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Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.11, pp.989-1005, 2011
jmmce.org Printed in the USA. All rights reserved
Corrosion Behaviour of Modified HVOF Sprayed WC Based Cermet
Coatings on Stainless Steel
, B. Hinton
, D. Gerrard
and K. Short
School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box
2476V, Melbourne, Vic. 3001, Australia
Aircraft Corrosion Control Group, Defence Science and Technology Organisation (DSTO),
Melbourne, Lorimer St, Vic 3207, Australia
Australian Nuclear Science Technology Organisation (ANSTO), Lucas Heights, New
Illawarra Rd, NSW, 2234, Australia
Corresponding Author: email@example.com
WC based cermet coatings have been considered as alternative replacements to the more
traditional hard chrome plating for improved surface properties of aircraft landing gear.
While these coatings are used in engineering applications requiring superior hardness and
improved wear resistance, little is known about the corrosion resistance.
In this study, three WC based composite coatings were deposited onto ferritic stainless steel
substrates using high velocity oxy fuel (HVOF) technology. Salt spray testing and
potentiodynamic scanning studies in a saline environment were conducted on the coatings.
Characterisation of the coating structure, composition and morphology, was carried out,
prior to and after corrosion testing, using optical microscopy, scanning electron microscopy
and EDX elemental analysis.
The results showed that poor corrosion performance was exhibited by all three coatings.
This was attributed to the high levels of porosity and the presence of micro-cracks within the
coatings, resulting in attack of the substrate directly by the saline environment and possible
galvanic coupling effects between the substrate and the coating. Dissolution and / or erosion
of specific phases within the coating, resulting in the formation of micro-channels and
increased number / size of voids may have accelerated corrosion of the system.
Key words: HVOF, WC, Structure, Characterisation, Salt Spray Testing, Potentiodynamic
990 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10, No.11
For many years corrosion protection of structural aircraft components such as the landing
gear, has been achieved with the deposition of a hard chrome electroplated coating. Hard
chrome plating provides a combination of good surface hardness, wear resistance and
adequate corrosion resistance. However, the carcinogenic effects of hexavalent chromium
salts associated with the plating process, has stimulated research into the use of “cleaner
technology” methods for coating production, as alternatives to chrome plating. One such
group of coatings of considerable interest are thermally sprayed WC based ceramic / metal
composite coatings. These coatings are selected primarily for applications requiring
enhanced wear resistance and surface hardness, and to a lesser extent, their corrosion
Studies on the behaviour of HVOF deposited WC coatings by Nascimento et al  showed
poor corrosion resistance, results being similar to those obtained for hard chromium
electroplated coatings, even though improved fatigue and abrasive wear characteristics were
observed. However, improved corrosion resistance was observed when metallic and other
binders were incorporated within the WC based matrix. Superior corrosion resistance for
HVOF sprayed WC – Co coatings on steel compared with electroplated hard chrome coatings
after long term atmospheric corrosion testing has been reported [1-3] and further
improvements, when exposed to seawater, were observed by Perry et al  when Cr was
added to the WC - Co based coating’s matrix. Reduced porosity with increasing levels of Co
binder, observed by Saenger et al  may be responsible for increased resistance. Significant
improvement in the corrosion behaviour of HVOF WC-Co coatings was observed after
modifying the coating composition with additions of NiCrAl and / or conducting a post melt
Corrosion studies conducted by Lekatou et al on WC-17Co coatings in both 0.5 M H
and 3.5% aqueous NaCl  revealed that active corrosion processes were induced as a result
of an inhomogeneous binder comprising of a Co(W,C) matrix with varying composition.
Active dissolution of the binder phase (Co) initially occurred during anodic polarisation in
0.5 M H
 In addition, potentiodynamic scans conducted in both solutions revealed
pseudopassivity was observed, which was thought to be associated with oxidation of the W,
Co and C. A comparison of the electrochemical corrosion resistance of HVOF sprayed WC-
17%Co and WC-17%Ni using polarisation and electro-impedance spectroscopy studies 
revealed that the improved corrosion behaviour associated with the WC-17Ni coating was
attributed to a combination of lower porosity of these coatings and better corrosion resistance
of the Ni binder, when deposited on mild steel substrates. Similar observations have been
reported for bulk WC-Ni composites and attributed to the corrosion resistant nature of the Ni
The electrochemical corrosion behaviour of HVOF sprayed Cr
–NiCr and WC–Ni coatings
under different process conditions using potentiodynamic scanning and EIS were studied by
Espallargas et al . The results showed that the Cr
–NiCr coatings showed superior
Vol.10, No.11 Corrosion Behaviour of Modified HVOF 991
corrosion resistance compared with the WC-Ni coatings, making them a good alternative to
conventional hard chrome coatings.
