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Modeling Hydrogen Absorption and HIC of Titanium Alloys under Yucca Mountain Repository Conditions

Zack Qin

and David W. Shoesmith

The University of Western OntarioLondon, Ontario, Canada

3rd

International Workshop on Long-Term Prediction of Corrosion in Nuclear Waste SystemsMay 14-18, 2007, The Pennsylvania State University, State College, Pennsylvania, USA

2

HUMBOLDTCOUNTY

PERSHINGCOUNTY

ELKOCOUNTY

WHITE PINECOUNTY

NYECOUNTY

LAN

DER

CO

UN

TY

EUR

EKA

CO

UN

TYCHURCHILL

COUNTY

WA

SHO

EC

OU

NTY

MINERALCOUNTY

STOREYLYON

DOUGLAS

ESMERALDACOUNTY LINCOLN

COUNTY

CLARKCOUNTY

LASVEGAS

INYO COUNTYCALIFORNIA

NELLISAIR FORCE

RANGE

NVTESTSITE

3

4

Engineered Barrier System (EBS)

5

Drip Shield (DS)

The primary function of the DS is to protect the waste package (WP) from seepage drips that could form high ionic strength aqueous solutions at high temperatures and lead to crevice corrosion (CC) on WP material.

6

DS Criteria to Protect WP from CC

CC initiates only after the establishment of aqueous conditions (T < Taq), and when T > TCC, the CC threshold temperature.

DS is required to provide protection for 2000 years on realistic conditions to 6000 years on more aggressive, but much less likely, conditions.

7

Ti-7 for construction of the shell for primary corrosion resistanceTi-28/29 for the support members for a combination of corrosion and mechanical stabilityExceptional corrosion resistance (25~40 nm/yr)Potentially susceptible to hydrogen induced cracking (HIC)

Titanium Alloys

ASTM Grade

Crystal structure

Composition(%wt)

Tensile (MPa)

Yield (MPa)

Modulus (104 MPa)

Ti-7 α (0.12~0.25) Pd 345 275 10.3

Ti-28 α/β 3Al-2.5V-(0.08~0.14) Ru 620 483 9.03

Ti-29 α/β 6Al-4V-(0.08~0.14) Ru 829 795 11.2

Composition and mechanical properties

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Hydride Layer

Dense Band

Metal

From AECL report 11608 From AECL report 11284

Surface hydride layer Hydrogen-induced cracking

Hydrogen Induced Cracking (HIC)Hydrogen ingress into Ti-alloys will result in formation of hydrides and fast crack growth which ultimately would lead to instantaneous failure.

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O2-

H2 O

Habs

H2

Ti4+

Ti

OxideGrowth

MetalOxidation

HydrideFormation

TiHx

TiO2

H+

Water Reduction

TiOOH OxideTransformation

AqueousEnvironment

PassiveOxide

Metal

Corrosion and Hydrogen Absorption Processes

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Failure Model for Ti Drip ShieldsDegradation modes: general corrosion (GC) and HIC

Corrosion occurs only when aqueous conditions are established:

Source of hydrogen:Only a small fraction of the hydrogen produced will absorb into the metal:

The absorbed hydrogen is uniformly distributed in the metal:

Ti + 2 H2

O →TiO2

+ 4 H

absorbedH

produce

mfm

Δ=

Δ

)( 0 cTiH

H hhAmC

−=ρ

T < Taq

The two processes are inextricably linked since the rate of hydrogen absorption is controlled by the GC rate.

4 HTi

fM

γ =

11

Hydrogen produced by the corrosion process will absorb into Ti. On the other hand, the hydrogen already absorbed will be released as the metal containing the hydrogen converts to oxide. There is also a possibility of hydrogen re-distribution between the metal and the oxide.

( ) cc

HTiH dhhh

mAdm ⎥⎦

⎤⎢⎣

⎡−

−+−=0

1 ηγγρ

Hydrogen Concentration

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎦

⎤⎢⎣

⎡−−

−=

−ηγ

ηγγ

0

)(11)(h

thtC cH

When η

= γ⎥⎦

⎤⎢⎣

⎡−−=

0

)(1ln)(h

thtC cH γ

12

H c

once

ntra

tion

CH

Pene

trat

ion

dept

h h c

Time

h0

CHIC

tHIC tGC

Failure by HIC always prior to failure by GC, i.e. tHIC < tGC

Sufficiently high stress intensity exists causing instant brittle failure once the absorbed hydrogen concentration exceeds a critical value

HICHICH CtC =)(

Failures by GC if corrosion penetrates the wall allowance ∫ =

GCt

GC hdtR0 0

Failure Criteria

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Monte Carlo SimulationsMonte Carlo simulation is an ideal tool for predicting EBS lifetimes, since it accounts for the uncertainties arising from using experimental databases measured over short time periods for long time predictions by a probabilistic approach.Monte Carlo code, EBSPA, currently in its fourth version, for performance assessment of EBS.

