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The Electrochemistry of Stress Corrosion Cracking in
Water-Cooled Reactor Coolant Circuits
Digby D. Macdonald
Center for Electrochemical Science and Technology
Pennsylvania State University
University Park, PA 16802
Presented at
OLI Systems Inc
Morris Plains, NJ
October 23, 2007
Background• Localized corrosion is characterized by non-homogeneous, quasi-stationary anodic
and cathodic partial current densities on a surface.• The anodic and cathodic regions are spatially separated and are coupled according to
the Differential Aeration Hypothesis (U. R. Evans, circa 1927).• The anode occurs in that region of the system that has minimal access to the
cathodic depolarizer (e.g., O2), whereas the cathode occurs in that region that has the greatest access to the depolarizer.
• In order to maintain charge balance a positive current flows through the solution from the local anode to the local cathode, where it is consumed by the reduction of the depolarizer.
• In the author’s opinion, the key to understanding and predicting stress corrosion cracking lies in developing an understanding of the origin and properties of the coupling current.
• These principles have been applied to predicting the accumulation of damage due to Intergranular Stress Corrosion Cracking (IGSCC) in sensitized Type 304 SS in BWR primary coolant circuits.
Ls
s
Figure 1. Coupling of crack internal and external environments.
The Coupling Current• Recognizes the important (and perhaps) dominant
role played by the external surfaces in SCC.• Can be readily measured with great accuracy.• The noise contained in the coupling current
enables direct interrogation of the processes that occur at the crack tip. To the authors knowledge, this is the only method that has been devised for direct interrogation.
• Has provided unprecedented information of crack tip dynamics in a variety of systems.
• Difficult to understand why the technique is not more widely used.
CreviceSolnCinTipCTIP RRZRZIEE
CreviceSolnCinTip
CTIP
RRZRZ
EE I
Etip
Ztip
Rin
ZC
Rcrevice
EC
Rsoln
CRACK TIP
CRACK FLANK
CRACK MOUTH
Crack Equivalent Electrical Circuit
Figure 2. Schematic of specimen used for measuring the coupling current. Note that the specimen, except for the crack, is electrically isolated from the environment by a PTFE coating so as to inhibit oxygen reduction from occurring on the surface. The electron current generated at the crack tip is then forced to flow through the ZRA to the external cathodes where oxygen reduction does occur.
Figure 3. Coupling current and stress intensity versus time for Type 304 SS in simulated BWR coolant at 250 oC. The specimen was equipped with one platinized nickel side cathode (see Figure 2).
Figure 4. Typical form of the noise in the coupling current for Type 304 SS with two Type 304 SS side cathodes in simulated BWR coolant at 250 oC and at a stress intensity of 27.5 MPam. Stainless steel cathodes.
Frequency of micro-fracture events versus stress intensity for IGSCC in sensitized Type 304 SS in water at 250 oC.
KI /MPa.m1/2
0 10 20 30 40 50 60
Fra
ctu
re F
req
uen
cy /
s- 1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Type 304 SS cathodesTitanium cathodes
KISCC
Proposed fracture mechanism, in which the crack advances in a series of brittle, micro fracture events at the crack front.
Brittle Micro Fracture Event Size
f = Frequency of brittle micro fracture event (~ 2 s-1)B = Width of specimen (1.27 cm)r = Radius of brittle micro fracture event.
~ 2-3μm
Given a grain size of 20 – 50 μm, there should be roughly 4 to 10 events in each package, which is in good agreement with experiment.The remarkable conclusion is that, in this case, the crack advances fracture event-by-fracture event with minimal overlap between events.
B
fr
dt
da 22
2/1
2
)/(
f
dtdaBr
Coupling Current
Assume that the current from any given micro fracture event decays as
The total current due to all of the micro fracture events that have occurred from zero time until the time at which the crack length is L is given by
To a good approximation, the coupling current can be simplified to
Setting L = 0.082 cm, B = 1.27 cm, r = 3 x 10-4 cm, and i0 and tf equal to 1-10 A/cm2 and 0.01 s (Ford, et.al.), respectively, the coupling current is calculated to be 68 – 683 μA, which is also in good agreement with experiment.
)()(
it
titi
n
f
o
2/2
1
2 )()(rBL
k
n
o
n
ft BLikriftII
)(1
)()1(2
BLikri
n
ftII n
o
n
f
t
The Critical Potential• A critical potential exists below which IGSCC does not
occur to any significant extent.• The critical potential is a function of a number of variables:
- Degree of sensitization
- pH- Temperature- Solution composition- Strain rate- Crack length
• For ECP < EIGSCC, failure occurs by mechanical overload, except in acidic solutions where cracking occurs by a hydrogen mechanism.
