high resolution characterization of corrosion and hydrogen pickup … · 2016. 7. 22. · •...
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Department or office title: change on Slide Master
High resolution characterization of corrosion and hydrogen pickup of Zr-Nb cladding alloys Jing Hu, Brian Setiadinata, Thomas Aarholt, Alistair Garner, Arantxa Vilalta-Clemente, Jonna Partezana, Philipp Frankel, Paul Bagot, Sergio Lozano-Perez, Angus Wilkinson, Michael Preuss, Michael Moody, Chris Grovenor • Department of Materials, University of Oxford, Parks Road, Oxford, UK • School of Materials, University of Manchester, Manchester, UK • Westinghouse Electric Company, 1332 Beulah Road, Pittsburgh, USA
MUZIC-2 (Mechanistic Understanding of Zirconium Corrosion and
Hydrogen Pickup) consortium
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Work on oxidation of Zr started in Oxford in 2007 with UK research council funding (MUZIC-1)
• In phase 1 we asked ourselves a set of questions
• Can Atom Probe tomography be applied to studying oxidation mechanisms?
• Can new techniques developed in electron microscopy give new insight?
• Are there completely new experimental approaches that are worth studying?
A brief introduction
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500 nm
oxide metal
Local Electrode Atom Probe
Datasets of
100s of
millions of
atoms
First APT analysis of oxidised Zr samples (Dan Hudson) -shows 3D shape of interface as well as identifying a ZrO stoichiometry
Zr (green),
ZrO (blue),
O2 (orange)
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3D visualisation by FIB sectioning (Na Ni)
FIB 3D imaging [1]
Several hundred
individual images 20
by 5 microns
combined to make an
image of the cracks
and of the
metal/oxide interface
morphology
[1] Ni ect. Corr Sci (2011)
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Interconnected porosity (Na Ni)
50 nm
Use Fresnel Imaging technique in TEM to visualise porosity evolution in Zr oxide
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Cartoon of the oxide structure
Pores at the
monoclinic ZrO2 grain
boundaries gradually
reach the metal/oxide
interface
20 nm
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Where is the protective oxide?
From these
observations, we
proposed that the
protective oxide
identified by
electrochemical
methods as the
region under the
interconnected
porosity [2]
[2] Na Ni Dphil thesis (2011)
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Can correlate specific oxidation
events with local microstructure
NanoSIMS observation of 18O/16O ratio [3]
High
resolution
SEM images
shows a
vertical crack
propagates
far into the
oxide
200 nm
SIMS imaging of 18O spiking
[3] Yardley, Ni ect. JNM (2013)
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Phase 2: Ab initio modelling of ZrO phases
Identifies a stable hexagonal ZrO phase [4]
[4]Nicholls et al 2014 Advanced Engineering Materials
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Phase 3: new questions
Since 2012 [MUZIC2] we have been asking different questions
• Is the ZrO phase this predicted hexagonal phase (and why do we care about ZrO)?
• What is the role of SPPs?
• Can we directly study H transport mechanisms in the oxide?
• Are there any even newer techniques that will give us new information?
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Why study RXA Zr-1.0Nb?
