ORNL/TM-5503
Temperature Gradient Compatibility
Tests of Some Refractory Metals and
Alloys in Bismuth and Bismuth-Lithium
Solutions |
R. DiStefano
B
J.
0. B. Cavin
Printed in the United States of America. Available from
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U.S. Department of Commerce
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Wy ‘
ORNL/TM-5503
Distribution
Category UC-76
Contract No. W-7405-eng-26
METALS AND CERAMICS DIVISION
TEMPERATURE GRADIENT COMPATIBILITY TESTS OF SOME REFRACTORY
METALS AND ALLOYS IN BISMUTH AND BISMUTH-LITHIUM SOLUTIONS
J. R. DiStefano and 0. B. Cavin
Date Published: November' 1976
Sl NOTICE — E
: eport was prepared as an account of work |
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i | mesearch and Development Administration, nor any of}
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.| or uscfuiness of any informati tus, product or
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)
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for the
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DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED Qfi_
&
Abstract .« « ¢ o« ¢ o o ¢ o o
Introduction . « « ¢« ¢« ¢ « &
Results o+ o o o ¢ ¢ o o ¢ o &
Quartz Tests - Group 1 .
Quartz Tests - Group 2 .
Metal Loop Tests in Bi—2,5%
Summary and Conclusions . . .
Acknowledgments . . . . . . &
REfETrenCeS « o o o o o o o o o o o o o o o o s
CONTENTS
-
iii
11
11
15
30
32
33
33
TEMPERATURE GRADIENT COMPATIBILITY TESTS OF SOME REFRACTORY
METALS AND ALLOYS IN BISMUTH AND BISMUTH-LITHIUM SOLUTIONS
J. R. DiStefano and 0. B. Cavin
ABSTRACT
Quartz, T-111 and Mo thermal—convection 1oop tests were
conducted at temperatures up to 700°C (100°C AT) to determine
the compatibility of several refractory metals/alloys with
bismuth and bismuth-lithium solutions for molten salt breeder
reactor applications. Methods of evaluation included weight
change measurements, metallographic examination, chemical and
electron microprobe analysis, and mechanical properties tests.
Molybdenum, T-111, and Ta—10% W appear to be the most promising
containment materials, while niobium and iron-based alloys
are unacceptable.
INTRODUCTION
A key feature in the conceptual design of the single-fluid molten
salt breeder reactor (MSBR) is the connecting chemical processing plant
to continuously remove protactinium and fission products from the fuel
salt (Fig. 1). Protactinium, the intermediate element in the breeding
chain between thorium and 233U, has a significant neutron capture cross
section and must be kept out of the core to obtain a good breeding ratio.
Rare-earth-element fission products are also neutron poisons and so must
be stripped from the fuel. A promising approach for MSBR fuel processing
uses liquid bismuth containing dissolved lithium and thorium as reductants
to extract protactinium and rare earth elements from fuel salt containing
both uranium and thorium (Fig. 2). 1In 1968 the chemical feasibility of
this process was demonstrated. .
' One of the requirements for the development of the reductive extraction
process is identifying materials that are compatible with both molten
flouride salts and bismuth-containing reductants. Hastelloy N (Ni—?% Cr-167%
'Mo—5% Fe) has excellent compatibility with molten salts at 500~700°C;?
however, it does not have good compatibility with bismuth. Of ten elements
tested, graphite, tungsten, and molybdenum appear most promising (Table 1)
and they are also compatible with molten flouride mixtures, Extensive
investigations of bismuth as a reactor coolant have shown that inhibitors
- such as magnesium, titanium, and zirconium are often required to reduce
the high corrosion rates of conventional materials in bismuth.?!
However, these types of inhibitors would not be readily acceptable for use
in the molten salt system because they would be eliminated from the bismuth
stream by several of the proposed steps in processing.
- ORNL-DWG 68-1185ER
PRIMARY
SALT PUMP
SECONDARY
NoBFy-NaF SALT PUMP
COOLANT SALT
PURIFIED
SALT
+— GRAPHITE -
MODERATOR
REACTOR
HEAT
EXCHANGER
566°C .
-
| -CHEMICAL
PROCESSING
PLANT
TLiF ~BeF, - Thiy - UF,
FUEL SALT e STEAM GENERATOR
TURBO-
GENERATOR
STEAM
Fig. 1. 8ingle-Fluid, Two-Region Molten-Salt Breeder Reactor.
N
ORNL-DWG 71-9004R
FUEL SALT
RECONSTITUTION
URANIUM PROTACTINIUM RARE EARTH
ELEMENTS
REMOVAL
REACTOR
REMOVAL REMOVAL
FISSION PRODUCTS
TO WASTE
Fig;'Z. - Simplified Flowsheet for Processing the Fuel in a Molten-
- Salt Breeder Reactor. . - ' , . o :
v}
Table 1. Solubilities of Some Elements in Bismuth at 600°C
Element Solubility, ppm -~ Reference
W -~ Not detectable 3
C | - <1 > , 4
Mo | <1 S 536
Be 6 - 6
v 45;110 . 11
Fei o =10 | - 5
Cr : 145 5
Nb o 100200 8
Co ' 590 9
Ni | 65,750 | 10
The low solubility of graphite, tungsten, and molybdenum in bismuth
has been confirmed in compatibility tests by other investigators, and, in
addition, rhenium, tantalum, tantalum alloys, and certain ceramic oxides
and carbides also appeared to be promising for bismuth containment.12_17
In a high temperature isothermal system thermodynamic equilibrium
among components is generally reached relatively quickly; the compatibility
of the system can often be predicted if phase diagrams of the components
are known. When there are thermal fluctuations ‘and/or gradients within
a system and the equilibrium constant for a corrosion reaction is
temperature dependent, continuing corrosion can occur. For example, 1if
iron dissolves in bismuth, and the solubility increases with temperature,
simultaneous dissolution and deposition of iron will occur in the system.