Figure 1: Measured and calculated (via the CEFM) crack growth rates for sensitized Type 304 SS in high temperature aqueous solutions as a function of ECP and conductivity. The citations refer to references in the original source [5].
1.5 mm/yr
0.06 mm/yr
Corrosion Potential (Vshe)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6Cra
ck
Gro
wth
Ra
te (
cm/s
) o
r C
ou
plin
g C
urr
ent
(A)
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
Crack growth rate
Coupling current
Figure 3: Calculated crack growth rate and coupling current for IGSCC in sensitized (DOS = 15 C/cm2) Type 304 SS in dilute NaCl solution (0.135 ppm Na) at 288 oC as a function of potential of the steel at the external surfaces remote from the crack mouth and as modeled using the Coupled Environment Fracture Model [3-5]. KI = 27 MPam, crack length = 0.5 cm,
crack width = 1.0 cm, crack mouth opening displacement = 5x10-4 cm, solution flow velocity = 100 cm/s, hydrodynamic diameter = 50 cm, 25 =
0.807 S/cm, 288 = 6.89 S/cm, and pH288 = 5.89.
Coupling Current (microamp)
2.0e-7 4.0e-7 6.0e-7 8.0e-7 1.0e-6 1.2e-6 1.4e-6 1.6e-6 1.8e-6
Cra
ck
Gro
wth
Ra
te (
cm
/s)
8.5e-7
9.0e-7
9.5e-7
1.0e-6
1.0e-6
1.1e-6
1.1e-6
1.2e-6
Figure 5: Relationship between the crack growth rate and coupling current for IGSCC in sensitized Type 304 SS in oxygenated (7.6 ppm, 2.38 x 10-4 m) sodium chloride (50 ppm, 8.62 x 10-4 m) solution at 250 oC.
Impact of Crack Length• Necessary to differentiate between mechanical crack length (MCL)
and electrochemical crack length (ECL). The former is the distance between the crack tip and the loading line, while the latter is the shortest distance between the crack tip and the external surface.
• CEFM predicts that the CGR decreases as the crack length increases because of the increase in the IR potential drop down the crack.
• CEFM also predicts that the critical potential shifts in the positive direction with increasing crack length.
• Predictions difficult to test using fracture mechanics specimen, because the ECL is constant.
• Predictions are confirmed by indirect evidence.
Corrosion Potential /Vshe
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Cra
ck G
row
th R
ate
/cm
.s-1
1e-10
1e-9
1e-8
1e-7
L=0.1cm L=0.5cm L=1.0cm L=2.0cm L=5.0cm L=10cm L=20cm L=50cm
COD=5x10-4 cm, Crack Width=1.0 cm, KI=27 MPa.m1/2,
V=100 cm/s, d=50 cm, T=288 oC, [H2]=10-4 ppb,
[H2O2]=10-4 ppb, [Na+]=1.35 ppbNa,
[H2SO4]=10-6 ppbS, [O2]=1 - 2.33x106 ppb,
288=2.69S/cm, 25=0.0618 S/cm,
Creep Rate =1.61x10-10 cm/s.
Corrosion Potential /Vshe
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Co
up
ling
Cu
rren
t /A
1e-16
1e-15
1e-14
1e-13
1e-12
1e-11
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
L=0.1cm L=0.5cm L=1.0cm L=2.0cm L=5.0cmL=10cm L=20cm L=50cm
COD=5x10-4 cm, Crack Width=1.0 cm, KI=27 MPa.m1/2,
V=100 cm/s, d=50 cm, T=288 oC, [H2]=10-4 ppb,
[H2O2]=10-4 ppb, [Na+]=1.35 ppbNa,
[H2SO4]=10-6 ppbS, [O2]=1 - 2.33x106 ppb,
288=2.69S/cm, 25=0.0618 S/cm,
Creep Rate =1.61x10-10 cm/s.
