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Spent Fuel and Waste Science and Technology
SNF Storage Canister Pitting and SCC Current Research at Sandia National Labs
SFWST MeetingMay 20, 2017
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration
under contract DE-NA0003525. SAND2018-5813 PE
Eric Schindelholz, Charles Bryan, Christopher AlexanderSandia National Laboratories
Spent Fuel and Waste Science & Technology Program
Spent Fuel and Waste Science andTechnology SNL Objectives
2
• Develop quantitative definition of brine properties on canister as function of environmental conditions to inform SCC models and laboratory studies
• Understand relationship between surface environment and damage distributions and rates
• Quantify impact of material and mechanical environment variability on corrosion and SCC processes
May 23, 2018 SFWST 2018
When and where on the canister and across storage sites do we have greatest risk of developing cracks?
Spent Fuel and Waste Science andTechnology
BeginStorage
Pit Initiation
Crack Initiation
Penetration
Incubation Pit Growth Crack GrowthStorage Time
SNL — Surface Environment, Brine StabilitySNL/OSU/UVA— Pitting initiation and growth, pit-to crack transition (experimental)CSM/SNL — Pitting initiation and growth (stress, microstructure effects
CSM — Pit-to-Crack Transition (Modeling)
NCSU (SNL) — crack growth
OSU (SNL) — crack growth SNL/LANL — mockup pitting/cracking
3
Canister SCC: Important Processes
SNL — SCC model framework
(1) Role of surface environment on pitting damage distributions and pit-crack transition
(2) Relationship between surface environment and electrochemical processes driving damage
(3) Pitting/SCC initiation propensity versus canister material state (microstructure, surface finish, stress/strain)
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Canister SCC: Important Processes
BeginStorage
Pit Initiation
Crack Initiation
Penetration
Incubation Pit Growth Crack Growth
Storage Time
SNL — Surface Environment, Brine Stability
SNL/OSU — Pitting initiation and growth,Pit-to crack transition (experimental)
CSM/SNL — Pitting initiation and growth (effect of stress)
CSM — Pit-to-Crack Transition (Modeling)
NCSU (SNL) — SCC growth rates
OSU (SNL) — SCC growth rates
SNL/LANL — mockup pit/crack
4
(1) Role of surface environment on pitting damage distributions and pit-crack transition
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Role of Surface Environment on Pitting Damage and Pit-Crack Transition
Knowledge Gaps:• Environmental condition (T, RH, salt load)
affect pit kinetics and pit morphology?• To what extent can we predict crack initiation
based on pit characteristics (shape, size)?
Impact:• Datasets for model development/validation• Relevance and role of pitting stage as function
of environment in SCC
Approach:• Parametric coupon-level pitting experiments in
ISFSI-relevant environments • Constant load marker band SCC tests in same
environments to determine corrosion features that act as crack initiation sites
Freq
uenc
y
Pit Depth
t2 t3
t1
Principle CurvatureMadison, 2014
Environment 1
Micromorphological characterization of pit-crack initiation sites
??Horner, 2011
Horner, 2011
Donahue, 2016
Pit number, size, shape distributions
K ≥ Kth
5May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
High throughput approach for building parametric pitting damage datasets
6
• 1 x 2” coupons loaded with artificial sea salt and exposed to fixed environmental conditions for up to 2 years
• 304H, mirror and 120 grit “mill” finish• 10 and 300 µg/cm2 artificial sea salt Inkjet printing for high-
throughput salt loading
Pit morphology analysis w/ optical profilometry
J. Locke, OSU
10 µg/cm2
300 µg/cm2
Freq
uenc
y
Pit Depth
Environment 1t2 t3
t1
500µm
500µm
55
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
7
1cm
1cm
1cm
1cm
1cm 1cm
1cm 1cm1cm
1cm
6 months
1 month
2 Weeks
1 Week
1cm
1cm
1cm
1cm
1cm
1cm
6 months
1 month
2 Weeks
1 Week
300µg/cm2 salt loading density samples after environmental exposure
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
1
300µg/cm2, 75%RH, 6 MonthsMill Finish
300µg/cm2, 40%RH, 6 MonthsMill Finish
1 2 3 2 31
No Pitting Observed2mm2m
m
10µg/cm2, 75%RH, 6 MonthsMill Finish
Spot 1Spot 1
• Threshold condition used to find pits: Area > 75µm2 , Depth > 8µm• Individual pits manually filtered from damage sites.