Under steady state conditions the solution or deposition rate equation for
a given location can be expressed.
J K( Cear = )
where J is the flux of material entering or leaving the liquid metal, X is
a rate constant, C,,. 15 the equilibrium solubility of the solute, and C
is the actual concentration of the solute. The driving force for either
dissolution or deposition is directly proportional to the concentration
differential that exists in the system at a particular location and the
appropriate rate constant for the rocess. Our data (Table 1) agree with
previous compatibility experiments'? that suggest that C is very low
for graphite and the refractory metals tungsten, rhenium, moiybdenum, and
tantalum. Niobium and iron are somewhat less promising, but still show
relatively low solubility in bismuth.
The reductive extraction process for protactinium removal uses
bismuth containing small concentrations of lithium, and the metal transfer
process for removing rare earth element fission products from the fuel
‘salt uses bismuth containing 2-3 wt %Z Li. Since the solubility of
materials in bismuth-lithium solutions could be appreciably different from
their solubility in pure bismuth (Table 1), tests had to be conducted in
the appropriate chemical processing solutions.
| Thermal convection loops used in these tests were fabricated from
quartz, T-111 (Ta—87% W-2%Z Hf), and molybdenum. The quartz loops were
satisfactory for testing in pure bismuth and in bismuth containing up to
0.01 wt Z lithium. No quartz loops with higher lithium concentrations
were operated because of the reaction between Si02 and lithium. Loops
fabricated from quartz were relatively inexpensive and could be operated
in an alr environment. Samples of the materials being tested were
suspended in the vertical hot- and cold-leg sections (Fig. 3). To prevent
the samples from floating, they were attached to a quartz rod that was
held at the top of each leg. Several types of sample geometries were
used, including flat tabs, cylindrical tubes, and cylindrical and sheet
tensile specimens.
- External heaters were placed on each leg to preheat the loop above
the melting point of bismuth or the Bi-Li solution. A pot containing the
- melt to be circulated was attached by a mechanical connector to the bottom
of the cold leg (Fig. 3.). Before filling, the loop was evacuated so a
pressure differential would force the liquid into the loop. During
operation, the metal line below the quartz was unheated to allow formation
of a solid plug to serve as a freeze valve. When the test was completed,
this section of the line was heated to allow the fluid to drain back into
the attached pot.
The design of the metal thermal convection loops was similar to that
of the quartz loops (Fig. 4). Two sections of 7/8-imn. OD X 0.050 in.
tubing were bent to form one vertical-horizontal segment (Fig, 4). The
segments were joined by making gas-tungsten-arc saddle welds at the top
and bottom of the vertical legs. Tubular sheet and tensile specimens
were suspended in each of the vertical legs. Wires linked the specimens
together to ridigly attach the specimen chain to the vertical legs at the
top and bottom.
The metal loops were used to test a bismuth-lithium alloy containing
2,5 wt Z Li. This alloy was prepared and purified in a molybdenum-lined
stainless steel container (labeled "transfer pot" in Fig. 4)., The lines
extending from the top of the loop surge tank were made of tantalum for
ease of fabrication and corrosion resistance. A commercially-produced
Nb—-1% Zr-type 316 stainless steel dissimilar metal joint connected the
transfer pot to the loop. The Nb—1% Zr end was welded to the tantalum
tube; a mechanical fitting connected the stainless steel end to the ,
transfer pot lines. Two types of heaters were used: The main hot-leg
heater contained a radiating tantalum element while standard Calrod
heaters were strapped to other portions of the loop (Fig. 4). The
molybdenum loop is shown (Fig. 5) prior to being installed in a vacuum
chamber where the test was runm. _
vl
*
Fig. 3.
and Cold Legs.
Y-93581
Quértz Thermal Convection LooP'w:Lth.Metal Samples 1in Hot
" ORNL-DWG 74-12762
TO VACUUM/ '
Ly THERMOCOUPLE WELL
THERMOCOUPLE WELL = TO vACUUM
CLOSED FILL LINE — SURGE TANK
MOUNTING FRAME ~
VERTICAL LEGS —___ ||
P
L
N } Lt HOT LEG HEATER
b
Ly
i
i
|
TEST SPECIMENS - L
TYPICAL 2 L
b
t
CALROD HEATERS
{3 LEGS)
A
,s
POST TEST h_,
DRAIN LINE -
OUMP TANK
@m
Fig. 4. Design of Metal Thermal Convection Loop Apparatus.
(i
TR
L5k
kA
¥
ER T
Blish LR
Vo
Molybdenum Thermal Convection Loop Prior to Test in
Fig. 5.
Vacuum Chamber.
After evacuation and baking out of the vacuum chamber, the entire
loop was heated to 500°C, since the melting point of Bi—2.5 wt % Li is
slightly less than 500°C (Fig. 6). As the transfer pot was heated to
600°C (900°C in the case of the molybdenum loop), the loop was evacuated,
and differential pressure was used to transfer the bismuth-lithium
solution into the loop. During operation the tantalum heater heated the
hot leg, but the required temperature differential was maintained with
little or no heat to the rest of the loop from the Calrod heaters.