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The EBSPA can calculate: a.

the cumulative probability of failures (CPF) of DS, WP, and EBS;b.

the responsibility ratio of individual components to overall EBS

failure; c.

the contribution

of different degradation modes to failure;d.

the effect of volcanic events on EBS lifetime.

EBSPA Code

104 105 10610-5

10-4

10-3

10-2

10-1

100

C

umul

ativ

e pr

obab

ility

Lifetime (year)

DS WS OL Lid WP EBS

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Flow Chart of DS Simulation

T < Taq

CH

(t) = ΣΔmH

(t) /[ρTi

A(h0

-hc

(t)] hc

= Σ RGC

(t) Δt

hc

(tGC

)

≥ h0 CH

(tHIC

) ≥ CHIC

tHIC

< tGC

GC failure HIC failure

GC HIC

No Yes

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0 0.5( , ) ( )H Hf T t f T t−=

H absorption depends both on temperature and time

Simulation Parameters

CHIC is the hydrogen concentration above which fast crack growth (brittle failure) occurs.

100 101 102 103 104 105 10610-5

10-4

10-3

10-2

10-1

100

T < 100°C

f H

time (year)

T > 100°C

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Parameter Distribution Values

Taq

(°C) N/A 120

RGC

(nm/year) Weibull scale factor = 22.4566, shape factor = 0.8057

Enhancement ×b N/A 1, 2, 5

CHIC

(ppmw) Uniform 100–300, 200–600

fH0

(T < 100°C) Uniform 0.001–0.01, 0.01–0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7

fH0

(T > 100°C) Uniform 0.01–0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7, 0.7–0.9

Simulation Parameters

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

Experimental Weibull

Cum

ulat

ive

Dis

trib

utio

n Fu

nctio

n

GC rate of Ti -DS (nm/yr)

•

Data from Lawrence Livermore National Lab fitted to a Weibull

distribution•

Enhancement b×RGC

Corrosion rate RGC

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Effective initial failure: CPF = 0.0001 (for ~10,000 DS)The higher fH0, the early the DS would fail, and the fewer DS that would survive after 1,000,000 years.The effect of CHIC is significant at higher fH0, but less so when fH0 is small.

Simulation Results: Effects of fH0

and CHIC

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

1x104

2x104

3x104

4x104

5x104

cHIC=100-300 ppm cHIC=200-600 ppm

Initi

al fa

ilure

(yrs

)

Max fH0

0.0 0.2 0.4 0.6 0.8 1.040

42

44

46

48

50

52

54

cHIC=100-300 ppm cHIC=200-600 ppm

106 y

ears

sur

viva

l (%

)

Max fH0

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Even for the most aggressive enhancement tested (b = 5), the DS would likely survive for more than the period required to protect the WP from CC on conservative conditions.

Simulation Results: Effect of RGC

103 104 105 10610-5

10-4

10-3

10-2

10-1

100

b = 1

2

realistic

Cum

ulat

ive

prob

abili

ty

Lifetime (year)conservative

5

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Conclusions

Probabilistic model based on anticipated repository environment and available data on the corrosion of titanium and its alloys.It considers general aqueous corrosion and HIC as the degradation modes; the model inevitably predicts failure by HIC prior to failure by general corrosion.Based on the model, the Monte Carlo code, EBSPA, was developed to predict the lifetime of DS, WP, and EBS.The most important parameters that affect HIC in the DS are fH, CHIC, and RGC. The sensitivities to these parameters were investigated by Monte Carlo simulations.

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Further ImprovementsHydrogen transport in titanium. The coupled absorption and diffusion equations could be solved by the finite element method using COMSOL Multiphysics.the formation of hydride layer near the DS surface, and its influence on hydrogen absorption.H absorption occurs locally at fault sites in the passive film, such as grain boundaries and mismatched phases.Assessment of galvanic coupling between the titanium surface and carbon steel structure components, a process that would be expected to accelerate the rate of hydrogen production and absorption.A shift in support of GC of titanium from water reduction to oxygen reduction, a process that does not produce hydrogen.

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Electrochemistry and Corrosion Studies at Western

http://publish.uwo.ca/~ecsweb

Thank YouAcknowledgements

This project was funded by the Natural Science and Engineering Research Council of Canada (NSERC)

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