Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt
The corrosion of nickel-molybdenum alloys and their constituents in a molten eutectic sodium fluoride-zirconium fluoride mixture has been studied by cyclic voltammetry, X-ray analysis, SEM and metallography. The dependence of the corrosion rate of nickel-molybdenum alloys in molten fluorides on the...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2005
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| Цитувати: | Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt / V.M. Azhazha, А.А. Аndriiko, А.S. Bakai,S.V. Volkov, S.V. Devyatkin, А.N. Dovbnya, S.D. Lavrinenko, А.А. Omelchuk, B.M. Shirokov, R.S. Shmegera // Вопросы атомной науки и техники. — 2005. — № 4. — С. 67-73. — Бібліогр.: 7 назв. — англ. |
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irk-123456789-805462016-04-14T10:19:59Z Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt Azhazha, V.M. Andriiko, A.A. Bakai, A.S. Volkov, S.V. Devyatkin, S.V. Dovbnya, A.N. Lavrinenko, S.D. Omelchuk, A.A. Shirokov, B.M. Shmegera, R.S. The corrosion of nickel-molybdenum alloys and their constituents in a molten eutectic sodium fluoride-zirconium fluoride mixture has been studied by cyclic voltammetry, X-ray analysis, SEM and metallography. The dependence of the corrosion rate of nickel-molybdenum alloys in molten fluorides on the time of soaking in melt and irradiation with an electron beam on an electron accelerator has been investigated. It has been shown that increasing the time of contact of nickel-molybdenum alloys with fluoride melt decreases the corrosion current density, and irradiation increases it in the greater extent the higher the electron beam energy. Intercrystalline corrosion is typical of the alloys of this composition. Методами циклічної вольтамперометрії, рентгенофазового, мікроскопічного аналізів, а також металографії досліджена корозія нікель-молібденових сплавів та компонентів, які входять до їхнього складу, в розплавленій евтектичній суміші фторидів натрію та цирконію. Досліджена залежність швидкості корозії зазначених сплавів від тривалості витримки в розплаві та опромінення пучком електронів на електронному прискорювачі. Показано, що збільшення тривалості контакту нікель-молібденових сплавів з фторидним розплавом зменшує густину струму корозії, а опромінення – збільшує її в тим більшій мірі, чим більша енергія електронного пучка. Для сплавів даного складу характерна міжкристалітна корозія. Методами циклической вольтамперометрии, рентгенофазового, микроскопического анализов, а также металлографии изучена коррозия никель-молибденовых сплавов и составляющих компонентов в расплавленной эвтектической смеси фторидов натрия и циркония. Исследована зависимость скорости коррозии никель-молибденовых сплавов в расплавленных фторидах от времени выдержки в расплаве и облучения пучком электронов на электронном ускорителе. Показано, что увеличение времени контакта никель-молибденовых сплавов с фторидным расплавом уменьшает плотность тока коррозии, а облучение – увеличивает ее в тем большей мере, чем больше энергия электронного пучка. Для сплавов данного состава характерна межкристаллитная коррозия. 2005 Article Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt / V.M. Azhazha, А.А. Аndriiko, А.S. Bakai,S.V. Volkov, S.V. Devyatkin, А.N. Dovbnya, S.D. Lavrinenko, А.А. Omelchuk, B.M. Shirokov, R.S. Shmegera // Вопросы атомной науки и техники. — 2005. — № 4. — С. 67-73. — Бібліогр.: 7 назв. — англ. 1562-6016 PACS 61.80-X; 81-40-WX http://dspace.nbuv.gov.ua/handle/123456789/80546 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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The corrosion of nickel-molybdenum alloys and their constituents in a molten eutectic sodium fluoride-zirconium fluoride mixture has been studied by cyclic voltammetry, X-ray analysis, SEM and metallography. The dependence of the corrosion rate of nickel-molybdenum alloys in molten fluorides on the time of soaking in melt and irradiation with an electron beam on an electron accelerator has been investigated. It has been shown that increasing the time of contact of nickel-molybdenum alloys with fluoride melt decreases the corrosion current density, and irradiation increases it in the greater extent the higher the electron beam energy. Intercrystalline corrosion is typical of the alloys of this composition. |
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Article |
| author |
Azhazha, V.M. Andriiko, A.A. Bakai, A.S. Volkov, S.V. Devyatkin, S.V. Dovbnya, A.N. Lavrinenko, S.D. Omelchuk, A.A. Shirokov, B.M. Shmegera, R.S. |
| spellingShingle |