Studies on the effect of post laser treatment on the corrosion properties of HVOF WC based
coatings [12, 13] revealed improved corrosion resistance after laser treatment. Such
improvement was attributed to a number of factors, to include reduction in the size and
number of pores / microcavities formed [12, 13], elimination of the formation of a discrete
splat-structure , the formation of a compact interface  and a reduced microgalvanic
driving force between the WC and the metal matrix .
Differences in the corrosion behaviour of HVOF thermally sprayed WC coatings with a
selection metallic binders were attributed to variations in the galvanic effects induced
between the WC phase and various metallic binder . Here, enhanced corrosion was
associated with binder materials showing more anodic characteristics.
In contrast, electrochemical corrosion tests conducted by Koon et al  revealed that
corrosion rate values were much higher for WC-Co-Cr and WC-CrC-Ni coatings, compared
with hard chrome coated stainless steel samples and stainless steel itself.
The literature has shown that while it is suggested that the use of binder materials with WC
may improve the corrosion resistance, this is dependant on the composition of the binder
material and that the results on their behaviour are conflicting. The purpose of this study was
to systematically investigate the corrosion behaviour of a series of HVOF sprayed WC based
composite coatings deposited on stainless steel, using standard salt spray testing and
electrochemical potentiodynamic scanning studies.
Rectangular plate 409 ferritic stainless steel specimens were sectioned from 2 mm thick
sheet, having dimensions 150 mm X 100 mm. Samples were shot blasted and degreased in
alcohol to remove any surface contaminants, prior to coating. Three modified WC based
cermet coatings were deposited onto the stainless steel plates using commercially available
high velocity oxygen fuel (HVOF) spraying facilities. Coatings, having thicknesses in the
range 250 – 300 µm, were of the type WC-12Ni, WC-20Cr
-7Ni and WC-10Co-4Cr. No
information was available about the specific spraying parameters (distribution and size of
powder feedstock, relative amounts, spraying velocity, etc), due to the proprietary nature of
the commercial process.
The corrosion behaviour of the WC based coatings was determined using salt spray (fog)
testing and electrochemical potential-current density (potentiodynamic scanning)
measurements. All salt spray (fog) tests were carried out in accordance with ASTM B 117
standards . Prior to insertion into the salt spray cabinet, samples were degreased in MEK.
Samples were inclined at a 45º angle, coating side upward, which were exposed to a salt fog
produced from 5% NaCl solution, with pH range from 6.5 to 7.2. The temperature of the
992 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10, No.11
chamber was maintained constant at 35ºC. Exposure times for coated samples ranged from
between 19 hours up to 558 hours. Samples were removed periodically from the chamber for
visual inspection and microscopy analysis.
Potentiodynamic scanning studies on coated and uncoated samples were conducted using a
Potentiostat. A conventional three - electrode cell set up consisting of the working electrode,
a saturated calomel electrode as the reference electrode and a platinum counter electrode was
used. Analysis of the results was achieved using the software Corrview. All tests were
carried out in 0.6 M sodium chloride (NaCl) solution at room temperature (23ºC). Here,
samples were scanned in the range -1200 mV to +200 mV in order to obtain full anodic and
cathodic polarisation curves. The results were presented as polarisation curves in the form E
vs. log I plots. The corrosion potential E
and the corrosion current I
were determined by
the Tafel extrapolation method.
Characterisation of the coatings surface and cross section, prior to and after salt spray testing,
was carried out using optical microscopy and scanning electron microscopy in conjunction
with EDX analysis.