To stop the test, the heaters were turned off and the bismuth-
lithjum solution was allowed to solidify. When the system had cooled,
the vacuum chamber was opened and the bottom of the cold leg cut off. A
drain tank was placed immediately under this portion of the cold leg, the
chamber re-evacuated, and the loop heated to approximately 600°C to
allow it to drain.
Samples generally_required cleaning before meaningful weight and |
dimensional measurements could be obtained. Several different techniques
removed the bismuth or Bi-Li that adhered to the samples. If the amount
were small, mechanically cleaning the samples was sufficient. A more
satisfactory technique was to amalgamate the bismuth by dipping the
sample into hot mercury (approximately 100—-150°C) and then removing the
amalgam mechanically.
Bismuth used in these studies was Grade 69 obtained from Cominco
American Corporation. Except for Quartz loops 1 and 2, the as~-received
material was first purified by bubbling hydrogen through the molten metal
for two hours at 350°C, in a molybdenum-lined container. Initially the
exit gas burned intermittently and had a reddish hue. Subsequently the
gas burned continuously and was almost colorless indicating that reaction
with hydrogen had ceased. If an alloy were required, solid lithium was
added to solid, purified bismuth, and the mixture was heated above the
melting temperature of the alloy. The phase diagram shows that the melting
temperature of the alloy varies with lithium concentration (Fig. 6).
Additionally, in the Group 1 series (see results section) the bismuth or
bismuth-1ithium solution was filtered prior to introduction into the loop
through a type 316 stainless steel filter having openings of 10 um.
Determinations of various impurities before and after the various treat-
ments (Table 2) showed no significant improvement in the purity of the
bismuth; however, visual observations of the exit gas flame and the
appearance of the bismuth indicated that some purification had occurred
during hydrogen firing.
The addition of more lithium to bismuth as required for the two metal
thermal convection loop tests was much more difficult because (1) the
melting temperature of the alloy is much higher, (2) the bismuth-lithium
reaction is strongly exothermic, and (3) during alloying there is a
‘tendency to form LizBi (Bi—9% Li), which melts at 1150°C. A special
apparatus was used to prepare Bi—2.5% Li for these tests (Fig. 7).
Lithium was first purified by hot gettering with zirconium foil at 800°C,
and the required amount transferred into a type 304 stainless steel
container. Sequential steps were then as follows:
1. Solid bismuth was loaded into a molybdenum-lined stainless steel
container and the container was sealed under argon.
ORNL-DWG 75-9963
WEIGHT PERCENT BISMUTH
60 80 9092 94 9% 97 28 99
1200 g MEFT T 7 |
1100 /M
R
W1/
/1N
8
\\
LiyBi
/
/ LiBi
TEMPERATURE (°C)
3
Q
8
'l 415°
by,
400 ' 300° _ \
| LiBi(H)
!
300 | 27
! 195.5)
200 1R 15) ]
T 475
(180% > _j,
5
100 T
o .
0O © 20 30 4 S € 70 8 %0 100
) ATOMIC PERCENT ISMUTH Bi
‘Fig. 6. Lithium-Bismuth Phase Diagram.
Table 2. Concentration of Impurities in Bismuth
Used in Quartz Loop Tests, ppnm
c:::‘;:fl‘n o er Fe Mn M Mo c Hy 0z
As-received <5 <5 <5 <5 <1 <20 <1 <6
| After hydrogefi ireatment 2 8 1l 3 | 0.1 - NDa. 5 9
Filtered through stainless 20 30 3 0.2 w* s 7
steel screen
aND = Not determined.
TO OIL BUBBLER-VENT
T
ORNL-DWG 74-12579
304SS LITHIUM FILL LINE
Tovi Ay TO JRGON AND BLOW BACK LINE
Vi
304SS THERMOCOUPLE WELL
30455~ 3 Va4 TO VACUUM
<1 N\ —
304584V, 1o ]
~"ARGON
T ’ '
: ml A ot
: / { o } } -
E ; HYDROGEN § / TO ARGON
1~ MOLYBDENUM TUBES |
YN et 4—304SS
2. : | THERMOCOUPLE
; MOLYBDENUM LINER | WELL |
? i=-304LSS
: PISMUTH / 4| | LITHIUM 4
iy i< 304s5S
e 4 in,———— 25 in—»
Fig. 7. System Used to Prepare and Purify Bi-Li Alloy.
ot
11
2. The bismuth container was evacuated and heated at 650°C.
3. Hydrogen was bubbled through the molten bismuth for approximately
24 hr.
4, The temperature of the bismuth was raised to 700°C; the lithium
was heated to 250°C in a separate container.
5. Lithium was transferred into the bismuth container by pressure
differential. Molten lithium was injected below the liquid
bismuth surface (Fig. 7).
After alloying a sample was taken and a typical analysis was
lithium-2.4%, hydrogen-l4 ppm, and oxygen-90 ppm.
Nominal operating conditions for all of the loop tests was 700°C
maximum hot leg temperature and a minimum cold leg temperature of 590—
625°C. For the quartz loop tests Cr-Al thermocouples were used; they
were located in wells protruding through the quartz into the liquid
stream. Thermocouples made of Pt, Pt—10%Z Rh were similarly inserted into
wells through the T-111 tubing wall to measure temperature in test CPML-1.
In test CPML-2 Pt, Pt—10% Rh thermocouples were again used, but they were
strapped to the outside surface of the molybdenum tubing.
All quartz loops were operated in an external air environment.
Refractory metal loogs were operated in a vacuum chamber at pressures of
107" to 10' Pa (10~° to 10~7 torr).
