effects of hydrogen peroxide on corrosion of stainless steel, (iv)

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (IV)Determination of Oxide Film Properties with Multilateral Surface Analyses

Takahiro MIYAZAWA1, Shunsuke UCHIDA1;�, Tomonori SATOH1, Yusuke MORISHIMA1,Tatsuya HIROSE1, Yoshiyuki SATOH1, Koichi IINUMA1, Yoichi WADA2,

Hideyuki HOSOKAWA2 and Naoshi USUI3

1Quantum Science and Energy Engineering Department, Graduate School of Engineering, Tohoku University,01 Aobayama, Aoba-ku, Sendai 980-8579

2Power and Industrial Systems R&D Laboratory, Hitachi, Ltd., 7-2-1, Ohmika-cho, Hitachi-shi, Ibaraki 319-12213Nuclear System Division, Hitachi, Ltd., 3-1-1, Saiwai-cho, Hitachi-shi, Ibaraki 317-8511

(Received July 11, 2004 and accepted in revised form November 21, 2004)

Corrosive conditions in BWRs are determined mainly by hydrogen peroxide (H2O2). Then, a high temperature,high-pressure H2O2 water loop was fabricated to identify the effects of H2O2 on corrosion and stress corrosion crack-ing of stainless steel.

By changing concentrations of H2O2 and O2, in situ measurements of electrochemical corrosion potential (ECP)and frequency dependent complex impedance of test specimens were carried out and then characteristics of oxide filmon the specimens were determined by multilateral surface analyses, i.e., laser Raman spectroscopy and secondary ionmass spectroscopy. The following points were experimentally confirmed.(1) The hematite ratio in the oxide films of the specimens exposed to H2O2 was expressed as a logarithmic function

of [H2O2]. The hematite ratio was measurable for 8 ppm O2, but negligibly small for 200 ppb O2.(2) H2O2 exposure led to thicker oxide layers than O2 exposure and Cr depletion did.(3) The oxide film thickness first increased as [H2O2] decreased from 100 to 10 ppb and then it decreased. This meant

that a large dissolution rate caused a thin oxide film in spite of the large growth rate of oxide film, while a lowgrowth rate caused a thinner oxide film at low [H2O2].

KEYWORDS: hydrogen peroxide, stress corrosion cracking, electrochemical corrosion potential, frequencydependent complex impedance, BWR type reactors, multilateral surface analyses, secondary ion mass spectros-copy, laser Raman spectroscopy

I. Introduction

Corrosive conditions in BWRs are determined mainly byhydrogen peroxide (H2O2), a radiolytic species in the reactorwater.1,2) To determine the effects of H2O2, O2 and theirmixture on electrochemical corrosion potential (ECP) oftype 304 stainless steel, the authors fabricated a high temper-ature, high-pressure water loop with a polytetrafluoroethyl-ene (PTFE) inner liner which minimized H2O2 decomposi-tion and lowered possible oxygen concentration.3–7)

From in situ measurements, i.e., ECP and frequencydependent complex impedance (FDCI), made in previouspapers,7–9) the authors confirmed the following.(1) The ECP and FDCI data of the specimens exposed to

100 ppb H2O2 were not affected by co-existing O2 withthe same level oxidant concentration and they were alsonot affected by pre-exposure to 200 ppb O2. From theviewpoint of ECP, this meant that corrosive conditionsof hydrogen water chemistry (HWC) were the same asthose of normal water chemistry (NWC).

(2) The low frequency semi-circles radii of the FDCI datafor the specimens exposed to 100 ppb H2O2 reached asaturation value which was much smaller than radii sat-uration values for specimens exposed to 200 ppb O2 andto 10 ppb H2O2.

(3) Smaller resistances of oxide dissolution and larger elec-tric resistances of the oxide film were obtained for thespecimens exposed to 100 ppb H2O2. This caused ECPto increase by shifting the anodic polarization curve ofstainless steel to the high potential side.

In this paper, characteristics of surface oxide film of stain-less steel specimens exposed to H2O2 and O2 at elevatedtemperature were examined by multilateral surface analysesfollowing the in situ measurements, i.e. ECP and FDCI,and then contributions of surface properties to corrosionbehaviors of stainless steel in BWR cooling water wereconsidered.