3. RESULTS AND DISCUSSION
Characterisation of the As Sprayed Coatings
Optical micrograph of the WC-12Ni, WC-20Cr
-7Ni and WC-10CoO-4Cr coating in cross
section are shown in Figs. 1 to 3 respectively. The coating thickness measured in this region
varied between 242 and 275 µm for WC-12Ni coating. Similar thicknesses were observed
for the WC-10Co-4Cr coating (typically in the range 261 to 286 µm); however the WC-
-7Ni coatings were much thinner, with coating thickness varying from 128 to 150
Fig. 1 Optical micrograph of WC-12Ni coating cross section
Vol.10, No.11 Corrosion Behaviour of Modified HVOF 993
Fig. 2 Optical micrograph of WC-20Cr
-7Ni coating cross section
Fig. 3 Optical micrograph of WC-10Co-4Cr coating cross section
The micrographs reveal a high level of porosity distributed uniformly across the whole of the
coating cross section. For the WC-12Ni coating, large voids were more evident near the
surface of the coating, though it is apparent that the coatings porosity extends throughout the
entire coating to the substrate. The overall porosity of this coating was approximately 15%,
though in certain areas this appeared to be higher. A lower value of approximately 8% was
observed in the WC-20Cr
-7Ni coating. In particular, extremely high levels of porosity
were noted in the WC-10Co-4Cr coating, the percentage porosity approximating 25% or
The scanning electron micrograph of the WC-12Ni coating surface, as shown in Fig. 4,
confirms the porous nature of these types of coatings. The surface topography is extremely
rough and suggests the presence of an inhomogeneous structure, containing both granular
dispersed and matrix phases separated by regions of high porosity / voids. Similar
topographies were evident on the other coatings. Further, the presence of micro-cracks were
evident within the WC-10Co-4Cr coating (Fig. 5).
994 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10, No.11
Fig. 4 SEI micrograph of WC-12Ni coating surface (X6000 mag)
Fig. 5 SEI micrograph of WC-10Co-4Cr coating surface showing the presence of fine cracks
It is suggested that this type of topography is associated with poor sintering methods during
the HVOF deposition of the coatings, resulting in incomplete melting of the binder material.
Further, the presence of the micro-cracks may have arisen as a result of thermal stresses being
set up during the spraying process. Overheating of the substrate, may have resulted in a
mismatch of thermal expansion between the substrate and the coating, leading to cracking /
inter-laminar debonding of the coating as a result of the residual stress build up, as suggested
by Stokes .
3.2 Salt Spray Test Results
Low powered optical micrographs of the WC-12Ni coating after 4.5 hours, 114 hours and
397 hours exposure in the salt spray cabinet can be seen in Figs. 6 to 8 respectively. In order
to ascribe some semi-quantitative data to these series of tests, the coating was considered to
be no longer functional as a barrier to corrosion at the onset of time to red rust (TTRR), when
corrosion of the substrate occurs through the coating. After 4.5 hours exposure in the salt
spray chamber, samples were already showing evidence of corrosion spots. Three different
Vol.10, No.11 Corrosion Behaviour of Modified HVOF 995
corroded regions were identified on the coating surface, indicating three various stages of
corrosion. Firstly, there was evidence of slight discolouration of the WC-based coatings
(initiation stages). More pronounced discolouration was identified as the light corrosion
stage. Finally, the advanced corrosion stage, as shown in Fig. 6, is characterised by the
formation of golden patina. It is possible that these regions of discolouration are associated
with the formation of nickel oxides for coatings containing nickel and chromium oxides for
coatings containing chromium. It should be emphasised that at this stage of testing, these
spots were associated with discolouration / corrosion of the coating only, with no evidence of
any contribution from the underlying substrate (absence of Fe based species).
Fig. 6 Optical micrograph of WC-12Ni coating after 4.5 hours exposure to the salt spray test
Time to red rust (TTRR) was first observed on the WC-12Ni coating surface after 114 hours
exposure in the salt spray chamber. Here, the red rust erupted from the coating and was
washed down the coated panel by the flow of salt condensation, leading to the formation of a
tail as shown in Fig. 7. This surface red rust became more apparent after 397 hours of
exposure, as observed in Fig. 8.
