effects of noble metal deposition upon corrosion behavior of structural materials in nuclear power...

Effects of Noble Metal Deposition upon Corrosion Behavior

of Structural Materials in Nuclear Power Plants, (I)Effect of Noble Metal Deposition with an Oxide Film on Type 304 Stainless Steel

under Simulated Hydrogen Water Chemistry Condition

Kazushige ISHIDA1;�, Yoichi WADA1, Masahiko TACHIBANA1,Hideyuki HOSOKAWA1 and Masato NAKAMURA2

1Power and Industrial Systems R&D Laboratory, Hitachi, Ltd., 7-2-1 Omika-cho, Hitachi-shi, Ibaraki 319-12212Power Systems, Hitachi Works, Hitachi, Ltd., 3-1-1 Saiwai-cho, Hitachi-shi, Ibaraki 317-8511

(Received March 25, 2005 and accepted in revised form July 6, 2005)

The effects of noble metal deposition under hydrogen water chemistry (HWC) condition on the features ofoxide film formed on structural components in a reactor were studied. Noble metal-deposited type 304 stainless steelspecimens with an oxide film were exposed to the simulated HWC condition, including co-existing Co radioactivity.Relationships between features of the oxide film which had two layers and the accumulation and distribution of Coradioactivity in the oxide film were established.

The outer layer of the oxide film which consisted of �-Fe2O3, Fe3O4 and NiFe2O4 was dissolved by noble metaldeposition and exposure to the HWC condition. The reasons for this were as follows. Solubility of �-Fe2O3, Fe3O4 andNiFe2O4 increased with the decrease of electrochemical corrosion potential. Dissolution of these compounds wasaccelerated by the anodic reaction of hydrogen which is catalyzed by noble metal.

Co radioactivity was mainly incorporated into the inner layer. This was caused by the substitution of radioactive Coions for ferrous ions in the oxide film, based on the observation that growth of the oxide film and oxidation of basemetal stopped in the HWC condition. The inner layer consisted of FeCr2O4 which is stable at low ECP and it did notdissolve. Co radioactivity was not incorporated into the outer layer because it dissolved.

KEYWORDS: water chemistry, stainless steel, electrochemical corrosion potential, oxide film, BWR typereactors, HWC, surface characterization, noble metal, stress corrosion cracking

I. Introduction

Electrochemical corrosion potential (ECP) is an index ofintergranular stress corrosion cracking (IGSCC) susceptibil-ity, which is remarkably reduced below an ECP of �230mVversus standard hydrogen electrode (SHE) in boiling waterreactors (BWRs).1,2) On the other hand, O2 and H2O2 areproduced by water radiolysis in the BWR core and theycause an increase in ECP.3)

Hydrogen injection into reactor water (i.e., hydrogenwater chemistry: HWC) is one of the methods to decreasethe concentrations of O2 and H2O2 and to lower the ECP.HWC has been applied to many BWRs.2) Recently, noblemetal deposition on the surface of the reactor structuralmaterials by chemical addition (NMCA) has been appliedto many US BWRs in order to enhance the effect of HWCon IGSCC mitigation.4,5) After NMCA, ECP of the reactorstructural materials was seen to decrease remarkably witha small amount of hydrogen injection.4,5)

However, the ECP decrease may change the features ofthe oxide film on the surface of the materials. The ECP de-crease also affects the corrosion behavior and the radioactiv-ity accumulation. Thermodynamic stabilities of the oxidefilm formed on the structural materials and in which radioac-tivity is accumulated may depend on water chemistry. In

fact, increases in electrical conductivity and the soluble orinsoluble radioactivity in reactor water after NMCA havebeen reported.6)

The oxide films on type 304 stainless steel (304SS) andtype 316 stainless steel (316SS) have been reported to con-sist of two layers, an outer layer and an inner layer.3,7) Theouter layer consisted of �-Fe2O3, NiFe2O4 and �-Fe3O4

for the oxidizing condition and Fe3O4 and NiFe2O4 for thereducing condition in 288�C water.3,7) The inner layer con-sisted of NiFe2O4, Fe3O4 and FeCr2O4 for both condi-tions.3,7) The character of the oxide film was changed bythe cyclic change of the oxidizing and reducing conditions.7)

The transformation of the oxide film of the noble metal-de-posited 304SS by coexisting Cu (15 ppb) and Zn (5 ppb) ionsand excess H2 (O2 200 ppb, H2 35 ppb) has been studied byusing transmission electron microscopy and energy disper-sive spectroscopy.8) It was reported that �-Fe2O3 formingthe outer layer was transformed into Fe3O4 or ZnFe2O4,Fe ions were released from the oxide film, and Cr was en-riched in the inner layer.8) For Co accumulation in the oxidefilm of the noble metal-deposited 316SS, the distribution ofCo in the oxide film with coexisting Co ions (3 ppb), no Znor Cu ions and excess H2 (O2 20 ppb, H2 100 ppb) was stud-ied using glow discharge lamp spectroscopy. It was reportedthat Co was distributed in the inner layer.9) However the ef-fects of noble metal deposition on the relationships betweenfeatures of the oxide film and the accumulation and distribu-tion of Co radioactivity in the oxide film are not clear.�Corresponding author, Tel. +81-294-52-9179, Fax. +81-294-52-

