Chromium depletion in relation with oxidized grain boundaries at the SCC crack tip

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Chromium depletion in relation with oxidized grain

boundaries at the SCC crack tip

J. Nguejio, J. Crepin, C. Duhamel, C. Guerre, F. Jomard

To cite this version:

J. Nguejio, J. Crepin, C. Duhamel, C. Guerre, F. Jomard. Chromium depletion in relation with oxidized grain boundaries at the SCC crack tip. EUROCORR 2016 - The European Federation of Corrosion, Sep 2016, Montpellier, France. �cea-02431818�

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CHROMIUM DEPLETION IN RELATION

WITH OXIDIZED GRAIN BOUNDARIES

AT THE SCC CRACK TIP

JOSIANE NGUEJIO (1,2)

JÉRÔME CRÉPIN (1) _{, CÉCILIE DUHAMEL}(1)

CATHERINE GUERRE (2) _{, FRANÇOIS JOMARD}(3)

2016/09/14 | PAGE 1

(1) : Den-Service de la Corrosion et du Comportement des Matériaux dans leur Environnement (SCCME), CEA, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France

(2) : MINES ParisTech, PSL Research University, MAT- Centre des matériaux, CNRS UMR 7633, BP 87 91003 Evry, France

(3) : Laboratoire GEMaC, UMR 8635 CNRS - Université De Versailles

2016/09/14 J. Nguejio

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| PAGE 2 J. Nguejio

INDUSTRIAL CONTEXT

Understanding this mechanism is a key issue for the extension of PWR operation time > 30 – 40 years

Pressurized Water Reactor (PWR)

Steam generator tubes : Nickel Base Alloy A 600-> Ni-15Cr-Fe Temperature 285 - 325 °C Pressure 155 bar Boren (H₃BO₃) 10 -1200 ppm Lithium (LiOH) 0,7 - 2,2 ppm H₂ 25 - 50 cm3_.kg-1_(TPN) O₂ < 5 ppb pH_300°C  7 Primary water conditions

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Cr-depletion

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CRACK PROPAGATION MODEL [1]

SCC propagation mechanism proposed by authors [1] 1. Chromium oxidation at the crack tip : weakens the grain boundary (GB)

2. Chromium depleted area of 20 nm in the grain and 200 nm along the GB 3. Chromium depletion is observed only in one side of GB

4. Assumptions :

 Strain may be different in the vicinity of the grain boundary

 Chromium diffusion is the rate limiting step of the SCC mechanism.

CONTEXT

[1] M. Sennour, P. Laghoutaris, C. Guerre, R. Molins, Journal of Nuclear Materials, 393 (2009) 254-266 Grain with higher

deformation-> defects

Grain with lower deformation

Chromium oxide

Cr-depleted area

~ 𝟐𝟎𝟎 𝒏𝒎

In the grain with higher deformation :

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CHROMIUM DIFFUSION : A KEY STEP OF THE SCC MECHANISM

CONTEXT

[1] D.D. Pruthi et al. Journal of Nuclear Materials 64 (1977) 206-210 [2] T.F. Chen et al. Materials Transactions 44 (2003) 40-46

[3] K. Monma, et al. The Japan Institute of Metals and Materials 28 (1964) 188-192 [4] B. Million, et al.Materials Science and Engineering 72 (1985) 85-100

[5] Moulin P., et al. Acta Metallurgica 77 (1979) : 1430-43.

10-28 10-26 10-24 10-22 10-20 10-18 10-16 10-14 10-12 10-10 10-8 10-6 6 10-4 8 10-4 1 10-3 1,2 10-31,4 10-31,6 10-31,8 10-3 2 10-3 D [c m²/s] (1/T) [K-1] 350°C 500°C 700°C 1000°C 6.10-18, 350°C 3.10-26, 350°C Grain boundary diffusion Lattice diffusion

Aim : determine which mechanism could explain the formation of Cr-depletion in one grain ahead of the crack tip ?

y = 2√Dt

-Grain boundary diffusion at 350°C  t = 3 days

-Lattice diffusion at 350°C  t = 1000 years

Accelerating factor for chromium diffusion at the SCC crack tip ? Study the lattice and grain boundary chromium diffusion at 350°C

Assumption :

Plasticity at the crack tip could enhance chromium diffusion.