RXA Zr-1.0Nb (Zr-1%Nb-0.01%Sn-0.1%Fe)
• RXA Zr-1.0Nb showed delayed transition than ZIRLO [5]
• Hydrogen pickup fraction (HPUF): 12% RXA Zr-1.0Nb vs 16% ZIRLO [6]
(Zr-1%Nb-1%Sn-0.1%Fe)
Transition
~140 day ~360 day
[5] Wei, J. et. al. (2012). Corrosion Engineering, Science and Technology. [6] Romero, J. et. al (2015). TopFuel 2015
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Why study RXA Zr-1.0Nb? – Even better neutron irradiation performance
~ 5.2 μm autoclave
~ 2.2 μm reactor
RXA Zr-1.0Nb in autoclave
RXA Zr-1.0Nb in reactor
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Why study annealed Zr-1.0Nb? – Heat treatment can change the corrosion and HPU
Recrystallised
Annealed
• Almost four time corrosion rate after the 720°C heat treatment. [5]
• Hydrogen pickup fraction (HPUF): 12% RXA Zr-1.0Nb vs 9% Annealed Zr-1.0Nb [6]
SPP: β-Nb →β-Zr
[5] Wei, J. et. al. (2012). Corrosion Engineering, Science and Technology. [6] Romero, J. et. al (2015). TopFuel 2015
Recrystallised X2 RXA Zr-1.0Nb in autoclave
RXA Zr-1.0Nb in reactor
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Sample Overview
Oxide
Metal
Pt protective layer
120-day H2O Early-trans
225-day (180-day H2O +45-day D2O) Mid-trans
360-day H2O Transition
3-day H2O Early
585-day (540-day H2O +45-day D2O) Post second-trans
540-day In reactor
1 um
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Oxide microstructure by TKD
RXA Zr-1.0Nb 360-day transition
• Equiaxed-columnar-equiaxed grain structure • Very organised microstructure [7]
2um Metal
Oxide
[7] Hu, J. Garner, A. et al. Micron 69, 35–42 (2015).
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Oxide microstructure by TKD
2um
360-day transition
585-day Post-transition
RXA Zr-1.0Nb neutron irradiated 540-day
• Equiaxed-columnar-equiaxed grain structure • Very organised microstructure • Fewer cracks • Longer columnar grains
Hu, J. et al. Micron 69, 35–42 (2015). Garner, A., Hu, J. et al, Acta Materialia, 99, 2015
RXA Zr-1.0Nb in autoclave
Wider, shorter grains Very few cracks
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ASTAR oxide phase analysis
120-day RXA Zr-1.0Nb 46-day annealed Zr-1.0Nb
• More tetragonal phase throughout the oxide ( especially far from crack)
• Non-uniform suboxide distribution, Left-thicker suboxide, right ( 2nd tran)- thinner. [9]
• Mostly monoclinc, 3% tetragonal phase scattered, mostly at M/O
• Saw-tooth suboxide along M/O
[4] Nicholls, R. J. et al. Adv. Eng. Mater. (2014).
[8] B. Puchala and A. Van der Ven (2013)
[9] Hu ect. TopFuel 2015
Hexagonal ZrO phase with P-62m symmetry
lattice parameters a=5.31 Å and c=3.20 Å[4,8,9]
2nd transition cracks
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• EELS mapping and TKD tells us that there are 2 kinds of Zr-O regions at the metal/oxide interface:
1) Hexagonal ZrO and 2) oxygen-saturated zirconium metal • Their thicknesses vary enormously with position and stage of
oxidation, but the combined region forms an uniform protective layer
Low loss EELS mapping +MLLS fitting*
*Electron Energy Loss Spectroscopy (EELS) + Multiple Linear Least Squares (MLLS) fitting
RXA Zr-1.0Nb 360-day transition
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Zr-O layer across M/O in RXA Zr-1.0Nb
This is not one line profile, this is hundreds of line profile across the mapping area of 10 um width.
• Saw-tooth shape suboxide + oxygen saturated Zr form an uniform protective layer
• Combined layer undergoes cycle growth, drops at transition
Transition
Exposure (days)
Thickness (um)
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Correlating Zr-O with instantaneous oxidation rate
ZIRLO
ZIRLO
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• SPPs are too far apart to directly influence transport processes
3D FIB
reconstruction
What is the role of SPPs?
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• Widely reported that SPPs gradually become amorphous and dissolve in the growing oxide
• This must dope the oxide locally with Fe2+, Cr3+ Nb5+
[10] Anada, H., Herb, B. J., Nomoto, K., Hagi, S., Graham, R. A., Kuroda, T. (1996)
Zirconium in the Nuclear Industry: Eleventh International Symposium, ASTM STP 1295, American Society for Testing Materials p. 74-93.