II. Experimental

1. Experimental SetupA schematic diagram of the high temperature, high pres-

sure hydrogen peroxide water loop is shown in Fig. 1.7)

Details of the experimental loop were shown in previouspapers.7–9) More than 93% of H2O2 remained at the auto-clave outlet, while more than 90% remained in the sampledwater.11) Major parameters of the experimental loop are list-ed in Table 1.

Chemical composition (mass%) of the specimens (type304 stainless steel, 10mm�10mm�1mm) is listed inTable 2. Specimens were not heat-treated and their surfaceswere polished mechanically with #2000 emery paper andbuffing. The experimental conditions are listed in Table 3.�Corresponding author, Tel. and Fax. +81-22-217-7911,

E-mail: [email protected]

Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 42, No. 2, p. 233–241 (February 2005)

233

ORIGINAL PAPER

The H2O2 concentration ([H2O2]) in the sampled water wasdetermined every few hours with a flow cell type H2O2 de-tector developed for the experiments.10,11) The H2O2 concen-tration could be monitored continuously with the in-line dis-solved O2 detector installed after the cooler in the loop.7)

2. In situ Measurements of Corrosion BehaviorsDetermination procedures of the effects of H2O2 and O2

on stainless steel specimens were of two types; the firstwas in situ responses at elevated temperature and the otherwas surface characterization of the specimens after being re-moved from the autoclave. ECP and FDCI of specimenswere measured in the autoclave. A schematic diagram ofthe impedance measurement system is shown in Fig. 2.

Major experimental procedures are shown in Table 4.(1) Electrochemical Corrosion Potential (ECP)

ECP was measured against the Ag/AgCl type external ref-erence electrode at both upper and lower locations in the au-toclave (Fig. 2).7–9) ECP measurement was automaticallycarried out every minute and the measured data were trans-ferred to the PC of the data acquisition system.(2) Frequency Dependent Complex Impedance (FDCI)

In order to measure FDCI of specimens, an alternatingvoltage with a sine wave pattern was supplied between thecounter and lower electrodes by changing frequency from0.1mHz to 100 kHz, the very small current between thespecimens was measured with a potentiostat (Toho-Giken,Model-2000) and then the frequency dependent currentwas analyzed with a frequency response analyzer (NFElectronic Instrument, Model S-5720).7–9)

3. Multilateral Surface AnalysesInstruments for multilateral surface analyses to examine

the oxide film characters are listed in Table 5.12,13) Chemicalforms of the oxide films were observed by X-ray diffraction(XRD)12) and laser Raman spectroscopy (LRS).12) Further-more, elemental composition and distribution through thefilm depth were measured by Rutherford back scatteringspectroscopy (RBS)14) and secondary ion mass spectroscopy(SIMS).15) X-ray photoelectron spectroscopy (XPS)16,17) wasapplied to obtain information on the nearest top surface ofthe oxide films.

In this paper, analytical results of LRS and SIMS are in-troduced. Some results of XRD, RBS, and XPS were shownpreviously.12,13) For LRS, a fine laser beam (diameter: 1 mm)of visible rays (wavelength: 632.8 nm) was irradiated ontothe specimen surface and the Raman shift of the scatteredlaser was measured to analyze chemical form of oxide at asurface layer. For SIMS, Csþ ions (energy: 3 keV) werebombarded onto the specimen surface to sputter oxide withina very small spot and the secondary ions from the sputteredarea were analyzed by mass spectroscopy. Isotope distribu-tion through depth was determined.

N2 gas (bubbling)

coolersampling lineDO

κ

H2O2 storage tank

coolerion exchangeresin column

regeneratingheat exchanger

autoclave with PTFE inner liner

main heater

main pump

make-up water tank

recirculation pump

pH

DO

pH: pHDO: dissolved oxygenκ: conductivity

(flow rate:10 l/h)

air

injection pump

PTFE inner liner autoclave(inner diameter: 30mm)

H2O2 detection

reference electrode (Ag/AgCl)

liquid junction

lower SUSelectrode

PTFE inner liner

PTFE inner liner

inletH2O2

injection system

upper SUSelectrode

heater

outlet

Fig. 1 Schematic diagram of high temperature, high pressure hydrogen peroxide water loop

Table 1 Major parameters for the experimental loop

Item Parameter Parameter range

Autoclave Temperature 553KPressure 8.0MPaFlow rate 2.8ml�s�1

Flow velocity 5.5 cm�s�1

Conductivity <20 mS/m[O2] 0–8,000 ppb[H2O2] 0–1,000 ppb

Feed water tank Temperature 280–300KPressure 0.1MPaConductivity <20 mS/m[O2] 0–8,000 ppb[H2O2] 0 ppb

Table 2 Chemical composition (mass%) of the specimens�

C Si Mn P S Ni Cr Fe

0.06 0.42 0.83 0.028 0.005 8.41 18.31 Bal.

