1/ Photoelectrochem-Shadow Corrosion 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 Photo-electrochemical Investigation of Radiation Enhanced Shadow Corrosion Phenomenon
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Photo-electrochemical Investigation of Radiation Fundamentals • Results ... understanding of radiation and electrochemical aspect of shadow corrosion. 9/ Photoelectrochem-Shadow
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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
2 /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
3 /Photoelectrochem-Shadow Corrosion
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
4 /Photoelectrochem-Shadow Corrosion
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
5 /Photoelectrochem-Shadow Corrosion
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
6 /Photoelectrochem-Shadow Corrosion
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
7 /Photoelectrochem-Shadow Corrosion
Theoretical Equilibrium Potentials for Water/Metal Oxides Relevant to BWR Chemistry at 288oC
• 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
9 /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• 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
10 /Photoelectrochem-Shadow Corrosion
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
11 /Photoelectrochem-Shadow Corrosion
Photoelectrochemical Measurement
•
Materials– Pure Zr, Zircaloy 2, 304 SS, X750– Preoxidized in 1.1ppm O2 , 300oC water,
• No galvanic current on Zry2/Zry2 couple• Anodic dissolution of Zr on Zry2/Zr couple
23 /Photoelectrochem-Shadow Corrosion
Effect of Electrode Separation Distance
• Separation distance ↓, galvanic corrosion ↑• May be due to water conductivity (resistance)
24 /Photoelectrochem-Shadow Corrosion
Galvanic Current of Zr Alloys – X750 Couples
Less susceptibility of GNF alloys to galvanic corrosion
Zircaloy 2 GNF-Ziron
25 /Photoelectrochem-Shadow Corrosion
• 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
26 /Photoelectrochem-Shadow Corrosion
Effect of Water Chemistry on Galvanic Current
With hydrogen in water, galvanic current ↓
27 /Photoelectrochem-Shadow Corrosion
Increase in electric conductivity of ZrO2 outer oxide by UV
High Temperature Impedance on Zirclaoy 2 Oxide
28 /Photoelectrochem-Shadow Corrosion
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
29 /Photoelectrochem-Shadow Corrosion
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-
30 /Photoelectrochem-Shadow Corrosion
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-
31 /Photoelectrochem-Shadow 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