Fig. 7 Optical micrograph of WC-12Ni coating after 114 hours exposure to the salt spray test
996 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10, No.11
Fig. 8 Optical micrograph of WC-12Ni coating after 397 hours exposure to the salt spray test
In contrast to the WC-12Ni coating, the formation of red rust spots on the surface of the WC-
-7Ni and WC-10Co-4Cr coatings occurred after only 19 hours exposure, as shown in
Figs 9 and 10.
Fig. 9 Optical micrograph of WC-20Cr
-7Ni coating after 19 hours exposure to the salt
Fig. 10 Optical micrograph of WC-10Co-4Cr coating after 19 hours exposure to the salt spray
3.3 Potentiodynamic Scanning Results
A comparison of the I
values for all three coatings compared with the uncoated
stainless substrate are shown in Figs. 11 and 12 respectively.
Vol.10, No.11 Corrosion Behaviour of Modified HVOF 997
Icorr Values for WC Composite Coatings on
Fig. 11 I
values for HVOF WC based composite coatings deposited on stainless steel
Ecorr Values for WC Composite Coatings on
Ecorr (mV) vs SCE
Fig. 12 E
values for HVOF WC based composite coatings deposited on stainless steel
All coatings exhibited much higher I
values (60 µA/cm
, 200 µA/cm
and 30 µA/cm
-7Ni and WC-10Co-4Cr respectively) compared with 0.85 µA/cm
for uncoated stainless steel. This would suggest that WC-20Cr
-7Ni coatings would be
expected to exhibit the highest corrosion rates, followed by WC-12Ni, WC-10Co-4Cr and
finally stainless steel showing the lowest corrosion rate.
Further, all coatings, displayed more negative E
values compared with the stainless steel,
stainless steel having a value of -260 mV, compared with -520 mV, -390 mV and -440 mV
for WC-12Ni, WC-20Cr
-7Ni and WC-10Co-4Cr respectively.
A typical E – log I plot for WC-10Co-4Cr deposited on stainless steel is shown in Fig. 13.
998 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10, No.11
Fig. 13 E – Log I plot for WC-10Co-4Cr deposited on stainless steel
3.4 Structural Characterisation of the Corroded Coatings
A scanning electron micrograph of the corroded WC-12Ni coating surface and corresponding
elemental mapping using EDX analysis are shown in Figs. 14 and 15 respectively. The SEI
image indicates cracking occurring between the matrix and binder particles. The elemental
maps indicate that, within the coating, a significant amount of Ni and Na were present, and to
a lesser extent W, C, Fe, O, Cr and Cl. Within the cracks themselves, reduced amounts of Ni
and Na, and to a lesser extent O and Cl, were observed, in addition to increased amounts of
W. Analysis of EDX spectra taken from both corroded and uncorroded WC-12Ni coatings
confirmed after corrosion, Ni content increased from 14.13 to 26.17 wt% and O content from
0.38 to 23.37 wt%, while a large reduction in the W content from 84.06 to 44.38 wt% was
observed. Further, small amounts of Fe (2.38 wt %), Cr (0.07 wt %), Na (2.78 wt %) and Cl
(0.9 wt %) were identified on the corroded coating surface .
The corroded surface of the WC-20Cr
-7Ni coating, as shown in Fig. 16, shows the
presence of quite large micro-cracks, as previously observed in the WC-12Ni coating. It is
postulated that the accelerated growth of these cracks, initiated during the spraying process,
may have been due to the combined effect of inherent residual stresses and corrosive
Vol.10, No.11 Corrosion Behaviour of Modified HVOF 999
environment, creating a situation analogous to stress corrosion cracking. Further work on
stress analysis needs to be carried out to confirm stress levels within the coating.
Fig. 14 SEI micrograph of corroded WC-12Ni coating (X 20,000 mag)
Fig. 15 EDX elemental maps of corroded WC-12Ni coating in Fig. 14 (X 20,000 mag)
1000 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10,
3.5 Discussion on the Corrosion Behaviour of WC Based Composite Coatings
The results from this study indicate that, overall, the (i) poor performance observed from the
salt spray studies and (ii) higher I
values when compared with the base material from the
electrochemical studies can be largely attributed to the high degree of porosity / presence of
voids and micro-cracks within the coating. It is likely that the coatings contain numerous
networks of continuous voids, pores and cracks that extend from the coating surface through
to the substrate. Such findings are in agreement with previously studies [14, 19].