*Not heat treated. Surface was polished mechanically by #2000 emery

paper and buff.

234 T. MIYAZAWA et al.

JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

Table 3 Experimental conditions

run experimental conditionsoxidant

(concentration)exposure time preconditioning

1 O2 (8,000 ppb) 300 h

2 O2 (200 ppb) 200 h

3 O2 (200 ppb) 200 h after 200 h exposure to 100 ppb of H2O2

4 H2O2 (100 ppb) 300 h

5 H2O2 (10, 5 and 1 ppb) 100 h after 300 h exposure to 100 ppb of H2O2

6 H2O2 (10 ppb) 300 h

7 H2O2 (5 ppb) 300 h

8 H2O2 (100 ppb) 200 h after 200 h exposure to 200 ppb of O2

9 O2 (200 ppb) and H2O2 (100 ppb) 200 h

: exposure time

GPIB

potentiostat

frequencyresponse analyzer

PC(LabVIEW)

VECPexternal reference electrode (Ag/AgCl)

lower SUS electrode*

PTFE lined autoclave

upper SUS electrode

counter electrode (SUS)*

* electrodes (10mm x 10mm x 1mm)

potentiostat

frequencyresponse analyzer

PC(LabVIEW)

I

Fig. 2 Schematic diagram of impedance measurement system

Table 4 Determination procedures of the effects of hydrogen peroxide on stainless steel

In situ/out loop Obtained information Major instruments

In-situ measurements Electrochemical corrosion potential External reference electrode+Electrometer(elevated temperature) Crack growth rate Compact tension test+Potential drop method

Dynamic behavior of oxide film Potentiostat+frequency analyzer(Frequency dependent complex impedance)

Out loop measurement Weight change Electric balance(ambient temperature) Surface properties Multilateral surface analysis instruments

Table 5 Instruments for multilateral surface analyses

InstrumentsIncidentbeam

Beamsize

Incidentcondition

Detectedparticles

Obtained information

Laser Raman He–Ne1 mm�

Wavelength: Scattered Chemical form of oxidespectroscopy (LRS) laser 632.8 nm laser (thin layers of surface)

Secondary ion massCsþ 500�500 mm2 Energy:

Cs clusterIsotope distribution

spectroscopy (SIMS) 3 keVs (through depth)

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (IV) 235

VOL. 42, NO. 2, FEBRUARY 2005

III. Experimental Results

1. In situ Measurements of Corrosion Behaviors(1) Electrochemical Corrosion Potential (ECP)

The specimen exposed to 100 ppb H2O2 reached ECP sat-uration in about 50 h, while that exposed to 200 ppb O2

showed strange variation of ECP during early exposureand then reached saturation. The increasing ECP was consid-ered due to oxide film growth until the saturated level wasreached.(2) Frequency Dependent Complex Impedance (FDCI)

Cole–Cole plot of measured FDCI between a pair of type304 stainless steel specimens exposed to 200 ppb O2 isshown in the previous paper.8,9)

As a result of equivalent circuit analysis, resistances ofoxide dissolution, Ra, and electric resistances of oxide film,Rf , were obtained.

Resistance of oxide dissolution, Ra, increased from lessthan 7 k��cm2 for the specimens exposed to 100 ppb H2O2

to 45, 90 and 250 k��cm2 for those exposed to 10, 5 and1 ppb, respectively. The relationship between H2O2 concen-tration and Ra is shown in Fig. 3.

Electric resistances of oxide film, Rf , obtained from Cole–Cole plots for the specimens exposed to 100 ppb H2O2 areshown in Fig. 4. Rf reached the terminal value in 50 h, whichwas not affected by co-existing O2. Stable and dense thin ox-ide film with high electric resistances (70 k��cm2) was de-veloped on the specimen surface, which caused higherECP. For complete separation of Ra and Rf values, directmeasurement of Rf under wet conditions at elevated temper-ature is desirable. The authors are preparing to undertakecontact resistant measurements for this purpose.