RESULTS
Compatibility results from these experiments can be grouped according
to the materials tested and the lithium concentration of the bismuth,
Bismuth and Bi—0.01% Li solutions (Groups 1 and 2) were circulated in
quartz loops containing samples of various metals (Table 3). A solution
of Bi—2.5% Li (Group 3) was circulated in a molybdenum loop (CPML-1) for
3000 hr and in a T-111 loop (CPML-2) for 8700 hr. 1In each case the
loops contalned samples of the same material as the loop itself. Alloys
investigated were based on molybdenum, tantalum, niobium, and iron.
When Nb or Fe-based alloys were included, the tests operated for relatively
short times before the flow stopped (Quartz tests — Group 1). All tests
using only molybdenum alloy or tantalum alloy samples completed their
scheduled operating period (up to 10,000 hr) (Quartz tests — Group 2).
Quartz Tests - Group 1
, Quartz loops 1 and 2 contained samples of TZM, tantalum, Nb—1 Zr and
niobium and operated for 18 and 23 hr respectively before flow stopped.
In loop 1 flow was stopped by a plug which formed at the top of the cold
leg. The quartz fractured at this point, exposing the bismuth to air.
The samples remianed immersed in bismuth, but the weight changes (Table 4)
may reflect exposure to air as well as mass~transfer effects, Loop 2
stopped flowing after 23 hr. Although a solid plug had again formed, it
did not fracture the quartz. Samples of the plug were analyzed spectro-
graphically and niobium was found to be the major constituent of the plug.
12
Table 3. Summary of Quartz Loop Tests (700°C max, ~100°C AT)
Time
Loop Samples Fluid * (hr)
, : : Group 1
1 TZM2/Ta/Nb/Nb~1% Zr Bi 185
2 TZM/Ta/Nb/Nb—1%Z Zr Bi 23
4 Nb/Nb—1% Zr - Bi 115,
9 Fe—5% Mo Bi 423
Group 2 _
3 Mo/T2M® Bi 3,000°
6 Ta/T-111 Bi 3,000°
10 Mo brazed with Bi 2,1002
. Fe-based alloys 3,000
17 Ta—10% W Bi 3,000°
7 Mo/TZM Bi—0.01% Li 3,000°
11 Mo Bi—0.01% Li 3,0002 |
12 T-111 Bi—0.01% Li 3,000°
13 Ta Bi—0.01% Li 3,000,
18 Ta—10% W Bi—0.01% Li 3,000°
15 Mo Bi—0.01% Li 10,000°
o]
TZM is Mo—0.5%Z Ti—0.1% Zr.
bFlow stopped.
cTerminated, samples dissolved.
d -111 is Ta—8%7 W—-2% Hf.
eCom.pleted.
Table 4. Weight Changes in Samples From Quartz Loop 12
Material Location We%iz;cgginge C?;;?i;gfiyzite
Tantalum Hot leg 0.4 —=195 |
TZM Hot leg -15.9 —-7,738
Nb*i% Zr Hot leg —54.3 '26’42fi;f}gn
Niobium Hot leg —31.0 15,084, ;
Tantalum Cold leg +0:3 !*Ifi61
TZM Cold leg —11.0 ;:i%%
Nb—1% Zr Cold leg 4.9 —2,385;
Niobium Cold leg -1.3 —633
@After 18 hr in bismuth at 700°C maximum
temperature differential.
temperature and 100°C
13
X-ray diffraction also indicated the presence of small amounts of
Bi203-Nb20s. To eliminate the possibility that oxygen contamination of
bismuth might be responsible for these results, bismuth used in subsequent
group 1 tests was purified by bubbling hydrogen through the molten metal
for 2 hr at 350°C and then filtering as described previously.
Loop 4 contained niobium and Nb—1Z%Z Zr samples and was operated at a
maximum temperature of 705°C with a temperature differential of 75°C for
a period of 115 hr before flow stopped. ZX-ray diffraction analysis of a
sample taken from the plug that had formed in the cold leg showed the
presence of several phases: bismuth, alpha zirconium, and several
unidentified phases. ' Niobium was not detected by x-ray diffraction
analysis, but the atomic radius of niobium is the same as bismuth and it
could occupy substitutional lattice sites in the bismuth unit cell without
affecting the lattice parameter of the bismuth. Spectrographic analysis
of a sample of the plug materials indicated that it contained 0.5% Nb,
100 ppm Zr, 300 ppm Ni, 500 ppm Fe, with the remainder bismuth. The iron,
nickel, and chromium probably dissolved in the bismuth during transfer
through stainless steel lines and filter during loading!of the loop.
The samples from this test are shown (Fig. 8). The Nb—1% Zr sample
from the hot leg was almost completely dissolved. Bismuth remained on
the samples after draining and was not successfully removed; therefore,
weight change data were not recorded.
Fig. 8. Niobium and Nb—1% Zr Samples From Quartz Thermal Convection
Loop 4.
K i
14
Photomicrographs of selected samples are shown (Fig. 9).. The niobium
-sample from the hot leg is thicker than it was originally; however, the
unattacked center section is less than 0.020 in. compared to an original
sample thickness of 0.027 in. In electron probe scanning images of this
sample (Fig. 10) the layer adhering to the niobium appears to be primarily
bismuth, but the particles within the bismuth are niobium.
In niobium—17% zirconium samples from hot- and cold-leg regions of
~loop 4 (Fig. 9) the cold-leg sample has a greater unaffected center section
(about 0,027 in.) than the hot-leg sample, and has greater total thickness
(about 0.040 in. compared with 0.030 in.). This suggests that there has
been considerable mass transfer of material from hot- to cold-leg sectioms.