Diffusion time too high (compared to 6000 seconds [1])

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Mechanical measurements during creep tests (Average strain rate)

EXPERIMENTAL APPROACH 0,1 1 10 100 1000 104 105 106 0 20 40 60 80 100 120 140 Diffusion profiles

After creep test at 350°C - 100 MPa , 4 hrs 50 Cr 52 Cr 58 Ni 60 Ni In te n si ty [cp s/ s] Depth [nm] Cr diffusion Cr / Ni interface Cr deposition Steps :

1. Materials

 Pure nickel Single crystal

-crystallographic orientation <101>

2. Chromium deposition



Sample polishing

 Chromium evaporation under vacuum -> 20 nm layer (powder 52_{Cr - 99,6% purity)}

3. Creep tests under secondary vacuum

 Both diffusion and plastic deformation

4. Diffusion coefficients

 Measured by Secondary Ion Mass Spectrometry (SIMS)

Diffusion profiling by SIMS (CAMECA IMS 7F – GEMaC laboratory)

3 3,5 4 4,5 5 0 20 40 60 80 100 0 4000 8000 1,2x104 1,6x104

Creep test at 350°C - 100 MPa

S tr e ss [M P a ] S tr a in [ % ] Time [s]  0  1  2

Primary creep Secondary creep Applied stress

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DIFFUSION ENHANCED BY PLASTICITY

MECHANICAL RESULTS : CREEP TESTS ON PURE NICKEL

Creep law

:

ε = C. exp −

ΔH_RTcreep

. σ

n

Power factor n ~ 7

Garofalo [1] : n = 5 – 6

For a given σ

:

ΔH

_creep

= −R

δ ln ε

δ

1 T

Loading at 500°C : 55-74 MPa

(yield stress 48 MPa)

Loading at 350°C : 74-100 MPa

(yield stress 60 MPa)

ΔH

_creep

= 125 kJ/mol

For T between 350°C – 500°C

Mechanical results are in good agreement with literature.

-18 -17 -16 -15 -14 -13 -12 17,8 17,9 18 18,1 18,2 18,3 18,4 18,5 y = -146 + 7,3 x R2= 0,97 y = -145 + 7,1 x R2= 0,93 500°C 350°C ln (d /dt) ln (, Pa) The same 

[1] Garofalo Franck « Fundamentals of Creep and Creep rupture in Metals » 1965 , p.104

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CHROMIUM DIFFUSION PROFILES OBTAINED BY SIMS

[1] Chetroiu, B. “Stress corrosion cracking of Alloy 600 in primary water of PWR: study of chromium diffusion” Thesis MINES ParisTech 2015

1. Diffusion enhancement is observed, when the applied load is over the yield stress

Accelerating effect of plasticity on diffusion at 500°C and 350°C ⟺

2. Calculation of apparent diffusion coefficients by the slope of diffusion profiles

𝜕2lnC 𝜕x2 = −

1 4 𝐃𝐚𝐩𝐩t

Thin film solution of 2nd _{Fick law}

-6 -5 -4 -3 -2 -1 0 1 0 5000 1x104 1,5x104 2x104 2,5x104 3x104 Effect of mechanical loading on diffusion at 500°C

 = 0 MPa t = 30 h [1]

 = 74 MPa t = 3 h Yield stress at 500°C ~ 48 MPa

x² [nm2] Ln (s ignal 52 Cr) -4 -3 -2 -1 0 1 0 2000 4000 6000 8000 1x104 Effect of mechanical loading on diffusion at 350°C

 = 0 MPa T = 400°C t = 1616 h [1]

 = 100 MPa t = 4 h Yield stress Ni at 350°C ~ 60 MPa

Ln (s

ignal

52

Cr)

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EUROCORR 2016 | 2016-09-14 0,1 10 1000 105 107 109 0 5x10-7 1x10-6 1,5x10-6 2x10-6 2,5x10-6 3x10-6 3,5x10-6

Enhanced diffusion factor in function of strain rate and temperature 350°C D a p p /D v d/dt [s-1 ] 500°C 2016/09/14 | PAGE 8 J. Nguejio

DIFFUSION ENHANCED BY PLASTICITY

APPARENT DIFFUSION COEFFICIENTS AND PLASTICITY

D

_app

D

_v

∝ B . ε

_p

plasticity-enhanced diffusion

𝐵 = 𝑓 1/𝑇 10-17 10-16 10-15 10-14 0 5x10-7 1x10-6 1,5x10-6 2x10-6 2,5x10-6 3x10-6 3,5x10-6

Diffusion under plastic deformation

D app d/dt [s-1 ] 500°C 350°C

D

_app

with 𝛆

The enhanced diffusion factor

Dapp

Dv _350°C ≫

Dapp

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| PAGE 9 J. Nguejio

𝐃

_𝛆

= f [ ε 𝑇, 𝜎 ]

then…

𝐃

_𝛆

depend on temperature and stress

_{For a given σ}

_:

_ΔH

ε = −R δ ln D_δ_{1 T} ε

ΔH

_ε

= 115 kJ/mol

The activation energy of chromium diffusion enhanced by plasticity is :

This value is lower than activation energy of lattice diffusion

-38 -37 -36 -35 -34 -33 17,8 17,9 18 18,1 18,2 18,3 18,4 18,5 The same  500°C 350°C ln (, Pa) ln ( D app )

ACTIVATION ENERGY OF CHROMIUM DIFFUSION ENHANCED BY PLASTICITY

What about diffusion enhanced by the plasticity in front of the SCC crack tip ?