2. Multilateral Surface Analyses(1) Laser Raman Spectroscopy

Surface characterization by LRS of the specimens ex-posed to 200 ppb O2 and 100 ppb H2O2 was carried out next.LRS patterns for standard specimens consisting of Fe3O4, �-Fe2O3, NiFe2O4 and FeCr2O4 and the specimens exposed toH2O2 and O2 are shown in Fig. 5.6) Clear peaks correspond-ing to hematite (�-Fe2O3) were observed for the specimens

exposed to H2O2 while they were indistinct for O2 exposure.In order to evaluate the existence ratio of 4 standard ox-

ides, i.e., Fe3O4, �-Fe2O3, NiFe2O3 and FeCrO4, in the ox-ide film on each specimen, the measured LRS data for thespecimen were compared with the calculated spectrum ob-tained by summing the suitably weighted standard patterns(Fig. 5) and then the weighting factor to give the minimumdiscrepancy between the measured and the calculated wasselected as the existence ratio (Fig. 6).

As a result of synthesizing the measured LRS data withthe standard LRS patterns, major chemical forms of oxidefilms of specimens for each exposure condition were ob-tained (Fig. 7).(2) Secondary Ion Mass Spectroscopy

The test specimens were sputtered with a 3 keV Csþ ionbeam, which provided high-resolution mass spectroscopynot only for metallic ions, but also oxygen and hydrogenions.12) Oxide layers with a constant O/Fe ratio were ob-served at the surfaces of the specimens (Fig. 8). The O/Feratio drastically decreased as the depth increased. The thick-ness of oxide film on the stainless steel exposed to H2O2 wasabout 0.3 mm, while that to O2 was about 0.2 mm.

The oxygen/iron (O/Fe) and chromium/iron (Cr/Fe) ra-tios obtained from the SIMS data for the specimens exposedto 200 ppb O2 and 100 ppb H2O2 for 200 h are shown inFig. 9. H2O2 caused thicker oxide layers than O2. One ofthe features of oxide film of the specimens exposed to H2O2

was Cr depletion in the oxide layers. Cr enrichment was ob-served in the oxide film on the specimens exposed to 200 ppbO2; this was expected from previous experience.12,13)

The oxygen/iron (O/Fe) and chromium/iron (Cr/Fe) ra-tios obtained from the SIMS data for the specimens exposedto 100 ppb, 10 ppb and 5 ppb H2O2 for 300 h are shown inFig. 10. Oxide layers with a constant O/Fe ratio were ob-served at the surfaces (Fig. 10(a)). The O/Fe ratio drasticallydecreased as the depth increased. The thickness of oxide filmon the specimen exposed to 100 ppb H2O2 for 300 h wasabout 0.3 mm, while it was about 0.5 mm for 10 ppb H2O2 ex-posure and 0.3 mm for 5 ppb H2O2 exposure. Increasing[H2O2] might enhance oxide dissolution to reduce oxide filmthickness, while decreasing [H2O2] might moderate corro-

0 20 40 60 80 100 120[H2O2] (ppb)

resi

stan

ce o

f ox

ide

diss

olut

ion,

Ra

(ohm

cm

2 )

106

105

104

103

102

101

Fig. 3 Relationship between H2O2 concentration and resistanceof oxide dissolution

10

8

6

4

2

0

elec

tric

res

ista

nce

of o

xide

film

, R

f(o

hm c

m2 )

x103

0 50 100 150 200 250exposure time (h)

100 ppb H2O2

100 ppb H2O2+200 ppb O2

Fig. 4 Electric resistance of oxide film obtained from Cole–Coleplots



sion due to less oxidant. Chromium depletion was observedfor each specimen exposed to H2O2.

