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ORNL-TM-5783.txt
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ORNL/TM-5783
Distribution
Category UC-76
Contract No. W-7405-eng-26 )
METALS AND CERAMICS DIVISION
COMPATIBILITY STUDIES OF POTENTIAL MOLTEN-SALT BREEDER
REACTOR MATERIALS IN MOLTEN FLUORIDE SALTS
J. R. Keiser
Date Published: May 1977
NOTICE —m ey
This report was prepared as an account of work
sponsored by the United States Government. Neither
the United States nor the United States Energy
Research and Development Administration, nor any of
their employees, nor any of their contractors,
subcontractors, or their employees, makes any
warranty, express or implied, or assumes any legal
liability or responsibility for the accuracy, completeness
or uscfulness of any information, apparatus, product or
process disclosed, or represents that its use would not
infringe privately owned rights,
NOTICE This document contains information of a preliminary nature,
It is subject to revision or correction and therefore does not represent a
final report.
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830 |
operated by ’ MA
UNION CARBIDE CORPORATION AT [ ”
for the
ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION
CUMENT IS UNLIMITED
DISTRIBUTION OF THIS DOCUMENT IS M g{“\
CONTENTS
ABSTRACT . . . ¢« v v ¢ v v 4 4 v & v o W
INTRODUCTION . . . . « « ¢« ¢« ¢ ¢ o« + &
EXPERIMENTAL METHODS . . . . . . .
EXPERIMENTAL RESULTS . . . . . . . .
Thermal-Convection Loop 21A .
Thermal-Convection Loop 23 . . . .
Thermal~Convection Loop 31 . .
Thermal-Convection Loops 18C and 24
Forced-Circulation Loop FCL-2B .
CONCLUSIONS . . . ¢ ¢ v v v v v ¢« o o &
ACKNOWLEDGMENTS . . . . « . . « « + « &
iii
COMPATIBILITY STUDIES OF POTENTIAL MOLTEN-SALT BREEDER
REACTOR MATERTALS IN MOLTEN FLUORIDE SALTS
J. R. Keiser
ABSTRACT
This report summarizes the molten fluoride salt compatibility
studies carried out during the period 1974—76 in support of
the Molten-Salt Reactor Program. Thermal-convection and forced-
circulation loops were used to measure the corrosion rate of
selected alloys. Results confirmed the relationship of time,
initial chromium concentration, and mass loss developed by
previous workers. The corrosion rates of Hastelloy N and
Hastelloy N modified by the addition of 1-3 wt % Nb were well
within the acceptable range for use in an MSBR.
INTRODUCTION
The purpose of this report is to summarize the corrosion studies
carried out for the Molten-Salt Reactor Program during the period
September 1974 through May 1976. These studies were intended to determine
the corrosion resistance of potential Molten-Salt Breeder Reactor contain-
ment vessel materials in molten fluoride salt. The nickel-base alloy
Hastelloy N was used successfully for the containment vessel of an
experimental molten-salt reactor, the Molten-Salt Reactor Experiment.
However, the discovery of irradiation embrittlement and grain boundary
embrittlement by the fission product tellurium led to a program to
develop an alloy that would be sufficiently resistant to the conditions.
To ensure that any new or modified alloy would have a high resistance
to corrosion by the fluoride salt, salt-metal corrosion studies were
made. Materials investigated include Hastelloy N, chromium and niobium
modifications of Hastelloy N, Inconel 601, and type 316 stainless steel.
The stainless steel was tested in LiF-BeF,; (65-35 mole %), and the other
alloys were tested in MSBR fuel salt, LiF-BeF;-ThF,-UF,
(72-16-11.7-0.3 mole 7).
