Photo-electrochemical Investigation of Radiation Enhanced Shadow Corrosion Phenomenon

Young-Jin Kim & Raul Rebak: GE-Global Research Center Y-P Lin, Dan Lutz, & Doug Crawford: GNF-A Aylin Kucuk & Bo Cheng: EPRI

16th Int. Symposium on Zirconium in the Nuclear Industry Chengdu, China

May 9-13, 2010

1/ Photoelectrochem-Shadow Corrosion 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 2/ Photoelectrochem-Shadow Corrosion Shadow Corrosion

Away from (a)X-750 spacer ring 6“ Elevation 500X ~4 micron oxide (b)(a)

Beneath 500X ~27 micron patch oxide (b) X-750 spacer ring No Elevation

From Adamson R. B., Lutz D. R., Davies J. H. Control blade shadow on channel (Chen & Adamson, 1994)

3/ Photoelectrochem-Shadow Corrosion Proposed Mechanisms in Literatures

Distance between Alloys Crevice Corrosion • Chen & Adamson, 1994 • Garzarolli et al,. 2001

• Etoh et al., 1999 Galvanic Corrosion • Chatelain et al., 2000 • In contact

• Lysell et al., 2001 • Lysell et al,. 2001, 2005 Local Radiolysis Beta Emission • Non-contact • Chen & Adamson, 1994 • Etoh et al., 1997 • Nanika and Etoh, 1996 • Ramasubramanian, 2004

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

4/ Photoelectrochem-Shadow Corrosion Galvanic Corrosion

Classical Case Requirement Shadow Corrosion

Yes Two dissimilar metals Yes/No in contact Yes Surface area No

Acathode >Aanode (Spacer-cathode) Yes Media Yes (with radiation) Shadow corrosion is not a typical GC

5/ Photoelectrochem-Shadow Corrosion Model Calculation of Water Radiolysis (GE/Harwell)

NWC HWC

H2 O2

H2 O2 O2

O2

C. Ruiz et al., BNES 6, Water chemistry in-core is always oxidizing V-2, p. 141 (1992) 6/ Photoelectrochem-Shadow Corrosion 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 Co3O4 CoO

-0.025- CuO/Cu2OCuO Cu2O -0.150- CuO/Cu

-0.280- Cu2O/Cu Cu2O BWR ECP Cu

-0.428- Fe2O3/Fe3O4 Fe2O3 Fe3O4 -0.474- Co3O4/Co

-0.530- H2O/H2 HWC

-0.580- NiO/Ni NiO Ni -0.625- CoO/Co CoO Co -0.765- Fe2O3/Fe

-0.812- Fe3O4/Fe Fe3O4 Fe -1.121- ZnO/Zn ZnO Zn

-1.322- Cr2O3/Cr Cr2O3 Cr 7/ Photoelectrochem-Shadow Corrosion 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

8/ Photoelectrochem-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 n-Type Semiconducting Film • The electrode potential of p-type oxides • The electrode potential of n-type oxides shifts in the anodic direction by the shifts in the cathodic direction by the photoexcitaton of the oxides photoexcitaton of the oxides – Hydrogen evolution by radiation – Oxygon evolution by radiation excited electrons excited holes

- - ++4e + e 4H 4H +4 O2+ O + O → H O → 2 2H 2 2 2

E corr2 2H+ Photoexcitation + 2e- → H 2 Anodic Current 2H+ + 2e- → Photoexcitation Ecorr1 H Ecorr1Cathodic Current 2 2H+ + 2e- → + Electrode Potential Electrode Potential H 4H 2 E + O2+ corr2 +4h → 2H2O

Log i Log i 9/ Photoelectrochem-Shadow Corrosion 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

Test Alloys Sn Fe Cr Ni Nb Si Zr Zircaloy 2 1.3 0.18 0.1 0.07 Balance Zircaloy 4 1.3 0.2 0.1 Balance GNF-Ziron 1.3 0.25 0.1 0.07 Balance GNF-NSF 1.0 0.35 1.0 Balance Zr+Fe+Ni 14.0 9.8 Balance Zr+Fe+Cr 28.1 26.1 Balance Zr+Fe+Ni+Si 13.0 8.7 1.2 Balance

10 / Photoelectrochem-Shadow Corrosion Photoelectrochemical Measurement

• Materials – Pure Zr, Zircaloy 2, 304 SS, X750 o – Preoxidized in 1.1ppm O2, 300 C water, • Electrolyte o – 0.01M Na2 SO4, 25 C TM o – 1.1ppm O2, HWC, NobleChem H2 O, 300 C • UV Source – EXFO-UV Omnicure Model S2000, 250-400nm • Measurements – Gamry Reference 600 – Corrosion potential – Electrochemical impedance – Galvanic current

11 / Photoelectrochem-Shadow Corrosion Oxide: NWC, 1month, 300oC

Zry2 X750

304SS

12 / Photoelectrochem-Shadow Corrosion Pt Pt Overlay

Ni Oxide Particles

Oxide Zry2

Epoxy Fine Grain/ Thin Oxide Layer

Ni(Fe,Cr)O4

Fe2O3 X750 Matrix

• Zry2: ZrO2 (n-type) • 304SS: Fe2 O3 (n-type), spinel (p-type) • X750: NiO+ spinel (p-type) Fe3O4, FeCr2O4 304SS 13 / Photoelectrochem-Shadow Corrosion Corrosion Potential of Zry2, 304SS & X750

UV “ON” : 304SS & X-750 OCP ↑, Zry2 OCP↓ 14 / Photoelectrochem-Shadow Corrosion Corrosion Potential of Intermetallic Alloys

