The calculated thickness of the oxide film

successfully reproduced the dependency seen in the

measured oxide film thickness. The concentration

of H2O2 that gave the maximum thickness of the

oxide film thickness was 20 ppb in the calculation,

which was larger than with experimental data. This

indicated that the oxide dissolution in the

calculation was too large. The calculated result for

the average diameter of the magnetite particles in

lower concentration of H2O2 did not agree with the

measured result. With lower oxidation Fe3O4

particles were generated though the precipitation

reaction between the dissolved ferrous ions and thewater. With lower concentration of H2O2 the

contribution of Fe3O4 being generated through the

Table 5: Major parameters used in this study to calculate the oxide film growth

Figure 14: Calculated results of the dependency of the properties of oxide film on [HFigure 14: Calculated results of the dependency of the properties of oxide film on [H2O

2]

100 101 102


0.0

0.1

0.2

0.3

0.4

0.5

thickness(m)

100 101 102


0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.5

thickness(m)

100

101

102

0

0.5

1.5

1.0


averagediameter(m)

100

101

102

0

0.5

1.5

1.0

0

0.5

1.5

1.0


averagediameter(m)

hematite

Mea. Cal.magnetite

hematite

Mea. Cal.magnetite

hematite

Mea. Cal.magnetite

Mea. Cal.Mea. Cal.Mea. Cal.

0.00

0.05

0.10

0.15

0.20

oxidant concentration (ppb)100

101

102

hematiteratio

(-)

0.00

0.05

0.10

0.15

0.20

oxidant concentration (ppb)100

101

102

hematiteratio

(-) Mea. Cal.Mea. Cal.Mea. Cal.

a) Oxide film thickness b) Average particle diameter c) Hematite ratio

Parameter(mol2/m4/s) (1/s) m(1/m2/s) h(1/m2/s) kg(1/s) (1/s) m(1/s) h(1/s)

100ppb 6.0x10-10 2.2x10-7 55.0 0.35 2.3x10-9 2.0x10-6 0.5 1.0x10-3

20ppb 1.2x10-10 8.8x10-9 55.0 0.35 1.1x10-8 2.0x10-6 0.02 4.0x10-5

10ppb 6.0x10-11 2.2x10-9 55.0 0.35 2.3x10-8 2.0x10-6 5.0x10-3 1.0x10-5

5ppb 3.0x10-11 5.5x10-10 55.0 0.35 4.5x10-8 2.0x10-6 1.3x10-3 2.5x10-6

Parameter(mol2/m4/s) (1/s) m(1/m2/s) h(1/m2/s) kg(1/s) (1/s) m(1/s) h(1/s)

100ppb 6.0x10-10 2.2x10-7 55.0 0.35 2.3x10-9 2.0x10-6 0.5 1.0x10-3

20ppb 1.2x10-10 8.8x10-9 55.0 0.35 1.1x10-8 2.0x10-6 0.02 4.0x10-5

10ppb 6.0x10-11 2.2x10-9 55.0 0.35 2.3x10-8 2.0x10-6 5.0x10-3 1.0x10-5

5ppb 3.0x10-11 5.5x10-10 55.0 0.35 4.5x10-8 2.0x10-6 1.3x10-3 2.5x10-6

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reaction of ferrous ion and water can be considered

significant and may have caused the gap between

the measured diameters and the calculated

diameters in lower concentration of H2O2. The

calculated hematite ratio successfully reproduced

the quantitative tendency seen in the measured data.

The calculated results of oxide film growth in

100 ppb and 10 ppb of H2O2 are given in Figure 15.

The calculated results basically agreed with the

measured data. There were large gaps between the

calculated results and the measured data for the

number density of hematite in 100 ppb and the

hematite ratio in 10 ppb. This indicated that the

basic equations or parameters for the hematite

formation used in this study were not suitable for

use in calculating the hematite formation in oxide

films. And unfortunately only limited data existson the change in hematite over exposure time. To

improve the precision of the hematite formation

calculations used in the model more data is

necessary. The calculated results for the change in

weight of the stainless steel agreed with the

measured data [14].

Conclusions

The following summarizes the conclusions of

this study;1) A double oxide layer with a tight inner oxide

layer and a porous outer oxide layer were

confirmed. The inner layer was formed through

direct oxidation. The outer layer consisted of the

oxide particles. The oxide particles were formed

by the precipitation of dissolved ferrous ions

released from the inner layer.