Small area scans used to measure pit distributions across surface
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Pit Depth Diameter
Time # of Pits
Avg.(µm)
Max(µm)
Avg.(µm)
Max(µm)
1 Week 4 15 21 24 352 Weeks 12 15 19 23 401 Month 16 15 22 29 526 Month 53 22 44 32 89
Combined pits
Single Pit
Small area scans used to measure pit distributions: 40% RH, 35oC
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Pit Depth Diameter
Time # of Pits Avg.(µm)
Max(µm)
Avg.(µm)
Max(µm)
1 Week 0 < 8 < 8 - -2
Weeks 0 < 8 < 8 - -
1 Month 0 < 8 < 8 - -
6 Month 29 20 41 24 82
Larger area scans needed for a statistically accurate damage
distribution
500µm
Dried Salt
Corroded zone
Small area scans used to measure pit distributions: 75% RH, 35oC
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
50.8mm
25.4
mm
5.7mm
6.4m
m
19mm
7mm 3
1
4
2• Samples split into four sections and
measurements made across each quadrant• Data filtering parameters:
• Depths > 8µm below zero• Areas > 75µm2
• Damage sites are the same depth as pits
19mm
1 quadrant showing output from image analysis software – Negative Control
7mm
DetectionLimit
Negative control output after using thresholding
conditions
Increasing Depth
Currently Developing Large Area Analysis Strategy
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Role of Surface Environment on Pitting Damage and Pit-Crack Transition
Knowledge Gaps:• Environmental condition (T, RH, salt load)
affect pit kinetics and pit morphology?• To what extent can we predict crack initiation
based on pit characteristics (shape, size)?
Impact:• Datasets for model development/validation• Relevance and role of pitting stage as function
of environment in SCC
Approach:• Parametric coupon-level pitting experiments in
ISFSI-relevant environments • Constant load marker band SCC tests in same
environments to determine corrosion features that act as crack initiation sites
Freq
uenc
y
Pit Depth
t2 t3
t1
Principle CurvatureMadison, 2014
Environment 1
Micromorphological characterization of pit-crack initiation sites
??Horner, 2011
Horner, 2011
Donahue, 2016
Pit number, size, shape distributions
K ≥ Kth
12May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Presentation or Meeting Title 13
Understand the pit morphology leading to SCC initiation
Variables:• Atmospheric exposure parameters and salt loading
density • Naturally occurring corrosion morphology
Method: SCC testing• Gauge length of longitudinal tensile bars will be loaded
with salt and corroded in a humidity controlled chamber.• Load salt and corrode side of coupons (red).• Remove from humidity chamber and print salt on face of
coupon (green).• Extra salt on face will contribute electrolyte to the crack tip
during propagation.
• Constant load with intermittent high R ripple fatigue loads during SCC tests to determine corrosion features that act as crack initiation sites.
J. R. Donahue and J. T. Burns, International Journal of Fatigue 91 (2016), 79-99.
Fracture surface of Custom 465-H950
J. Locke, T. Weirich, OSU
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Canister SCC: Important Processes
BeginStorage
Pit Initiation
Crack Initiation
Penetration
Incubation Pit Growth Crack Growth
Storage Time
SNL — Surface Environment, Brine Stability
SNL/OSU — Pitting initiation and growth,Pit-to crack transition (experimental)
CSM/SNL — Pitting initiation and growth (effect of stress)
CSM — Pit-to-Crack Transition (Modeling)
NCSU (SNL) — SCC growth rates
OSU (SNL) — SCC growth rates
SNL/LANL — mockup pit/crack
14
(2) Relationship between surface environment and electrochemical processes driving damage
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
How do Surface Conditions Impact Electrochemical Processes Driving Corrosion/SCC?