8803, E-mail: kazushige [email protected]

Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 42, No. 9, p. 799–808 (September 2005)

799

ORIGINAL PAPER

The purposes of this work were to study the effects of no-ble metal deposition and exposure to HWC condition on fea-tures of the oxide film such as its composition, weight andmorphology, and to establish relationships between featuresof the oxide film and the accumulation and distribution of Coradioactivity in the oxide film.

II. Experimental

1. Procedures and ApparatusThe surfaces of 304SS specimens (2 cm�1 cm�0.1 cm)

were polished using 600-grid emery paper and degreasedwith acetone. Then all specimens were exposed to 280�Cwater containing O2 (300–400 ppb) for 330 h (flow rate0.01 cm/s) in order to form an oxide film on the surface(pre-oxidization). Next some of the specimens were exposedat 150�C to noble metal solutions containing O2 (200 ppb)for 24 h (flow rate 1 cm/s). Concentration distribution of no-ble metal in the oxide film of 304SS has been reported to dif-fer between Pt and Rh.10) This difference may affect featuresof the oxide film and the accumulation and distribution of Coradioactivity in the oxide film. So, in this treatment, three no-ble metal solutions were used, Pt (100 ppb) solution added asNa2Pt(OH)6, Rh (100 ppb) solution added as Na3Rh(NO2)6,and Pt (100 ppb) and Rh (100 ppb) mixture added asNa2Pt(OH)6 and Na3Rh(NO2)6. The temperature chosenand the concentrations of noble metal used were based on lit-erature data.6,9,11) The solutions were injected into the loop(Fig. 1) just before the autoclave so as not to decomposein the feed water piping. Finally all specimens, includingthe specimens without noble metal deposition, were exposedfor 1,000 h to the simulated HWC condition: flow rate,0.6 cm/s; 280�C water containing 10 ppb O2 and 50 ppbH2 with co-existing 4Bq/kg 58Co in the form of CoSO4.Non-radioactive Co, that is, 59Co was also injected at thesame time in the form of CoSO4 in order to prevent all

58Co from being adsorbed on the pipe wall because radioac-tive 58Co concentration was very low (3:4�10�6 ppb). Theconcentration of non-radioactive Co was 0.1 ppb which isthe same order as the concentration in actual BWR water.12)

The concentration of non-radioactive Co was so small thatnon-radioactive Co would not affect the features of the oxidefilm.

Pre-oxidization, noble metal deposition and exposure tothe simulated HWC condition were carried out using thehigh temperature and high pressure water loop shown inFig. 1. It was a re-circulation loop in which used waterwas regenerated and returned to the pure water tank for fur-ther reuse. Demineralized pure water with an electrical con-ductivity<0.01mS/m at 25�C was stored in the reservoirtank. N2, O2 and/or H2 gases were bubbled in to control dis-solved oxygen and hydrogen concentrations. After the waterwas heated to the desired temperature, it was supplied to theautoclave. Chemical agents such as noble metal ions, Coions and Co radioactivity were injected into the loop in frontof the autoclave. Each treatment was carried out using a dif-ferent re-circulation loop because noble metal deposition onthe autoclave inner surfaces may change the water quality.

2. Sample Characterization AnalysisAnalyses were made for the deposited amount of noble

metal, the features of the oxide film such as its composition,weight and morphology, and the accumulation and distribu-tion of Co radioactivity in the oxide film.

The deposited amounts of noble metals after noble metaldeposition and after exposure to the simulated HWC condi-tion were measured by chemical dissolution of the oxide filmusing aqua regia. The noble metal concentrations in that so-lution were analyzed by graphite furnace atomic absorptionspectroscopy (AAS) (Hitachi, Ltd., Polarized Zeeman Atom-ic Absorption, Z-5000).