The occurrence of red rust spots after 114 hours for the WC-12Ni, and 19 hours for the other
coatings, was due to rusting of the stainless steel substrate, due to ingress of the NaCl
solution through channels associated with the porosity and voids within the coating, as
evidenced in Figs. 9 to 12. In addition, the presence of a network of micro-cracks (Fig. 13),
may have also have contributed further to the poor corrosion resistance of the coating as these
micro-cracks may have allowed NaCl to seep in through the coating and attack the stainless
steel substrate. In particular the enlarged micro-cracks observed on the WC-20Cr
coating as shown in Fig. 16 is supported by previous studies (10) which suggested that the
overall corrosion resistance of a WC-CrC-Ni coating was inferior to that of a WC-Co-Cr
coating due to the presence of micro-cracks acting as infiltration paths for the solution. Such
a mechanism may also be responsible for the higher I
value observed for the WC-20Cr
Fig. 16 SEI micrograph of corroded surface of WC-20Cr
-7Ni showing enlarged micro-
The lack of red rust spots on the WC-12Ni coating, prior to 114 hours exposure time was
possible due to the fact that stainless steel initially showed high resistance to corrosion and /
or the coating was acting as a barrier to corrosion. The onset of time to red rust after only 19
hours for the other two coatings can be attributed to similar mechanisms, accelerated times
occurring as a result of the higher levels of porosity for the WC-10CO-4Cr coating and the
-7Ni coating being much thinner. The observed improvement in the WC-12Ni
coating’s resistance to salt spray testing may be due to the presence of the Ni acting as an
inert binder, as observed by Aw et al .
In addition, the increase in corrosion rates (higher I
values) and more negative E
for the coatings, when compared with the stainless steel, can be attributed to the formation of
Vol.10, No.11 Corrosion Behaviour of Modified HVOF
a galvanic cell resulting in accelerated corrosion of the more active components of the
coating system, as observed by Cho et al .
It is further suggested that accelerated corrosion of the substrate via voids and cracks may
occur due to dissolution of certain phases within the coating, resulting in increased formation
of voids and continuous channels through the coating allowing for further ingress of the
The observed significant reduction in the amount of W and increase in the amount of Ni
present in the corroded WC-12Ni coating may be attributed to (i) the dissolution of tungsten
during salt spray testing and / or (ii) the migration of Ni species at the surface after testing.
The significant amounts of both Ni and O within the coating surface, suggest the presence of
NiO as a corrosion product. Appreciable levels of Na and to a lesser extent, Cl, confirm the
presence of salt residue from the testing. The presence of Fe and Cr species at the surface
cannot rule out selective leaching of these species from the base material. Increased levels of
W within the cracks, compared to the coating, may give support to the theory of Ni products
forming on the surface, thus masking out the W, as opposed to W dissolution. Further
characterisation using XRD and other techniques is recommended to confirm the above
Metallographic cross sections of the corroded WC-10Co-4Cr coating after exposure to salt
spray testing are shown in Figs. 17 and 18. It is evident that the increased presence of
continuous micro-channels (Fig. 17) and large voids (Fig. 18) occurred during their exposure
to the salt spray. Here, the formation of micro-channels and voids can be attributed to both
mechanical and chemical effects, whereby (i) dissolution of soluble phases within the coating
and / or (ii) erosion of certain phases within the coating may have occurred.
Fig. 17 Optical micrograph of corroded WC-10Co-4Cr coating cross section showing the
presence of micro channels
1002 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10,
Fig. 18 Optical micrograph of corroded WC-10Co-4Cr coating cross section showing large
The corroded surface of the WC-10Co-4Cr coating, as shown in Fig. 19, consists of the WC
matrix, Co and Cr binders, salt deposits and oxide corrosion products. EDX analysis of the
surface revealed an increase in the concentration of Cr (2.9 to 4.0 wt%) and Co (9.4 to 18.7
wt%) and a decrease in the W species (87.3 to 70.2 wt%), when compared with the as
sprayed coating. Small amounts of Na and Cl (< 1 wt%) was also recorded. This would
suggest, as for the WC-12Ni coating, dissolution of the WC phase, the formation of Co and
Cr oxides and deposits of salt on the surface has occurred after exposure to the salt spray.