UV “ON” : SPP CP ↑, behaving as cathode over anodic Zr 15 / Photoelectrochem-Shadow Corrosion Galvanic Corrosion Test

Zry2-Pt

Zry2-X750

Zry2-304SS

Zry2-Zry2

Zry2-Zr

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

Negative current flow indicates the anodic dissolution of Zr 16 / Photoelectrochem-Shadow Corrosion Electrochemical Impedance of Zircaloy 2 Oxide

Preoxidized Surface in 300oC H2O 0.01M Na2SO4, 25oC 1.3E+07

Zircaloy 2-No UV 1.0E+07

7.5E+06

5.0E+06 Imaginary Part, ohm Part, Imaginary

2.5E+06 Zircaloy 2-UV

0.0E+00

0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07 2.5E+07 Increase in electric conductivityReal Part, ohm of ZrO2 by UV 17 / Photoelectrochem-Shadow Corrosion High Temperature UV Test System

Test Electrode Pt Electrode H2O In Temperature Couple

Chemical Injection H2O Out

Test Specimen

Reference X Electrode H2O Drain line

Sapphire Window for UV Guideline

18 / Photoelectrochem-Shadow Corrosion o ECP of 304SS, X750, Zry2 & Pt in 300 C H2O

PWR: in-core BWR: in-core

BWR: ECP difference between Zry2 and spacer 19 / PWR: Similar ECPs between Zry2 and spacerPhotoelectrochem-Shadow Corrosion o ECP of Zircaloy 2 & X750 in 1.1ppm O2 at 300 C 0.25 o High Purity Water, 1.1ppm O2, 300 C

0.15 X750

0.05

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

ECP, V(SHE) -0.05 Zircaloy 2

-0.15

-0.25 0 5 10 15 20 25 30 35 40 45 50 Immersion Time, minute UV ↓ Zry2 ECP & ↑ X750 ECP 20 / UV ↑ ECP difference between Zry2 & spacerPhotoelectrochem-Shadow Corrosion Galvanic Corrosion Test

21 / Galvanic current: Pt > X-750 > 304 SSPhotoelectrochem-Shadow Corrosion Galvanic Corrosion Test

• No galvanic current on Zry2/Zry2 couple 22 / • Anodic dissolution of Zr on Zry2/Zr couplePhotoelectrochem-Shadow Corrosion Effect of Electrode Separation Distance

• Separation distance ↓, galvanic corrosion ↑ • May be due to water conductivity (resistance) 23 / Photoelectrochem-Shadow Corrosion Galvanic Current of Zr Alloys – X750 Couples Zircaloy 2 GNF-Ziron

Less susceptibility of GNF alloys to galvanic corrosion 24 / Photoelectrochem-Shadow Corrosion Effect of Water Chemistry on ECP

• A Higher Galvanic corrosion between Zry2 & spacer in an early oxidation

• Occurrence of shadow corrosion early in life 25 / • With hydrogen in water, ΔECP ↓ Photoelectrochem-Shadow Corrosion Effect of Water Chemistry on Galvanic Current

With hydrogen in water, galvanic current ↓ 26 / Photoelectrochem-Shadow Corrosion High Temperature Impedance on Zirclaoy 2 Oxide

Increase in electric conductivity of ZrO2 outer oxide by UV 27 / Photoelectrochem-Shadow Corrosion Zircaloy 2 Oxide formed for 3 month in NWC

Matrix

Columnar grain

Equiaxed grains

Pt from FIB

28 / Photoelectrochem-Shadow Corrosion Photo-Excitation at Fuel Cladding/Spacer (Contact between two Alloys)

• ZrO2 (n-type film) – The holes migrate to the Radiation surface, reacting with an hv donor state while the ZrO electron moves to the 2 e- Zircaloy e- NiO X-750 backside contact. Photocurrent – Anodic photocurrent

• NiO (p-type film) In-core H2O – The electron migrates to the surface and reacts with • Zry2: Low corrosion potential, anodic oxidized chemical species in Dissolution, high corrosion rate the electrolyte • X-750: High corrosion potential, cathodic reaction, low corrosion rate – Cathodic photocurrent • Electron transfer from Zry2 to X-750 Mechanism: Galvanic Corrosion Corrosion potential difference between two alloys 29 / Photoelectrochem-Shadow Corrosion Photo-Excitation at Fuel Cladding/Spacer (No Contact between two Alloys) Radiation Chemistry • Water Radiolysis - - Radiation – eaq , OH, H, O, H2, O2, H2O 2, OHaq , + hv Haq • Photoemission of Electron ZrO 2 - e - – Electron transfer to MO/H2O - e* aq - Zircaloy e - NiO X-750 interface (e ) OH → OH – Emitted electron (e*-) into water – Formation of hydrated electron (e -) aq In-core H2O • Electron Scavengers - - – eaq + N2O → N2O - - • No physical contact of two alloys – eaq + NO → NO - - • Presence of radical species and impurities – eaq + H2O → H + OH - - • Electron transfer through water by radicals – 2eaq + H2 → 2OH

Hypothetic Mechanism: Radical Induced Corrosion 30 / Electron transfer by radical species;Photoelectrochem-Shadow ionic current Corrosion 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

31 / Photoelectrochem-Shadow Corrosion Acknowledgement

• Financial Support – EPRI (EPRI Contract EP-P26485/C12727) – GNF-A • Technical Contribution – Corrosion & Electrochemistry Lab, GE-GRC – Surface Characterization Lab, GE-GRC

32 / Photoelectrochem-Shadow Corrosion