The oxide film compounds were identified by comparison

Pure water tank

Chemicalfeed tank

Circulationpump

Pump

Condenser

Demineralizer

Pressure regulator

EC O2 H2Sensors

Demineralizer

Heat exchanger

Preheater

Autoclave

O2, H2, N2 gases

Pump

Fig. 1 Schematic of high temperature and high pressure water loop

800 K. ISHIDA et al.

JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

of the Raman spectra of the standard samples using a Ramanspectroscopy (Renisaw, plc, Raman Microscope system3000). The standard samples were �-Fe2O3, �-Fe2O3,Fe3O4, NiO, NiFe2O4, Cr2O3 and FeCr2O4. Their selectionwas based on the elements involved in the material andavailable thermodynamic data.13)

Purity of powder oxide samples �-Fe2O3, Fe3O4,NiFe2O4, Cr2O3 and NiO was 99.9% and that of powder�-Fe2O3 sample was 99% (Kojundo Chemical LaboratoryCo., Ltd.). Purity of FeCr2O4 was not known. Five to sixpoints on the specimen surfaces were analyzed because thelaser spot was so small (about 10 mm) that only one measure-ment would not provide data on the typical compounds ofthe oxide film. �-Fe2O3 was identified by its characteristicpeaks (227, 246, 299, 412, 499, 612 cm�1). Fe3O4

(669 cm�1) and FeCr2O4 (681 cm�1), NiFe2O4 (700 cm�1)were identified by their characteristic peak and its half width.NiFe2O4 was identified by its peak at 486 cm�1.

To get the composition of the oxide film, it was dissolvedin two steps: (1) ultrasonic cleaning and cathodic electroly-sis; and (2) chemical dissolution using alkaline potassiumpermanganate solution and ammonium citrate solution. Dis-solved metals came from the outer layer in the first step andfrom the inner layer in the second step.14) Concentrations ofmetal ions such as Fe, Ni and Cr in the solution were ana-lyzed by AAS.

In order to measure the amount of weight change duringexposure to the simulated HWC condition, a specimen wasweighed before and after exposure. The amount of theweight change was defined as

(Amount of weight change) ¼ Wo �Wi; ð1Þ

where Wi: Weight before exposure to the simulated HWCcondition

Wo: Weight after exposure to the simulated HWCcondition.

The morphology and the element segregations of theoxide film before and after exposure to the simulatedHWC condition were observed by scanning electron micro-scope and energy dispersive X-ray spectroscopy (SEM-EDX) (SEM: Hitachi, Ltd., N-2460, EDX: Horiba, Ltd.,EMAX-5770).

The deposited amount and distribution of Co radioactivitywere measured with a germanium gamma-ray detector(Seiko EG&G Co., Ltd., GEM-15190-P). To find this distri-bution, Co radioactivity was measured twice: after no treat-ment and after ultrasonic cleaning and cathodic electrolysis.The distribution of Co radioactivity was defined by Eqs. (2)and (3). Deposited amount of Co radioactivity was comparedusing the relative amount defined in Eq. (4):

(Co radioactivity involved in outer layer) ¼ At � Ac ð2Þ(Co radioactivity involved in inner layer) ¼ Ac ð3Þ(Relative amount of Co radioactivity)

¼ (At of each specimen)

=(At of specimens with no noble metal deposited) ð4Þ

where At: The measured value of Co radioactivity after notreatment (total Co radioactivity)

Ac: The measured value of Co radioactivity afterultrasonic cleaning and cathodic electrolysis.

III. Results

1. Deposited Amount Change of Noble MetalDeposited amounts of noble metal on and in the oxide film

after the treatment of noble metal deposition and after expo-sure to the simulated HWC condition are shown in Fig. 2.The decrease in Pt due to exposure to the simulated HWCcondition was small for Pt-deposited specimens in the100 ppb Pt solution (Pt-dep.). The deposited amount of Ptdid not change in the region of variation of deposited amountfor Pt- and Rh-deposited specimens in the 100 ppb Pt and100 ppb Rh mixed solution (Pt+Rh-dep.). The variation ofdeposited amount including analysis error was about 3%for Pt and 12% for Rh. Rh fell to 40 and 60% of the initialamount for Rh deposited specimens in the 100 ppb Rh solu-tion (Rh-dep.) and Pt+Rh-dep., respectively. The amount ofnoble metal which is needed to decrease ECP to about�500mV vs. SHE is more than 4�10�3 g/m2.15) So, theamount of noble metal was enough to decrease ECP about�500mV vs. SHE even after the exposure to the simulatedHWC condition for all noble metal-deposited specimens.

2. Identification of the Oxide Film CompositionRaman spectra of standard samples and compounds found

in the specimen surfaces are shown in Fig. 3. Two Ramanspectra for compounds found in the specimen surfaces areshown. In all specimen surfaces, NiO, Cr2O3 and �-Fe2O3

were not present.For the no noble metal-deposited specimen (No-NM dep.),

�-Fe2O3, Fe3O4, NiFe2O4 and FeCr2O4 were identified be-fore and after exposure to the simulated HWC condition.This agreed with literature findings.3,7) On the other hand,for noble metal-deposited specimens, the �-Fe2O3 peak be-came small after exposure to the simulated HWC condition.Based on the appearance of the peak around 700 cm�1,

0.00

0.02

0.04

0.06

0.08

0.10

0.12: Pt: Rh

Before exposure

After exposure

0.14

Ptdep.