Crack Growth Rate /cm.s-1
1e-10 1e-9 1e-8 1e-7 1e-6
Co
up
ling
Cu
rren
t /A
1e-16
1e-15
1e-14
1e-13
1e-12
1e-11
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
L=0.1cm L=0.5cm L=1.0cm L=2.0cm L=5.0cm L=10cm L=20cm L=50cm
COD=5x10-4 cm, Crack Width=1.0 cm, KI=27 MPa.m1/2,
V=100 cm/s, d=50 cm, T=288 oC, [H2]=10-4 ppb,
[H2O2]=10-4 ppb, [Na+]=1.35 ppbNa,
[H2SO4]=10-6 ppbS, [O2]=1 - 2.33x106 ppb,
288=2.69S/cm, 25=0.0618 S/cm,
Creep Rate =1.61x10-10 cm/s.
Application to Practical Systems
• Boiling Water Nuclear Reactors.• IGSCC in weld sensitized Type 304 SS in
the primary coolant circuit.• Problem has cost the utilities hundreds of
millions to billions of dollars through lost production.
• Damage can now be predicted using the CEFM.
• Fourteen reactors modeled to date.
Where and When Does IGSCC Occur
SS Heat Transport Components
–Ex-Vessel
–In-Vessel
Three Factors Together
–High Stress
–An Aggressive Environment
–A Susceptible Material
Flow path of the primary coolant circuit of a BWR having external coolant pumps (GE Model VI).
Connections How all the topics fit together
Thermal-Hydraulic Data
Dose Rate Profiles
Initial Conditions
Velocity, Temperature,
& Plant Data
& Steam Quality
Neutron & Gamma
Water Radiolysis
Radiolytic Effects
Chemical Reactions
Fluid Convection
Corrosion Potential
Crack Growth Rate
SpeciesConcentrations
Methodology
• Radiolysis of Water in BWR Heat Transport Circuit– Chemical/ionic composition of bulk water
• Electrochemical Corrosion Potential– Charge/mass transfer at metal/water interface
• Crack Growth Rates in Components– Electrochemical Dissolution Of Metal
Radiolysis of Water in the Heat Transport
Circuit• Balance of Species: LOSS + GAIN = 0
– Radiolytic Yield– Chemical Kinetics (Reactants & Products)– Boiling (Liquid and Gas Exchange)– Convection
(G G
100NG G
100N)FrdV [ k C C C k C ]dV
[d(uC )
dxdV] (mC mC )dV 0.
ig g
V
in n
Vsm s m
m 1
N
i si ss 1
N
s 1
N
ii i
gi i
fg
~
Electrochemical Corrosion Potential
(ECP)• R/O Reactions at Metal/Solution Interface
• • Charge and Mass
Transfer
• Conservation of Charge: Net Current is Zero
• Mixed Potential, ECP
H 2H 2e
O 4H 4e 2H O
H O 2H 2e 2H O
2+ -
2+ -
2
2 2+ -
2
ie e
1i
1i
e1i
eR/ O
(E E )/ b (E E )/ b
0,R/ O i,f
(E E )/ b
i,r
(E E )/ b
R/ Oe
a R/ Oe
c
R/ Oe
a R/ Oe
c
i (E) i (E) 0R/ O,jj 1
n
corr
ALERT Code
• Developed from an earlier code DAMAGE-PREDICTOR that predicted only state points properties.
• ALERT predicts both state point properties and damage integrated over a specified Corrosion Evolutionary Path (CEP).
• ALERT and its companion code for BWRs with internal pumps, REMAIN, are the only codes in existence, to the author’s knowledge, that predict integrated damage.
• Codes currently lack an adequate model for crack nucleation.
Two Types of Calculations
• State points defined for a given set of conditions - [O2], [H2O2], [H2], T, pH, flow velocity, stress intensity factor, crack length, etc. Provides a “snap-shot” in time.
• IGSCC damage (crack length) for a specific point in the coolant circuit integrated along the corrosion evolutionary path.
“Double blind” test of ECP prediction at Leibstadt in Switzerland
Corrosion Potential – Leibstadt, Switzerland
Crack Growth Rate – Leibstadt, Switzerland
Case Study• GE Model 6 BWR.• Path to future state specified.• Normal Water Chemistry / Hydrogen Water Chemistry.• Catalytic Coatings or Inhibiting Dielectric Films (e.g.,
ZrO2).• Calculates Radiolysis Product Concentrations, ECP,
Crack Growth Rate, and Accumulated Damage over the Plant Evolutionary Path.
• Modeled a crack on the inside of the core shroud adjacent to the H-3 weld near the top.
• ALERT takes into account the dependence of crack growth rate on crack length.
• First and only instance where a prediction of this type has been made by anyone.
