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Role of cavity formation and corrosion
on SCC Initiation of CW TT690
August 9-13, 2015
Institute of Nuclear Safety System
Koji Arioka, T. Miyamoto, T. Yamada, T. Terachi
QMN-3 2012 , June 10-16, 2012 ( Idaho falls, USA )
1 1
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
Outline
2
Crack initiation of CW TT690 in air
Unexpected rapid IG-crack occurred caused by cavity formation.
Initial IG-crack initiated inside materials just below the surface.
Process of cavity formation
Dependence of stress gradient ( ∂σ / ∂x ) on population of cavity.
Diffusion of CW induced vacancies driven by ( ∂σ / ∂x ).
SCC initiation of CW TT690 in PWR water
SCC initiated from shallow (~0.1mm) pre-crack in PWR water.
Effect of corrosion and HELP by absorbed H in PWR water.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
Crack Initiation
• Test materials
20%CW TT 690 ( Grain size : ~100 μm )
• Test Environments
PWR water : 360˚C, 320˚C
Air : 320 ~ 475 ˚C
• Static Constant load : K : 35-40 MPa m1/2 3
0.5 mm R
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
4
2. 8mm
Crack seems to initiate from inside material just beneath surface.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
Crack initiation at the notch (20%CW TT 690, 425°C x 15,771 h in air)
Engineering importance When crack initiate from the surface by this mechanism, crack penetrate almost immediately to significant depth, such as 2 to 3 mm.t
5
Temperature dependence on crack initiation
20%CW TT 690
450 400 360 320 (°C) 100 year
gas water
K. Arioka, Corrosion, 71 (2015): p.401.
Careful attention should be paid on SCC initiation after LTO.
10 year
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
20%CW TT 690
1.E-10
1.E-08
1.E-06
1.E-04
1.E-02
0.0013 0.0014 0.0015 0.0016 0.0017 0.0018
1/ Temp ( K-1 )
1/ C
rack
init
iati
on ti
me
( h-1
)
1.E-10
1.E-08
1.E-06
1.E-04
1.E-02
Crac
k G
row
th r
ate
(mm
/se
c )
Crack Initiation in Air SCC Initiation in PWR Crack Growth in air Crack Growth in H2 gas Crack Growth in Ar gas Crack Growth in air ( Lab. )
Correlation between crack initiation and growth (20%CW TT 690)
6
Similar slope of initiation and growth in gas
Similar slope suggested that cavity formation seem to play a role both in crack initiation and growth of CW TT690.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
450 400 360 320 (°C)
Crack Initiation
Crack growth
20%CW TT690
Role of cavity formation on initiation
7
How cavity form at GB ?
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
8
( Sc / S0 ) xi : Area fraction of cavities ( population )
( Measured by image-processing )
Distribution of cavities (20%CW TT690, 460, 2,771 h in air)
2 µm 50 µm
Grain boundary
Cavities (black)
Carbides (light grey)
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
9
Role of Cavity on GB Bonding Strength ( 20% CW TT 690 , 410˚C x 5,036 h in air ( no crack ) )
(a) (a)
(b) 20 μm
1 µm
Initial crack seem to initiate at the peak population of cavities.
(b)
K. Arioka, Corrosion, 71 ( 2015 ) : p.401.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
Mechanism of Cavity Formation
10
Key Processes
Rate limited Processes
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
: Cavity
11
Cavities seem to be formed result from diffusion of vacancy induced by cold work driven by stress gradient.
∂C /∂t = D ( ∂2C /∂x2 + ( Ω / RT ) ( C∂2σ/∂x2) )
Cavity
Assumed Processes of Cavity formation
: Cold work induced vacancy (Cv)
Cv =ηεn ε : Strain H.G. van Beuren, Acta Metal, 3 ( 1955 ) : p.519.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
0
0.5
1
1.5
2
0 0.4 0.8 1.2
Vacancy concentration (at.%)
Dist
ance
from
surf
ace
of n
otch
( m
m )
1000 h2000 h5000 h8000 h
Change in distribution of vacancies with time
∂C / ∂t = D ( ∂2C / ∂x2 + ( Ω / RT ) ( C ∂2σ / ∂x2) )
Calculated condition
Grain size : 10 μm
D (320) : 4.5x10-24 m2/s
8000 h 2000 h
12
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
13
SCC initiation in PWR primary water
Additional key processes
in water ?
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
14
Corrosion ahead of SCC-tip
( 30%CW TT690, 17,119h at 360 in PWR )
• Corrosion occurred ahead of SCC-tip.
• Water seems to penetrate through
cavities from ahead of SCC-tip.