Loop 9, which contained Fe—5% Mo samples, operated for 423 hr before
a power failure stopped the test. After 216 hr one of the samples from
the hot leg came loose from its wire holder and floated to the top of the
bismuth. When the loop operation was stopped after 423 hr this was the
only sample recovered; all other samples apparently had dissolved.
Analysis of a sample of the bismuth from the top of the cold leg determined
that it contained 13% iron and 0.018%7 molybdenum. The unheated area of the
cold leg above the samples was the coldest portion of the loop and is
probably where deposition occurred. If so, bismuth flowing past the cold-
_Y-132369
s C.035 INCHES —
030 m T wox 1000 m 1
- -Iz‘ig. 9. Niobium and Nb—1Z Zr Samples From Quartz Thermal Convection
Loop 4. ' ‘ N
15
Y -1002%0
Back-Scattered Elecirons
Fig. 10. Electron Probe Scanning Images of Niobium Bot Leg Sample
From Quartz Thermal Convection Loop 4.
leg samples could continue to have a strong dissolution effect. Static
capsule tests of this alloy for 600 hr at 650°C in bismuth did not show
detectable dissolution, thus indicating a strong influence of the rate
constant X (Eq. 1) on mass transfer of iron in temperature gradient systems
involving bismuth.
Quartz Tests - Group 2
Ten quartz loops were tested with samples of tantalum (alloys) and/or
molybdenum (alloys).. Loops 3, 6, 10, and 16 circulated bismuth while the
remaining six loops circulated Bi—0.01% Li. All of these loops were
operated for 3000 hr except for loop 10, which was operated 2100 hr,
and loop 15, which was operated 10,000 hr.
Loop 10 contained molybdenum samples that had been brazed together
with iron-based alloys. Four different compositions were used:
Braze Alloy Designation Nominal Composition (wt %)
16 M Fe—0% Mo—4% C-1% B
35 M Fe—~15% Mo—4% C—1% B
36 M Fe—25%Z Mo—4%Z C—1%Z B
42 M Fe—15% Mo—4% C1% B—5Z Ge
16
The loop operation was stopped after 2100 hr and weight changes of the
samples were measured (Table 5). They were very small weight changes, and
analysis of the bismuth gave little evidence that dissolution had occurred.
Iron and molybdenum concentrations were less than 3 ppm, and carbon was
&-13 ppm. However, metallographic examination showed that each of the
braze alloys contained an outer surface layer (Fig. 11). A more detailed
examination was made of the samples brazed with alloys 16 M and 35 M.
For the alloys before test no outer surface layer was visible (Fig. 12).
An electron beam microprobe analysis determined the distribution of iron,
molybdenum, and bismuth in the braze after test. In alloy 16 M the outer
surface layer was rich in iron; the white particles contained 88% bismuth,
5% molybdepum, and less than 0.57 iron (Fig. 13). An area adjacent to
the white particles contained 41% bismuth, 23% molybdenum, and 4% ironm,
while the dark spots were predominantly iron (about 70%Z). 1In alloy 35 M
the surface layer appears to be subdivided into two segments (Fig. 14)
with the outermost segment rich in molybdenum and the inner segment
predominantly iron. Bismuth has completely penetrated the braze (Fig. 15).
Thus, although there was little evidence of mass transfer, bismuth did
react with the iron-based braze alloys. -
The effect of lithium in bismuth on the corrosion rates of Mo/TZM,
Ta/T-111, and Ta—10% W is shown in Tables 6, 7, and 8. Maximum corrosion
rate (weight loss) of molybdenum in Bi—0. 01% Li was 16.9 mg/cm®-yr compared
with 1.64 mg/cm?eyr in pure bismuth. Tantalum and T-111 also showed higher
rates of mass transfer in Bi~0.01%Z Li, but Ta—10%Z W showed the opposite
effect — a higher corrosion rate in bismuth compared with Bi-0. 01/ Li,
where all the samples exhibited small weight gains.
Table 5. Weight Cfi;nges in Molybdenum—~Braze Samples Exposed to Bismuth
in Quartz Thermal Convection Loop Tests
Conditions: 700°C Max Hot Leg Temperature; 95°C Temperature Differential
Weight Change (mg)
Braze Alloy
Hot Leg Cold Leg
16 M (Fe—4%Z C-1%Z B) | - -1.6 Sample not included
35 M (Fe4% C-1% B—15% Mo) —4,2 +12.2
36 M (Fe—4% C—1% B—25% Mo) ~16.6 +5.0
42 M (Fe4% C-1%Z B—-15% Mo—5%Z Ge) 0.2 +2.2
Y-106373
Alloy 16 M (Fe-4C ~18) i Alloy 36M (Fe-25Mo-4C ~18)
. NS ~
Alloy 35M (Fe-15Mo0-4C -18) Atloy 42M (Fe-15Mo~4C -1B-5Ge)
Fig. 11. Brazed Molybdenum Samples After Exposure to Bismuth in
Quartz Thermal Convection Loop Test 10,
.
18
Y=-132372
ith Alloy 16M
Brazed wi
ith Alloy 35M
Mo Brazed wi
Brazed Molybdenum Samples Before Test in Quartz Loop 10.
Fig. 12.
’
t
Lighter
Ke
|
Fe
B
19
Electron Beam Scanning Images of Molybdenum Sample Brazed
Backscattered Electrons
13.
