22.39 Reactor Design, Operation, and Safety
Environmental Degradation
Fundamentals
R. G. Ballinger
Professor
Massachusetts Institute of Technology
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
1
22.39 Reactor Design, Operation, and Safety
Thermodynamics
2
22.39 Reactor Design, Operation, and Safety
Corrosion Cell Schematic
V
e-
i
i
-
+
Cathode
Anode
Oxidation
H Reduction
3
22.39 Reactor Design, Operation, and Safety
Standard Cell
V
e-
i
i
-
+
Cathode
Anode
Oxidation
H2
Standard Conditions
• 1 Atm
• pH = 0
• Unit Activity
• 25°C
Pt
H Reduction
Standard Potential (E0)
E0, H = 0
4
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.
5
22.39 Reactor Design, Operation, and Safety
Schematic of Corrosion Reactions
M + H 2O → M aqz + + ze −
Metal (M)
M + nH 2O → MOnn− + 2nH + + ne −
H+
M2+
ee-
H+
H+
H+
H+
H2
H+
H+
Figure by MIT OCW.
M + H 2O → MO + 2 H 2O + 2e −
M aqz + + H 2O → MO + H +
2 H + + 2e − → H 2
2 H 2O + 2e − → H 2 + 2OH −
O2 + 2 H 2O + 4e − → 4OH −
6
22.39 Reactor Design, Operation, and Safety
Basic Relationships
lL + mM + ... → qQ + rR + ...
ΔG = (qGQ + rGR + ..) − (lGL + mGm + ..)
Anode(Oxidation)
+ ne −
−
M →M
+ ne
Cathode(Reduction)
M
ne −
+
• G = Free Energy (J/mol)
• n = # Equivalents Involved
(# electrons)
−
+ ne → M
−
2 H + 2e → H 2 ( g )
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22.39 Reactor Design, Operation, and Safety
Key Relationships
ΔG = -nFE
RT ( ActivityPr oducts )
E = E0 −
ln
nF ( ActivityRe ac tan ts )
pH = -log [H+]
• 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
8
22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Fe-H2O @ 25°C
Figure removed due to copyright restrictions.
9
22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Cr-H2O @ 25°C
Figure removed due to copyright restrictions.
10
22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Ni-H2O @25°C
Figure removed due to copyright restrictions.
11
22.39 Reactor Design, Operation, and Safety
Pourbaix Diagram: Ti-H2O @ 25°C
Figure removed due to copyright restrictions.
12
22.39 Reactor Design, Operation, and Safety
Fe-Cr-Ni System @ 25°C
Corrosion
Immunity
Corrosion
Passivation
Immunity
Passivation
Corrosion
Passivation
Immunity
13
22.39 Reactor Design, Operation, and Safety
Combined Fe-Cr-Ni @ 25°C
Passivation?
14
22.39 Reactor Design, Operation, and Safety
Kinetics
15
22.39 Reactor Design, Operation, and Safety
Schematic “Evans” Diagram
+ +2e
(+)
Potential, E (v) vs. SHE
(-)
2H
→
H2
ER, Cath.
2H +
+
2e -→
H
ηCathodic
2
βCathode
ECorr
n+ +ne
M
M→
ER, Anode
M n+
+n
e -→
βAnode
ηAnodic
M
i0, Anode. i0, Cath.
Current Density (A/cm2)
i Corrosion
16
22.39 Reactor Design, Operation, and Safety
Schematic of Passive Behavior (Anode)
Transpassive
(+)
Potential, E(v) vs. SHE
(-)
iL
Passive
EPassive
n+ +
M→M
ne-
Active
iPassive
iCritical
Current Density (log), (A/cm2)
17
22.39 Reactor Design, Operation, and Safety
Schematic of Anodic & Cathodic InteractionsInterplay
io, Noble Cathode
(+)
Potential, E(v) vs. SHE
(-)
Noble Cathode
Redu
ction
ECorr, Passive
Active Cathode
Reac
Flow, T
tion
il, C1
il, C2
il, C3
il, C4
EPassive
ECorr, Active
n+ +
M→M
ne-
iPassive
iCritical iL, Anode
Current Density (log), (A/cm2)
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22.39 Reactor Design, Operation, and Safety
Key Kinetics Relationships
i
η = β log
i0
RT
iL
η =−
ln
nF iL − i
DnF
iL =
c
δt
D = D0e
−
Q
RT
• 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 O Concentration)
• Compositions (Microstructure)
• Stress
• Radiation Dose, Dose rate, Radiation Type
„
„
2
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22.39 Reactor Design, Operation, and Safety
The Role of Electrochemical
Processes 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.
22
22.39 Reactor Design, Operation, and Safety
PASSIVATING
CRACK
FLANKS
REGION I
CRACK TIP/METAL
INTERFACE
ANODIC OR CATHODIC PRECIPITATES
REGION II
METAL
MASS
TRANSFER
REGION III
CRACK ENCLAVE
LOCAL DEPLETIONIN INTERFACE
REGION
DIFFUSION OF
METAL OR IMPURITY
ATOMS TO GRAIN BOUNDARY
PRECIPITATE AT
TIP/METAL INTERFACE
MASS TRANSFER
GRAIN
BOUNDARY
MAJOR ELEMENT
DEPLETION
BARE
SURFACE
IMPURITY
SEGREGATION
METAL DISSOLUTION
HYDROGEN REDUCTION
PRECIPITATE/MATRIX
CELL
Model For EAC Process
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22.39 Reactor Design, Operation, and Safety
IMPORTANT PHENOMENA IN
REGION 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 IN
REGION 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 IN
REGION 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 ASSISTED
CRACKING 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 rate
Corrosion 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 PHASES
Gamma Prime (Ni (Al,Ti))
3
Gamma Double Prime (Ni Nb)
3
Eta (Ni Ti)
3
Laves (Fe Ti...)
2
MC Carbides
M C Carbides
7
3
M C Carbides
23
6
MnS Inclusions
Oxide Inclusions
Delta (Ni Nb)
3
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22.39 Reactor Design, Operation, and Safety
IMPORTANT PHASE
CHARACTERISTICS
• 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 Modes
1. 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 Modes
1.
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 Corrosion
‹ Predictable
‹ Slow (Normally)
Localized Corrosion
‹ “Unpredictable”
‹ Potentially Very Rapid
‹ Can 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)
K = f (a,σ , geometry )
Defects “Become”
da
n
=
Δ
C
K
( )
Cracks
dn
‹ Respond to Stress
⎡ Qg ⎛ 1
1 ⎞⎤
da
• Multiple Cracks Link
Up
„
Higher Driving Force
(K) for “Linked System
= exp ⎢ − ⎜ −
⎜
dt
⎢⎣ R ⎝ T Tref
K → K C → Unstable
⎟ ⎥ α ( K − K th )
β
⎟
⎠ ⎥⎦
• “Main” Crack
Propagates to Failure
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22.39 Reactor Design, Operation, and Safety
Crack “History”
Unstable
Crack Growth
(Exceed KCrit)
Crack Length (a)
Unacceptable NDE Sensitivity
“Adequate” NDE Sensitivity ??
a0
Ideal NDE Sensitivity
Time
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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 Concentrations
2. Avoid Galvanic Couples
3. Avoid Sharp Bends of Velocity Changes
in Piping Systems
4. Design Tanks for Complete Draining
5. To Weld or Not to Weld?
6. Design to Exclude Air
7. Avoid Heterogeneity
8. Design for Replacement
39