Photo-electrochemical Investigation of Radiation Fundamentals • Results ... understanding of...

1 /Photoelectrochem-Shadow Corrosion

Young-Jin Kim & Raul Rebak: GE-Global Research CenterY-P Lin, Dan Lutz, & Doug Crawford: GNF-A

Aylin Kucuk & Bo Cheng: EPRI

16th Int. Symposium on Zirconium in the Nuclear Industry Chengdu, ChinaMay 9-13, 2010

Photo-electrochemical Investigation of Radiation Enhanced Shadow

Corrosion Phenomenon


Outline•

Introduction– Shadow Corrosion Phenomenon at Plants– Literature Review– BWR Water Chemistry

•

Objectives•

Photoelectrochemistry– Fundamentals

•

Results & Discussion– Measurements at low and high temperatures

– Corrosion potential– Electrochemical impedance– Galvanic current

– Proposed Mechanisms•

Summary

•

Acknowledgement


Shadow Corrosion

~4 micron oxide500X

~27 micron patch oxide500X

(a)

(b)

(b)(a)

From Adamson R. B., Lutz D. R., Davies J. H.

Away fromX-750 spacer ring

Beneath X-750 spacer ring

Control blade shadow on channel (Chen & Adamson, 1994)

6“ Elevation

No Elevation


Proposed Mechanisms in Literatures

Distance between Alloys•

Chen & Adamson, 1994

•

Etoh et al., 1999•

Chatelain et al., 2000

•

Lysell et al., 2001

Beta Emission•

Chen & Adamson, 1994

•

Nanika and Etoh, 1996

Crevice Corrosion•

Garzarolli et al,. 2001

Galvanic Corrosion•

In contact

•

Lysell et al,. 2001, 2005Local Radiolysis•

Non-contact

•

Etoh et al., 1997•

Ramasubramanian, 2004

Comprehensive overall review by Adamson, “Shadow Corrosion” in Corrosion Mechanisms in Zirconium Alloys”, A. N. T. International, Sweden, 2007


Galvanic Corrosion

Classical Case Requirement Shadow Corrosion

Yes Two dissimilar metals in contact

Yes/No

Yes Surface area

Acathode >Aanode

No

(Spacer-cathode)Yes Media Yes

(with radiation)

Shadow corrosion is not a typical GC


Model Calculation of Water Radiolysis (GE/Harwell)

C. Ruiz et al., BNES 6, V-2, p. 141 (1992)Water chemistry in-core is always oxidizing

H2 O2

O2

NWC HWC

H2 O2

O2


Theoretical Equilibrium Potentials for Water/Metal Oxides Relevant to BWR Chemistry at 288oC

Redox Potential(Volts, SHE)_ Couple________ Stable Species___________ Conditions

+0.320 O2/H2O NWC

-0.022 Co3O4/CoO Co3O4CoO

-0.025 CuO/Cu2O CuOCu2O

-0.150 CuO/Cu

-0.280 Cu2O/Cu Cu2OCu

-0.428 Fe2O3/Fe3O4 Fe2O3Fe3O4

-0.474 Co3O4/Co

-0.530 H2O/H2

-0.580 NiO/Ni NiONi

-0.625 CoO/Co CoOCo

-0.765 Fe2O3/Fe

-0.812 Fe3O4/Fe Fe3O4Fe

-1.121 ZnO/Zn ZnOZn

-1.322 Cr2O3/Cr Cr2O3Cr

Redox Potential(Volts, SHE)_ Couple________ Stable Species___________ Conditions

+0.320 O2/H2O NWC

-0.022 Co3O4/CoO Co3O4CoO

-0.025 CuO/Cu2O CuOCu2O

-0.150 CuO/Cu

-0.280 Cu2O/Cu Cu2OCu

-0.428 Fe2O3/Fe3O4 Fe2O3Fe3O4

-0.474 Co3O4/Co

-0.530 H2O/H2

-0.580 NiO/Ni NiONi

-0.625 CoO/Co CoOCo

-0.765 Fe2O3/Fe

-0.812 Fe3O4/Fe Fe3O4Fe

-1.121 ZnO/Zn ZnOZn

-1.322 Cr2O3/Cr Cr2O3Cr

HWC

-

-

-

-

-

-

-

-

-

-

-

-

-

-

BWR ECP


Objective

• To establish a laboratory test condition for simulating the shadow corrosion

• To perform basic research for a better understanding of radiation and electrochemical aspect of shadow corrosion


