environmental degradation fundamentals · 2020. 7. 10. · environmental degradation fundamentals...
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22.39 Reactor Design, Operation, and Safety
Environmental Degradation Fundamentals
R. G. BallingerProfessor
Massachusetts Institute of Technology
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22.39 Reactor Design, Operation, and Safety
Outline• Thermodynamics of Corrosion: Pourbaix Diagrams
• Corrosion Kinetics
Polarization Diagrams
Corrosion Rate and Corrosion Potential
Passivation
• Design Implications
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22.39 Reactor Design, Operation, and Safety
Thermodynamics
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22.39 Reactor Design, Operation, and Safety
Corrosion Cell Schematic
V
i
Anode
Cathode
e- i
Oxidation H Reduction
+-
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22.39 Reactor Design, Operation, and Safety
Standard CellV
i
Anode
Cathode
e- i
Oxidation H Reduction
+-H2
Pt
Standard Conditions• 1 Atm• pH = 0• Unit Activity• 25°C
Standard Potential (E0)E0, H = 0
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22.39 Reactor Design, Operation, and Safety
Table of Standard Potentials for various Electrode Reactions removed due to copyright restrictions.
Data referenced to Latimer, W. Oxidation Potentials. Prentice-Hall, 1952.
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22.39 Reactor Design, Operation, and Safety
Schematic of Corrosion Reactions
2
2
2
2
2
2 2
2 2
2
2
2 2
2 2 2
2 4 4
zaq
nn
zaq
M H O M ze
M nH O MO nH ne
M H O MO H O e
M H O MO H
H e H
H O e H OH
O H O e OH
+ −
− +
−
+
+ −
−
−
+ → +
+ → +
+ → + +
+ → +
+ →
+ → +
+ + →
Metal (M)
H2
M2+
H+
e-
e-
H+H+
H+
H+
H+ H+
Figure by MIT OCW.
2 2
−
+
−
−
+
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22.39 Reactor Design, Operation, and Safety
Basic Relationships
2
... ...( ..) ( ..)
(Oxidation)
(Reduction)
2 2 ( )
Q R L m
ne
ne
lL mM qQ rRG qG rG lG mG
Anode
M M neCathode
M ne MH e H g
−
−
+ −
−
+ −
+ + → + +Δ = + + − + +
→ +
+ →
+ →
• G = Free Energy (J/mol)• n = # Equivalents Involved (# electrons)
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22.39 Reactor Design, Operation, and Safety
Key Relationships
Pr0
Re tan
( )ln( )
oducts
ac ts
ActivityRTE EnF Activity
= −
ΔG = -nFE
• G = Free Energy (J/mol)• n = # Equivalents Involved (# electrons)• F = Faraday’s Constant (96,500 J/eq)• E = Potential (V)• Eo = “Standard” Potential (V), Potential @ 25°C, Unit Activity, pH = 0• R = Gas Constant (Appropriate Units)• Activity = ~ Concentration (mol/l) for Dilute Solutions
(Activity Coefficient X Concentration for Concentrated Solutions
pH = -log [H+]
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22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Fe-H2O @ 25°C
Figure removed due to copyright restrictions.
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22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Cr-H2O @ 25°C
Figure removed due to copyright restrictions.
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22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Ni-H2O @25°C
Figure removed due to copyright restrictions.
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22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Ti-H2O @ 25°C
Figure removed due to copyright restrictions.
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22.39 Reactor Design, Operation, and Safety
Fe-Cr-Ni System @ 25°C
Corrosion
Passivation
ImmunityImmunity
Passivation
Corrosion
Corrosion
Immunity
Passivation
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22.39 Reactor Design, Operation, and Safety
Combined Fe-Cr-Ni @ 25°C
Passivation?
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22.39 Reactor Design, Operation, and Safety
Kinetics
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22.39 Reactor Design, Operation, and Safety
Schematic “Evans” Diagram
Current Density (A/cm2)
Potential, E (v) vs. SHE
ECorr
(+)
(-)
ER, Cath.