Reactor Design Information
Reactor Regions# Name Output Mesh (cm)1 Core_Channel 102 Core_Bypass 103 Upper_Plenum 104 Mixing_Plenum 105 Upper_Downcomer 106 Lower_Downcomer 107 Recirculation 1008 Jet_Pump 109 Bottom_of_the_Lower_Plenum 1010 Top_of_the_Lower_Plenum 10
Coolant Flow Path
Flow Mixing Table
From To
Core
_Cha
nnel
Core
_Byp
ass
Uppe
r_Pl
enum
Mixi
ng_P
lenu
m
Uppe
r_Do
wnco
mer
Lowe
r_Do
wnco
mer
Recir
cula
tion
Jet_
Pum
p
Botto
m_o
f_th
e_Lo
wer_
Plen
um
Top_
of_t
he_L
ower
_Ple
num
Core_Channel 0.901Core_Bypass 0.099
Upper_Plenum 0.85Mixing_Plenum 1
Upper_Downcomer 1 0.695Lower_Downcomer 1
Recirculation 0.305Jet_Pump 1
Bottom_of_the_Lower_Plenum 1Top_of_the_Lower_Plenum 1 1
Feedwater 0.15
Control Sum 1 1 1 1 1 1 1 1 1 1
Geometry, Thermo-Hydraulics, Radiation Levels, Stresses, Cracks,
Coatings
State Point Data
Path T Flow Steam Void H.D. Dose Rate Surface Coating Initial CriticalVelocity Velocity Fraction Diameter Neutron Gamma Catal (Exchange Current Density Modifier) K Crack K Crack
(cm) (C) (cm/s) (cm/s) -- (cm) (R/s) (R/s) H2O2 H2 O2 H2O2 MPa*m^1/2 cm MPa*m^1/2 cm
0.0 276.7 217.6 257.8 0.000 1.595 2.43E+04 5.02E+03 1 1 1 1 15 0.315.2 276.9 217.6 257.8 0.000 1.595 2.43E+04 5.02E+03 1 1 1 1 15 0.330.5 279.6 217.9 258.2 0.000 1.595 9.15E+04 1.89E+04 1 1 1 1 15 0.345.7 280.6 218.5 258.9 0.000 1.595 1.33E+05 2.74E+04 1 1 1 1 15 0.361.0 281.7 220.2 260.9 0.005 1.595 1.54E+05 3.17E+04 1 1 1 1 15 0.376.2 282.9 225.1 266.9 0.023 1.595 1.97E+05 4.06E+04 1 1 1 1 15 0.391.4 284.3 238.5 283.9 0.081 1.595 3.26E+05 6.74E+04 1 1 1 1 15 0.3
106.7 285.6 257.3 308.3 0.150 1.595 3.79E+05 7.82E+04 1 1 1 1 15 0.3121.9 286.8 280.5 339.1 0.223 1.595 4.04E+05 8.34E+04 1 1 1 1 15 0.3137.2 287.7 307.2 375.6 0.295 1.595 4.17E+05 8.60E+04 1 1 1 1 15 0.3152.4 288.4 336.1 416.8 0.360 1.595 4.23E+05 8.73E+04 1 1 1 1 15 0.3167.6 288.9 366.1 461.4 0.418 1.595 4.25E+05 8.76E+04 1 1 1 1 15 0.3182.9 289.2 396.3 508.4 0.468 1.595 4.25E+05 8.77E+04 1 1 1 1 15 0.3199.1 289.5 426.1 557.2 0.511 1.595 4.25E+05 8.76E+04 1 1 1 1 15 0.3213.4 289.7 455.0 607.1 0.547 1.595 4.23E+05 8.73E+04 1 1 1 1 15 0.3228.6 289.8 482.7 657.6 0.578 1.595 4.20E+05 8.67E+04 1 1 1 1 15 0.3243.3 289.9 509.0 708.2 0.605 1.595 4.17E+05 8.60E+04 1 1 1 1 15 0.3259.1 289.9 533.8 758.8 0.628 1.595 4.12E+05 8.50E+04 1 1 1 1 15 0.3274.3 290.0 557.2 809.2 0.648 1.595 4.08E+05 8.42E+04 1 1 1 1 15 0.3289.6 290.0 580.0 859.3 0.666 1.595 4.06E+05 8.39E+04 1 1 1 1 15 0.3304.8 290.0 602.4 908.3 0.683 1.595 4.06E+05 8.37E+04 1 1 1 1 15 0.3320.0 290.1 624.0 956.3 0.698 1.595 3.98E+05 8.20E+04 1 1 1 1 15 0.3335.3 290.1 644.8 1001.4 0.711 1.595 3.76E+05 7.77E+04 1 1 1 1 15 0.3350.5 290.0 662.6 1041.5 0.722 1.595 3.33E+05 6.87E+04 1 1 1 1 15 0.3365.8 289.9 677.6 1073.8 0.730 1.595 2.60E+05 5.36E+04 1 1 1 1 15 0.3381.0 289.7 683.1 1087.8 0.734 1.595 9.24E+04 1.91E+04 1 1 1 1 15 0.3