Impact:1) Relevance and accessible limits of
existing deterministic damage models 2) Conditions under which initial salt
chemistry, RH and T can be used to predict kinetics and damage
3) Inform laboratory tests- salt loading, time maximum pit size crack growth rate (?)
NaCl on steel
MacDonald, 1991Chen et al. 2008
Schindelholz, 2014
15
Approach:• Define physical/chemical electrolyte
properties during corrosion• Quantify impact on electrochemical
corrosion processes controlling rate
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Prediction of Maximum Pit Size from Brine Characteristics and
Electrochemical Kinetics
16
Assumptions:1. Continuous brine layer2. Hemispherical pit3. Cathodic and anodic kinetics independent of
time (fixed electrolyte conditions)
Max. cathode current
ln 𝐼𝐼𝑐𝑐,𝑚𝑚𝑚𝑚𝑚𝑚 =4𝜋𝜋𝜋𝜋𝑊𝑊𝐿𝐿∆𝐸𝐸𝑚𝑚𝑚𝑚𝑚𝑚
𝐼𝐼𝑐𝑐,𝑚𝑚𝑚𝑚𝑚𝑚+ 𝑙𝑙𝑙𝑙
𝜋𝜋𝜋𝜋𝑟𝑟𝑚𝑚2 ∫𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸𝑐𝑐𝑟𝑟 𝐼𝐼𝑐𝑐 − 𝐼𝐼𝑝𝑝 𝑑𝑑𝐸𝐸
∆𝐸𝐸𝑚𝑚𝑚𝑚𝑚𝑚
Brine conductivity
Brine layer thickness
Cathodic Kinetics
𝐼𝐼𝑚𝑚𝑟𝑟
= 1
𝐼𝐼𝑐𝑐,𝑚𝑚𝑚𝑚𝑚𝑚
Challenge:Information on electrochemical parameters lacking for expected canister brine conditions (electrolyte thickness and chemistry)
Chen, Kelly, 2010
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Measurement of Electrochemical Parameters in Analog Electrolyte Brines as f(T,RH, salt load, chemistry)
17
Rotating disc electrode to simulate brine layer thickness > 1 µm
ln 𝐼𝐼𝑐𝑐,𝑚𝑚𝑚𝑚𝑚𝑚 =4𝜋𝜋𝜋𝜋𝑊𝑊𝐿𝐿∆𝐸𝐸𝑚𝑚𝑚𝑚𝑚𝑚
𝐼𝐼𝑐𝑐,𝑚𝑚𝑚𝑚𝑚𝑚+ 𝑙𝑙𝑙𝑙
𝜋𝜋𝜋𝜋𝑟𝑟𝑚𝑚2 ∫𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸𝑐𝑐𝑟𝑟 𝐼𝐼𝑐𝑐 − 𝐼𝐼𝑝𝑝 𝑑𝑑𝐸𝐸
∆𝐸𝐸𝑚𝑚𝑚𝑚𝑚𝑚0.001
0.01
0.1
1
10
100
1000
20 30 40 50 60 70 80 90 100
Thic
knes
s, µ
m
RH%
25 ºCa.
seasaltg/m2
4.02.01.00.50.20.1
Cl
Thic
knes
s/µm
Cathodic polarizationcurves
RH/%
Conc
entr
atio
n/m
SNL/UVA
Spent Fuel and Waste Science andTechnology
Cracking Susceptibility Based on Maximum Pit Size Predictions
acrit
Example: KISCC = 5 Mpa·m1/2, σ= 500 MPa
Conceptual calculations of hemispherical maximum pit size derived from cathodic kinetics measured in NaCl brines
Need to understand relevant limits of numerous assumptions including:• lab = field Echem parameters?