Rhdep.

Pt+Rhdep.

Am

ount

of

nobl

e m

etal

/g·m

-2

: Pt: Rh

Fig. 2 Deposited amount of noble metal on and in oxide film be-fore and after exposure to the simulated HWC condition

Effects of Noble Metal Deposition upon Corrosion Behavior of Structural Materials in Nuclear Power Plants, (I) 801

VOL. 42, NO. 9, SEPTEMBER 2005

100 200 300 400 500 600 700 800 900

wave numbers/cm-1

standard sample

No-NM dep. after HWC

Pt-dep. after HWC

Rh-dep. after HWC

Pt+Rh-dep. after HWC

Inte

nsity

(ar

bitr

ary

unit)

No-NM dep. before HWC

NiFe2O4

FeCr2O4

Fe3O4

α-Fe2O3

Cr2O3

NiO

γ -Fe2O3

Fig. 3 Raman spectra of the standard samples and compounds found in specimen surfaces



presence of Fe3O4 (669 cm�1), FeCr2O4 (681 cm�1) andNiFe2O4 (700 cm�1) were confirmed. Since the NiFe2O4

peak at 486 cm�1 diminished after exposure to HWC condi-tion, dissolution of NiFe2O4 was also caused by the noblemetal deposition.

The metal amounts involved in oxide films of the speci-mens are shown in Fig. 4. For No-NM dep., Fe was enrichedin the outer layer and Cr was enriched in the inner layer aswere reported previously.3) For noble metal-deposited speci-mens, the Fe amount decreased remarkably in the outer layerand decreased slightly in the inner layer compared to No-NM dep. The Ni amount in the outer layer also remarkablydecreased and that in the inner layer decreased only in thecase of Pt-dep. The amount of Cr in the inner layer was near-ly equal to that of No-NM dep.

The amount of compound in the oxide films is shown inFig. 5, which is calculated from the data shown in Fig. 4, as-suming that Fe, Ni and Cr exist as Fe3O4, NiFe2O4 andFeCr2O4. Iron oxides were present as �-Fe2O3 and Fe3O4

(Fig. 3) but their abundance ratio was not known. So the ironoxide was assumed to be Fe3O4 in this calculation. The con-tent ratios of iron in Fe3O4 and �-Fe2O3 were 0.72 and 0.70,respectively. The difference was so small that the assump-tion did not seem to affect the calculation.

For noble metal-deposited specimens, the amount of ironoxide calculated as Fe3O4 and NiFe2O4 in the outer layerwas remarkably decreased. NiFe2O4 and FeCr2O4 in the in-ner layer decreased slightly except for Pt-dep.

3. Weight ChangeThe amounts of weight change of the specimens are

shown in Fig. 6. For the No-NM dep., the weight decreasedslightly. From this result, it was concluded that growth of theoxide film and oxidation of base metal stopped in the simu-lated HWC condition. On the other hand, for noble metal-deposited specimens, the weight decreased remarkably foreach specimen. Amount of weight change for Rh-dep. was

smaller than that for Pt-dep. or Pt+Rh-dep.Total amount of oxide film was estimated as about

3 g/m2 by using the results of Fig. 5. So, the amount of ox-ide film might decrease to 1/2–2/3 of that before exposureto the simulated HWC condition.

4. Morphology of the Oxide FilmSEM-EDX images of the oxide film surfaces before and

after exposure to the simulated HWC condition are shownin Fig. 7. For No-NM dep., the morphology change wassmall before and after exposure to the simulated HWC con-dition. But for each noble metal-deposited specimen, the

No-NMdep.

Ptdep.

Rhdep.

Pt+Rhdep.

2.5

2.0

1.5

1.0

0.5

0.0

3.0A

mou

nt o

f m

etal

/ g·

m-2

Innerlayer

Outerlayer

: Fe: Ni: Cr

Involved inouter layer

: Fe: Ni: Cr

Involved ininner layer

Fig. 4 Amount of metal involved in oxide film

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

No-NMdep.

Ptdep.

Rhdep.

Pt+Rhdep.

Outerlayer

Innerlayer

:Fe3O4

:NiFe2O4

:FeCr2O4

:Fe3O4

:NiFe2O4

:FeCr2O4

Involved inouter layer

Involved ininner layer

4.0

4.5

Am

ount

of

com

poun

ds /

g·m

-2

Fig. 5 Calculated amount of compounds in oxide film

No-NMdep.

Ptdep.

Rhdep.

Pt+Rhdep.

0.0

-0.4

-0.8

-1.2

-1.6

-2.0

Am

ount

of

wei

ght c

hang

e / g

·m-2

Fig. 6 Amount of weight change during exposure to the simulatedHWC condition



number of particles decreased and the face of some particleswas locally dissolved by exposure to the simulated HWCcondition.