Azhazha, V.M. Andriiko, A.A. Bakai, A.S. Volkov, S.V. Devyatkin, S.V. Dovbnya, A.N. Lavrinenko, S.D. Omelchuk, A.A. Shirokov, B.M. Shmegera, R.S. Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt Вопросы атомной науки и техники |
| author_facet |
Azhazha, V.M. Andriiko, A.A. Bakai, A.S. Volkov, S.V. Devyatkin, S.V. Dovbnya, A.N. Lavrinenko, S.D. Omelchuk, A.A. Shirokov, B.M. Shmegera, R.S. |
| author_sort |
Azhazha, V.M. |
| title |
Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt |
| title_short |
Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt |
| title_full |
Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt |
| title_fullStr |
Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt |
| title_full_unstemmed |
Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt |
| title_sort |
corrosion of irradiated ni-mo alloys in sodium fluoride - zirconium melt |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2005 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/80546 |
| citation_txt |
Corrosion of irradiated Ni-Mo alloys in sodium fluoride - zirconium melt / V.M. Azhazha, А.А. Аndriiko, А.S. Bakai,S.V. Volkov, S.V. Devyatkin, А.N. Dovbnya, S.D. Lavrinenko, А.А. Omelchuk, B.M. Shirokov, R.S. Shmegera // Вопросы атомной науки и техники. — 2005. — № 4. — С. 67-73. — Бібліогр.: 7 назв. — англ. |
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Вопросы атомной науки и техники |
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2025-07-06T04:33:16Z |
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2025-07-06T04:33:16Z |
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| fulltext |
PACS 61.80-X; 81-40-WX
CORROSION OF IRRADIATED Ni-Mo ALLOYS IN SODIUM FLUO
RIDE - ZIRCONIUM FLUORIDE MELT
V.M. Azhazha1, А.А. Аndriiko2, А.S. Bakai1,S.V. Volkov3, S.V. Devyatkin3,
А.N. Dovbnya1, S.D. Lavrinenko1, А.А. Omelchuk3, B.M. Shirokov1, R.S. Shmegera 3
1National Science Center Kharkov Institute of Physics and Technology, Akademicheskaya
str. 1, 61106 Kharkov, Ukraine, e-mail: bakai@kipt.kharkov.ua;
2National Technical University “KPI”, Department of Chemistry, Pobedy avenue 37, 03056
Kiev-56, Ukraine, e-mail: andriiko@xtf.ntu-kpi.kiev.ua;
3Vernadskii Institute of General and Inorganic Chemistry, Ukrainian National Academy
of Science, Palladin avenue 32/34, 03680 Kiev 142,Ukraine, e-mail: devyatkin@ion
c.kar.net, omelchuk@ionc.kar.net
The corrosion of nickel-molybdenum alloys and their constituents in a molten eutectic sodium fluoride-zirconi
um fluoride mixture has been studied by cyclic voltammetry, X-ray analysis, SEM and metallography. The depen
dence of the corrosion rate of nickel-molybdenum alloys in molten fluorides on the time of soaking in melt and irra
diation with an electron beam on an electron accelerator has been investigated. It has been shown that increasing the
time of contact of nickel-molybdenum alloys with fluoride melt decreases the corrosion current density, and irradia
tion increases it in the greater extent the higher the electron beam energy. Intercrystalline corrosion is typical of the
alloys of this composition.
INTRODUCTION
Nickel- and molybdenum- base alloys, which are
known as Hastelloy (commercial name), have been
widely used in various branches of science and technol
ogy, in particular in accelerator-driven nuclear reactors,
which operate on molten salt fuel compositions [1]. The
development of methods for the control of the corrosion
resistance of these alloys to fluoride melts and the eluci
dation of the effect of different factors (radiation expo
sure, alloy composition, time of contact with molten
salts, etc) on corrosion rate are important not only scien
tific but also practical tasks since they make it possible
to determine the critical service life of structural materi
als.
This paper presents results of determining the corro
sion rate of nickel-molybdenum alloys by cyclic
voltammetry in a molten eutectic sodium fluoride-zirco
nium fluoride mixture. This mixture is resistant to ra
dioactive irradiation and is recommended as a carrier of
isotopes that are to be transmuted in accelerator-driven
nuclear reactors. The corrosion resistance of nickel-
molybdenum alloys of different composition at different
time of contact with fluoride melt under different elec
tron beam irradiation conditions was investigated. The
results of estimating the corrosion rate of individual al
loy constituents (nickel, molybdenum, iron, niobium,
chromium) and the change in the mechanical properties
of the alloys under investigation, which has been carried
out by nanoindentation analysis, are given [2].
EXPERIMENTAL
Two alloys of Hastelloy-N type of the following
composition (Table 1) were prepared and investigated.