Page 37
surrounding oxide. It is generally agreed that SPP in the oxide film readily oxidise [70]. This
process is illustrated in Figure 13. Metallic precipitates do not oxidise until they are completely
surrounded by the oxide [115], before this their metallic character is retained within the oxide
[116]. EDX showed that Ni and Cr also diffused away from precipitates in the oxide, although at
a lesser rate than Fe [89]. Anada et al. [117] reported that a transformation from columnar grains
to equiaxed grains was observed predominantly around the oxidised precipitates and that
oxidation of the precipitates was the cause of an accelerated corrosion.
Figure 13: Schematic of the effect of iron diffusion from intermetallics precipitates in the oxide
film on Zircaloy-4 [117].
What is the role of SPPs?
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SPP analysis from Atom Probe Tomography
Zr metal
Suboxide 100 nm
Nb Zr
Fe
ZrO
• dislocations at suboxide/metal interface.
decorated with Fe atoms • β-Nb particles: Containing ≈ 85at% Nb and ≈13at% Zr at the core. Fe segregates to interface. This Fe segregation is rapidly lost to the oxide as the SPPs oxidise
Line profile through SPP-metal interface
(Brian Setiadinata)
[9] Hu ect. TopFuel 2015
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Beta-Nb (smaller) + Zr-Fe-Nb SPP (larger) Both rather stable in oxide
RXA Zr-1.0Nb in autoclave RXA Zr-1.0Nb in reactor
Beta-Nb + Zr-Fe-Nb SPP Fe dissolves under irradiation Cr dissolves slower j
EDX mapping on SPPs in RXA Zr-1.0Nb
Hu ect. TopFuel 2015
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EELS study of SPP oxidation state
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EELS study on SPP oxidation state
XANES data also shows a similar trend. SAMAKOTO et al Topfuel (2012)
Gradual oxidation of Nb from 2+ to 5+ from M/O to top surface
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Where should we worry about doping?
Protective Oxide
Both Nb and Fe released from the SPPs may contribute to doping the oxide or reducing space charge build up
H+
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2H-
3D SIMS profiling of deuterium
Depth profile of top 1.2 um of 1.7 um oxide layer Deuterium segregation to horizontal cracks and to linear features through the oxide thickness?
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APT of deuterium at oxide grain boundaries
D+ ions OD+ ions Grain boundary map
Similar observations in: Sundell, G et al (2015) Direct observation of hydrogen and deuterium in oxide grain boundaries in corroded Zirconium alloys. Corrosion Science 90:1-4.
50 nm
Zr 2.5% Nb CANDU sample (provided though MUZIC2 project)
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Porosity study: Fresnel imaging
RXA Zr-1.0Nb in
autoclave 225-day
100nm from the metal-
oxide interface:
a) interconnected pores parallel to M/O
b) vertically interconnected pores along the columnar oxide grain boundaries
a) b)
Underfocus:
Overfocus:
Oxide growth direction
[9] Hu ect. TopFuel 2015
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Porosity in ex-reactor and autoclave
Oxide growth direction
RXA Zr-1.0Nb in reactor
• Very little porosity in reactor vs in autoclave
Annealed Zr-1.0Nb in autoclave • Lots of interconnected porosity
along grain boundaries vs RXA Zr-1.0Nb in autoclave and in reactor
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Conclusions
Equiaxed oxide
Columnar oxide
Suboxide
Metal
RXA Zr-1.0Nb with better oxidation resistance and lower HPU. Compared with ZIRLO and annealed Zr-1.0Nb, it has: • Fewer cracks, more organised and longer
columnar grain structure • Porosity along equiaxed and columnar
GBs. Porosity content is much lower in neutron irradiated RXA Zr-1.0Nb.
• Two types of SPPs, β-Nb and Zr-Nb-Fe and gradual oxidation of Nb from 2+ to 5+ from M/O to top surface.
• Thicker Zr-O region when oxidation rate is slower
• New experimental techniques can help understand the oxidation and HPUF performance of Zr fuel clad alloys.