1 μm
100 nm (a)
(a)
Oxide
100 nm
O
AES Analysis of Oxygen
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
Hydrogen-enhanced localized plasticity (HELP)
( 20%CW TT690, 360˚C ( Ar gas H2 gas ) )
Rates of cavity formation may be affected by absorbed H.
20%CW TT690 ( 360C )
0
0.02
0.04
0.06
0.08
0 1000 2000 3000
time ( h )
Disp
lace
men
t ( m
m ) Ar gas H2 gas
Switch to H2
H2 gas Ar gas
HELP
15
16
Prediction of SCC initiation in PWR water
Major topics for prediction 20%CWTT 690
450 400 360 320 (°C) 100 year
gas water
K. Arioka, Corrosion, 71 (2015): p.401.
Environmental factors should be examined for prediction of SCC.
10 year
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
> 30,000 h
Examination of fracture surface
and cross sectional view after tests
17
3 mm
Observation of fracture surface
Observation of Cross sectional view
SCC initiation of 20%CW TT690 in PWR water
Shallow Pre-crack : 0.14 mm (~ 1 grain size )
Low K condition : K=24 MPa m1/2
Long test period : 20,653 h at 360
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
1 μm
SCC initiation in PWR water
( 20%CW TT690, 20,653h at 360 ) Pre-crack : ~ 0.14 mm
K=24 MPa m1/2
18 Clear cavities were observed after 20,653h in water.
Cavities
(black) Carbides
(light gray)
80 μm
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
SCC initiation in PWR water
( 20%CW TT690, 20,653h at 360 )
19
• Many shallow IGC covered by oxide were observed.
• IGC without oxide also were observed ahead of SCC tip.
IGC covered by oxide
IGC without oxide
Pre-crack : ~ 0.14 mm
K=24 MPa m1/2
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
1 μm
SCC initiation in PWR water
( 20%CW TT690, 20,653h at 360 )
20
Many cavities were observed ahead of pre-crack after
20,653h although it is shorter than predicted time in air.
1 μm
Cavities (black) Carbides
(light gray)
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
Ni C Cr
21
AES analysis on crack surface
( 20%CW TT690, 20,653h at 360 in PWR water )
Light grey particles are chromium rich carbides.
Carbides (light gray)
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
SCC initiation in PWR water
( 20%CW TT690, 20,653h at 360 )
22 Some of pores seem to be prints of carbides.
Carbides (light gray)
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
SCC initiation in PWR water
( 20%CW TT690, 20,653h at 360 )
23
• Additional pores were observed on inner side of pores (cavities).
• The reason why carbides were removed without deformation are
assumed to result from cavity formation adjacent to carbides.
• Pores seem to consist of “print of carbides with cavities”,
“cavities”, and “small dimple”.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
(a)
(a)
Role of cavity on SCC initiation in PWR water
24
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
σ
Cavity Carbide
SCC-tip
Role of cavities
Cavities weaken bonding strength at grain boundaries in mechanically.
• Cavities play a role of pathway for water to penetrate from SCC-tip.
Water
25
Effect of corrosion in water on SCC initiation
Clear cavities were recognized after 20,653h at 360°C in PWR water.
Results seem to suggest rapid SCC initiation in water rather than in air.
Operating temperature
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
TT 690 450 400 360 320 (°C)
100 year
gas water
10 year
Summary (1/2) 1. Unexpected rapid cavity formation did occur CW TT 690 prior to
Crack Initiation and SCC growth.
2. GB bonding strength seems to be weaken by cavity formation at
grain boundary during long term operation.
3 Cavity formation seems to occur result from the diffusion of cold
worked induced vacancies driven by stress gradient even at
L & HWR operating temperature.
Careful attention should be paid about the change in material
properties, such as GB bonding strength and local chemical
composition, to assess SCC Initiation and growth after long term
operation in addition to the electro-chemical reactions in water. 26
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
4. Shorter SCC initiation seems to occur in PWR water than in air result from the effect of corrosion in water near cavities at grain boundaries after long term operation.
Electro-chemical reaction in water, such as IG-corrosion and HELP seems to affect on SCC initiation during long term incubation.
Much more careful and fundamental studies are crucial for precise prediction of SCC initiation of SG tubing beyond 60 years.
27
Summary (2/2) 17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )
28
Acknowledgement
• Authors wish to acknowledge the financial support of
KEPCO.
• Author greatly acknowledge Dr. Roger W. Staehle for
his thoughtful encouragement, and helpful suggestions.
• Author also thanks Drs. Robert L. Tapping and Michel
Wright in CNL for their helpful discussions.
17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )