Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy
Time dependencies for the parameters (current, voltage, inner resistance) of shortcut galvanic cell Hastelloy-carbon material have been studied in NaF-ZrF₄ eutectic melt at 600…650°C. Analysis of these data, together with the data on polarization of separate electrodes at open circuit conditions, sh...
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| Zitieren: | Corrosion processes in zirconium fluoride-sodium fluoride melts.study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy / А.S. Bakai, S.V. Volkov, V.M. Azhazha, А.А. Аndriiko, А.А. Omelchuk, M.I. Buryak, B.M. Voronin, S.D. Lavrinenko // Вопросы атомной науки и техники. — 2005. — № 4. — С. 62-66. — Бібліогр.: 3 назв. — англ. |
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Bakai, A.S. Volkov, S.V. Azhazha, V.M. Andriiko, A.A. Omelchuk, A.A. Buryak, M.I. Voronin, B.M. Lavrinenko, S.D. 2015-04-18T19:25:02Z 2015-04-18T19:25:02Z 2005 Corrosion processes in zirconium fluoride-sodium fluoride melts.study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy / А.S. Bakai, S.V. Volkov, V.M. Azhazha, А.А. Аndriiko, А.А. Omelchuk, M.I. Buryak, B.M. Voronin, S.D. Lavrinenko // Вопросы атомной науки и техники. — 2005. — № 4. — С. 62-66. — Бібліогр.: 3 назв. — англ. 1562-6016 PACS 81.40-WX https://nasplib.isofts.kiev.ua/handle/123456789/80545 Time dependencies for the parameters (current, voltage, inner resistance) of shortcut galvanic cell Hastelloy-carbon material have been studied in NaF-ZrF₄ eutectic melt at 600…650°C. Analysis of these data, together with the data on polarization of separate electrodes at open circuit conditions, showed that shortcut current is related to the rate of corrosion of the alloy contacted with carbon, if Zr metal is absent in the system. In presence of Zr, the current corresponds to the rate of zirconium reaction with the melt (related to unit surface of the Hastelloy electrode), and can be used for studies of dissolution kinetics of Zr metal in the melt. It was found that contact of the alloy with carbon material results in the increase of corrosion rate both in pure eutectics and with the additives of LaF₃. Presence of Zr metal suppresses the corrosion process. Вивчена зміна в часі параметрів (струм, напруга, внутрішній опір) короткозамкненого гальванічного елемента хастелой/вуглецевий матеріал в евтектичному розплаві NaF-ZrF₄ при 600-650°C. Аналіз цих даних в сукупності з даними поляризаційних вимірювань на окремих електродах при розімкненому колі показав, що струм короткого замикання відповідає швидкості корозії сплаву в контакті з вуглецем при умові, якщо металічний Zr в системі відсутній. В присутності Zr цей струм визначається швидкістю взаємодії цирконію з розплавом (віднесеної до одиниці поверхні хастелою), що може бути використано для дослідження кінетики розчинення цирконію в розплаві. Встановлено, що контакт сплаву з вуглецем посилює корозію як в чисто евтектичному розплаві, так і з добавленням LaF₃. Присутність металічного Zr пригнічує корозійний процес. Исследована временная зависимость параметров (ток, напряжение, внутреннее сопротивление) короткозамкнутого гальванического элемента хастеллой/углеродный материал в эвтектическом расплаве NaF-ZrF₄ при 600-650°C. Анализ этих данных, в совокупности с данными поляризационных измерений на отдельных электродах при разомкнутой цепи, показал, что ток короткого замыкания соответствует скорости коррозии сплава в контакте с углеродом при условии, если металлический Zr в системе отсутствует. В присутствии же Zr этот ток определяется скоростью взаимодействия циркония с расплавом (отнесенной к единице поверхности хастеллоя), что может быть использовано для исследования кинетики растворения циркония в расплаве. Установлено, что контакт сплава с углеродом усиливает коррозию как в чисто эвтектическом расплаве, так и с добавками LaF₃. Присутствие же металлического Zr подавляет коррозионный процесс. The work was supported by STCU Project #294. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy Корозійні процеси в розплаві фторидів цирконію і натрію. Дослідження короткозамкненого елемента C/(ZRF₄ – NAF)ЕВТ/Хастеллой Коррозионные процессы в расплаве фторидов циркония и натрия. Исследование короткозамкнутого элемента C/(ZRF₄ – NAF)ЕВТ/Хастеллой Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy |
| spellingShingle |
Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy Bakai, A.S. Volkov, S.V. Azhazha, V.M. Andriiko, A.A. Omelchuk, A.A. Buryak, M.I. Voronin, B.M. Lavrinenko, S.D. |
| title_short |
Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy |
| title_full |
Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy |
| title_fullStr |
Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy |
| title_full_unstemmed |
Corrosion processes in zirconium fluoride-sodium fluoride melts.Study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy |
| title_sort |
corrosion processes in zirconium fluoride-sodium fluoride melts.study of shortcut cell c/(zrf₄ – naf)eut/hastelloy |
| author |
Bakai, A.S. Volkov, S.V. Azhazha, V.M. Andriiko, A.A. Omelchuk, A.A. Buryak, M.I. Voronin, B.M. Lavrinenko, S.D. |
| author_facet |
Bakai, A.S. Volkov, S.V. Azhazha, V.M. Andriiko, A.A. Omelchuk, A.A. Buryak, M.I. Voronin, B.M. Lavrinenko, S.D. |
| publishDate |
2005 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Корозійні процеси в розплаві фторидів цирконію і натрію. Дослідження короткозамкненого елемента C/(ZRF₄ – NAF)ЕВТ/Хастеллой Коррозионные процессы в расплаве фторидов циркония и натрия. Исследование короткозамкнутого элемента C/(ZRF₄ – NAF)ЕВТ/Хастеллой |
| description |
Time dependencies for the parameters (current, voltage, inner resistance) of shortcut galvanic cell Hastelloy-carbon material have been studied in NaF-ZrF₄ eutectic melt at 600…650°C. Analysis of these data, together with the data on polarization of separate electrodes at open circuit conditions, showed that shortcut current is related to the rate of corrosion of the alloy contacted with carbon, if Zr metal is absent in the system. In presence of Zr, the current corresponds to the rate of zirconium reaction with the melt (related to unit surface of the Hastelloy electrode), and can be used for studies of dissolution kinetics of Zr metal in the melt. It was found that contact of the alloy with carbon material results in the increase of corrosion rate both in pure eutectics and with the additives of LaF₃. Presence of Zr metal suppresses the corrosion process.
Вивчена зміна в часі параметрів (струм, напруга, внутрішній опір) короткозамкненого гальванічного елемента хастелой/вуглецевий матеріал в евтектичному розплаві NaF-ZrF₄ при 600-650°C. Аналіз цих даних в сукупності з даними поляризаційних вимірювань на окремих електродах при розімкненому колі показав, що струм короткого замикання відповідає швидкості корозії сплаву в контакті з вуглецем при умові, якщо металічний Zr в системі відсутній. В присутності Zr цей струм визначається швидкістю взаємодії цирконію з розплавом (віднесеної до одиниці поверхні хастелою), що може бути використано для дослідження кінетики розчинення цирконію в розплаві. Встановлено, що контакт сплаву з вуглецем посилює корозію як в чисто евтектичному розплаві, так і з добавленням LaF₃. Присутність металічного Zr пригнічує корозійний процес.
Исследована временная зависимость параметров (ток, напряжение, внутреннее сопротивление) короткозамкнутого гальванического элемента хастеллой/углеродный материал в эвтектическом расплаве NaF-ZrF₄ при 600-650°C. Анализ этих данных, в совокупности с данными поляризационных измерений на отдельных электродах при разомкнутой цепи, показал, что ток короткого замыкания соответствует скорости коррозии сплава в контакте с углеродом при условии, если металлический Zr в системе отсутствует. В присутствии же Zr этот ток определяется скоростью взаимодействия циркония с расплавом (отнесенной к единице поверхности хастеллоя), что может быть использовано для исследования кинетики растворения циркония в расплаве. Установлено, что контакт сплава с углеродом усиливает коррозию как в чисто эвтектическом расплаве, так и с добавками LaF₃. Присутствие же металлического Zr подавляет коррозионный процесс.
|
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/80545 |
| citation_txt |
Corrosion processes in zirconium fluoride-sodium fluoride melts.study of shortcut cell C/(ZrF₄ – NaF)EUT/Hastelloy / А.S. Bakai, S.V. Volkov, V.M. Azhazha, А.А. Аndriiko, А.А. Omelchuk, M.I. Buryak, B.M. Voronin, S.D. Lavrinenko // Вопросы атомной науки и техники. — 2005. — № 4. — С. 62-66. — Бібліогр.: 3 назв. — англ. |
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2025-11-24T11:37:30Z |
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PACS 81.40-WX
CORROSION PROCESSES IN ZIRCONIUM FLUORIDE – SODIUM FLU
ORIDE MELTS.
STUDY OF SHORTCUT CELL C/(ZrF4 – NaF)EUT/HASTELLOY
А.S. Bakai1, S.V. Volkov2, V.M. Azhazha1, А.А. Аndriiko3, А.А. Omelchuk2, M.I. Buryak2,
B.M. Voronin2, S.D. Lavrinenko1
1National Science Center Kharkov Institute of Physics and Technology,Akademicheskaya
str. 1, 61106 Kharkov, Ukraine, e-mail: bakai@kipt.kharkov.ua ;
2Vernadskii Institute of General and Inorganic Chemistry, Ukrainian National Academy of
Science, Palladin avenue 32/34, 03680 Kiev 142,Ukraine, e-mail: omelchuk@ionc.kar.net;
3National Technical University “KPI”, Department of Chemistry, Pobedy avenue 37,
03056 Kiev-56, Ukraine, e-mail: andriiko@xtf.ntu-kpi.kiev.ua
Time dependencies for the parameters (current, voltage, inner resistance) of shortcut galvanic cell Hastelloy-car
bon material have been studied in NaF-ZrF4 eutectic melt at 600…650oC. Analysis of these data, together with the
data on polarization of separate electrodes at open circuit conditions, showed that shortcut current is related to the
rate of corrosion of the alloy contacted with carbon, if Zr metal is absent in the system. In presence of Zr, the current
corresponds to the rate of zirconium reaction with the melt (related to unit surface of the Hastelloy electrode), and
can be used for studies of dissolution kinetics of Zr metal in the melt. It was found that contact of the alloy with car
bon material results in the increase of corrosion rate both in pure eutectics and with the additives of LaF3. Presence
of Zr metal suppresses the corrosion process.
INTRODUCTION
It is known that Ni-Mo alloy (trademark Hastelloy)
are relatively stable in zirconium fluoride molten mix
tures and can be used as the material for technical appli
cations of such melts, in particular, in nuclear power
technologies. That is why studies of its corrosion resis
tance in such media are of practical interest.
Corrosion of Hastelloy has been studied earlier by
the electrochemical technique, X-ray analysis, SEM and
metallography [1]. Corrosion currents were found to be
about 10-2 mA/cm2 depending on the preliminary heat
treatment and irradiation. This is fairly good stability for
most practical applications.
The above mentioned data correspond to the behav
ior of the alloy contacted with the melt only. However,
the metal in some important cases is in long-term con
tact with graphite or other carbon material. One can ex
pect the differences in corrosion resistance of the alloy
because of the formation of electrochemical couple
working as a corrosion shortcut galvanic cell. This pa
per presents some results on the corrosion behavior of
Hastelloy in contact with carbon material. Processes in
pure ZrF4-NaF eutectic melt and in the presence of the
LaF3 and Zr metal additives were studied.
EXPERIMENTAL
Linear sweep voltammetry was used for studies of
corrosion processes at separate electrodes. The voltam
mograms were taken by means of the PC governed Po
tentiostat PI-50 at scan rate 2 mV/s vs. classy carbon
reference electrode (RE) in case of the alloy and vs.
Hastelloy (H) RE when C electrode was studied.
Long-term experiments with the electrochemical cell
C/(ZrF4 – NaF)eut/H were performed in glassy carbon
or graphite (СС composite) crucibles where the walls
served as a C electrode (~35 cm2 area was in contact
with the melt). Hastelloy plate of 2cm2 area was used as
second electrode.
The open circuit voltage was measured with high in
put resistance electronic milivoltmeter V7-21. Mil
iampermeter with low inner resistance was used for
measurement of shortcut current of the cell. The cell’s
resistance was checked with AC bridge R5083, frequen
cy was 100 kHz.
RESULTS AND DISCUSSION
Several electrochemical reactions should be consid
ered in the analysis of the processes in the cell:
1. Reduction of zirconium (IV) to low valence com
pounds. Though the process is a complex multi-step
one [2], we denote it by the simplified reaction:
Zr(IV) +2e- = Zr(II) (1)
This is the main cathode reaction on both electrodes.
The reaction (1) could be reversible. That is, we should
take into account the reverse (anode) reaction after the
accumulation of significant amount of low valence
species in the melt:
Zr(II) = Zr(IV) +2e- (1,a)
2. The anode process on the alloy electrode is the elec
trochemical oxidation of the metal:
Me = Men+ + ne-, (2)
where Me = Ni (main component) or Mo, Cr, Ti, Fe
(doping metals).
_______________________________________________________________________________
62 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 62-66.
mailto:bakai@kipt.kharkov.ua
3. No reductant species other than carbon are present in
the system at the initial time period; then the oxida
tion of carbon should be the main anode reaction at
C electrode of the cell. Basically, it must be the elec
trochemical fluorination of the carbon1:
С + xF- = CFx + xe-/. (3)
The reactions (1) and (2) proceed with equal rates in
open circuit conditions. The polarization curves of the
electrode plotted in semi-logarithm scale can be used for
calculation of stationary potentials and corrosion cur
rents. Fig. 1 shows an example of such plot.
-400 -300 -200 -100 0 100 200 300
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
lgi
кз
=-0.8 (mA/cm2)
S17
lgi
c
=-2.08 mA/cm2
lg
i,
m
A
/c
m
2
E, mV
Fig. 1. Typical corrosion diagram of Hastelloy
(vs. glassy carbon RE) in a fresh melt ZrF4 – NaF,
600oC. The arrow shows an expected shortcut current
(see below)
The curves in Fig. 1 were taken vs. GC reference
electrode. It was noticed that, in freshly prepared melt,
the stationary potential, which can be determined at in
tersect point of the linear branches of cathode and anode
curves, attains a value close to -200 mV. In course of
the long-time exposure of the melt in carbon container,
the potential gradually shifts to more positive values
(Fig. 2).
-300 -200 -100 0 100 200 300
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0
139_2_back
lgic=-0.74 mA/cm2
lg
i,
m
A
/c
m
2
E, mV
Fig. 2. Corrosion diagram of the alloy sample after prolonged
treatment in the melt for several days
Gradual displacement of the stationary potential to
ward positive values was also found in the experiments
where OCV of the cell was measured for several days
(Fig. 3). Disregarding the transient processes after melt
1 The side reaction С + О2- = СО + 2е- is also possible if the
presence of oxide compounds in the melt cannot be neglected;
this possibility should not effect our analysis of the problem.
ing the electrolyte, one can see almost linear time drift
of the potential to negative side.
0 1 2 3 4 5 6 7 8 9 1011121314151617181920
-0,6
-0,5
-0,4
-0,3
-0,2
-0,1
0,0
E
, V
Hours
Fig. 3. Stationary potential of the alloy vs. GC mea
sured in daytime for 4-day period. The transients after
the vertical lines result from the termination
of the process when the electrolyte was repeatedly crys
tallized and melted again
In our opinion, such drift of the potential vs. GC elec
trode results rather from the change of GC potential
than the metal one. Polarization curves of carbon elec
trode vs. the alloy RE, Fig. 4, support this conclusion.
-1000 -800 -600 -400 -200 0 200 400 600 800 1000 1200
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
lg
|i|
, m
A
/c
m
2
E vs "X", mVFig. 4. Semi-logarithm plot of the polarization curves of GC
electrode vs. H RE
Linear parts of the dependencies in Fig. 4 intercept at
the potential value near +200 mV vs. H, which is con
sistent with the stationary potential difference obtained
from the polarization curves of the alloy vs. GC elec
trode, Fig. 1. However, the current densities on GC
electrode are much higher, which evidences essentially
larger rates of the electrochemical reactions at this elec
trode. These reactions in a fresh electrolyte have to be
the oxidation of carbon (3) and reduction of zirconium
(IV) (1).
Upon a time, such processes on the carbon electrode,
with its large surface area and high rates of electro
chemical transformations, should result in the accumu
lation of Zr(II) low valence species in the bulk and, ac
cordingly, to development of the parallel reaction of its
anode oxidation (1a). It means that, in open circuit con
ditions, the potential of GC electrode should approach
the value of equilibrium potential of Zr(IV)/Zr(II) cou
ple, which, in turn, is the more negative the higher is the
concentration of Zr(II) species in the electrolyte.
Thus, the potential of GC electrode becomes more
negative due to the accumulation of low valence Zr(IV)
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63 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 62-66.
reduction products in the melt. It manifests itself experi
mentally as the drift of the potential of alloy vs. GC to
ward more positive values comparatively to the starting
point (fig. 2,3).
The potentials of both electrodes approach each
other when the cell is shortened. The potential differ
ence then should be determined by the value of shortcut
current and inner resistance of the cell:
Δϕ=I sh R . (4)
The inner resistance was periodically checked in
course of the experiments. The results are given
in Fig. 5.
0 200 400 600 800 1000
0
10
20
30
40
50
0
2
4
6
8
10
12
R
, O
hm
Time, hours
∆ϕ
R
∆
ϕ,
m
V
Fig. 5. Time dependencies of inner resistance and
ohmic drop on the cell (-)С/(ZrF4-NaF)eut/H(+) in long-
time continuous experiments. Temperature 600оС
As follows from the data above, the cell’s resistance
is changing from tenth to several tens of mV in a com
plex manner.
Meanwhile, the potential difference, calculated from
experimental data by the formula (4), is of a few mV
only. Since the value of OCV is of 2 orders higher, we
can neglect the ohmic potential drop considering the po
tentials equal for both electrodes when the cell is short
ened.
Turning back to the polarization curves of separate
electrodes (Fig. 1-3), two important points must be em
phasized. Fist, as it was already mentioned, partial cur
rent densities at stationary potential are much larger for
GC than for the H electrode. Second, the surface area
for GC is also much larger (the ratio of GC to H sur
faces in the experiments was 12-15). Thus, it is evident
that shortening the cell practically could not effect the
potential of GC, while the potential of H electrode
would change and attain the value almost the same as
the open circuit potential of GC electrode.
From these considerations we can estimate the short
cut current from the polarization curves of H electrode
vs. C as the current at zero potential. Analysis of the
data available shows that shortcut current density (per
unit surface of H electrode) should be about
100…200 mcА/сm2 (for example, 160 mcA/cm2 for the
sample of Fig. 1). The direct measurements, which are
represented below, confirm this conclusion.
Fig. 6 shows the time dependence of shortcut cell op
erated continuously for about 2 months. The dynamics
of system is rather complicated – non-linear oscillations
are observed with the period about 200 hours. As it was
shown in [3], such behavior of an electrochemical sys
tem may result from the effect of feedbacks between
sub-systems (electrodes), which are interconnected via
the bulk of the electrolyte.
0 200 400 600 800 1000 1200
100
150
200
250
300
350
400
450
500
550
I,
m
c
A
t, hours
X108
Fig. 6. Time dependence of shortcut current of the cell
(-)Х108/ZrF4 – NaF/C(+), Temperature 600оС, SC = 35 sm2,
SH = 2.5 cm2
In such situation, the time constant of the interac
tion, which approximately corresponds to the period of
oscillations (if the conditions exist for its development),
can be determined from the formula
T≈ 2 ⋅π⋅δ⋅V
D⋅S c⋅S a
, (5)
where V is a bulk volume of the electrolyte, δ the thick
ness of diffusion layer; D the diffusion coefficient, and
Sc, Sa the surface areas of electrodes.
The calculations by this formula show that, in case of
“common” diffusion through the diffusion layers in
molten electrolytes (the order of values: δ~ 10-2 cm
D~10-5 cm2/s), the oscillation period should be about 10
hours, which is much less than observed by an order of
value.
Reasons for such disagreement could be an increase
of diffusion distance (δ) or decrease of diffusion coeffi
cient D or both of these. We assumed that the diffusion
processes could be retarded due to its occurrence not
only in the melt but in solid also.
To check this assumption, we prepared a cross-section
of the electrode after durable exposition in melt (Fig. 7).
Fig. 7. SEM photograph of cross-section after durable expo
sure in ZrF4 – NaF + 6,7%LaF3 melt
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64 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 62-66.
In Fig. 7 one can see, under the layer of frozen elec
trolyte, a thin (~1mcm) film of reaction products, which
could affect the process. However the investigation of
the composition of alloy along the cross-section evi
dences another most probable reason (Fig. 8).
As follows from the data of Fig. 8, the doping metals
(Cr, Al, Ti) are oxidizing rather than the main
component Ni. Hence, its diffusion to the surface should
effect the overall kinetics.
0 5 10 15 20 25 30 35
76
78
80
82
84
86
88
90
0
2
4
6
8
10
12
Ni
C
on
te
nt
o
f N
i,
w
t.%
Distanse from the edge, mcm
C
on
te
nt
o
f M
o,
C
r,
Fe
, A
l,
Ti
, w
t.%
Ti, Al
Fe
Cr
Mo
Fig. 8. Composition of the alloy after durable treatment
in the melt along the cross-section of Fig.7.
The data obtained by X-ray microanalysis
It can be confirmed by comparing of the rate of diffu
sion with the observed corrosion current:
i
nF
=D dC
dx
, (6)
where C is the concentration of oxidized species and i is
mean corrosion current density. The derivative in the
right of (6) can be estimated from the data of Fig.7 as a
slope of concentration profile, and average current from
its time dependency. The calculations give
D = 1,5*10-10 см2/с, which is consistent with common
values for the diffusion in solids.
Thus, the dynamics of the system is determined by
complicated set of processes including slow transport of
the alloy’s components from the bulk metal to the sur
face. Development of mathematic model of the system
and its theoretical analysis is necessary for deeper in
sight.
Considering the shortcut current of the cell as an oxi
dation current of Hastelloy (reaction 2), one can calcu
late the weight loss of the sample from total charge con
sumed, which can be found as the integral of current-
time dependencies. For the data of Fig.6, such calcula
tion gives Q=339 mAh and ∆m=320 mg for the reaction
of Cr oxidation. Such estimation is rough because other
metals (Ti, Al) should also oxidize.
Additions of lanthanum fluotide to the electrolyte do
not change the qualitative character of the process
(Fig. 9).
This is expectable, because La metal is very chemi
cally active and its fluoride could hardly ever be re
duced thus affecting the processes in the cell.
The behavior of the cell is somewhat different in case
when CC composite carbon material is used instead of
glassy carbon (Fig. 10). After 12 days of continuous
work, the sign of electrodes changed and the metal elec
trode became positive.
Higher chemical activity and larger real surface of the
CC material comparatively to GC is evidently the rea
son for this. The reduction of Zr(IV) (1с) in this case is
then faster and higher is the rate of accumulation of Zr
intermediates in the melt. Thus, potential of C electrode
moves far to the negative values and becomes eventual
ly more negative than the Hastelloy.
0 50 100 150 200 250 300 350 400 450 500
0
50
100
150
200
250
300
350
I,
m
cA
Time, hours
X17
X23-2
Fig. 9. Time dependencies of shortcut currents
for the cells (-)Х17/ZrF4 – NaF/C(+)
and (-)Х23-2/ZrF4–NaF+6,7%LaF3/C(+).
The conditions are similar to that of Fig. 5
100 200 300 400 500
-20
-10
0
10
20
30
40
50
60
I,
m
cA
Time, hours
X115
Fig. 10. Shortcut current of the cell Х108/ZrF4 –
NaF/C, where С – СС composite material
After the signs change, reduction reaction prevails at
H electrode and corrosion process is suppressed.
The carbon electrode becomes an anode of the cell
from the very beginning (the sign “-“) if Zr metal is
added to the melt. Zirconium is deposited on the bottom
of carbon crucible, which is also the C electrode of the
cell. Being an active metal, it dissolves anodically:
Zr = Zr(IІ) +2e-. (7)
Conjugated cathode reaction (1) proceeds in parallel.
Hence, total process at C electrode is the dissolution of
zirconium metal, which passes into the melt in form of
intermediate valence species.
The process at H electrode is mainly the cathode reac
tion (1), the corrosion reaction can be neglected. Its rate
is:
I 1
H=i1
H S H , (8)
where i1
H is current density, S H is the surface area
of H electrode.This I 1
H is the same shortcut current,
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65 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 62-66.
which we can measure. It is equal to algebraic sum of
the currents related to the reactions (1) and (6) at C elec
trode.
Since the potential difference between the two elec
trodes in shortcut cell Δϕ=IR is a few mV only (see
above), we can consider the reaction (1) to proceed at
the same potentials on both electrodes. Hence, the cur
rent density of this process at both electrodes should be
practically the same:
i1
H=i 1
C=i1 . (9)
Then, following conclusion comes from the above:
The H electrode serves as a probe for direct rate mea
surement of the reaction of Zr metal with the melt at its
dissolution:
I R
S C
=I 1
S H
1
S C
(10)
or, taking into account that Sc >> S x ,
I R≃
I
S H
S C . (11)
That is, measuring the shortcut current allows for direct
determination of reaction rate of Zr metal with the melt
according to the equation:
Zr + Zr(IV) = 2Zr(IІ). (12)
Figs. 11 and 12 present the data for two electrolytes,
one of them containing additives of LaF3.
20 40 60 80 100 120 140 160
-200
0
200
400
600
800
1000
1200
1400
I,
m
cA
Time, hours
X23-3
Fig. 11. Time dependence of shortcut current for the
cell (-)С/ZrF4 – NaF + Zr/Х23(+). Zirconium content
3,5%, temperature 600оС, electrode surface
areas as in Fig. 6
0 10 20 30 40 50
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
I,
m
cA
Time, hours
X23-1
Fig. 12. Chortcut current for similar cell,
Fig. 9, with 5% LaF3 and 7,5% Zr
In spite of the oscillations, the current tends to de
crease in both cases. Seemingly, the process (11) is over
in 50 – 100 hours. The question upon the character of
formed products is still open. Seems that they are hardly
soluble in the melt and form solid deposits both on the
metal (which is proved experimentally) and on the car
bon surfaces.
CONCLUSIONS
As follows from the results of this work, measure
ments of shortcut currents of the cell Hastelloy/melt/car
bon, together with analysis of polarization curves of
separate electrodes, can be used as a method for investi
gation of the electrochemical reactions in Hastelloy –
fluoride melt – carbon system. Following conclusions
can be drawn:
− corrosion of Hastelloy enhances in the melt ZrF4 –
NaF (eut) if the alloy is in contact with carbon mate
rial;
− the shortcut current corresponds to the rate jf corro
sion, which can be used to estimate the depth of cor
rosion from these data;
− additions of lanthanum fluoride to zirconium fluo
ride – sodium fluoride melt does not significantly ef
fect the corrosion processes;
− zirconium metal reacts with the melt. If it is present
in the system, measurement of shortcut currents per
mits to study the rate of this reaction in a long-term
process.
The work was supported by STCU Project #294.
REFERENCES
1. S.V. Devyatkin, S.V. Volkov, А.А.Omel’chuk,
A.S. Bakai. Electrochemical behaviour Ni-Mo alloys
and alloy additions under anodic polarization in NaF–
ZrF4 melts //Euchem 2004 Molten Salts Conference
Proceedings. 20-25 June 2004. Wroclaw 2004,
p. 154–159.
2.А.А. Аndriiko, О.I. Boiko Studies of intervalence
equilibriums in ionic melts by electrochemical methods
//Ukrainian Chem. Journ. 1987, v. 53, #12,
p. 1279–1286.
3.A.A. Andriiko, E.V. Panov, A.P. Mon’ko. Solid film
at the anode influences the electrorefining of polyvalent
metals. Bifurcation analysis of the problem //J. Solid
State Electrochemistry. 1998, v. 2. p. 198–203.
КОРРОЗИОННЫЕ ПРОЦЕССЫ В РАСПЛАВЕ ФТОРИДОВ ЦИРКОНИЯ И НАТРИЯ. ИССЛЕДОВАНИЕ КО
РОТКОЗАМКНУТОГО ЭЛЕМЕНТА C/(ZRF4 – NAF)ЕВТ/ХАСТЕЛЛОЙ
_______________________________________________________________________________
66 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 62-66.
А.С. Бакай, С.В. Волков, В.М. Ажажа, А.А. Андрійко, А.А. Омельчук, Н.И. Буряк, Б.М. Воронин,
С.Д. Лавриненко
Исследована временная зависимость параметров (ток, напряжение, внутреннее сопротивление) короткозамкнутого гальванического
элемента хастеллой/углеродный материал в эвтектическом расплаве NaF-ZrF4 при 600-650oC. Анализ этих данных, в совокупности с
данными поляризационных измерений на отдельных электродах при разомкнутой цепи, показал, что ток короткого замыкания соответ
ствует скорости коррозии сплава в контакте с углеродом при условии, если металлический Zr в системе отсутствует. В присутствии же
Zr этот ток определяется скоростью взаимодействия циркония с расплавом (отнесенной к единице поверхности хастеллоя), что может
быть использовано для исследования кинетики растворения циркония в расплаве. Установлено, что контакт сплава с углеродом усили
вает коррозию как в чисто эвтектическом расплаве, так и с добавками LaF3. Присутствие же металлического Zr подавляет коррозион
ный процесс.
КОРОЗІЙНІ ПРОЦЕСИ В РОЗПЛАВІ ФТОРИДІВ ЦИРКОНІЮ І НАТРІЮ. ДОСЛІДЖЕННЯ КОРОТКОЗА
МКНЕНОГО ЕЛЕМЕНТА C/(ZRF4 – NAF)ЕВТ/ХАСТЕЛЛОЙ
О.С. Бакай, С.В. Волков, В.М. Ажажа, О.О. Андрійко, А.О. Омельчук, М.І. Буряк, Б.М. Воронін,
С.Д. Лавриненко
Вивчена зміна в часі параметрів (струм, напруга, внутрішній опір) короткозамкненого гальванічного елемента хастелой/вуглецевий
матеріал в евтектичному розплаві NaF-ZrF4 при 600-650oC. Аналіз цих даних в сукупності з даними поляризаційних вимірювань на
окремих електродах при розімкненому колі показав, що струм короткого замикання відповідає швидкості корозії сплаву в контакті з
вуглецем при умові, якщо металічний Zr в системі відсутній. В присутності Zr цей струм визначається швидкістю взаємодії цирконію з
розплавом (віднесеної до одиниці поверхні хастелою), що може бути використано для дослідження кінетики розчинення цирконію в
розплаві. Встановлено, що контакт сплаву з вуглецем посилює корозію як в чисто евтектичному розплаві, так і з добавленням LaF3.
Присутність металічного Zr пригнічує корозійний процес.
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67 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2005. №.4.
Серия: Физика радиационных повреждений и радиационное материаловедение (87), с. 62-66.
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