Table 1
Chemical composition of nickel-molybdenum alloys
Alloy Chemical composition, wt.%
Ni Mo Cr Fe Ti Al Other elements
composition
A base 11…12 6.5…7.5 ≤1.5 ≤ 0.5 ≤ 0.8 Mn<0.5; Si<0.15;
composition B base 11…12 6.2…7.2 ≤1.5 ≤ 0.5 ≤ 0.8 Mn<0.5; Si<0.15; Nb-0.5;
Y-0.05
The difference in the chemical composition of these
alloys was that in the alloys of composition A, 0.3 wt. %
chromium was replaced by niobium, and 0.05 wt. % yt
trium was added (composition B).
To fabricate the alloys of the chosen composition,
the high-purity initial components were used with a low
content of interstitial impurities. The basic components
of the alloys were nickel, molybdenum chromium, iron,
titanium, niobium, manganese that were refined from
undesirable impurities, using the physical techniques.
Since those metals have very different properties, it
is utterly impossible to remove impurities from them,
using only one technique. Refining of nickel, molybde
num, niobium, titanium and iron warranted employment
of the electron-beam melting (EBM) technique. Refin
ing of chromium and aluminum made for high-vacuum
________________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 67-73.
67
mailto:bakai@kipt.kharkov.ua
annealing. Refining of manganese was made by the vac
uum distillation method. The alloy metal component re
fining resulted in the following information.
Nickel. The EBM was made in the ultra-high-vacu
um facility “UPM-1” [3,4]. The vacuum system of the
facility is composed two diffusion vapor oil pumps with
the nitrogen protection and two getter ion pumps “GIN-
5”. The boundary vacuum of the facility is 10-6 Pa, the
vacuum maintained within the limits of 10-2…10-5 Pa in
the course of the casting. The casting was made via the
classic alloy dot-remelting method in the following
way: heating>melting>casting> crystallization. Elec
trolytic nickel was used as initial material that called for
double EBM. Schematic of the EBM facility is given in
Fig. 1.
In the course of the EBM, the process began of gas
release from nickel at different stages of the casting. To
monitor the process, the mass-spectrometer “МХ-
7304А” was used connected to the facility through a de
vice named “Dilutor” [6]. The element composition of
the processed nickel and metal upon completion of the
EBM was measured, using “ЕhMAL-2” energy mass-
analyzer.
The EBM refinement resulted in production of high-
purity nickel samples. The content of the main impuri
ties in nickel samples prior to and after the EBM is giv
en in Table 2.
1. Fig. 1. Schematic of EBM facility, using the
oil-free pumping system: 1 – facility frame;
2 – electron beam gun; 3 – bending magnet
system; 4 – remeltable electrode; 5 – ingot;
6 – crystallizer; 7, 8 – electrode positioning
mechanism; 9,10 – electrode withdrawal
mechanism; 11 – getter ion pump; 12 – dif
fusion pump; 13 – pre-evacuation pump;
14-20 – vacuum gates;
21 – manometric gauges
Table 2
Content of impurities in initial and post-EBM nickel samples
Impurity, wt.% Fe Co Si Cu As Sb P Bi
Initial 0,002 0,0026 0,00003 0,0017 0,00004 0,00003 0,0001 0,00004
Post 2nd EBM 0,0017 0,0009 0,00003 0,0017 0,00001 0,00003 0,00007 0,00004
Impurity, wt.% Zn Sn Al Pb Cd Mg Se Cl
Initial 0,0041 0,00005 0,00009 0,00007 0,00005 0,00005 0,0014 0,0005
Post 2nd EBM 0,0008 0,00005 0,00006 0,00007 0,00005 0,00004 0,00027 0,0002
As a result of the refinement, there was a decrease in
the content of iron, cobalt, phosphorus, aluminum, mag
nesium; the content of arsenic, zinc, selenium and chlo
rine decreased considerably. The double EBM resulted
in production of nickel with the purity 99.994 wt. % [4].
The examination of nickel microstructure upon com
pletion of the EBM indicated that it differed substantial
ly from structure of the initial metal. An agglomeration
of impurities was observable in nickel after the first re-
melting on the grain boundaries that was absent after the
second EBM re-melting, which bespeaks the effective
ness of nickel refinement, using the EBM.
Molybdenum. After the EBM, the content of metal
impurities decreased 10-to-30-fold. The removal of sili
con was insignificant. Tungsten impurities were not re
movable. The principal purification was made of the
gaseous impurities: oxygen, nitrogen and hydrogen.
Niobium. The initial material for the casting was
niobium of the brand “NB-1”. The content of metal im
purities in niobium after two consecutive EBMs was as
follows: Al – 0,004; Fe – 0,0001; Cr < 0,001; Ni <
0,0004; Si – 0,005; Cu – 0,0006; Ca < 0,003 wt. %.
Titanium. The initial material for the casting was ti
tanium sponge “ТG-90”. A titanium ingot with the puri
ty 99.99 wt.% was produced via the EBM method.
Iron. Armco iron rods were used as initial material
for the casting. The EBM of iron was carried out via the
alloy dot-remelting. The Brinelle hardness of initial
Armco iron rods was 830 MPa, decreasing to 624 MPa
after the remelting. The iron purity level was deter
mined to a considerable degree by the content of nickel
and cobalt.
Chromium. The main impurities in chromium were
iron, silicon, aluminum, nickel and interstitial impurities
________________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 67-73.
68
(nitrogen, oxygen and carbon). High-vacuum annealing
of chromium samples at the temperature 1200 °С for 5
hours resulted in a decreased content of the interstitial
impurities, probably, by 10 times.
In order to introduce the volatile components into
the alloy, aluminum and manganese, in a more assured
way, the arc casting facility was used priorly to obtain
manganese-aluminum alloys.
The voltammetric investigations were carried out in
hermetically sealed reactors made of stainless steel un
der dry argon in a three-electrode electrolytic cell by
cyclic voltammetry on a PC controlled potentiostat
“Elektroflex” type EF 453. A ZrF4(49.5 mol. %)-NaF
(50.5 mol. %) melt was used as the electrolyte. The melt
was placed in a glassy carbon crucible, which was used
as the auxiliary electrode. The working electrodes were
samples of Hastelloy (compositions (A and B) heat-
treated under different conditions in a molten NaF–ZrF4
at 650 °C (100…700 hours) and the alloys constituents
(Ni, Fe, Mo, Cr, Nb). The potential sweep rate at the
working electrode was 10 mV/s. The working electrode
potential was recorded with respect to an unpolarized
glassy carbon electrode.
Samples of alloys which were subjected in a melt of
the above composition to a long (700 hours) irradiation
at 650 °C on a LUE-10 (LINAC-10) linear electron ac
celerator with 10 MeV energy and 5 kW power (current
density 0.5 mA/cm2) were also investigated.
To assess the effect of electron irradiance on nickel-
molybdenum alloys, 6 specimens of alloy of preset
composition were put in each ampule made of a carbon-
carbon composite filled with a molten eutectic sodium
fluoride- zirconium fluoride mixture. The electron beam
energy at the entrance to the first sample was estimated
to be 5066 eV/atom and at the exit from the last sample
64 eV/atom [5]. Corrosion current at different surfaces
of the same sample (electron beam entry-exit) were also
estimated. To this end, one of the surfaces of the alloy
under investigation was covered with boron nitride
paste.
The mechanical investigations were carried out by
the nanoindentation method on a Nano Indenter – II de
vice (MTS System) [2].
RESULTS AND DISCUSSION
The corrosion rate is determined by metal (Mi) an
odic dissolution current:
Mi
0 → Mi n+ + ne- (1)
and can be calculated if the metal (My) ion cathodic re
duction current is known:
My
m+ + ne- → My
(m-n)+ (2)
provided that the total current traversing the cell is zero
[6]. Since the experiments were performed in an inert
atmosphere, the only cathodic depolarizer (oxidant) in
the fluoride melt under investigation can be ions of zir
conium or one of the alloy constituents that transferred
into the melt by the exchange reaction:
zMx
0 + aZrF4 → zMxFa + aZrF4-z . (3)
The investigation carried out showed that almost all
voltammograms exhibit in the cathodic region a wave
which precedes zirconium ion discharge (Fig. 2), there
fore the portion of the voltammetric curve (section ac)
corresponding to the cathodic process at the most posi
tive potential was employed in the calculations of the
corrosion rate of the samples under investigation. In the
anodic region, all voltammograms exhibited a wave of
anodic dissolution of the sample under investigation
(section ab), which was followed by a current drop (sec
tion bd, due to a surface passivation) followed by an in
crease in current (section df due to a pit formation). To
calculate the corrosion current of the samples under in
vestigation in the anodic region, the initial portion of the
voltammetric curve (section ab) was used.
Fig. 2. Voltammogram of a nickel-molybdenum alloy
(composition B, sample 120) in a molten eutectic sodi
um fluoride-zirconium fluoride mixture at 650 0C
Examination of the anodic (section ab) and cathodic
(section ac) branches of volammograms in semilog co
ordinates allows one to determine the conditions under
which the total current traversing the cell is zero and to
calculate the corrosion current density of the sample un
der investigation (Fig. 3).