ER, Anode
i0, Cath.i0, Anode. i Corrosion
ηCathodic
ηAnodicM→Mn+ +ne-
M n++ne -→M
H2→2H+ +2e-
2H ++2e -→H2
βCathode
βAnode
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22.39 Reactor Design, Operation, and Safety
Schematic of Passive Behavior (Anode)
Current Density (log), (A/cm2)
Potential, E(v) vs. SHE
(+)
(-)
Transpassive
Passive
Active
EPassive
M→Mn++ ne-
iL
iPassive iCritical
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22.39 Reactor Design, Operation, and Safety
Schematic of Anodic & Cathodic Interactions-Interplay
Current Density (log), (A/cm2)
Potential, E(v) vs. SHE
(+)
(-) M→Mn++ ne-
iPassive iCritical
EPassive
iL, Anode
ECorr, Passive
Noble Cathode
Active Cathode
ECorr, Active
io, Noble Cathode
il, C1 il, C2 il, C3 il, C4
Flow, TReduction Reaction
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22.39 Reactor Design, Operation, and Safety
Key Kinetics Relationships
0
0
log
ln L
L
L
QRT
ii
RT inF i i
DnFi ct
D D e
η β
η
δ−
=
= −−
=
=
• β = “Tafel” Slope• i = Current density• io = Exchange Current Density (A/cm2)• R = Gas Constant (appropriate units)• n = # Equivalents (electrons) transferred• F = Faraday’s Constant (96,500 C/eq)• η = Overvoltage (V)• D = Diffusion Coefficient (cm2/sec)• Do = Constant• Q = Activation Energy (Units consistent with R)• T = Temperature (°K)• c = Concentration (M)• δ = transference #• t = Surface Layer (in solution) Thickness (cm)
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22.39 Reactor Design, Operation, and Safety
Key Variables• Temperature
15°C ~ 2X in Rates• Concentrations
M, Hydrogen, Oxygen, Contaminants• Flow Velocity• Potential (Dominated by O2 Concentration)• Compositions (Microstructure)• Stress• Radiation Dose, Dose rate, Radiation Type
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22.39 Reactor Design, Operation, and Safety
The Role of ElectrochemicalProcesses in Environmental
Degradation
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22.39 Reactor Design, Operation, and Safety
OBSERVATIONS• From an electrochemical point of view all
structural materials are composites.• Electrochemical differences can result in
accelerated electrochemical reactions.• If these reactions occur environmentally
assisted attack may be promoted.• In these situations both anodic and cathodic
processes must be considered.
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22.39 Reactor Design, Operation, and Safety
PRECIPITATE/MATRIX CELL
BARE SURFACE
SEGREGATIONIMPURITY
MAJOR ELEMENT DEPLETION
REGION III
REGION I
REGION II
CRACK TIP/METALINTERFACE
METAL
CRACK ENCLAVE
PRECIPITATE AT TIP/METAL INTERFACE
GRAIN BOUNDARY
ANODIC OR CATHODIC PRECIPITATES
PASSIVATING CRACK FLANKS
MASS TRANSFER
MASS TRANSFER
HYDROGEN REDUCTION
DIFFUSION OFMETAL OR IMPURITY
ATOMS TO GRAIN BOUNDARYLOCAL DEPLETIONIN INTERFACE
REGION
METAL DISSOLUTION
Model For EAC Process
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22.39 Reactor Design, Operation, and Safety
IMPORTANT PHENOMENA INREGION 1
• Creation of fresh metal by crack propagation.• Galvanic coupling between matrix and
precipitates.• Metal dissolution and other anodic reactions.• Hydrogen reduction or other cathodic
reactions.• Mass transfer to or from the crack enclave due
to diffusion, convection or ion migration.• Crack extension, by mechanical or chemical or
electrochemical means.• Hydrogen assisted crack growth.
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22.39 Reactor Design, Operation, and Safety
IMPORTANT PHENOMENA INREGION II
• Precipitation at grain boundaries.• Minor/major element segregation.• Near grain boundary element depletion
or accumulation.• Development of plastic zone due to
crack propagation.