SIMS depth profiles of oxygen and chromium in the testspecimens with pre-exposure to O2 are compared inFig. 11. The oxygen/iron (O/Fe) ratios through depth for

the specimens exposed to just 200 ppb O2, to just 100 ppbH2O2 and to 100 ppb H2O2 after 200-h exposure to 200ppb O2 are compared in Fig. 11(a). It was observed that lateexposure to 100 ppb H2O2 determined the oxide film thick-ness, which was thicker than that to exposed to just

Fe3O4

α-Fe2O3

NiFe2O4

FeCr2O4

00.20.40.60.81.01.2

inte

nsit

y (

-)

00.20.40.60.81.01.2

inte

nsit

y (

-)

200 300 400 500 600 700 800wave number (cm-1)

00.20.40.60.81.01.2

inte

nsit

y (

-)

00.20.40.60.81.01.2

inte

nsit

y (

-)

200-hour exposure to 100 ppb H2O2

200-hour exposure to 200 ppb O2

00.20.40.60.81.01.2

inte

nsit

y (

-)

200 300 400 500 600 700 800wave number (cm-1)

00.20.40.60.81.01.2

inte

nsit

y (

-)

00.20.40.60.81.01.2

inte

nsit

y (

-)

300-hour exposure to 5 ppb H2O2

(a) Standard oxides (b) Oxides on the specimens

Fig. 5 Laser Raman spectra

200 500400300 800600 7000

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Fe2O3 NiFe2O4

FeCr2O4

Fe3O4

wave number (cm-1)

inte

nsity

(-)

H2O2

100ppb

(calculated)

Fig. 6 Comparison of the measured spectrum and the calculated obtained by summing the suitably weighted standardpatterns



200 ppb O2 (Fig. 11(a)). Late-exposure to 100 ppb H2O2

caused Cr depletion in the oxide film, while exposure to just200 ppb O2 caused Cr enrichment in the oxide (Fig. 11(b)).

SIMS depth profiles of oxygen and chromium in the testspecimens with pre-exposure to 200 ppb O2 after 200-h ex-posure to 100 ppb H2O2 are compared with those exposedto just 200 ppb O2, just 100 ppb H2O2 and a mixture of100 ppb H2O2 and 200 ppb O2 in Fig. 12. Exposure to100 ppb H2O2, even if it was either only H2O2 exposure,H2O2 pre-exposure, H2O2 late-exposure or simultaneous ex-posure with O2 and H2O2, caused thicker oxide films than

that exposure to just 200 ppb O2 (Fig. 12(a)) and causedCr depletion in the oxide film (Fig. 12(b)). The late exposureto 200 ppb O2 caused a thinner oxide film than exposure tojust 100 ppb H2O2, while the mixed exposure caused thickerfilm.

IV. Discussion

As [H2O2] was lowered from 100 to 5 ppb, the saturatedradii of the low frequency semi-circles increased continuous-ly and the terminal ECP remained at the same level up to10 ppb and then decreased a little at 5 ppb (Fig. 13). The in-creasing radii meant increasing resistances of oxide dissolu-tion. Both dependences of ECP and saturation of the radii on[H2O2] were not affected by co-existing [O2] with the samelevel oxidant concentration.

As a result of water radiolysis evaluation for BWR cool-ing water, it was assumed that 100 ppb H2O2 co-existed with200 ppb O2 under NWC, while 10 ppb H2O2 existed withouta measurable concentration of O2 under HWC. This meantthat corrosive conditions of HWC were the same as thoseof NWC from the viewpoint of ECP, though thickness andcharacteristics of oxide films were different for both condi-tions.

Ratios of hematite in oxide films on the specimens ex-posed to O2 and H2O2 environments are given expressedas a function of oxidant concentration in Fig. 14. The hem-

Fe3O4

α-Fe2O3

FeCr2O4

NiFe2O4

0 0.2 0.4 0.6 0.8 1

run 1 8ppm O2 (300 h)

run 5 Decreased H2O2 after 300-hour exposure to 100 ppb H2O2

run 6 10 ppb H2O2 (300 h)

run 7 5 ppb H2O2 (300 h)

run 8a 200 ppb O2 (200 h)

run 3a 100 ppb H2O2 (200 h)

run 8b 100 ppb H2O2 (200 h) after 200-hour exposure to 200 ppb O2

run 3a 100 ppb H2O2 (200 h)

run 2 200 ppb O2 (200 h)

run 3b 200 ppb O2 (200 h) after 200-hour exposure to 100 ppb H2O2

run 9 mixture of 200 ppb O2 and 100 ppb H2O2 (200 h)

run 9 mixture of 200 ppb O2 and 100 ppb H2O2 (200 h)

run 4 100 ppb H2O2 (300 h)