0.0 279.0 28.2 0.0 0.000 3.020 4.25E+04 4.38E+04 1 1 1 1 27.5 0.5381.0 286.0 28.2 0.0 0.000 3.020 4.25E+04 4.38E+04 1 1 1 1 27.5 0.5
Plant OperationPower and Water Chemistry History
Operation Periods Feedwater % Rated Impurities (ppb)Months Days Hours Power H2 O2 Flow Temp Cl- SO4-- CO3--
8 20% 0 0.053 20% 52% 6 6 1516 50% 0 0.053 50% 69% 4 4 1024 80% 0 0.053 70% 87% 3 3 832 100% 0 0.053 100% 100% 2 2 5
1 100% 1 0.053 100% 100% 2 2 52 100% 1 0.053 100% 100% 2 2 53 100% 1 0.053 100% 100% 2 2 56 100% 1 0.053 100% 100% 2 2 59 100% 1 0.053 100% 100% 2 2 512 100% 1 0.053 100% 100% 2 2 515 100% 1 0.053 100% 100% 2 2 518 100% 1 0.053 100% 100% 2 2 520 100% 1 0.053 100% 100% 2 2 520 8 20% 0 0.053 20% 52% 6 6 1520 16 50% 0 0.053 50% 69% 4 4 1020 24 80% 0 0.053 70% 87% 3 3 820 32 100% 0 0.053 100% 100% 2 2 521 30% 1 0.053 100% 100% 2 2 521 8 20% 0 0.053 20% 52% 6 6 1521 16 50% 0 0.053 50% 69% 4 4 1021 24 80% 0 0.053 70% 87% 3 3 821 32 100% 0 0.053 100% 100% 2 2 522 90% 1 0.053 100% 100% 2 2 5
Input File: State Point Data10 44Core_ChannelCore_BypassUpper_PlenumMixing_PlenumUpper_DowncomerLower_DowncomerRecirculationJet_PumpBottom_of_the_Lower_PlenumTop_of_the_Lower_Plenum
0 0 0.901 0 0 0 0 0 0 0 0 0 0.099 0 0 0 0 0 0 0 0 0 0 0.85 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0.695 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0.305 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0
0 0 0 0.15 0 0 0 0 0 0
0.00 276.67 217.58 257.81 0 1.595 2.43E+04 5.02E+03 1 1 1 1 15 0.3 15.24 276.85 217.58 257.81 0 1.595 2.43E+04 5.02E+03 1 1 1 1 15 0.3 30.48 279.63 217.90 258.19 0 1.595 9.15E+04 1.89E+04 1 1 1 1 15 0.3 45.72 280.63 218.52 258.92 0 1.595 1.33E+05 2.74E+04 1 1 1 1 15 0.3 60.96 281.71 220.15 260.86 0.005 1.595 1.54E+05 3.17E+04 1 1 1 1 15 0.3 76.20 282.85 225.05 266.91 0.023 1.595 1.97E+05 4.06E+04 1 1 1 1 15 0.3 91.44 284.27 238.48 283.93 0.081 1.595 3.26E+05 6.74E+04 1 1 1 1 15 0.3 106.68 285.64 257.32 308.27 0.15 1.595 3.79E+05 7.82E+04 1 1 1 1 15 0.3 121.92 286.80 280.54 339.09 0.223 1.595 4.04E+05 8.34E+04 1 1 1 1 15 0.3 137.16 287.71 307.16 375.60 0.295 1.595 4.17E+05 8.60E+04 1 1 1 1 15 0.3 152.40 288.39 336.05 416.77 0.36 1.595 4.23E+05 8.73E+04 1 1 1 1 15 0.3
Output: Chemistry, ECP, CGR
1.Core_Channel
X T pH OH- H2O2 HO2 HO2- O2 O2- H2 O2g H2g ECP REDOX CGR Cond25 COND(cm) (C) ppb ppb ppb ppb ppb ppb ppb ppm ppm mV SHE mV pm/s uS/cm uS/cm
0 276.7 5.6 50 18 0 2 184 1 1 --- --- 205 169 368 0.11 3.9110 276.8 5.6 48 61 1 7 174 4 3 --- --- 239 186 487 0.11 3.920 277.7 5.6 46 104 1 12 168 5 5 --- --- 245 189 506 0.11 3.930 279.6 5.6 44 185 1 20 160 7 10 --- --- 249 189 515 0.11 3.8940 280.3 5.6 42 276 1 28 156 7 15 --- --- 252 190 527 0.11 3.8850 280.9 5.6 41 356 1 35 155 8 20 2.9 0.4 252 189 528 0.11 3.8860 281.6 5.5 40 421 2 41 155 9 24 13.4 2.4 254 190 534 0.1 3.8870 282.4 5.5 39 474 2 45 153 9 26 14.1 2.9 255 192 538 0.1 3.8780 283.2 5.5 38 518 2 48 150 9 27 14.4 3.2 258 195 550 0.1 3.8790 284.2 5.5 38 558 2 51 147 9 24 16.2 3.6 263 201 564 0.1 3.86