o
Previous reserachers!»? have measured the corrosion resistance of
Hastelloy N and two types of stainless steel in various fluoride salt
mixtures., Their work has shown a multicomponent alloy is corroded by
the oxidation and removal of the least noble component. For Hastelloy N,
the least noble component is chromium. Fluoride salts can oxidize
chromium by reaction with impurities in the salt such as HF, NiF,, and
FeF, and by reaction with constituents of the salt. Impurity reactions
expected are
2HF + Cr = Cr¥-> + Hz (l)
and
FeFo + Cr = CrF» + Fe . (2)
The salt constituent UF, can give the reaction:
2UF, + Cr = CrF, + 2UF3; . (3)
If salt containing UF4 and a small amount of impurities is put into
a Hastelloy N system in which the salt circulates nonisothermally,
chromium will initially be removed from all parts of the system because
of reaction with both impurities and UF,. The impurity reactions are
expected to go to completion fairly rapidly so that they will have an
effect only on the short-time corrosion results. On the other hand,
since the equilibrium constant for reaction (3) is a function of tempera-
ture, this reaction provides a means for the continuous transfer of
'J. W. Koger, Alloy compatibility with LiF-BeF, Salts Containing
ThFy and UF, ORNL/TM-4286 (December 1972).
. ’J. H. DeVan, Effect of Alloying Additions on Corrosion Behavior of
Vickel-Molybdemum Alloys in Fused Fluoride Mixtures M.S. Thesis, University
of Tennessee, August 1960.
chromium from the hotter sections of the system to the cooler sections.
The amount of chromium, AY, removed from a unit area of surface can bhe
shown to be:
where B is a temperature-dependent constant, 'y is the initial concentra-
tion of chromium in the alloy, and I is the diffusivity of chromium in
the alloy. The limiting step for this mass transfer has been shown
to be the diffusion of chromium in the metal when Hastelloy N is the
alloy considered. If other strong fluoride formers — such as Ti, Nb,
or Al — are present in the alloy, mass transfer of these elements would
be expected to occur by the same mechanism as discussed above,
EXPERIMENTAL METHODS
Our corrosion studies have been carried out in five thermal-
convection loops and one forced-circulation locp. Figures 1 and 2
show schematic drawings of these loops. These loops circulate salt
around a syvstem across which a temperature gradient is maintained. For
most of these loops the temperature limits were maintained at 704 and
566°C (1300 and 1050°F), the proposed maximum and minimum temperatures
for the fuel salt of an MSBR. Other important features of these loops
are the removable corrosion specimens and the accesses to the salt to
permit insertion of electrodes for controlled-potential voltammetry.
Voltammetric measurements, which were made by members of the Analytical
Chemistry Division, allowed us to make on-line determinations of the
oxidation potential and corrosion product concentration of the salt.
Detailed explanations of voltammetry are available elsewhere.3s"
The operating conditions for each of the loops are described in Table 1.
M. W. Rosenthal, P. N. Haubenreich, P. B. Briggs, Comps., 7The
Development Status of Molten Salt Breeder Reactorse., ORNL-4812
(August 1972) pp. 153-57.
“J. R. Keiser, J. H. DeVan, and D. L. Manning, The Corrosion Resist-
ance of Type 31€ Stainless Steel to Li,BeF,, ORNL/TM-5782 (April 1977).
ORNL-DWG 68-398TR3
STANDPIPE
CLAMSHELL
HEATERS
SAMPLER
VALVES —
FLUSH
TANK DUMP
TANK
Fig. 1. Schematic of Thermal-Convection
Loop. Scale is 0.15 m. Height shown is 0.76 m.
CRNL-DWG 70-5632R
N
Ry
\i)
FREEZE VALVE |
(TYPICAL) —__
AIR
CORROSION SPECIMENS
(1175°F }
RESISTANCE HEATED SECTION NO.1
THERMOCOUPLE
WELL
Yo-in.OD x 0.042-in. WALL
BALL VALVE HASTELLOY N
HEATER LUGS (TYPICAL) RESISTANCE HEATED SECTION NO. 2
COOLER NO.4
FLOW RATE = ~4gpm
VELOCITY = ~10 fps IN Y%-in. TUBING
REYNOLDS NO. = 6600 TO 14,000
SALT PU
0w i
MP 1] 2
CORROSICON
SPECIMENS
FREEZE VALVES
CORROSION
COOLER NO.2 SPECIMENS
v
FiLL. AND DRAIN TANK
705°C (1300°F)
DRAIN AND FiLL LINE
(&\ s, 1/4-in.0D X 0.035-in. WALL
N
THERMOCOUPLE WELL
Fig. 2. Schematic of Molten-Salt Forced-Convection Corrosion Loop MSR-FCL-2 b.
Flow rate = 0.25 liter/sec; velocity ® 3 m/sec in 13-mm tubing; loop tubing is
13-mm OD by 1.07-mm wall; drain and f£ill line is 6.4~mm OD by 0.9-mm wall.
Table 1.
. Loop
Loop Material
21A Hastelloy N
23 Inconel 601
31 Type 316 Stainless Steel
24 Hastelloy N
i8C Hastelloy N
FCL-2B Hastelloy N
Loop Operating Conditions
Operating
oy Q
Speci?en Salt Yemperature, "C
Material e e e e
Max Min
Thermai-C- ve ! Zoun Leons
Hastelloy N, MSBR /04 366
1%-Nb-mod Hastelloy N Fuel
Inconel 601 MSER S04 566
Fuel
Graphite MSBR 677 550
Fuel
Type 316 Stainless Steel Li,BeF, 649 493
/%-Cr-mod Hastelloy N, MSBR 704 566
127-Cr-mod Hastelloy N, Fuel
3.47-Nb-mod Hastelloy N
10%2-Cr-mod Hastelloy N, MSBR 04 566
15%-Cr-mod Hastelloy N Fuel
Forced-Circualation Loop
Hastelloy N, MSBR 704 566
1%Z-Nb-mod Hastelloy N Fuel
Analytic method
development
Base—-line corrosion data
Measure corrosion rate
ot high-Cr alloy
(Inconel 601)
Investigate raphite
Uy Reaction
Measure corrosion rate
of type 316 stainless
steel in potential
coolant salt
Investigate effect of
Be addition to salt
Measure corrosiun rate
of modified Hastelloy N
alloys
Measure corrosion rate
of modifled Hastelloy N
alloys
Base-line ceorrosion data
in high-velocity salt
9
EXPERIMENTAL RESULTS
Thermal-Convection Loop 21A
Hastelloy N loop NCL 21A was the first loop to be put into operativa
when the Molten-Salt Reactor Program was resumed in 1974. As such, the
loop was used to obtain base-line corrosion data for Hastelloy N and to
provide a test bed for voltammetry measurements of MSBR fuel salt. The
voltammetry results showed that the oxidation potential as reflected
by the U(IV)/U(LIL) ratio remained quite high throughout the 17.5 months
of operation. In fact, with a U(IV)/U(III) ratio of about 10* loop 21A
contained the most oxidizing salt of all the loop experiments.
The first specimens used in this loop were made of Hastelloy N
and were removed for examination about every 2500 hr. Figure 3 shows
the weight change as a function of the exposure time for these specimens
for up to 10,000 hr in salt. From this figure the rate of the weight
change clearly decreased with time. Figure 4 shows that the change
in weight varies as the square root of time, as predicted from Eq. (4).
ORNL-DWG 77-3820
T
566°C i1
_-——'_Q'_-——-—-
1 /.——-—-——_-—F' l
635°C .
0
o~
E 4
)
o 4fi<\\
E N
: ®
Q-2 S
\.
~Jo4°C
\"
0 2000 4000 6000 BOOO 10,000
EXPOSURE TIME (hr)
Fig. 3. Weight Change vs Exposure Time for Hastelloy N Specimens
Exposed to MSBR Fuel Salt at 566, 635, and 704°C in Thermal-Convection
Loop 21A.
ORNL-DWG 76-4843
AM(mg/cn?)
°
0 20 40 60 80 100
4
(EXPOSURE TIME)”2 (VRp)
Fig. 4. Weight Change vs Square Root of Exposure Time for
Hastelloy N Specimen Exposed to MSBR Fuel Salt at 690°C in Thermal-
Convection Loop 21A.
Specimens that had been exposed for 7500 hr in the hottest and coldest
parts of the loop were examined metallographically. As is evident in
Fig. 5 the pitting on the higher temperature specimen was limited to
about 5 um, indicating that the effect of the salt was relatively mild.