Fig. 7 Major chemical forms of oxide films of specimens

0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6depth (µm)

O/F

e ra

tio (

-)

8 ppm O2

100 ppbH2O2

200-hour exposures

Fig. 8 Depth profile of oxygen in the test specimens measured bySIMS



atite ratio of the specimens exposed to H2O2 was expressedas a logarithmic function of [H2O2]. The hematite ratio for aspecimen with 8 ppm O2 exposure was measurable, but thatfor 200 ppb O2 exposure was negligibly small. Hematite hasoften been observed on test specimens taken from the

primary cooling systems of operating BWR plants. Somedeveloped due to H2O2 exposure, and some was from fueldeposits.

The inverse of the resistance of oxide dissolution is in pro-portion to dissolution rate of oxide. The relationship between

0

0.2

0.4

0.6

0 0.1 0.2 0.3 0.4 0.5depth (µm)

Cr/

Fe R

atio

(-)

200 ppb O2

100 ppb H2O2

200-hour exposures

(b) Cr/Fe ratio

0 0.1 0.2 0.3 0.4 0.5depth (µm)

0

0.2

0.4

0.6

0.8

1.0

O/F

e R

atio

(-)

200 ppb O2

100 ppb H2O2

200-hour exposures

(a) O/Fe ratio

Fig. 9 Depth profiles of oxygen and chromium in the test specimens measured by SIMS (comparison of specimensexposed to O2 and H2O2)

0

0.2

0.4

0.6

0.8

1.0

0 0.1 0.2 0.3 0.4 0.5 0.6

depth (µm)

O/F

e ra

tio (

-)

5 ppb

100 ppbH2O2

10 ppb

300-hour exposures

(a) O/Fe ratio

5 ppb

100 ppb H2O2

10 ppb

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6

depth (µm)

Cr/

Fe r

atio

(-)

300-hour exposures

(b) Cr/Fe ratio

Fig. 10 Depth profiles of oxygen and chromium in the test specimens measured by SIMS (comparison of specimensexposed to different concentrations of H2O2)

0

0.2

0.4

0.6

0.8

1.0

0 0.1 0.2 0.3 0.4 0.5

depth (µm)

O/F

e ra

tio (

-)

200 ppbO2

100 ppb H2O2

200 ppb O2 100 ppb H2O2

200-hour exposures

(a) O/Fe ratio

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5

depth (µm)

Cr/

Fe r

atio

(-) 200 ppb O2

100 ppb H2O2

200 ppb O2

100 ppb H2O2

200-hour exposures

(b) Cr/Fe ratio

Fig. 11 Depth profiles of oxygen and chromium in the test specimens measured by SIMS (Effects of pre-exposureto O2)



oxidant concentration and inverse of resistances of oxide res-olution is shown in Fig. 15. The thickness of oxide film firstincreased as [H2O2] decreased from 100 to 10 ppb and thendecreased (Fig. 10). A large dissolution rate might cause athin oxide film even if a large growth rate of oxide film

was expected at that concentration. When [H2O2] droppedbelow 10 ppb, growth rate of oxide film decreased to causethe thinner oxide film. It is generally understood that Cr-richoxide provided a protective film on the stainless steel. H2O2

exposure caused Cr depletion and formation of a high elec-tric resistance oxide film on the specimens and at the sametime, high dissolution rate. It is considered that the oxidelayers on the specimens exposed to H2O2 are divided intotwo: the outer layers with high dissolution rate and the innerlayers with high electric resistance. The inner layers are toothin to show Cr enrichment, which would develop into basemetal by a dry corrosion mechanism. The authors are prepar-ing to undertake weight loss measurements to confirm disso-lution of oxide film on the specimens exposed to 100 ppbH2O2.