Fig.
regions indicate higher concentration of listed element.
o
—t
o
o
o
-
N
4
:
o
=
i
=
3
g
0
o
K
o
+
5
o
12
G
&l
H
Q
4
&
~—
B
o
2
5
=
20
Mola ' FeKa
Fig. 14. Electron Beam Scanning Images of Molybdenum Sample Brazed
With Alloy 35M After Exposure to Bismuth in Quartz Loop 10. Lighter
regions indicate higher concentration of listed element.
21
-Y=-106001
Molybdenum
Base Metal
Blsmufh la X—Rays
‘Fig. 15. Electron Beam Sg,anning Image of Molybdenum Sample Brazed
With Alloy 35M After Exposure to Bismuth in Quartz Loop 10, Bismuth has
completely penetrated the b¥aze alloy.
e
22
Table 6. Comparison of Corrosion Rates of Molybdenum and TZM
Samples Exposed to Bismuth or Bi—0.01% Li in Quartz
Thermal Convection Loop Tests for 3000 hr
Corrosion Rate (mg/cmZeyr)
Location - Temperature —
' (°C) Loop 3 Loop 7 Loop 11
Bismuth B1-0.01% Li Bi—0.01% Li
Hot Leg 630 . +0.53 . 48.65 +3.10
640 +1.72% -0.36% —1.14
660 ~1.64 ~16.9 —3.80
690 +1.58 | -1.74 -9.70
Cold Leg 680 +3.80 +3,56 —0.03
660 -1.02 . ~0.58 +0.26
640 +0.90% +0.59% +0.44
615 +1.43 0o +1.34
VA sample.
Table 7. Comparison of Corrosion Rates of Tantalum and T-111 Exposed to
Bismuth and Bi—0.01% Li for 3000 hr in Quartz Thermal Convection Loops
Corrosion Rate (mg/cm?¢yr)
Temperature
Location (°C) Loop 6% Loop 13b Loop 12°
(Ta/T-111, Bismuth) (Ta, Bi—0.01% Li) (T-111, Bi—0.01% Li)
Hot Leg 630 +13.73¢ - -16.8 +0.82
660 #0.759 418 —2.28
660 ' —47.16 ' —6.5 +1.08
690 ~79.79 ~137 -13.6
Cold Leg 680 ~17.50 —68.9 —0.85
660 . —24.40 —47.7 +0.70
640 +0.15¢ —46.1 | —1.46
615 +0.29¢ ~112.4 +0.44
220 ppk tantalum in bismuth after test; less than 2 ppm before test.
b30—-150 ppm tantalum in bismuth after test; less than 1 ppm before test.
€20-800 ppm tantalum in bismuth after test; 3 ppm before test.
T-111 samples, others were tantalum samples.
23
Table 8. Comparison of Coxrrosion Rates of Ta—10% W Samples Exposed to
Bismuth or Bi—0.01% Li in Quartz Loop Tests for 3000 Hr
Corrosion Rate (mg/cm?eyr)
| Location : Te???;?ture Loop 17 Loop 18
J (Bismuth) (Bi—0.01% Li)
Hot Leg 630 —1.40 : +0.22
640 ~3.65 +0,20
660 a +0.26
| 690 - a —0.37
Cold Leg 680 | ~2.86 +0.46
' 660 —1.43 40,13
640 —2.48 - 40.35
615 —1.23 +0.13
aSample damaged during bismuth removal.
-
The metallographic appearance of Mo and TZM samples from loops 3 and
7 is shown (Figs. 16 and 17)., Hot-leg samples of molybdenum from both of
these tests show grain boundary corrosion to about 0.0005 in. Little or
no attack on TZM was noted. There was a layer of extremely fine grains at
; the surface of the molybdenum which were attacked. intergranularly.
| grains were probably caused by residual cold work from machining and
| subsequent recrystallization at the test temperature. If these grain
boundaries picked up iron during machining, preferential attack might be
expected.
These
Room temperature mechanical property tests of the molybdenum samples
showed their properties to be changed only slightly from those of the
as-received samples (Table 9). Metallographic examination of the tensile
- samples showed there was no effect of the intergranular attack of the fine
surface grains. Chemical analysis of the melts (Table 10) showed slightly
higher molybdenum in Bi—0.01% Li after testing. In general, the results
indicate that the addition of 0.01% Li to bismuth had little effect on
its compatibility with molybdenum.
The alloy Ta—10% W showed similar resitance to both ‘bismuth and
Bi—0.01% Li (Fig. 18). A hot-leg sample (from loop 17) exposed to
- bismuth was slightly corroded to a depth of approximately 0.0005 in.,
but there was no visible corrosion of any of the specimens exposed to
! Bi—0.01% Li. Chemical analyses of the melts after test were similar to
: : those before test, indicating that Ta—10% W has good corrosion resistance
to both fluids.
24
Y-99457
Control Hot Leg Cold Leg
Mo
Fig. 16. Molybdenum and TZM Samples From Quartz Loop 3.
Fig. 17. Molybdenum and TZM Samples From Quartz Loop 7.
25
Table 9. Effectvof:E2posure to Bi—0.01%Z Li in Quartz Thermal Convection
Loops on the Room Temperature Tensile Properties of Refractory Metals
7 Material
Sample location - Molybdenum Tantalum | CT-111 Ta—10Z2 W
‘ ’ uTs® Elong. urs? Elong. - yrs? Elong. urs? Elong.
(10? psi) (2) (10% psi) (1) (10% psi) ) (10% psi) )
As-received 110 - 8.7 &4 24 94 10 84 14
Hot leg 690°C 11 11.3 47 23 52 0 87 8
Hot leg 660°C 111 8.0 47 26 61 0 92 10
Cold leg 660°C 112 11.3 50 26 88 1 88 7
Cold leg 640°C 112 11.3 47 25 47 0 89 7
aUltimate tensile stress.
