Role of cavity formation and corrosion on SCC Initiation ...

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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 17 th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )

Transcript of Role of cavity formation and corrosion on SCC Initiation ...

Page 1: Role of cavity formation and corrosion on SCC Initiation ...

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 )

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17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )

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Outline

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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 )

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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

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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

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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 )

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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)

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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

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Role of cavity formation on initiation

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How cavity form at GB ?

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( 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)

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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 )

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Mechanism of Cavity Formation

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Key Processes

Rate limited Processes

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: Cavity

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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 )

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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

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SCC initiation in PWR primary water

Additional key processes

in water ?

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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

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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

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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

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> 30,000 h

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Examination of fracture surface

and cross sectional view after tests

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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

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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

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SCC initiation in PWR water

( 20%CW TT690, 20,653h at 360 )

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• 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 )

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1 μm

SCC initiation in PWR water

( 20%CW TT690, 20,653h at 360 )

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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)

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Ni C Cr

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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)

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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)

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SCC initiation in PWR water

( 20%CW TT690, 20,653h at 360 )

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• 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”.

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(a)

(a)

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Role of cavity on SCC initiation in PWR water

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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

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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

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TT 690 450 400 360 320 (°C)

100 year

gas water

10 year

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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

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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.

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Summary (2/2) 17th International Conference on Environmental Degradation, August 9-13, 2015 ( Ottawa, Canada )

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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 )