• Electrolyte and surface attributes are constant or changes due to corrosion or other processes are inconsequential
• Effects of Material features (microstructure) and stress/strain are negligible
18May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Canister SCC: Important Processes
BeginStorage
Pit Initiation
Crack Initiation
Penetration
Incubation Pit Growth Crack Growth
Storage Time
SNL — Surface Environment, Brine Stability
SNL/OSU — Pitting initiation and growth,Pit-to crack transition (experimental)
CSM/SNL — Pitting initiation and growth (effect of stress)
CSM — Pit-to-Crack Transition (Modeling)
NCSU (SNL) — SCC growth rates
OSU (SNL) — SCC growth rates
SNL/LANL — mockup pit/crack
19
(3) Pitting/SCC initiation propensity versus canister material state (microstructure, surface finish, stress/strain)
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
Knowledge Gap: Relationship between canister-relevant material characteristics (microstructure, stress/strain) and relative pit/crack susceptibility
Approach:• Exposing mockup plates loaded with MgCl2 (>
400 µg/cm2 ) to 4 % RH, 80O C• Characterize pit and crack distribution over
course of exposure • Postmortem characterization of pit and crack
geometry in relation to stress and material20
Vibrothermography crack detection method -courtesy M. Remillieux, LANL
cracks 80 kHz
What is pitting/SCC initiation propensity given materials features/state?
250 mmImpact:1) Knowledge of canister-relevant extreme case
corrosion/SCC behavior 2) Inform laboratory experiment design and
extrapolation of lab data to field conditions3) Benchmark, inform SCC models
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
EPRI, 2017fieldlab
Where and when to focus inspection?
How representative are lab conditions?
Environmental control of damage distribution and rates?
What are relevant and accessible model limits?
When and where on the canister and across storage sites do we have greatest risk of
developing cracks?
H2O, O2, CO2, HCl…
Salt loads and distributions, temperature, RH
Surface/atmospheric chemistry, RH variation
Benchmarking datasets, bounding limits, test assumptions, model confidence
21
Variations in canister surface environment, material properties, and stress
May 23, 2018 SFWST 2018
Spent Fuel and Waste Science andTechnology
EXTRAS
Date Presentation or Meeting Title 22
Spent Fuel and Waste Science andTechnology
Canister SCC: Important Processes
BeginStorage
Pit Initiation
Crack Initiation
Penetration
Incubation Pit Growth Crack Growth
Storage Time
SNL — Surface Environment, Brine Stability
SNL/OSU — Pitting initiation and growth,Pit-to crack transition (experimental)
CSM/SNL — Pitting initiation and growth (effect of stress)
CSM — Pit-to-Crack Transition (Modeling)
NCSU (SNL) — SCC growth rates
OSU (SNL) — SCC growth rates
SNL/LANL — mockup pit/crack
23November 15, 2017 EPRI Extended Storage Collaboration Program
What are the physical and chemical properties of electrolyte brines on surfaces as function of time and environmental parameters?
Spent Fuel and Waste Science andTechnology
Approach:• Post-exposure surface analyses of coupons from
pitting experiments:• TOF-SIMS, MicroRaman/FTIR, Auger
Spectroscopy• Cathodic kinetics of 304 in analog surface
chemistries (sea-salt brines, carbonate brines)• Establish variance in max pit size model predictions
due to evolving electrolyte, extend knowledge to CSM SCC electrochemical model
Inkjet printing for high-throughput salt loading
ISFSI-Relevant Conditions Extent of electrolyte coverage
and chemistry distribution
Cathodic and anodic kinetics in analog surface chemistries
TOF-SIMS
Chen and Kelly, 2010
NaOH
NaCl 1 mm
SEM-EDS
Raman
24November 15, 2017 EPRI Extended Storage Collaboration Program
How do Surface Conditions Impact Electrochemical Processes Governing Corrosion/SCC?
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