Table 10. Composition of Bismuth and Bi—0.01% Li Melts Before and After
Exposure to Molybdenum and TZM in Quartz Loops 3, 7, and 11
Concentration (ppm)
Loop Time Sampled - ‘ : _
c H N 0 ‘Li Mo -~ Ni si Ta Zr
'3 Beforetest 10 5 2 8 0.1 <0.5 30 8 3 0.3
After test: 9 <1 <1 2 0.1 <0.5 30 8 3 0.3
7 Before test 9 1 2 100 0.5 70 8 0.3
After test 30 1 77 60 4 70 10
11 Before test 22 <l <l 8 9 2 30 <30 3 <50
| 26 1 <1 30 100 <30 40 <30 <200 <50
fiAftef test.
26
Cold Leg: =1.2 mg/cm”/yr
Hot Leg: 6.4 mg/cm’/yr
fi “ICRONS
0,005 nnmmsoém L
Before Test Hot Leg: 40.26 mg/cm?/yr Cold Leg: +0.13 mg/cm?/yr
Fig 18. Ta—10% W Samples Before and After Exposure to Bismuth
(Quartz Loop 17 — Upper Pictures) and Bi—0.01% Li (Quartz Loop 18 — Lower
Pictures). Maximum hot leg temperature was 700°C and minimum cold leg
temperature was 600°C.
Weight changes in tantalum and T-111 specimens were greater when they
were exposed to Bi—0.01%Z Li than when exposed to bismuth and posttest
chemical analyses (Table 11) indicated there was a greater concentration
of tantalum in the melts containing 0.01% lithium. However, metallographic
examination of the samples did not show any significant differences
(Fig. 19). Weight changes in T-111 were small and there was little evidence
of corrosion, although surface and grain boundary corrosion from
0.001-0.002 in. occurred in tantalum.
The effect of time on the mass-transfer rate in the molybdenum-
Bi—0.01% Li system is indicated by comparison of the results from loops 11
and 15. Weight change data are presented in Table 12. Corrosion rates as
indicated by maximum welight loss in mg/cm? *yr generally decreased with
time. Tripling the time decreased the maximum corrosion rate by a factor
of about 1.5. However, the net weight loss obtained by summing the
welght changes of all the specimens increased from 549 mg to 1754 mg, a
3.2-fold increase. After 10,000 hr, corrosion of the molybdenum was
still less than 0.0005 in. deep (Fig. 20).
27
Table 11. Composition of Bismuth and Bi—0.01% Li Melts Before and After
Exposure to Tantalum/T-111 in Quartz Loops 6, 13, and 12
Concentration,ppm
Loop Time
‘ c H N 0 Li Fe Ni Cr Ta W Hf
6 Before test 36 . €1 <1 2 , 3 70 200 <2 <1 <1
After test <20 <1 <1 4 3 70 50 <15 <1 <1
13 Before test 16 <1l <1 16 127 30 100 150 5 <3 <3
After test 28 <2 <1. 20 g0 340 50 90 80 <3 <3
12 Before test 15 <1 <1 13 82 30 100 150 3 <3 <3
After test 23 <2 <1 20 80 40 60 50 250 <3 <3
Y-103346
As Annealed Hot Leg Celd Leg
Ta
T
Fig. 19. Tantalum and T-111 Specimens Before and After Exposure to
Bismuth in Quartz Thermal Convection Loop 6.
28
Table 12. Comparison of Weight Change and Corrosion Rate in Molybdenum
Samples Exposed to Bi—0.01% Li in Quartz Thermal Convection
Loops for 3,000 and 10,000 hr
Weight Change, mg/cm2 Corrosion rate, mg/cmz'yr
Sample Temperature 2 5
Location (°0) Loop 11 Loop 15 Loop 11 Loop 15
(3,000 hr) (10,000 hr) (3,000 hr) (10,000 hr)
Hot leg 630 —0.39 +0.17 ~1.14 +0.15
640 +1.06 —0.23 +3.10 —0.20
660 —1.30 ~4.10 —3.80 —3.59
690 -3.32 -7.08 —9.70 —6.20
Cold leg 680 —0.01 —1.47 —0.03 -1.28
660 +0.09 —0.55 +0.26 —0.48
640 +0.15 —0.39 +0.44 —0.34
615 +0.46 4+0.07 +1.34 +0.06
82-5 ppm molybdenum in bismuth after test; 2 bpm before test.
<100 ppm molybdenum in bismuth after test; <100 ppm before test.
Hab@#fl&.qkmvw
%0190, wichons 3
200X
_ 0,005 INCHES 0310' — :o.Easl
Before Test "Hot Leg: =3.6 mg/cm?/yr Cold Leg: 40.6 mg/em’/yr
Fig, 20. Molybdenum Samples Before and After Exposure to Bi—0.01% Li
for 3000 hr (loop ll-upper) and 10,000 hr (loop 1l6-lower). Lower samples
were etched by anodizing.
4
¢
29
When the corrosion rates of tantalum alloys and molybdenum were
compared (Table 13) after 3000 hr exposure to Bi—0.01% Li, Ta—10Z W
showed the greatest resistance to corrosion, while tantalum showed the
least resistance. Metallographically Ta—10%Z W, T-111, and molybdenum
were all corroded to <0.0005 in. deep compared with 0.001 to 0.002 in.