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Diffusion enhanced by plasticity at the crack tip causes Cr-depletion

PLASTICITY AT THE SCC CRACK TIP

Correlation between crack propagation rate and the strain rate at the SCC crack tip (by Le Hong)

On A600 at 360°C

Literature :

ε

_ct

~ 1. 10

−7

s

−1

Le Hong, S., et al. (1999)." 9th International Conference on Environmental Degradation of Materials in Nuclear power systems Newport Beach: p.115.

The strain rate estimated by the study is in good agreement with the strain rate at the SCC crack tip

0 4 x 10-17 8 x 10-17 1,2 x 10-16 1,6 x 10-16 2 x 10-16 2,4 x 10-16 2,8 x 10-16 3,2 x 10-16 0 5x10-8 1x10-71,5x10-72x10-72,5x10-73x10-73,5x10-74x10-7 Lattice diffusion at 350°C

Diffusion enhanced by plasticity at 350°C

d/dt [s-1 ] D app ( cm ²/ s) 3.3.10-7 s-1 1.2.10-7 s-1 3.10-8 s-1

Diffusion coefficient in front of the SCC crack tip is between :

9. 10

−17

_{− 4. 10}

−16

_cm

2

_/s

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2016/09/14 | PAGE 11 J. Nguejio 𝐶𝑟 𝑥, 𝑡 = 𝐶𝑖 + 𝐶₀− 𝐶𝑖) × erf 𝑥 2 𝐷𝐶𝑟× 𝑡 − erf 𝑘𝑐 2𝐷𝐶𝑟 erfc 𝑘𝑐 2𝐷𝐶𝑟 Wagner [1] _{calculation at 350°C}

AT THE CRACK TIP

Cr : 8%

[2]

Literature Cr-depleted area :

20 nm in 6300 sec

The minimum diffusion coefficient in the interval : 𝐶_{𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒} ~ 0 % 𝑎𝑡

Cr-depleted area : 20 nm

[1] Wagner, C. (1952). Journal of the Electrochemical Society 99(10): 369-380.

[2] M. Sennour, P. Laghoutaris, C. Guerre, R. Molins, Journal of Nuclear Materials, 393 (2009) 254-266

0 0,04 0,08 0,12 0,16 0 10 20 30 40 50 60

Cr depletion in Ni-Cr alloy by Wagner modelling

% at

Cr

Depth from the interface [nm] 9,0.10-17 cm2/s In te rf a ce mé ta l/o xyd e 4,0.10-16 cm2/s 𝐃_{𝐜𝐫𝐚𝐜𝐤−𝐭𝐢𝐩} = 9. 10−17_{− 4. 10}−16 _cm2_/s

The maximum diffusion coefficient in the interval 𝐶_{𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒} ~ 10 % 𝑎𝑡 Cr-depleted area : 50 nm 0 0,04 0,08 0,12 0,16 0 10 20 30 40 50 60

Cr depletion in Ni-Cr alloy by Wagner modelling

% at

Cr

Depth from the interface [nm]

In te rf a ce mé ta l/o xyd e D 3 = 1,6.10 -16 cm2/s

Exact value of 𝐃𝐜𝐫𝐚𝐜𝐤−𝐭𝐢𝐩 for : 𝐶𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒~ 8 % 𝑎𝑡

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CHROMIUM DIFFUSION AND SCC CRACK PROPAGATION

[1] M. Sennour, P. Laghoutaris, C. Guerre, R. Molins, Journal of Nuclear Materials, 393 (2009) 254-266

[2] Lorho, N. (2013). "SCC susceptibility in Alloy 600 with different strain histories." Thèse de l'Université de Manchester.

ΔH_{diffusion−plasticity} = 115 kJ/mol ΔHSCC−propagation = 130 kJ/mol

[2]

Oxygen diffuses towards the crack

2. The moving dislocations accelerate the chromium diffusion in the grain 1, with a higher strain rate ε₁ > ε₂)

1. A plastic zone ( ε_ct = 1. 10−7 s−1), is generated in front of the crack tip.

Diffusion enhanced by plasticity at the crack tip causes Cr-depletion

Plasticity could explain the Cr-depleted area in grain 1

Activation energy

The SCC crack propagation is linked to the mechanism of the chromium diffusion enhanced by the plasticity.