For the Pt-dep, sub-micron particles (which look brighterthan their surroundings) were observed on the oxide film af-ter noble metal deposition as shown in Fig. 8, and they stillremained after exposure to the simulated HWC condition.The point analyses using EDX confirmed that the particlesincluded Pt.

For the Rh-dep. after exposure to the simulated HWC con-dition, a strange deposition (a mesh-like deposition) was ob-served (on the top left in Fig. 9(a)). It included Rh accordingto the EDX mapping shown in Fig. 9(b).

5. Deposited Amount and Distribution of Co Radioactiv-ityRelative amounts of Co radioactivity of the specimens and

the distribution of Co radioactivity are shown in Fig. 10. For

After exposureBefore exposure

Pt-dep.

No-dep.

Rh-dep.

Pt+Rh-dep.

5µm

Fig. 7 SEM images of surfaces of specimens before and after exposure to simulated HWC condition



the No-NM dep., Co radioactivity was accumulated both inthe inner and outer layers as was already reported.14) Onthe other hand, the deposited amount of Co radioactivityof each noble metal deposited specimen decreased to about40% of that of No-NM dep. and the amount of Co radioac-tivity distributed in the outer layer also remarkably decreas-ed. The effect of the kind of the noble metal was small.

IV. Discussion

1. Deposited Amount of Noble MetalPlatinum and rhodium are reported to deposit as Pt0 and

PtIV, and RhIII, respectively. And PtIV and RhIII are reducedto the metallic state in the excess H2 condition.10) At lowECP such as �500mV vs. SHE, Pt and Rh exist as metalwhich is the stable state in neutral and 280�C water.

For Pt, PtIV is reduced to Pt via PtII, and PtII has the sol-ubility of 10�18 ppm in neutral and 25�C water according toPourbaix diagrams.16) So Pt did not dissolve during the re-duction reaction and remained as shown in Fig. 2. On theother hand, for Rh, RhIII is reduced to Rh via RhI, and RhI

has the solubility of ppm order in neutral and 25�C wateraccording to Pourbaix diagrams.17) So Rh dissolved duringthe reduction reaction and the deposited amount significantlydecreased (Fig. 2).

2. Features of Oxide FilmThe features of the oxide film for No-NM dep. scarcely

changed after exposure to the simulated HWC condition.This agreed with the literature observation.7) On the otherhand, the features of the oxide film for each noble metal-de-posited specimen changed remarkably. For each of them, theweight loss was caused by the dissolution of Fe and Ni,mainly from the outer layer. Cr enrichment in the inner lay-er, which has been reported in the literature,9) was the directresult of the dissolution of the inner layer Fe. The amounts of

1µm

967312

PtNiFeCr

metal composition / atomic %

178111

PtNiFeCr

metal composition / atomic %

Fig. 8 EDX point analysis of sub-micron particles of the Pt-dep.specimen

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Rel

ativ

e am

ount

of

58C

o / a

.u.

No-NMdep.

Ptdep.

Rhdep.

Pt+Rhdep.

: outer layer: inner layer

Fig. 10 Deposited amount and distribution of 58Co in oxide film

(a) SEM image

(b) EDX mapping data

20µm

Fig. 9 EDX mapping data of surfaces of Rh-dep. specimens afterexposure to simulated HWC condition



�-Fe2O3 and NiFe2O4 also decreased or diminished. Thesephenomena, that is, the combined effects of noble metaldeposition with HWC must be attributed to the differenceof the thermodynamic properties of each compound involvedin the oxide film.

Thermodynamic properties are affected strongly by theECP. In this work, ECP during the pre-oxidization condition(300–400 ppb O2) was estimated to be about 0mV vs. SHEand ECP during the simulated HWC condition (10 ppb O2

at inlet) was estimated to be about �350 to �500mV vsSHE for each specimen based on the literature data.3,15)

These ECP changes cause the thermodynamic stabilitychange. The dominant iron oxide compounds at about0mV vs. SHE and �500mV vs. SHE are �-Fe2O3 andFe3O4, respectively.13) The solubilities of NiFe2O4 andFe3O4 at ECP 0mV vs. SHE are about 3 to 5 orders as largeas those at ECP �500mV vs. SHE.13) On the other hand, thesolubility of FeCr2O4 at ECP 0mV vs. SHE is about 3 orderssmaller than that at ECP �500mV vs. SHE.13) From thesethermodynamic properties, it was reasonable to considerthat �-Fe2O3, Fe3O4 and NiFe2O4, or Ni and Fe dissolvedby exposure to the simulated HWC condition.

In the pre-oxidation condition, the outer layer was report-ed to consist of �-Fe2O3, NiFe2O4 and Fe3O4, and the innerlayer was reported to consist of FeCr2O4 and NiFe2O4.