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22.39 Reactor Design, Operation, and Safety
IMPORTANT PHENOMENA INREGION III
• Mass transfer into and out of the crack by diffusion, convection and ion migration.
• Oxygen reduction on passive or active crack walls.
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22.39 Reactor Design, Operation, and Safety
ENVIRONMENT ASSISTEDCRACKING MECHANISMS
• Stress Corrosion Cracking
• Hydrogen Embrittlement
• Intergranular Attack• Corrosion Fatigue
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22.39 Reactor Design, Operation, and Safety
KEY VARIABLES• Grain Boundary Morphology.• Electrochemical Activity of the Grain Boundary.• Fresh Metal Exposure Rate.• Reaction Kinetics.
Film formation rateCorrosion currents
• Galvanic Couples Between Grain Boundary Phases.
• Crack Tip pH.• Crack Tip Potential in Relation to Reversible
Hydrogen Potential
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22.39 Reactor Design, Operation, and Safety
TYPICAL PHASESGamma Prime (Ni3(Al,Ti))
Gamma Double Prime (Ni3Nb)Eta (Ni3Ti)
Laves (Fe2Ti...)MC CarbidesM7C3 CarbidesM23C6 CarbidesMnS Inclusions
Oxide InclusionsDelta (Ni3Nb)
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22.39 Reactor Design, Operation, and Safety
IMPORTANT PHASECHARACTERISTICS
• Is it anodic or cathodic with respect to other phases or matrix?
• Does it exhibit active or passive behavior?
• What are the kinetics of passivation?
• Corrosion current density?
• Exchange current density?
• Solubility of metal ions?
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22.39 Reactor Design, Operation, and Safety
Design Implications
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22.39 Reactor Design, Operation, and Safety
Materials Selection Considerations
• Applicability• Suitability• Fabricability• Availability• Economics• Compromise
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22.39 Reactor Design, Operation, and Safety
General Material Failure Modes1. Overload
2. Creep Rupture
3. Fatigue
4. Brittle Fracture
5. Wastage
6. Environmentally Enhanced
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22.39 Reactor Design, Operation, and Safety
Environmentally Enhanced Failure Modes1. General Corrosion
2. Localized Corrosion
Galvanic
Pitting
Crevice Corrosion
Stress Corrosion Cracking
Hydrogen Embrittlement
Corrosion Fatigue
Intergranular Attack
Erosion-Corrosion
Creep-Fatigue Interaction
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22.39 Reactor Design, Operation, and Safety
Key Point• Big Difference Between General & Localized Corrosion
General CorrosionPredictableSlow (Normally)
Localized Corrosion“Unpredictable”Potentially Very RapidCan be Multi-Phenomena (Pitting leading to Crack Initiation)
Significant Design Implications
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22.39 Reactor Design, Operation, and Safety
How Do Things “Fail” (Sometimes)• Crack Initiation
Often Multiple Sites (Pitting)Defects “Become”Cracks
Respond to Stress• Multiple Cracks Link
UpHigher Driving Force (K) for “Linked System
• “Main” Crack Propagates to Failure
( )
( )
( , , )
1 1exp
n
gth
ref
C
K f a geometryda C Kdn
Qda K Kdt R T T
K K Unstable
β
σ
α
=
= Δ
⎡ ⎤⎛ ⎞= − − −⎢ ⎥⎜ ⎟⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦→ →
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22.39 Reactor Design, Operation, and Safety
Crack “History”
Time
Crack Length (a)
a0
UnstableCrack Growth(Exceed KCrit)
Unacceptable NDE Sensitivity
Ideal NDE Sensitivity
“Adequate” NDE Sensitivity ??
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22.39 Reactor Design, Operation, and Safety
Indian Point R2C5 Crack
Photos removed due to copyright restrictions.
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22.39 Reactor Design, Operation, and Safety
General Design “Rules”
1. Avoid Stress/ Stress Concentrations2. Avoid Galvanic Couples3. Avoid Sharp Bends of Velocity Changes
in Piping Systems4. Design Tanks for Complete Draining5. To Weld or Not to Weld?6. Design to Exclude Air7. Avoid Heterogeneity8. Design for Replacement