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

*jing.hu@materials.ox.ac.uk jing.cynthia.hu@gmail.com

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

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)

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)

Interconnected porosity (Na Ni)

50 nm

Use Fresnel Imaging technique in TEM to visualise porosity evolution in Zr oxide

Cartoon of the oxide structure

Pores at the

monoclinic ZrO2 grain

boundaries gradually

reach the metal/oxide

interface

20 nm

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)

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)

Phase 2: Ab initio modelling of ZrO phases

Identifies a stable hexagonal ZrO phase [4]

[4]Nicholls et al 2014 Advanced Engineering Materials

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?

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

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

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

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

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).

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

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

• 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

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)

Correlating Zr-O with instantaneous oxidation rate

ZIRLO

ZIRLO

• SPPs are too far apart to directly influence transport processes

3D FIB

reconstruction

What is the role of SPPs?

• 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?

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

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

EELS study of SPP oxidation state

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

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+

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?

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)

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

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

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.