3,7)

These compounds were identified by Raman spectroscopy.For noble metal-deposited specimens, the tendency of de-creasing �-Fe2O3 and NiFe2O4 amounts was confirmed bythe composition analyses (Figs. 4 and 5) and the Ramanspectra (Fig. 3). From these considerations and experimentalresults, dissolutions of Fe and Ni were considered to becaused by the thermodynamic stability change due to theECP change.

On the other hand, for No-NM dep., the decrease inweight and the change in compound were negligible as con-firmed by Raman spectra (Fig. 3). From these results, thedissolution ratios of �-Fe2O3, NiFe2O4 and Fe3O4 basedon the equilibrium change would be small. So, the depositednoble metal was considered to accelerate the thermodynamicproperty change. Noble metal was reported to be a catalystfor the hydrogen oxidation reaction shown in Eq. (5).18)

The reductive dissolution reactions of �-Fe2O3, NiFe2O4

and Fe3O4 as shown in Eqs. (6)–(8) were considered to beaccelerated by the anodic reaction of hydrogen;7)

H2 ¼ 2Hþ þ 2e� ð5Þ�-Fe2O3 þ 6Hþ þ 2e� ¼ 2Fe2þ þ 3H2O ð6ÞFe3O4 þ 8Hþ þ 2e� ¼ 3Fe2þ þ 4H2O ð7ÞNiFe2O4 þ 8Hþ þ 2e� ¼ Ni2þ þ 2Fe2þ þ 4H2O: ð8Þ

In these reactions ferric ion is reduced to ferrous ion. The re-duction reaction of ferric ion is thermodynamically reasona-ble below �0:1V vs. SHE.18,20) So, the reduction reaction of�-Fe2O3, NiFe2O4 and Fe3O4 as shown in Eqs. (6)–(8) werethermodynamically reasonable. The amount of hydrogenwas enough not only to react with oxygen but also oxidecompounds under the experimental conditions of this work.The dissolution of the face of the crystal as shown in Fig. 7was considered to be proof of these ideas because these

forced dissolutions may occur near the deposited noblemetal. The dissolution based on the equilibrium changemay occur uniformly for each part or edges of the particles.

It should be noted that iron in FeCr2O4 is already divalentand that the chromium is trivalent and not reduced, so the in-ner layer consisting of FeCr2O4 would not be dissolved.13,20)

Slight dissolution of Ni in the inner layer for Rh-dep. andPt+Rh-dep. may occur because Ni was involved in the Croxide, e.g. (Fe,Ni)Cr2O4. The reason why Ni in the innerlayer was only dissolved for Pt-dep. was not clarified in thiswork.

3. Relationship between Co Radioactivity Accumulationand the Features of the Oxide FilmCo radioactivity is considered to be incorporated into the

oxide during formation of the oxide film and/or by thesubstitution of Co radioactivity for ferrous ions in the oxidefilm via reactions such as the equations:21–23)

Outer layer: �-Fe2O3 þ 58Co2þ þ H2O

¼ 58CoFe2O4 þ 2Hþ ð9ÞFe3O4 þ 58Co2þ ¼ 58CoFe2O4 þ Fe2þ ð10Þ

Inner layer: FeCr2O4 þ 58Co2þ ¼ 58CoCr2O4 þ Fe2þ: ð11Þ

Under the conditions of this work, Co radioactivity wasincorporated into the oxide by the substitution of Co radio-activity for ferrous ions in the oxide film because the growthof the oxide film stopped in the simulated HWC condition inwhich Co radioactivity was injected. The inner layer wasconsidered to consist of Fe3O4, NiFe2O4 and FeCr2O4, andthe outer layer was �-Fe2O3, Fe3O4 and NiFe2O4 based onthe results of Figs. 3–5 and the literature.3,7)

From these ideas, the results shown in Fig. 10 could beexplained. Both the inner layer and outer layer includedsome compounds such as �-Fe2O3, Fe3O4 and FeCr2O4 intowhich Co radioactivity is incorporated. So, Co radioactivitywould be incorporated into both the inner and outer layersfor No-NM dep.

For noble metal-deposited specimens, Co radioactivitywould not be incorporated into the outer layer because it dis-solved during exposure to the simulated HWC condition. Onthe other hand, Co radioactivity would be incorporated intothe inner layer because it mainly consisted of FeCr2O4

which did not dissolve.

4. Effects of the Kind of Noble MetalThe effects of the kind of noble metal on the features of

the oxide film and the accumulation and distribution of Coradioactivity in the oxide film were also studied by usingthree solutions: Rh solution, Pt solution and Pt+Rh solutionin the noble metal deposition step.