~

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2016/09/14 J. Nguejio | PAGE 14

A600 – exposed in hydrogenated steam at 480°C, 130h

G. Bertali, F. Scenini, M.G. Burke, 16th_{Environmental degradation, (2013).}

The author assumes that the depletion is due to the

diffusion induced grain boundary migration phenomenon (DIGM)

Cr – DEPLETION ALSO OBSERVED WITHOUT PLASTICITY

Cr-depletion is due to DIGM

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EXPERIMENTAL APPROACH AND RESULTS

Effect of GB misorientation

The DIGM is observed only for high angle grain boundaries (>15°) [1].

Oxidation is associated with the migration of the GB.

[1] Balluffi, R. and J. Cahn, Mechanism for diffusion induced grain boundary migration. Acta Metallurgica, 1980. 29: p. 493-500

Heat treatments are performed :

on A600 – polycrystal (1 mm)

at 500°C, 30 hrs in vacuum

Grain boundary observations : SEM/FIB and TEM in cross-section

20 µm LAGB (10°) HAGB (27°)

Oxidized GB

1 µm LAGB (10°) GB remained straight 1 µm HAGB (27°) GB migration

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2016/09/14 | PAGE 16 J. Nguejio TEM CHARACTERIZATIONS

D

_{moving−GB Cr}

~

D

_{stationary−GB Cr)} (6.10-13_{cm²/s at 500°C}_{by Pruthi)}

Pruthi, D. D. (1977). "Diffusion of chromium in Inconel 600." Journal of Nuclear Material 64: 206-210.

D

_{mobile−GB Cr} can be extrapolated at 350°C.

Migrated grain

boundary leaves in

its wake a

Cr-depleted area.

TEM bright field (BF) image

EFTEM 0 5 10 15 20 0 2000 4000 6000 8000 1 x 104 Conc. Cr (at.)

Fit with Wagner Law

Conc.

Cr (a

t.)

Distance from the surface (nm)

At 500°C, D moving-GB (Cr) = 2.10 -13 cm²/s Cr-depletion depth : 3 µm

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CHROMIUM DEPLETION ALONG THE INTERGRANULAR OXIDE PENETRATION

TEM CHARACTERIZATIONS

D

_{moving−GB Cr}

~

D

_{stationary−GB Cr)}

(6.10-13_{cm²/s at 500°C}_{by Pruthi)}

D

_{mobile−GB Cr} can be extrapolated at 350°C.

0 5 10 15 20 0 20 40 60 80 100 Wagner calculation at 350°C

At 350°C

Cr-depletion depth : 60 nm Conc. Cr (a t.)

Distance from the surface (nm)

10 nm

At the SCC crack tip [1]

Cr

₂

O

₃

200 nm

20 nm

The DIGM can also account for the Cr-depleted area at the SCC crack tip.

At 500°C

10°

At 350°C

[1] M. Sennour, P. Laghoutaris, C. Guerre, R. Molins, Journal of Nuclear Materials, 393 (2009) 254-266

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CONCLUSION

1. Cr – depletion at the crack tip



Without plasticity, lattice and grain boundary diffusion coefficients cannot explain

the asymmetrical depletion observed in one grain at the crack tip.



Enhanced diffusion by plasticity is observed at 500°C and 350°C.



Strain rate estimated in this study at 350°C is very close to the known strain rate at

the SCC crack tip.



Activation energy of chromium diffusion enhanced by the plasticity, is also in good

agreement with the activation energy of the SCC crack propagation.



Plasticity-enhanced diffusion can explain the dimension and the asymmetrical

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| PAGE 19

CONCLUSION

2. Cr – depletion along the intergranular oxide penetration



This study showed that the DIGM is observed only for HAGBs, known to be more

susceptible to SCC.



DIGM can explain the Cr-depletion along the intergranular oxide penetration

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2016/09/14 . Nguejio | PAGE 20

PROSPECTS

 To evidence the DIGM phenomenon in PWR conditions (A600 - 340°C, 1000 hrs)

 Coupling plasticity and DIGM

FS = 0.41 FS = 0.47

16

14 Perform creep tests on A600 (grain size ~ 1 mm) : study the asymmetrical diffusion

EBSD map

Strain field measured by digital image correlation

𝛆 = 𝟒 % 𝛆 = 𝟏 %

Cr map

Ni map

O map

Cr-depletion

IG oxidation 300 nm

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Thanks !

