Post on 17-Feb-2018
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
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
8 /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
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,
•
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
12 /Photoelectrochem-Shadow Corrosion
Oxide: NWC, 1month, 300oCX750Zry2
304SS
13 /Photoelectrochem-Shadow Corrosion
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)
14 /Photoelectrochem-Shadow Corrosion
Corrosion Potential of Zry2, 304SS & X750
UV “ON” : 304SS & X-750 OCP ↑, Zry2 OCP↓
15 /Photoelectrochem-Shadow Corrosion
Corrosion Potential of Intermetallic Alloys
UV “ON” : SPP CP ↑, behaving as cathode over anodic Zr
16 /Photoelectrochem-Shadow Corrosion
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
17 /Photoelectrochem-Shadow Corrosion
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
18 /Photoelectrochem-Shadow Corrosion
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
19 /Photoelectrochem-Shadow Corrosion
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
20 /Photoelectrochem-Shadow Corrosion
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
21 /Photoelectrochem-Shadow Corrosion
Galvanic Corrosion Test
Galvanic current: Pt > X-750 > 304 SS
22 /Photoelectrochem-Shadow Corrosion
Galvanic Corrosion Test
• 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
32 /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