Following the 10,000-~hr exposure of standard Hastelloy N, loop 21A
was used to test specimens of 1Z-Nb-modified Hastelloy N (experimental
heat 522). Only a short exposure was achieved with these specimens
before a power supply malfunction terminated the experiment, but the
short-time corrosion results (Table 2) compare very favorably with
results for standard Hastelloy N.
B Y-134311
=
*t
Fig. 5. Hastelloy N Exposed to MSBR Fuel Salt for 7500 hr at
(a) 704 and (b) 566°C. 500x.
Table 2. Hastelloy N Corrosion Rate Measurements from Loop 21A
Corrosion Rate, mg cmleyear‘l, at Each
Total - Exposure Temperature
Alloy Exposure . uXxposu npe
X "(hr) 566°C 635°C ,'_704°c
- ~ Standard 10,009 +1.17 +0.39 —3.09
-
1Z Nb Modified 1,004 0.0 —0.25 © =3.30
e e i iy
10
Théfm&léConvection'Loqp 23
The observation that the high-chromium alloy Inconel 601 (23 wt Z Cr)
resisted intergranular attack by tellurium led to the construction of a
thermalfconvection loop to determine how severe the corrosion by MSBR
fuel salt would be. After the new lobp was filled with salt, voltammetric
techniQues were used to follow the change in the U(IV)/U(III) ratio
as an indication of the extent of the initial reaction between chromium
and.UFu. The U(IV)/U(III) ratio decreased very rapidly, dropping to
about 40 within a fefirdays; meaning that considerable reaction was
probably occurring betweeh the salt and this Inconel 601 loop. Inconel
601 specimens exposed 721 hr all showed a weight loss, and that shown
bythe,hOttthképe¢ipen-was‘véiy iarge_(>30 mg cm~ > year~').. Furthermore,
the material lost by the hottest specimens was not removed umiformly from
the surface, but resulted in the formation of the porous surféce structure
shown in Fig. 6. As shown in Fig. 7; electron microprobe examination
of this specimen showed high thorium concentration in the pores. Since
the only known source of thorium.was the ThFy contained in the salt, the
salt likely penetrated the pores. Continuous line scans made with the
microprobe for the elements Ni, Cr, and Th, shown in Fig. 8, clearly show
the depletion of chromium near the surface. These results provide
further evidence of the presence of thorium in the pores. Diffusion
calculations provide another piece of evidence indicating that salt
must have penetrated the pores. Based on the bulk chromium concentration
of 23 wt %, microprobe measurements® determined a chromium concentration
of 6.6 wt Z at the surface of the specimen in Fig. 6. From these concentra-
tions, a depletion depth of 80 yum taken from Fig. 8, and diffusion values
taken from Evans, Koger, and DeVan,® we calculate that the exposure time
wofild have to be néarly 1000 times greater to attain this concentration
profile as a result of bulk diffusion alone at 704°C. To obtain this
SR. S. Crouse, ORNL, Private Communication, July 1975.
, SR. B. Evans III, J. W. Koger, and J. H. DeVan, Corrosion in
Polyt@ermal Loop Systems II. A Solid State Diffusion Mechanism With
and Without Liquid Film Effects, ORNL-4575, Vol. 2 (June 1971).
11
500x%,
at 704°C.
Incone1 601 Exposed to MSBR Fuel Salt for 721 hr
6._
Fig.
Y-131294
Bac’ksCaflér‘ed_ Electrons
Xays
Electron-Beam Scanning Images of Inconel 601 Exposed to MSER
720 hr
ThMe
Fig
at 704°C.
~
-
7
Salts for
=y
i
'
'
t
:
12
ORNL-DWG 77-3819
NN
sl
N4
T
»
NORMALIZED CONCENTRATION
Th
L/ P2 e R
0 20 40 €0 80
DEPTH FROM SURFACE (um)
Fig. 8. Microprobe Continuous Line Scan Across Corroded Area in
Inconel 601 Exposed to MSBR Salt for 720 hr at 704°C.
profile with an exposure time of 721 hr the exposure temperature would
have to be about 1000°C, nearly 300°C higher than it was. Thus, to
establish the gradient that was observed, salt was most likely present
in the pores to provide a short—circuit'path for removal of the chromium.
Examination of a specimen from the coldest part of the loop revealed
surface deposits,'shown in Fig. 9, which were identified by microprobe
analysis as chromium. The conclusion from this test is that Ihconé1'601
would be unsuitable for use in a molten-salt breeder reactor.
Fig. 9. Inconel 601 Exposed to MSBR Fuel Salt for 721 hr at 570°C. 500x,
13
It is expected that the lower limit for the U(IV)/U(III) ratio
in an MSBR will be determined by the conditions under which the reaction
4UF3 + 2C = 3UF, + UC; (5)
proceeds to the right. Because the U(IV)/U(III) ratio of the salt
in loop 23 had decreased to less than 6, we decided to try to reproduce
the results of Toth and Gilpatrick,7 shown in Fig. 10, which predict
that UC, should be stable at the lowest temperatures that could be
maintained in this loop, 545-550°C. However, graphite specimens ex-
posed to the salt for 530 hr did not show any evidence of UC,. Since
the specimens used were made of pyrolytic graphite, the high density of
the material likely limited contact of the salt and graphite. The exper-
iment was repeated by exposing a less dense graphite for 530 hr,
L. M. Toth and L. O. Gilpatrick, The Equilibrium of Dilute UFj
Solutions Contained in Graphite, ORNL/TM-4056 (December 1972).
ORNL-DWG 7212321
TEMPERATURE {*C)
500 550 600 650 700
. /
i/
3 /I &
///i ]
o
o
]
o
o
o
N
i
H
- N
N
1
O
[¥7]
log,y @
R=UFy /{UFy + UF,)
\d
N
/T
130 125 420 445 110 105 100
1000/ (ok)
Fig. 10. Equilibrium Quotients, & = (UFa)H/(UFu)a, Versus Temperature
for UC, + 3UFq(d = 4UF3(q) + 2C in the Solvent LiF-BeF,-ThFy,
(72-16-12 mole Z;.
14
then checking the graphite surface for the presence of a new phase by
x-ray diffraction. A new phase was found, and it was tentatively identi-
fied by 0. B. Cavin as UO;. If indeed this compound was U0, it probably
resulted from a uranium fluoride-water reaction, but it could have come
from the hydrolysis of UC, that had been formed by reaction (5). On
the basis of information from L. M. Toth® that nucleation of UC, under
our operating conditions could be very slow, a longer exposure was
undertaken. Two specimens of the less dense graphite were exposed for
about 3000 hr at a minimum temperature of 555°C in salt in which the
U(IV)U/(III) ratio had dropped to about 4. However, x-ray analysis of
these specimens showed no evidence of a phase other than the salt and
graphite. No further investigations were carried out because of the
termination of the program.
Thermal-Convection Loop 31
Thermal-convection loop 31 is constructed of type 316 stainless steel,
has type 316 stainless steel specimens, and has been used for corrosion
measurements with one of the altermnative coolant salts, LiF-BeF:
(66=34 mole %). For the first 1000 hr of operation this loop was used
to gather base-line corrosion data with as-received salt, which contained
a relatively high concentration of impurity FeF;. As shown in Fig. 11,
fairly significant weight changes occurred in the specimens, especially
during the first 500 hr. Metallographic examination of specimens from
the hottest and coldest positions showed, respectively, pitting and
deposition, as is apparent in Fig. 12. Electron microprobe examination
of the deposits indicated they were predominately iron, and we expect
that the deposition occurred as a result of reaction (2). Bulk salt
analyses and voltammetric measurement of the FeF, and CrF, concentrations
of the salt during the first 1000 hr support this idea.
To learn if addition of a reductant to the salt would decrease
the impurity level and consequently lower the corrosion rate, beryllium
was added to the salt. New specimens were then inserted, and the corrosion
rate was measured for stainless steel in this "'reducing' salt. As long
®L. M. Toth, private communication.
15
Y-137150
ORNL-DWG 76-3496
q_—_
& AS RECEIVED" SALT
X AFTER BE ADDITION
—_..u._“__—q-—__-—_—
§ %
C
i |