Josiane NGUEJIO : josiane.nguejio@cea.fr

Commissariat à l’énergie atomique et aux énergies alternatives

Centre de Saclay| 91191 Gif-sur-Yvette Cedex

Acknowledgments

Fabrice GASLAIN (Centre des Matériaux) Vladimir ESIN (Centre des Matériaux) Régis CLEMENT (Centre des Matériaux) Loic NAZE (Centre des Matériaux) Laure MARTINELLI (CEA)

Marc MAISONNEUVE (CEA)

| PAGE 21

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Diffusion in pre-deformed material : diffusion treatments after tensile tests

Samples

conditions

Diffusion

treatments

Non-deformed

30h, 500°C

𝜀

_𝑝

= 4 %

30h, 487°C

𝜀

_𝑝

= 20 %

30h, 487°C

No significant effect of the pre-deformation on chromium diffusion.

Pre-deformation can not explain the asymmetric oxidation and depletion along grain boundary.

Single crystal Ni 10-28 10-26 10-24 10-22 10-20 10-18 10-16 10-14 10-12 10-10 10-8 6 10-4 8 10-4 1 10-3 1,2 10-31,4 10-31,6 10-31,8 10-3 2 10-3 Ni pré-déformé - Chetroiu15

Ni sous charge (élastique) - Chetroiu15

Ni sous charge (plastique) - Chetroiu15

D [c m²/s] (1/T) [K-1] A600 - (Pruthi77) A600 - Monma (1964) A600 - (Chetroiu15) Nickel - Million (1985) Ni- (Chetroiu15) A600 - (Chen03) 350°C 500°C 700°C 1000°C D (Ni-Cr) = 1.10 -22 cm2/s D CSC (A600) = 6.10 -16 cm2/s "Pointe de fissure" 350°C (D Extrapolé)

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Diffusion at 500°C

Mechanical state Strain rate

(10-7s-1) 𝐃𝐚𝐩𝐩 (10-17cm²/s) Enhanced diffusion 𝐃𝐚𝐩𝐩 𝐃𝐯 𝟓𝟎𝟎°𝐂) Without plasticity - 4.0 ± 0.2 1.0 30 MPa [1]

(below the yield stress) - 4.6 ± 1.0 1.1

Plasticity 55 MPa 3.8 5.9 ± 0.6 1.5 Plasticity 60 MPa 5.6 7.7 ± 0.1 1.9 Plasticity 65 MPa 16 58 ± 1.6 4.5 Plasticity 70 MPa 19 100 ± 8.0 25 Plasticity 74 MPa 33 420 ± 10 105 Diffusion at 350°C

Mechanical state Strain rate

(10-7s-1) 𝐃_𝐚𝐩𝐩 (10-17cm²/s) Enhanced diffusion 𝐃𝐚𝐩𝐩 𝐃𝐯 𝟑𝟓𝟎°𝐂) Without plasticity [1] (400°C, 1616 hrs) - 0.003 -Plasticity 74 MPa 0.3 5.7 ± 2.0 8.105 Plasticity 84 MPa 1.2 11 ± 0.1 1.106 Plasticity 100 MPa 3.3 20 ± 1.6 2.106 ANNEX

APPARENT DIFFUSION COEFFICIENTS

0,1 10 1000 105 107 109 0 5x10-7 1x10-61,5x10-6 2x10-62,5x10-6 3x10-63,5x10-6 350°C D  /D v d/dt [s-1 ] 500°C

D

_app

D

_v

∝ B . ε

_p Accelerating factor plasticity-enhanced diffusion

B = f 1/T

temperature And/or strain rate

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2016/09/14 J. Nguejio | PAGE 24 2016/09/14 _{J. Nguejio} | PAGE 24

Migrated grain

boundary leaves in

its wake a

Cr-depleted area.

EDX spectrum images of the migrated GB.

TEM bright field (BF)

image

DIGM Mechanism

Steps of mechanism :

1. Intergranular diffusion

2. Kirkendall effect (vacancies flow) along the GB

3. Lead to the self-sustaining climb of GB dislocations

4. Steps of climb cause the GB migration 5. Depleted area is formed during the

migration MET CHARACTERIZATIONS AFTER HEAT TREATMENT AT 500°C

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2016/09/14 J. Nguejio | PAGE 25

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2016/09/14 J. Nguejio | PAGE 26 M. Sennour, P. Laghoutaris, C. Guerre, R.

Molins, Journal of Nuclear Materials, 393 (2009) 254-266

Cr-depletion in one grain

200 nm Vcrack = 1 mm/an

Step time of cracking ~ time of oxidation = 6300 sec