100 285.1 5.5 37 584 2 53 151 9 22 18.1 3.8 267 206 581 0.1 3.85110 285.9 5.5 37 594 2 54 155 9 19 20.1 3.9 270 209 588 0.1 3.85120 286.7 5.5 38 594 2 54 155 9 17 21.5 3.8 272 212 595 0.1 3.84130 287.3 5.5 38 587 2 54 152 8 16 22.2 3.6 273 213 598 0.1 3.83140 287.8 5.5 38 575 2 53 146 8 15 22.4 3.5 275 216 601 0.1 3.83150 288.3 5.5 38 562 2 53 140 8 14 22.2 3.3 275 216 600 0.1 3.83160 288.6 5.5 39 548 1 52 134 8 13 21.9 3.2 276 217 606 0.1 3.82170 288.9 5.5 39 535 1 51 128 7 12 21.4 3.0 277 218 606 0.1 3.82180 289.2 5.5 40 522 1 50 123 7 11 20.8 2.9 277 219 604 0.1 3.82190 289.3 5.5 40 511 1 49 118 7 11 20.4 2.8 278 220 608 0.1 3.82200 289.5 5.5 40 501 1 49 114 7 10 19.9 2.7 278 220 609 0.1 3.82210 289.6 5.5 40 492 1 48 110 7 10 19.2 2.6 278 221 609 0.1 3.81220 289.7 5.5 41 483 1 48 105 6 9 18.7 2.5 279 221 615 0.1 3.81230 289.8 5.5 41 475 1 47 102 6 9 18.2 2.4 280 222 616 0.1 3.81240 289.9 5.5 41 468 1 47 99 6 8 17.7 2.3 280 223 618 0.1 3.81250 289.9 5.5 41 461 1 46 96 6 8 17.3 2.3 281 224 621 0.1 3.81260 289.9 5.5 42 455 1 46 93 6 8 16.9 2.2 281 224 624 0.1 3.81270 290.0 5.5 42 449 1 46 91 6 8 16.6 2.2 281 224 622 0.1 3.81280 290.0 5.5 42 444 1 45 88 6 7 16.2 2.1 281 224 625 0.1 3.81290 290.0 5.5 42 439 1 45 86 6 7 15.9 2.1 282 225 628 0.1 3.81300 290.0 5.5 42 435 1 45 84 5 7 15.6 2.0 282 226 628 0.1 3.81
Coolant Chemistry1.Core_Channel
X H2O2 O2 H2(cm)
0 18 184 110 61 174 320 104 168 530 185 160 1040 276 156 1550 356 155 2060 421 155 2470 474 153 2680 518 150 2790 558 147 24
100 584 151 22110 594 155 19120 594 155 17130 587 152 16140 575 146 15150 562 140 14160 548 134 13170 535 128 12180 522 123 11190 511 118 11200 501 114 10210 492 110 10220 483 105 9230 475 102 9240 468 99 8250 461 96 8260 455 93 8270 449 91 8280 444 88 7290 439 86 7300 435 84 7310 431 82 7320 427 81 7330 422 79 7340 416 77 6350 408 76 6360 397 74 6370 382 73 6380 358 71 6381 356 71 6
Core Channel
1
10
100
1000
0 50 100 150 200 250 300 350 400
Location (cm)
Co
nc
en
tra
tio
n (
pp
b)
H2O2
O2
H2
Corrosion Potential1.Core_Channel
X ECP(cm) SHE
0 20510 23920 24530 24940 25250 25260 25470 25580 25890 263
100 267110 270120 272130 273140 275150 275160 276170 277180 277190 278200 278210 278220 279230 280240 280250 281260 281270 281280 281290 282300 282310 283320 283330 283340 284350 284360 283370 282380 282381 281
0
50
100
150
200
250
300
0 50 100 150 200 250 300 350 400
Location (cm)
EC
P (
mV
SH
E)
Crack Growth Rate1.Core_Channel
X CGR(cm) pm/s
0 00 368
10 48720 50630 51540 52750 52860 53470 53880 55090 564
100 581110 588120 595130 598140 601150 600160 606170 606180 604190 608200 609210 609220 615230 616240 618250 621260 624270 622280 625290 628300 628310 632320 635330 634340 637350 639360 637370 632380 629381 629
Crack Growth Rates
0
100
200
300
400
500
600
700
0 50 100 150 200 250 300 350 400 450
Location (cm)
CG