Fig. 3. Calculation of the corrosion current density of a
sample of alloy of composition A after 400 hours of
isothermal soaking in a sodium fluoride-zirconium flu
oride melt at 650 °C
Knowing the corrosion current density, one can cal
culate the corrosion rate K m=
ic AM
nF
(g/m2⋅h) or
K k=
K m
d M
(mm/yr) [6], where AM is the atomic mass
of metal, n is the number of electrons involved in the
________________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 67-73.
69
electrode process, F is Faraday constant, dM is the densi
ty of the metal under investigation.
The investigations carried out showed that the cur
rent rise on the anodic branch (section ab) of voltammo
grams of nickel-molybdenum alloys is observed in the
potential range corresponding to nickel ionization in flu
oride melts [7], and that the number of electrons in
volved in the electrode process is two. Therefore in the
calculations of the corrosion rate of these alloys we as
sumed that n=2, and that AM and dM are the atomic mass
and density of nickel respectively. An examination of
the voltammograms obtained showed that iron ioniza
tion is a two-electron process, chromium ionization a
three-electron process, niobium ionization a five-elec
tron process, and molybdenum ionization a six-electron
process. The results of the calculation of corrosion rates
are listed in Table 3.
Table 3
Corrosion rate of nickel-molybdenum alloys and their constituents
Sample
Corrosion
current densi
ty ic,mA/cm2
Corrosion rate
Km, g/m2⋅h Kk, mm/yr
Notes
Composition A 0.07 0.77 0.76 Without heat treatment
Composition A 0.017 0.19 0.18 After heat treatment
Composition A 0.023 0.25 0.25 100 h of isothermal soaking in a fluoride
melt at 650°С without irradiation
–«»– 0.01 0.11 0.11 200 h –«»–
–«»– 0.006 0.066 0.065 500 h–«»–
–«»– 0.00002 0.0002 0.0002 700 h–«»–
Composition B 0.026 0.29 0.28 404 h of isothermal soaking in a fluoride
melt at 650°С
Composition A 0.018 0.20 0.19
700 h of isothermal soaking in a fluoride
melt at 650°С in the case of irradiation
with an electron beam(5066 eV/atom)
Composition A 0.0074 0.081 0.080 –«»– 64 eV/atom
Composition B 0.02 0.22 0.22 –«»– 5066 eV/atom
Composition B 0.0004 0.0044 0.0043 –«»– 64 eV/atom
Mo 0.0016 0.0095 0.008 Without heat treatment
Ni 0.025 0.27 0.27 –«»–
Fe 5.2 54.2 60.37 –«»–
Nb 9.5 107.0 67.47 –«»–
Cr 1000 6466.3 7911.3 –«»–
From the data obtained it follows that the corrosion
resistance of the metals investigated decreases in the or
der: Mo, Hastelloy, Ni, Fe, Nb, Cr. The corrosion rate of
Hastelloy (composition A and B) depends on the time of
contact with fluoride melt and is the lower the longer
the time of isothermal soaking in the melt. This is due to
the fact that corrosion products appear at the interface
with time, which shift the stationary potential towards
more positive values. Besides, the corrosion products
formed passivate the surface of the samples under inves
tigation. The surface passivation of Hastelloy is evi
denced by the shape of voltammograms in the range of
high anodic potential values (Fig. 2). It was noted that
the section df, which characterizes the presence of pit
ting corrosion, is found not in all voltammograms. This
section is absent on voltammograms of the samples that
underwent preliminary heat treatment (Fig. 4, curve 2)
and long isothermal soaking in a molten sodium fluoride
-zirconium fluoride mixture (Fig. 4, curves 3 and 4).
Fig. 4. Voltammograms of Hastelloy (composition A) at
different heat treatment conditions: (1) without heat
treatment, (2,3,4) after heat treatment: the time of
isothermal soaking in a molten eutectic sodium fluo
ride-zirconium fluoride mixture at 650 °C is 0, 100 and
500 h respectively
The voltammograms of the samples that did not un
dergo heat treatment exhibit no passivation region
(curve 1) even after reaching current densities of over
4 A/cm2. The heat treatment sequence and conditions
were as follows: heating and soaking for an hour at
1100 °C, water quenching at room temperature, anneal
ing at 675 °C for 50 h under argon. It is non-adherence
to the heat treatment conditions that is apparently re
sponsible for the appearance of df sections (pitting dis
solution), which were found for individual samples of
compositions A and B (samples 24, 120, 106).