The tendency of the effects of noble metal deposition onthe features of the oxide film did not differ among Pt, Rhand Pt+Rh. However, for Rh-dep., the amount of weightchange was smaller than that for Pt-dep. or Pt+Rh dep.The �-Fe2O3 peak was detected in the Raman spectrum.From the above discussions, the effect of Rh on the outerlayer of the oxide film by exposure to the simulated HWCcondition appeared to be different from that of Pt or Pt+Rh.



However, the remaining amount of noble metal after ex-posure to the simulated HWC condition for Rh was smallerthan that for Pt-dep. or Pt+Rh-dep. The hydrogen oxidationreaction rate of Eq. (5) depends on the amount of noble met-al, so the decrease of the oxide film by the reductive disso-lution reactions of Eqs. (6)–(8) also depended on the amountof noble metal. Consequently, the amount of noble metalwas also one of the factors. It was not clarified in this workwhether the difference was caused by the kinds or amount ofnoble metal.

V. Conclusion

Noble metal-deposited type 304 stainless steel specimenswith an oxide film were exposed to the simulated hydrogenwater chemistry (HWC) condition, including co-existing Coradioactivity. The effects of noble metal deposition and ex-posure to HWC condition on the features of the oxide filmsuch as composition, weight and morphology were studied.The relationships between features of the oxide film andthe accumulation and distribution of Co radioactivity inthe oxide film were established. Results were as follows.

The oxide film had an outer layer which consisted of �-Fe2O3, Fe3O4 and NiFe2O4. The outer layer was dissolvedby noble metal deposition and exposure to the HWC condi-tion. The reasons were as follows. Solubility of �-Fe2O3,Fe3O4 and NiFe2O4 increased depending on the decreaseof ECP. Dissolution of these compounds was also accelerat-ed by the hydrogen oxidation reaction which is catalyzed bythe noble metal. The low dissolution of the inner layer wasattributed to the solubility of FeCr2O4 which was the maincomponent of the inner layer; the solubility decreased de-pending on electrochemical corrosion potential (ECP).

Cobalt radioactivity was mainly incorporated into the in-ner layer of the oxide film and this was caused by the substi-tution of radioactive Co ions for iron ions in the oxide film,based on the observation that growth of the oxide film andoxidation of base metal stopped in the HWC condition.The inner layer consisted of FeCr2O4 which is stable atlow ECP and it did not dissolve. Co radioactivity was not in-corporated into the outer layer because it dissolved.

The effects of the kind of noble metal were also studied byusing three solutions: Rh solution, Pt solution and Pt+Rh so-lution in the noble metal deposition step. A difference wasobserved in the degree of dissolution of the outer layer butnot in the accumulation of Co radioactivity. The remainingamount of noble metal after exposure to the simulatedHWC condition for Rh was also smaller than that for Pt-dep. or Pt+Rh-dep. So, it was not clarified in this workwhether the difference was caused by the kind or the amountof noble metal.

Abbreviations and Nomenclature

304SS: Type 304 stainless steel316SS: Type 316 stainless steelAAS: Graphite furnace atomic absorption spectroscopyBWR: Boiling water reactorEC: Electrical conductivityECP: Electrochemical corrosion potential

HWC: Hydrogen water chemistryIGSCC: Intergranular stress corrosion crackingNMCA: Noble metal deposition on the surface of reactor struc-tural materials by chemical additionNo-NM dep.: No noble metal deposited specimenPt-dep.: Pt-deposited specimens in the 100 ppb Pt solutionPt+Rh-dep.: Pt- and Rh-deposited specimens in the 100 ppb Ptand 100 ppb Rh solutionpre-oxidization: Treatment to form an oxide film on the surfaceRh-dep.: Rh-deposited specimens in the 100 ppb Rh solutionSEM-EDX: Scanning electron microscope and energy dispersionof X-ray spectroscopySHE: Standard hydrogen electrodeAc: Measured value of Co radioactivity after ultrasonic cleaningand cathodic electrolysisAt: Measured value of Co radioactivity after no treatment (totalCo radioactivity)Wi: Weight before exposure to the simulated HWC conditionWo: Weight after exposure to the simulated HWC conditionppm: mg/kgppb: mg/kg

References

1) M. E. Indig, et al., ‘‘Evaluation of in-reactor inter-granularstress corrosion cracking via electrochemical measurement,’’Int. Symp. on Environmental Degradation of Materials inNuclear Power Systems—Water Reactors, Monterey, Califor-nia, Sept. 9, 1985, p. 411 (1985).

2) R. L. Cowan, et al., ‘‘Experience with hydrogen water chem-istry in boiling water reactors,’’ Water Chemistry of NuclearWater Reactor Systems 4, Bournemouth UK, Oct. 13–17,1986, p. 29 (1986).

3) Y. J. Kim, ‘‘Analysis of oxide film formed on Type 304stainless steel in 288�C water containing oxygen, hydrogen,and hydrogen peroxide,’’ Corrosion, 55, p. 81 (1999).