While free energy data predicts the formation of CoO and Cr
and possibly Co(OH)
being the most energetically favoured (having free energies of -212.3, -252.9 and -105.6
kcal/mol respectively), the formation of soluble chloride species, such as CrCl
cannot be discounted as they have negative free energy values of -85.09, -116.18 and -
64.48 kcal/mol respectively.
Fig. 19 SEI micrograph of corroded surface of WC-10CO-4Cr
While it has been postulated that increased corrosion may occur due to dissolution and / or
erosion of phases within the coating, Warsono  observed a reduction in the corrosion
activities at the sites where the initial formation of red rust spots was observed, resulting in
fewer observed changes in the surface appearance with increased exposure time. It is
suggested that here, corrosion was reduced due to (i) the build up of corrosion products
within the pores and (ii) the corrosion products associated with the stainless steel acting as a
protective layer, thus forming a barrier between the salt solution and the stainless steel
substrate. Further, the formation of nickel oxide, as evidenced from EDX analysis, revealing
significantly higher levels of O and Ni compared with other regions, may have provided
Vol.10, No.11 Corrosion Behaviour of Modified HVOF
additional protection. It has been suggested  that a nickel oxide layer can behave as a
passive layer, thus protecting the coating from further corrosion.
The results of the study here are similar to observations by Nascimento et al , for HVOF
deposited WC coatings on AISI 4340 high strength steel, who suggested the lack of
protection can be attributed to the high content of pores. However, it should be emphasised
that the WC-12Ni coating in the current study had not completely corroded after 558 hours
exposure, while the WC coating was completely corroded after only 72 hours exposure .
This may be attributed to the fact that the high strength steel was less likely to be corrosion
resistant compared to the stainless steel.
1. Three WC based composite coatings (WC-12Ni, WC-20Cr
-7Ni and WC-10Co-4Cr)
were deposited on 409 ferritic stainless steel substrates using high velocity oxy fuel (HVOF)
2. Salt spray testing revealed extensive corrosion (time to red rust) was observed after 114
hours for the WC-12Ni coating and only 19 hours for both the WC-20Cr
-7Ni and WC-
3. The poor corrosion performance was attributed to the high levels of porosity and the
presence of micro-cracks within the coatings, resulting in attack of the substrate directly by
the saline environment. This was possibly due to incomplete sintering between the WC
matrix and the binders during the spraying process.
4. Potentiodynamic scanning studies revealed poor corrosion performance of the coatings
when compared with the stainless steel substrate, with increased I
values and more
values. This was attributed to the poor structure and possible galvanic coupling
effects between the substrate and the coating.
5. Corrosion of the coating substrate system may have been accelerated by dissolution and /
or erosion of specific phases within the coating, resulting in the formation of micro-channels
and increased number / size of voids.
1. To deposit coatings with greater variation in the spraying variables in order to
systematically study the effect of process parameters on the corrosion behaviour and optimise
the system in terms of reduced porosity / enhanced coating integrity
2. To expand the range of corrosion tests to include techniques such as ac impedance in order
to further characterise and optimise the coating system
1004 L.P. Ward, B. Hinton, D. Gerrard and K. Short Vol.10,
3. To conduct post deposition treatments (annealing, laser treatment) in order to reduce
4. To employ a greater range of characterisation techniques, such as XPS, in order to provide
a deeper understanding into the corrosion / active dissolution mechanisms occurring
This work was carried out as part of a research collaborative program between RMIT
University and Defence, Science and Technology Organisation (DSTO). The authors would
like to acknowledge G. Biddle from the Surface Materials Analysis Research & Training
Group, Department of Applied Physics RMIT University for conducting the SEM and EDX
analysis and F. Sciacca, T. Firouzian and R. Warsono for their contributions to the corrosion
studies as part of their undergraduate final year projects. Additionally, the authors would like
to acknowledge support provided for part of this overall program by the Australian Institute
of Nuclear Science and Engineering (AINSE - grant no. AINGRA04180P).
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