The samples that were subjected to long electron
beam irradiation in a molten eutectic sodium fluoride-
zirconium fluoride mixture also retain the passivity re
gion (Fig. 5); it was noted, however, that the potential
range with the lowest current density is the narrower the
more intense the irradiation to which the samples were
subjected.
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70
Fig. 5. Voltammograms of Hasteloy (composition A)
after irradiation with an electron beam with energy:
1 – 5066 eV/atom; 2 – 64 eV/atom in a molten eutectic
sodium fluoride-zirconium fluoride mixture at 650 °C
for 700 h
The investigations carried out showed that the
voltammetric method enables one to assess the influ
ence of the electron beam irradiance of samples in a flu
oride melt. For example, when a sample of composition
B is irradiated with an electron beam with an energy of
5066 eV/atom in a fluoride melt at 650 °C for 700h, the
corrosion current density is 0.02 mA/cm2, and at
64 eV/atom it is 0.0004 mA/cm2.
It was noted that the surfaces of the same sample are
characterized by different corrosion rates. At the en
trance of electron beam to a Hastelloy plate 0,3 mm in
thickness, the corrosion current density is on an average
an order of magnitude higher than that at the exit. For
example, if one of the surfaces of the sample under in
vestigation is coated with boron nitride, and a voltam
mogram of the uncoated surface is recorded, the corro
sion rate at the entrance of electron beam
(5066 eV/atom) for the sample of composition B is esti
mated to be 0,17 mA/cm2 and at the exit 0,01 mA/cm2.
The dependence of the corrosion rate of samples on
the heat treatment conditions, the presence of passiva
tion regions in the anodic potential range, and the ab
sence of pronounced pit formation regions allow one to
conclude that the corrosion of these alloys is of inter
crystalline character. This conclusion agrees with the re
sults of X-ray phase analysis and metallography. Ac
cording to the results of an X-ray phase analysis, the
diffractograms of alloys of compositions A and B,
which underwent only the above heat treatment without
contact with the molten sodium fluoride-zirconium fluo
ride mixture, exhibited only the beginning of formation
of phases that are high in nickel (low-intensity lines of <
5%) of the intensity of line (111) due to the nickel base
with d=2.10…2.11 Е (Ni3Mo of orthorhombic system)
and d=1.84…1.88 Е (Ni3(Al,Ti) of cubic system)).
Isothermal soaking in fluoride melts at 650 °C increases
the overall annealing (aging) time of the alloy and
makes for a change in its structure and appearance of
various secondary phases. After isothermal soaking in a
fluoride melt for more than 100 h, the secondary phases
NiAl, Ni2Al3, NiCr are identified. The secondary phases
manifest themselves most clearly after 500- hour soak
ing in a fluoride melt. The phase Ni3Mo is most dis
cernible in the alloy. In the samples subjected to irradia
tion in a fluoride melt, only the phase Ni3Mo is identi
fied most clearly.
The results of an X-ray phase analysis make it possi
ble in a certain measure to explain the relation between
the observed corrosion rates and heat treatment condi
tions. The alloy sample (Fig. 4, curve 1) not subjected to
heat treatment is virtually a homogeneous alloy (solid
solution), whose constituents retain individual electro
chemical properties, i.e. the alloy constituents with the
most negative potential (Al, Ti, Cr, Fe, etc) must dis
solve in the first place, and the constituents with more
positive electrode potential (Ni, Mo) must accumulate
as adatoms on the surface. It was shown above (Figs.
2,3) that the equilibrium potential of the base of the al
loy (nickel) is more positive than the alloy corrosion po
tential (-210 mV). In this case, nickel microcrystals may
form on the alloy surface owing to the high mobility of
adatoms; these crystals, however, do not create dense
homogeneous coating and do not make for its passiva
tion. On the contrary, the alloy corrosion rate must even
increase due to the formation of the galvanic couple
nickel-metal with a high negative potential (Cr, Fe). The
trend of the recorded voltammetric curve (Fig. 4, curve
1) speaks in favor of this corrosion mechanism. Even at
high current densities (>4 A/cm2), the surface of such a
sample does not change to passive state. The results of
the X-ray phase analysis showed that the samples of
Hastelloy subjected to heat treatment are no longer ho
mogeneous alloys. They are virtually heterophase com
positions. The formation of a secondary phase (which is
more electronegative relative to the main phase) termi
nates approximately after 500-hour annealing in a fluo
ride melt at 650 °C. Since the amount of the secondary
phase compounds is small relative to the main phase,
and they separate out mainly at the grain boundaries of
the main phase, a more stable continuous layer of the
base of the alloy remains on the surface after their disso
lution. It is this fact that causes the presence of passiva
tion regions in voltammograms, a shift of corrosion po
tential towards more electropositive values (Fig. 4,
curve 4) and hence a decrease in the overall corrosion
rate (Table 3). The shift of potential towards more posi
tive values must be the greater the higher the bond ener
gy in the intermetallic compounds formed. According to
the results of the X-ray phase analysis, the passivation
of the surface is also aided by increase in its adhesion to
the sodium fluoride – zirconium fluoride melt. It was
noted that after 700-hour contact with the sodium fluo
ride – zirconium fluoride melt, the phase 7NaF·6ZrF4 is
identified on the surface of all samples. The same effect
persists on irradiation with an electron beam.