V. Conclusions

The conclusions are summarized as follows.(1) Peaks corresponding to Fe3O4, �-Fe2O3, NiFe2O3 and

FeCrO4 were obtained from laser Raman spectroscopy(LRS) data. Clear peaks corresponding to hematite (�-

0.1

0

-0.1

-0.2

105

104

103

0 20 40 60 80 100[H2O2] (ppb)

EC

P (

V-S

HE

)

radi

us o

f co

mpl

ex im

peda

nce

low

cyc

le s

emi-

circ

le (

ohm

cm

2 )

radius

ECP

Fig. 13 Relationship between [H2O2], ECP and frequencydependent complex impedance at 200-h exposure

0

0.2

0.4

0.6

0.8

0 0.1 0.2 0.3 0.4 0.5

depth (µm)

Cr/

Fe r

atio

(-)

200 ppbO2

100 ppb H2O2

100 ppb H2O2 200 ppb O2

mixture (100 ppb H2O2 + 200 ppb O2)

200-hour exposures

(b) Cr/Fe ratio

0

0.2

0.4

0.6

0.8

1.0

0 0.1 0.2 0.3 0.4 0.5

depth (µm)

O/F

e ra

tio (

-)

200 ppb O2

100 ppb H2O2

100 ppb H2O2

200 ppb O2

mixture (100 ppb H2O2 + 200 ppb O2)

200-hour exposures

(a) O/Fe ratio

Fig. 12 Depth profiles of oxygen and chromium in the test specimens measured by SIMS (Effects of pre-exposure toH2O2 and mixture of H2O2 and O2)

0.12

0.10

0.08

0.06

0.04

0.02

0100 101 102 103 104

oxidant concentration (ppb)

α-Fe

2O3

ratio

(-) H2O2

O2

Fig. 14 Relationship between oxidant concentration and �-Fe2O3

ratio of oxide films

[H2O2] (ppb)

1/R

a(a

rbitr

ary

unit)

10-1 100 101 102

10-1

10-2

10-3

10-4

10-5

10-6

: constant concentration : transient exposure

Fig. 15 Relationship between oxidant concentration and the in-verse of resistances of oxide resolution



Fe2O3) were observed for the specimens exposed toH2O2 while they were indistinct for O2 exposure.

(2) The hematite ratio in the oxide films of the specimensexposed to H2O2 was expressed as a logarithmic func-tion of [H2O2]. The hematite ratio for a specimen with8 ppm O2 exposure was measurable, but that for200 ppb O2 exposure was negligibly small.

(3) The depth profiles of O/Fe and Cr/Fe ratios obtainedfrom the secondary ion mass spectroscopy (SIMS) datafor the specimens exposed to 200 ppb O2 and to 100 ppbH2O2 showed that H2O2 caused thicker oxide layersthan O2 and there was Cr depletion in the oxide layersfor the former.

(4) Exposure to 100 ppb H2O2, even if it was either onlyH2O2 exposure, H2O2 pre-exposure, H2O2 late-exposureor simultaneous exposure with O2 and H2O2, caused Crdepletion in the oxide film.

(5) The thickness of oxide film first increased as [H2O2] de-creased from 100 to 10 ppb and then decreased. As a re-sult of combined evaluation of SIMS depth profile dataand Cole–Cole plots, it was concluded that large disso-lution rate might cause a thin oxide film even if a largegrowth rate of oxide film was expected at high [H2O2],while a low growth rate of oxide film caused a thinneroxide film at low [H2O2].

Nomenclature

[H2O2], [O2]: Concentrations of hydrogen peroxide and oxygen(ppb)

Ra: Resistance of oxide dissolution (��cm2)Rf : Electric resistance of oxide film (��cm2)

Abbreviations

BWR: Boiling water reactorECP: Electrochemical corrosion potentialFDCI: Frequency dependent complex impedanceGP-IB: General purpose interface bathHWC: Hydrogen water chemistryIGSCC: Intergranular stress corrosion crackingLRS: Laser Raman spectroscopyNWC: Normal water chemistryPTFE: PolytetrafluoroethyleneRBS: Rutherford back scattering spectroscopySIMS: Secondary ion mass spectroscopyXPS: X-ray photoelectron spectroscopyXRD: X-ray diffraction

Acknowledgments

The authors wish to express their sincere thanks to Dr. K.

Mabuchi of the Hitachi Research Laboratory, Hitachi, Ltd.,for his kind advice on electrochemical impedance measure-ment. They also wish to express their thanks to Mr. Y.Murayama of Canon, Inc., Mr. J. Sugama of Tohoku ElectricPower Co. and Mr. N. Yamashiro of IAI Corporationfor their enthusiastic contribution to loop experiments andin-situ measurement of corrosion behavior.

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effects of hydrogen peroxide on corrosion of stainless steel, (iv)

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