4) S. Hettiarachchi, ‘‘First application of NobleChem� to anoperating BWR,’’ 1998 JAIF Int. Conf. on Water Chemistryin Nuclear Power Plants, Kashiwazaki Japan, Oct. 13–16,1998, p. 155 (1998).

5) S. Hettiarachchi, ‘‘NobleChem� for commercial power plantapplication,’’ Int. Conf. of Water Chemistry in Nuclear ReactorSystems, Avignon France, April 22–26, 2002, Paper No. 59(2002).

6) A. D. Odell, R. J. Scholz, ‘‘Benefits, impacts and effects on acommercial nuclear power plant due to noble metal chemicaladdition,’’ Water Chemistry of Nuclear Reactor Systems 8,Bournemouth UK, Oct. 22–26, 2000, p. 122 (2000).

7) Y. J. Kim, ‘‘Characterization of the oxide film formed on Type316 stainless steel in 288�C water in cyclic normal and hydro-gen water chemistries,’’ Corros. Sci., 51, p. 849 (1995).

8) Y. J. Kim, ‘‘Transformation kinetics of oxide formed on noblemetal treated 304SS in 288 water,’’ Corrosion 2001, PaperNo. 01136, (2001).

9) H. Inagaki, et al., ‘‘Effect of noble metal addition on cobaltdeposition upon stainless steel in high temperature water,’’Water Chemistry of Nuclear Reactor Systems 8, BournemouthUK, Oct. 22–26, 2000, p. 148 (2000).

10) Y. Wada, et al., ‘‘Characterization of noble metal depositionon Type 304 stainless steel,’’ 10th Int. Symp. on Environmen-tal Degradation of Materials in Nuclear Power Systems—Water Reactors, Lake Tahoe, Nevada, Aug. 5–9, 2001.

11) S. Hettiarachchi, et al., ‘‘NobleChem� technology for life ex-



tension of BWRs—Field experiences,’’ Water Chemistry ofNuclear Reactor Systems 8, Bournemouth UK, Oct. 22–26,2000, p. 116 (2000).

12) S. Uchida, et al., ‘‘An empirical formula predicting shutdowndose rate of the recirculation pipes in boiling water reactor,’’Nucl. Sci. Eng., 69, 78 (1979).

13) Y. Hemmi, et al., ‘‘Electrochemical considerations regardinggeneral corrosion of materials in BWR primary circuit,’’ J.Nucl. Sci. Technol., 31, 1202 (1994).

14) K. Mochizuki, et al., ‘‘Verification test on zinc injectionadjusted to Japanese BWR water chemistry,’’Water Chemistryof Nuclear Reactor Systems 8, Bournemouth UK, Oct. 22–26,2000, p. 142 (2000).

15) Y. Wada, et al., ‘‘Effectiveness analysis of noble metal treat-ment to mitigation of intergranular stress corrosion crackingof stainless steel in BWR reactor water environment,’’ 11thInt. Symp. on Environmental Degradation of Materials in Nu-clear Power Systems—Water Reactors, Stevenson, Washing-ton, Aug. 10–14, 2003, p. 488 (2003).

16) M. Pourbaix, Atlas of Electrochemical Equilibria in AqueousSolutions, Pergamon Press, New York, p. 350 (1966).

17) M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous

Solutions, Pergamon Press, New York, p. 378 (1966).18) Y. J. Kim, ‘‘Electrochemical interactions of hydrogen, oxygen

and hydrogen peroxide on metal surfaces in high temperature,high purity water,’’ 8th Int. Symp. on Environmental Degrada-tion of Materials in Nuclear Power Systems—Water Reactors,Amelia Island, Florida, Aug. 10–14, 1977, p. 641 (1997).

19) K. Dinov, et al., ‘‘Solubility of magnetite in high temperaturewater and an approach to generalized solubility computa-tions,’’ J. Nucl. Mater., 207, 266 (1993).

20) D. Cubicciotti, T. O. Passell, Computer Calculated PotentialpH Diagrams to 300�C, Volume 2 Handbook of Diagram,NP-3137, EPRI, (1983).

21) D. H. Lister, ‘‘The transport of radioactive corrosion productsin high-temperature water, II. The activation of isothermalsteel surfaces,’’ Nucl. Sci. Eng., 59, 406 (1976).

22) D. H. Lister, ‘‘Mass transfer in the contamination of isothermalsteel surfaces,’’ Nucl. Sci. Eng., 61, 107 (1976).

23) M. Haginuma, et al., ‘‘Effect of metal ion addition on cobaltaccumulation reduction and its thermodynamic evaluation,’’1998 JAIF Int. Conf. on Water Chemistry in Nuclear PowerPlants, Kashiwazaki, Japan, Oct. 13–16, 1998, p. 122 (1998).



effects of noble metal deposition upon corrosion behavior of structural materials in nuclear power...

Documents