The results of the voltammetric and X-ray phase in
vestigations agree with the conclusions drawn from
metallographic and microscopical analyses (Fig. 6).
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Fig. 6. SEM micrograph: cross-section of a sample of
composition A (sample 64) soaked in a NaF-ZrF4 melt
at 650 °C under irradiation with an electron beam of
5066 eV/atom energy for 700 h
Secondary phase compounds are mainly at the grain
boundaries of the main phase. On the surface of the
samples subjected to electron irradiation there are traces
of “etching”. According to the results of SEM, the sur
face of the sample is destroyed chiefly through the
”etching” of secondary phase compounds at the grain
boundaries of the main phase, the etching rate being the
higher the higher the electron beam energy. For exam
ple, in a sample of composition A (sample 25), the
thickness of the damaged layer at the entrance of elec
tron beam (5066 eV/atom) was 60…100 µm and at the
exit (opposite side) 40…50 µm. In a sample of compo
sition A (sample 64), the thickness of the damaged layer
at the entrance of beam was in the case of irradiation
with electron beam with lower energy (64 eV/atom)
15…30 µm and at the exit 10…15 µm. The state of the
surface of a sample of composition A (sample 25) at the
entrance of electron beam (5066 eV/atom) is shown in
Fig. 6. One can clearly see corrosion cracks on the sur
face of lateral microsection. An X-ray microanalysis has
been performed to the right and left of the circle, as
shown in the photograph, depthward with a step of 1 µ
m.
The results are listed in Table 4. The quantitative
composition of the sample with coordinate (0 µm) cor
responds to the composition on the grain surface but not
in the crack. The results obtained allow one to conclude
that corrosion damages are mainly of intercrystalline
character. The sample is destroyed at grain boundaries,
where the melt penetrates. The arrow indicates a future
crack.
Table 4
Results of the X-ray microanalysis of the surface of a sample of composition A (sample 24) at the entrance of
electron beam (5066 eV/atom) after 700 h at 650 °C in a sodium fluoride-zirconium fluoride melt
Element Percentage of the alloy constituents (at.%) on the right of crack at a distance of (µm):
0 1 2 3 4 6
Ti 0.55 0.44 0.48 0.46 0.62 0.48
Si 0.31 0.25 - - 0.37 0.36
Cr 7.37 7.62 7.42 7.67 7.95 7.87
Fe 2.35 1.69 1.61 1.74 1.68 1.57
Ni 79.42 78.88 78.26 76.94 77.41 76.87
Mo 4.53 9.11 10.33 11.29 10.66 10.56
Zr 0.58 - - 0.59 - -
Percentage of the alloy constituents (at.%) on the left of crack at a distance of (µm):
Ti 0.45 0.47 0.62 0.60 0.47 0.41
Si 0.41 0.22 0.22 0.34 0.39 0.37
Cr 6.99 7.44 7.86 7.83 7.81 7.70
Fe 1.92 1.66 1.55 1.49 1.68 1.55
Ni 78.89 78.42 77.58 77.33 77.09 77.55
Mo 6.33 9.83 10.40 10.63 10.84 10.74
Zr 0.65 - - - - -
The nanohardness H and elastic modulus E of the
samples under investigation were determined by nanoin
dentation analysis on a Nano Indenter II device with a
Berkovich indenter by the Oliver and Pharr technique
[2] from the depth of impression at a maximum load of
10mN (1 gf). The results of the hardness test showed
that either irradiation or salt solutions did not practically
affect the mechanical properties of the Hastelloy sam
ples. The results obtained are listed in Table 5. The rec
ommended alloy compositions can be used in the design
of molten-salt reactors.
Table 5
Results of the nanoindentation analysis (t=20 °C) of the mechanical properties of Hastelloy
Sample under investiga
tion
Beam energy
(eV/atom).