Photoelectrochemistry of Oxide Surface• If light of a suitable energy, hv, is absorbed by the oxide films, electrons can be

excited from occupied electronic states into unoccupied ones: hv → e- + h+

• Excited electrons and holes affect the corrosion processes

p-Type Semiconducting Film• The electrode potential of p-type oxides

shifts in the anodic direction by thephotoexcitaton of the oxides

– Hydrogen evolution by radiation excited electrons

Elec

trod

e Po

tent

ial

Log i

2H+ + 2e- → H2

PhotoexcitationEcorr1

Ecorr2

2H2O → O2+4H++4e-

2H+ + 2e- → H2

n-Type Semiconducting Film• The electrode potential of n-type oxides

shifts in the cathodic direction by thephotoexcitaton of the oxides

– Oxygon evolution by radiation excited holes

Elec

trod

e Po

tent

ial

Photoexcitation

Ecorr1

Ecorr2

2H2O → O2+4H++4e-

2H+ + 2e- → H2

2H2O+4h+ → O2+4H+

Log i

Anodic Current

Cathodic Current


Test Alloys Sn Fe Cr Ni Nb Si ZrZircaloy 2 1.3 0.18 0.1 0.07 BalanceZircaloy 4 1.3 0.2 0.1 BalanceGNF-Ziron 1.3 0.25 0.1 0.07 BalanceGNF-NSF 1.0 0.35 1.0 BalanceZr+Fe+Ni 14.0 9.8 BalanceZr+Fe+Cr 28.1 26.1 Balance

Zr+Fe+Ni+Si 13.0 8.7 1.2 Balance

Zr Specimens

•

Zr Alloys (Zry2, Zry4, GNF-Ziron, & GNF-NSF)– Annealed at 1050oC in Ar, immediately quenched in water– Etched in a 5% HF + 45% HNO3 + 50% H2 O solution

•

Intermetallic Alloys (Zr+Fe+Ni, Zr+Fe+Cr, & Zr+Fe+Ni+Si)– Manufactured at GE GRC by arc melting process– In the shape of irregular cast piece and no surface pretreatment


Photoelectrochemical Measurement

•

Materials– Pure Zr, Zircaloy 2, 304 SS, X750– Preoxidized in 1.1ppm O2 , 300oC water,

•

Electrolyte– 0.01M Na2 SO4, 25oC– 1.1ppm O2 , HWC, NobleChemTM H2 O, 300oC

•

UV Source– EXFO-UV Omnicure Model S2000, 250-400nm

•

Measurements– Gamry Reference 600 – Corrosion potential– Electrochemical impedance– Galvanic current


Oxide: NWC, 1month, 300oCX750Zry2

304SS


Pt Overlay

X750 Matrix

Fine Grain/Thin Oxide Layer

Ni(Fe,Cr)O4

Ni Oxide Particles

Fe2O3

Fe3O4, FeCr2O4 304SS

Epoxy

OxideZry2

Pt

• Zry2: ZrO2 (n-type)• 304SS: Fe2 O3 (n-type), spinel (p-type)• X750: NiO+ spinel (p-type)


Corrosion Potential of Zry2, 304SS & X750

UV “ON” : 304SS & X-750 OCP ↑, Zry2 OCP↓


Corrosion Potential of Intermetallic Alloys

UV “ON” : SPP CP ↑, behaving as cathode over anodic Zr


UV “ON” : Positive current flow indicates the anodic dissolution of Zircaloy 2Negative current flow indicates the anodic dissolution of Zr

Galvanic Corrosion Test

Zry2-Pt

Zry2-X750

Zry2-304SS

Zry2-Zr

Zry2-Zry2


0.0E+00

2.5E+06

5.0E+06

7.5E+06

1.0E+07

1.3E+07

0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07 2.5E+07Real Part, ohm

Imag

inar

y Pa

rt, o

hm

Zircaloy 2-No UV

Zircaloy 2-UV

Preoxidized Surface in 300oC H2O0.01M Na2SO4, 25oC

Increase in electric conductivity of ZrO2 by UV

Electrochemical Impedance of Zircaloy 2 Oxide


High Temperature UV Test System

X

Test Electrode

TemperatureCouple

ReferenceElectrode H2O Drain line

Sapphire Windowfor UV Guideline

Test Specimen

Pt Electrode

ChemicalInjectionH2O Out

H2O In


ECP of 304SS, X750, Zry2 & Pt in 300oC H2 O

PWR: in-core BWR: in-core

BWR: ECP difference between Zry2 and spacerPWR: Similar ECPs between Zry2 and spacer