2) Fe3O4 particles covered the surface of stainless

steel after 5 hours of exposure in higher

concentrations of H2O2 and -Fe2O3 particles

increased after 30 hours of exposure.

3) Large -Fe2O3 particles formed in higher

concentrations of H2O2. -Fe2O3 particles formed

through the oxidation of Fe3O4 particles and

increased in size because of their lower

dissolution rate.4) A modified double oxide layers model was

developed.

5) The oxide film thickness, the average diameter,

the number density of outer oxide particles, the

hematite ratio and the change in weight were

calculated to confirm the suitability of the model.

The calculated results were qualitatively agreed

with measured data. More data and consideration

was needed about on the generation and growth of

-Fe2O3.

Acknowledgement

The study has been supported by the Japan

Society for the Promotion of Science (JSPS) [A

Grant-in-Aid for Scientific Research: Subject No.

16360467 (2004-2006)]. As a result of transfer of

the chief researcher of the program from Tohoku

University to the Japan Atomic Energy Agency

(JAEA) in April 2005, the experimental facilities

were moved. The authors express their sincere

thanks to the JSPS and the JAEA for supporting the

experiments

Nomenclatures

A1 - A4: weighting factor of-Fe2O3, Fe3O4,FeCr2O4 and NiFe2O4 in the calculation offitting spectrum

C: ferrous ion concentration

Cb: ferrous ion concentration in bulk waterCm, Ch: number density of magnetite and hematite

particles

0 25050 100exposure time (h)150 200

0.05

0. 10

0.15

0.00

hematiteratio(-) 0.20

0 25050 100exposure time (h)150 200

0.05

0. 10

0.15

0.00

hematiteratio(-) 0.20

10ppb

Mea. Cal.100ppb

10ppb

Mea. Cal.100ppb

10ppb

Mea. Cal.100ppb

Figure 15: Calculated results of the dependency of the properties of oxide film on exposure time

b) Hematite ratio

0 250 100exposure time (h)150 200

0.5

1.0

1.5

0.0weightchange(g/m2) 2.0

50

10ppb

Mea.[14] Cal.

100ppb

10ppb

Mea.[14] Cal.

100ppb

10ppb

Mea.[14] Cal.

100ppb

c) Weight change

0 25050 100exposure time (h)

numberdensity(1/m2)

150 200

hematite

Mea. Cal.magnetite100ppb

10ppb magnetite

a) Number density

1011

1014

1013

1012

0 25050 100exposure time (h)

numberdensity(1/m2)

150 200

hematite


10ppb magnetitehematite


10ppb magnetitehematite


10ppb magnetite

a) Number density

1011

1014

1013

1012

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Cs: saturated concentration of ferrous ion in

boundary layer (1.0x10-4 mol/m3)

(0.0 mol)

fm, fh : constant determined by the densities andmolecular weight of magnetite andhematite.

(7.7x10-2, 6.1x10-2 respectively)kg: generation rate coefficient of oxide particlesk, km, kh: mass transfer rate of ferrous ion,

magnetite particles and hematite particlesthrough boundary layer

(8x10-3 1/s, 6x10-2 1/s, 6x10-2 1/s respectively)M: thickness of inner oxide layerrm, rh: average radius of magnetite and hematite

particles

S1 - S4: standard spectrum of-Fe2O3, Fe3O4,FeCr2O4 and NiFe2O4 in the calculation offitting spectrum

Sf: Calculated fitting spectrum of the measured

Raman spectrum using standard spectrumof the standard oxide

t: exposure timeTm, Th: total mole concentration of magnetite and

hematite layer per square meter.

Wm: initial weight of magnetite particles just after

generation (2.5x10-20 mol)

: corrosion rate coefficient of direct oxidation

: constant (5x107 m6/mol2)

m, h: growth rate coefficient of oxide particles

m, h: density of magnetite and hematite particles(5.2x10

3kg/m

3, 5.1x10

3kg/m

3respectively)

: generation rate coefficient of hematiteb: thickness of boundary layer (5x10

-4 m)

in: release rate of ferrous ion from inner layer

m, h: release rate of ferrous ion from oxideparticles

Abbreviations

BWR: boiling water reactor

CE: counter electrode

ECP: electrochemical corrosion potential

FDCI: frequency dependent complex impedance

LRS: laser Raman spectroscope

PTFE: polytetrafluoroethylene

SEM: scanning electron microscope

SIMS: secondary ion mass spectroscope

WE: working electrode

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