R (
pm
/s)
Power
0%
20%
40%
60%
80%
100%
120%
0 20 40 60 80 100 120 140
Time (months)
Po
wer
Feedwater Hydrojen Injection
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 20 40 60 80 100 120 140
Time (months)
[H2]
(p
pb
)
Corrosion Evolutionary Path
Plant Evolutionary Path, cont’d
O2
0
50
100
150
200
250
0 20 40 60 80 100 120 140
Time (months)
O2
Co
nce
ntr
atio
ns
(pp
b)
H2
0
5
10
15
20
25
30
0 20 40 60 80 100 120 140
Time (months)
H2
Co
nc
en
tra
tio
n (
pp
b)
Feedwater Hydrojen Injection
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 20 40 60 80 100 120 140
Time (months)
[H2]
(p
pb
)
Plant Evolutionary Path, cont’d
H2
0
5
10
15
20
25
30
0 20 40 60 80 100 120 140
Time (months)
H2
Co
nc
en
tra
tio
n (
pp
b)
ECP
0
50
100
150
200
250
300
350
400
450
0 20 40 60 80 100 120 140
Time (months)
EC
P (
mV
SH
E)
Crack Growth Rates
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100 120 140
Time (months)
CG
R (
pm
/s)
H2O2
0
50
100
150
200
250
300
350
400
450
500
0 20 40 60 80 100 120 140
Time (months)
H2
O2
Co
nc
en
tra
tio
n (
pp
b)
Plant Evolutionary Path, cont’d
Crack Growth Rates
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100 120 140
Time (months)
CG
R (
pm
/s)
Stress Intensity Factor
0
10
20
30
40
50
60
0 20 40 60 80 100 120 140
Time (months)
KI
MP
a m
^1
/2
Crack Propagation
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80 100 120 140
Time (months)
Cra
ck
De
pth
(c
m)
Accumulated Damage, Core Shroud
Stress Intensity Factor
0
10
20
30
40
50
60
0 20 40 60 80 100 120 140
Time (months)
KI
MP
a m
^1
/2
Crack Propagation
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80 100 120 140
Time (months)
Cra
ck
De
pth
(c
m)
Crack History
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 20 40 60 80 100 120
Time (months)
Cra
ck
De
pth
(c
m)
Normal Water Chemistry
Hydrogen Water Chemistry
Normal to Hydrogen Water Chemistry
Comparison with plant data – BWR in Taiwan
What Can Be Done To Reduce IGSCC Damage in BWRs?
• Enhance the rate of the hydrogen electrode reaction that occurs on the external surface (Noble Metal Chemical Addition® – introduced by GE to enhance HWC).
• Inhibit redox reactions (e.g., O2 and H2O2 reduction) on external surface.
• Use more resistant alloy (Type 304K SS instead of Type 304 SS) – applicable to new plants.
• Catalytic decomposition of H2O2 – shown by ALERT to help but not to be totally effective.
• Ultra low conductivity operation – the required conductivity probably cannot be achieved in an operating plant.
Figure 4: ECP variation in the heat transport circuit of a BWR employing general catalysis (exchange current density multiplier 104) under NWC ([H2]FW=0.0 ppm) and HWC ([H2]FW=1.0 ppm) conditions. Note that the
multiplier employed in the calculation is considered to be extreme and probably could not be achieved with deposited noble metal coatings. The value is used for illustrative purposes only.