Elastic modulus E, GPa Nanohardness H, GPa
On the surface In the bulk On the surface In the bulk
Sample A after heat treat
ment 0 266±16 266±16 4.7±0.3 4.7±0.3
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72
Sample A after 700-hour
contact with fluoride melt
0 219 219 3.0±2.2 4.9±0.3
64 242±35 268±8 5.7±0.3 4.8±0.2
5066 229±35 256±13 5.7±0.5 5.1±0.3
Sample B after heat treat
ment 0 297±11 297±11 6.9±0.3 6.9±0.3
Sample B after 700-hour
contact with fluoride melt
0 251±5 253±6 4.0±0.5 6.9±0.3
64 240±13 255±7 7.0±0.3 6.9±0.3
5066 242±17 255±9 6.9±0.2 6.9±0.3
CONCLUSIONS
The investigations carried out show that intercrys
talline corrosion is typical of nickel-molybdenum alloys
of the above-mentioned compositions. The corrosion re
sistance of the investigated samples due to surface pas
sivation is the higher the longer exposure in molten
NaF–ZrF4 mixture. The Hastelloy surface is eroded
mainly through the “etching” of the secondary phases at
the grain boundaries of the main phase. Surface passiva
tion is not typical of alloys that did not undergo heat
treatment.
Irradiation of the alloy surface by 10 MeV an elec
tron beam increases the corrosion rate.
The voltammetric method is usefull to control the
corrosion resistance of Hastelloy depending on irradia
tion.
The investigation of the mechanical properties of
Hastelloy by nanoindentation analysis show that either
irradiation or long contact with fluoride melt did not
practically affect their mechanical properties.
Alloys of the proposed compositions can be used in
the design of molten-salt nuclear power plants.
REFERENCES
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dov, А.O. Omelchuk, V.V. Rozhkov, V.F. Zelenskiy.
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6–11, 2004.
6.L. Kis. Kinetics of Electrochemical Metal Dissolution.
Budapest: “Akademiai Kiado”, 1988, p. 272.
7.A. Robin and J. de Lepinay. Electrochemical study of
the anodic dissolution of Iron and Nickel in molten LiF-
NaF-KF eutectic at 600 °C using convolutional voltam
metry //Electrochimica Acta (37). 1992, #13,
p. 2433–2436.
КОРРОЗИЯ ОБЛУЧЕННЫХ Ni – Mo СПЛАВОВ В РАСПЛАВЕ ФТОРИДОВ НАТРИЯ
И ЦИРКОНИЯ
В.М. Ажажа, А.А. Андрийко, А.С. Бакай, С.В. Волков, С.В. Девяткин, А.Н. Довбня, С.Д. Лавриненко,
А.А. Омельчук, Б.М. Широков, Р.С. Шмегера
Методами циклической вольтамперометрии, рентгенофазового, микроскопического анализов, а также
металлографии изучена коррозия никель-молибденовых сплавов и составляющих компонентов в расплавленной
эвтектической смеси фторидов натрия и циркония. Исследована зависимость скорости коррозии никель-молибденовых
сплавов в расплавленных фторидах от времени выдержки в расплаве и облучения пучком электронов на электронном
ускорителе. Показано, что увеличение времени контакта никель-молибденовых сплавов с фторидным расплавом
уменьшает плотность тока коррозии, а облучение – увеличивает ее в тем большей мере, чем больше энергия
электронного пучка. Для сплавов данного состава характерна межкристаллитная коррозия.
КОРОЗІЯ ОПРОМІНЕНИХ Ni-Mo СПЛАВІВ В РОЗПЛАВІ ФТОРИДІВ НАТРІЮ ТА ЦИРКОНІЮ
В.М. Ажажа, О.О. Андрійко, О.С. Бакай, С.В. Волков, С.В. Дев’яткін,
О.М. Довбня, С.Д. Лавриненко, А.О. Омельчук, Б.М. Широков, Р.С. Шмегера
Методами циклічної вольтамперометрії, рентгенофазового, мікроскопічного аналізів, а також металографії дослі
джена корозія нікель-молібденових сплавів та компонентів, які входять до їхнього складу, в розплавленій евтектичній
суміші фторидів натрію та цирконію. Досліджена залежність швидкості корозії зазначених сплавів від тривалості витри
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73
мки в розплаві та опромінення пучком електронів на електронному прискорювачі. Показано, що збільшення тривалості
контакту нікель-молібденових сплавів з фторидним розплавом зменшує густину струму корозії, а опромінення –
збільшує її в тим більшій мірі, чим більша енергія електронного пучка. Для сплавів даного складу характерна міжкри
сталітна корозія.
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