ECP of Zircaloy 2 & X750 in 1.1ppm O2 at 300oC

UV ↓ Zry2 ECP & ↑ X750 ECPUV ↑ ECP difference between Zry2 & spacer

-0.25

-0.15

-0.05

0.05

0.15

0.25

0 5 10 15 20 25 30 35 40 45 50Immersion Time, minute

ECP,

V(S

HE)

High Purity Water, 1.1ppm O2, 300oC

Zircaloy 2

UV "On" UV "On" UV "Off"UV "Off"UV "Off"

X750



Galvanic current: Pt > X-750 > 304 SS



• No galvanic current on Zry2/Zry2 couple• Anodic dissolution of Zr on Zry2/Zr couple


Effect of Electrode Separation Distance

• Separation distance ↓, galvanic corrosion ↑• May be due to water conductivity (resistance)


Galvanic Current of Zr Alloys – X750 Couples

Less susceptibility of GNF alloys to galvanic corrosion

Zircaloy 2 GNF-Ziron


• A Higher Galvanic corrosion between Zry2 & spacer in an early oxidation• Occurrence of shadow corrosion early in life

• With hydrogen in water, ΔECP ↓

Effect of Water Chemistry on ECP


Effect of Water Chemistry on Galvanic Current

With hydrogen in water, galvanic current ↓


Increase in electric conductivity of ZrO2 outer oxide by UV

High Temperature Impedance on Zirclaoy 2 Oxide


Columnar grain

Equiaxed grains

Matrix

Pt from FIB

Zircaloy 2 Oxide formed for 3 month in NWC

Columnar grain

Equiaxed grains

Matrix

Pt from FIB


Photo-Excitation at Fuel Cladding/Spacer (Contact between two Alloys)

•

ZrO2 (n-type film)– The holes migrate to the

surface, reacting with an donor state while the electron moves to the backside contact.

– Anodic photocurrent•

NiO (p-type film)– The electron migrates to the

surface and reacts with oxidized chemical species in the electrolyte

– Cathodic photocurrent

Zircaloy

ZrO2

NiO X-750

In-core H2 O

Radiation

Photocurrent

e-

hv

• Zry2: Low corrosion potential, anodic Dissolution, high corrosion rate• X-750: High corrosion potential, cathodic reaction, low corrosion rate• Electron transfer from Zry2 to X-750

Mechanism: Galvanic CorrosionCorrosion potential difference between two alloys

e-


Photo-Excitation at Fuel Cladding/Spacer (No Contact between two Alloys)

Radiation Chemistry•

Water Radiolysis– eaq

-, OH, H, O, H2 , O2 , H2 O2 , OHaq-,

Haq+

•

Photoemission of Electron– Electron transfer to MO/H2 O

interface (e-) – Emitted electron (e*-) into water– Formation of hydrated electron (eaq

-)•

Electron Scavengers– eaq

- + N2 O → N2 O-

– eaq- + NO → NO-

– eaq- + H2 O → H + OH-

– 2eaq- + H2 → 2OH-

Hypothetic Mechanism: Radical Induced Corrosion Electron transfer by radical species; ionic current

ZircaloyZrO2

NiO X-750

In-core H2 O

Radiation

e*-

hv

• No physical contact of two alloys• Presence of radical species and impurities• Electron transfer through water by radicals

e- eaq-

OH → OH-


Summary•

Photoelectrochemical Effect– UV light is a useful tool to understand the fundamental mechanism of radiation enhanced

shadow corrosion in high temperature water – UV light Increases the corrosion potential difference between Zr alloys and other alloys

(e.g., Alloy X-750, 304 SS, and Pt)– UV light enhances the galvanic corrosion (shadow corrosion) between two different alloys– The electric conductivity of ZrO2 is affected by UV– Interface of SPPs and Zr matrix may be susceptible to galvanic corrosion– No (less) galvanic corrosion in PWR condition: No shadow corrosion

•

Shadow Corrosion Mechanisms– When two alloys are in contact

– Galvanic corrosion by potential difference– When two alloys are not in contact

– Radical induced corrosion (proposed)– Experimental data is needed

•

Fundamental Method to Mitigate Shadow Corrosion– Minimize the corrosion potential difference between two alloys


Acknowledgement

•

Financial Support– EPRI (EPRI Contract EP-P26485/C12727) – GNF-A

•

Technical Contribution– Corrosion & Electrochemistry Lab, GE-GRC– Surface Characterization Lab, GE-GRC

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