Figure 5: Crack growth rate variation in the heat transport circuit of a BWR employing general catalysis (exchange current density multiplier 104) under NWC ([H2]FW=0.0 ppm) and HWC ([H2]FW=1.0 ppm)
conditions. Note that the multiplier employed in the calculation is considered to be extreme and probably could not be achieved with deposited noble metal coatings. The value is used for illustrative purposes only.
Figure 6: ECP variation in the heat transport circuit of a BWR employing general inhibition (exchange current density multiplier 10-4) under NWC ([H2]FW=0.0 ppm)
and HWC ([H2]FW=1.0 ppm) conditions. Note that the multiplier employed in the
calculation is arbitrarily chosen and represents a highly resistive dielectric film.
Figure 7: Crack growth rate variation in the heat transport circuit of a BWR employing general inhibition (exchange current density multiplier 10-4) under NWC ([H2]FW=0.0 ppm) and HWC ([H2]FW=1.0 ppm). Note that the multiplier
employed in the calculation is arbitrarily chosen and represents a highly resistive dielectric film.
Figure 8: Calculated ECP and IGSCC crack growth rate for sensitized Type 304 SS according to the MPM and CEFM, respectively, corresponding to the experimental conditions summarized in Figure 9. Other model parameters are given in Ref. [22].
Figure 9: Inhibition of IGSCC in Type 304SS by a dielectric ZrO2 coating
on the specimen external surfaces. The conductivity of the solution was 220 μS/cm at 25 oC and the solution was saturated with O2 at ambient
temperature ([O2] = 40 ppm). KI = 25 MPa√m, T = 288 oC. The crack
growth rate for the uncoated specimen is 4x10-7 cm/s while that for the coated specimen is < 2x10-8 cm/s.
SUMMARY AND CONCLUSIONS
•Intergranular Stress Corrosion Cracking (IGSCC) is primarily electrochemical in nature as are the most effective mitigation measures.
• The coupling current provides a wealth of information concerning events that occur at the crack tip. •The critical potential for IGSCC in sensitized Type 304 SS in high temperature aqueous solutions can be accounted for in terms of the dependence of the coupling current on the potential at the external surface remote from the crack mouth.
•The critical potential depends on the crack length, with EIGSCC becoming more positive with increasing length. All cracks must eventually die when EIGSCC > ECP.
Summary and Conclusions (cont.)• Shifting the ECP to a sufficiently negative value reduces the
coupling current and hence the IGSCC growth rate to the extent that it becomes negligible, signifying the critical potential.
• The coupling current and IGSCC growth rate may be reduced by
decreasing the charge transfer impedance at the external surface provided that the molar ratio of H2 to O2 is greater than 2 (as achieved in hydrogen water chemistry), by increasing the charge transfer impedance at the external surface regardless of the molar ratio of H2 to O2, or by increasing the resistivity of the external environment (lowering the conductivity).
• These strategies are a direct manifestation of the fact that stress
corrosion cracking falls within the framework of the differential aeration hypothesis, which requires strong coupling between the crack internal and external environments via the coupling current.
Summary and Conclusions (cont.)• The CEFM has been combined with the radiolysis model,
RADIOCHEM, and with the Mixed Potential Model, MPM, to produce powerful BWR simulation codes, DAMAGE-PREDICTOR, ALERT, and REMAIN. These codes predict coolant chemistry, ECP, and state point crack growth rate for any location in the coolant circuit and, in the case of ALERT and REMAIN, also predict accumulated damage along the Corrosion Evolutionary Path (CEP).
• Comparison of the predictions with plant data from the Leibstadt
BWR (Switzerland) and from a BWR in Taiwan reveals excellent agreement.
An even more advanced BWR code is under development,
We have developed a somewhat similar code for predicting the electrochemistry of Pressurized Water Reactor (PWR) primary coolant circuits.
ACKNOWLEDGEMTS
The author gratefully acknowledges the contributions made to this work by the following:
• Professor Mirna Urquidi-Macdonald, Pennsylvania State University.
• Dr. George R. Engelhardt, OLI Systems Inc and Adjunct Professor, Pennsylvania State University.
• Dr. T.-K. Yeh, National Tsing-Hua University, Taiwan, former student.
• Dr. Iouri Balachov, SRI International, Menlo Park, CA.
• Amit Jian, former MS student at Penn State.
• HanSang Kim, Penn State, current Ph.D. student.
• Department of Energy and SRI International for financial support.