- in tantalum. When the mechanical properties of the materials before and
after test were compared (Table 9), three of the materials, Ta—10% W,
tantalum, and molybdenum, showed little change, but the strength and
ductility of T-11l1l were significantly reduced. Loss of ductility in T-111
‘was also observed when the alloy was exposed to unalloyed bismuth. No
significant changes in the mechanical properties of T-1l1 were found when
the alloy was heated in argon for 3000 hr at 700°C
In studying the behavior of T-111, Inouye and Liu18 found that
~ relatively small additioms of oxygen can cause. ‘embrittlement, if oxygen
is added at 1000°C or lower. For example, the addition of 620 ppm
oxygen to T-111 at 1000°C will cause severe embrittlement (5% elongation),
but as small as 370 ppm will cause embrittlement if it is added at 825°C.
Samples of T-111 from these quartz loop tests picked up from 100500 ppm
during exposure at 600~700°C, and this may have caused embrittlement of
the alloy. The source of oxygen,invthese tests is presumed to be the
quartz loop material. The melts were slightly higher in oxygen following
the tests (Table 12), indicating that oxygen was being fed to the bismuth
or Bi-O 017 Li from the quartz or some outside source.
Table 13. Comparison of Corrosion Rates of Materials
Exposed to Bi—0.01% Li for 3000 hr in Quartz
‘Corrosion Rate, mg/cm?eyr
Temperature
Location - (°0) Loop 13 - Loop 18 - Loop 12 Loop 11
| (ta) (Te=10Z.W) (T-111) = (Mo)
Hot leg 630 —16.8 +0.22 - +0.82 ~1.14
" 640 - —41.8 40.20 ~-2,28 +3.10
660 T —. 5 ~40.26 +1.08 —3.80
690 - -137 -0.37 -13.6 -95.70
Cold leg 680. —68.9 +0.46 —0.85 —0.03
o 660 - —47.7 - 40.13 40.70 +40.26
615 =461 40.13 H0.44 +1.34
30
Metal Loop Tests in Bi—2.5%Z Li - Group 3
Two all-metal loop tests were operated with Bi—2.57 Li. One loop
(of T-111) operated for 3000 hr while the second (of molybdenum) operated
for 8700 hr. - Both loops had specimens located in the two vertical legs.
Maximum temperature was 700°C and the AT during operation was approximately
100°C. The tests were conducted within an ion—pumped high-vacuum
enclosure.
Maximum weight loss in the T-111 loop was 2.73 mg/em?® (7.97 mg/em?eyr).
There was very slight (less than 0.0005 in. deep) metallographic evidence
~or corrosion, and room-temperature tensile tests showed no changes in
mechanical properties of posttest samples compared with the starting
material. The oxygen content of the T-111 samples averaged 240 ppm after
test compared with 60 ppm before test. However, in addition to bismuth
exposure, the T-111 samples and loop were both annealed in vacuum for
~2 hr at 1400°C prior to test. That the samples were not brittle
after test suggests that much of the oxygen increase occurred during the
vacuum anneal. In contrast to the effect on T-11ll at lower temperatures,
the addition of 100~200 ppm at 1400°C did not embrittle the samples.19 At
lower temperatures oxygen in T-111 preferentially associates with hafnium
as extremely fine hafnium-oxygen zones that are coherent with the
matrix.2? At temperatures below 800°C coarsening of the hafnium—oxygen
zones is sluggish, but at high temperatures coarsening readily occurs
and the alloy is more ductile in this condition. Samples of brittle
T-111 from the quartz loop tests were heat-treated for 1 hr at 1400°C
and ductility was restored as determined by room temperature bend tests.
Specimens from the quartz loops were 0.020-in.-thick flat samples compared
with 0.125-in.~diam tensile bars in this test. If oxygen were
homogeneously distributed in the samples, specimen geometry probably did
not contribute to the difference in ductility that was noted.
The molybdenum loop operated for 8700 hr. Weight changes for
specimens from hot and cold leg sections of the loop were recorded
(Fig. 21). The maximum weight loss was 3.62 mg/cm®? (approximately
3.6 mg/cm? *yr or 0.15 mil/year assuming uniform surface removal), and
occurred in the specimen located in the maximum temperature section at
the top of the hot leg. Deposition occurred in all cold leg samples
and in the first three samples in the heated section. The maximum
corrosion rate of 3.62 mg/cm?® *yr 1s about that found in previous quartz
loop tests that circulated Bi—100 ppm lithium for 3000 hr (Table 7) and
indicates that mass transport of molybdenum is relatively insensitive
to the 1ithium content of the bismuth,
Metallographic examination of samples from the molybdenum loop
revealed small amounts of dissolution and deposition of molybdenum in
the hot and cold leg samples respectively. Intergranular corrosion
occurred to depths less than 0.0005 in. (Fig. 22).
Mechanical properties (Table 14) of selected specimens from the hot
leg were, on the average, more ductile and weaker than those from the
cold leg. However, the properties of all specimens fall within a normal
range for this material,
i
!
¥
WEIGHT CHANGE (mg/cm?)
Fig. 21.
Fig. 22. Metallographic
31
ORNL-DWG M-10730
SAMPLE NUMBER
8 9o 4 2 20 8 #© 14 12 8
249 *I 'i'!
T T T T T 1T 17171 (L O T
1.6 —.\ .
[ e DEPOSITION
08 [~ (WEIGHT GAINS)
o ’
-08 - DISSOLUTION » _]
* (WEIGHT LOSSES)
-16 N —
6 I
[ BOTTOM OF / BOTTOM OF
-2.4 | HOT LEG 600°C ) ; COLD LEG 600" C —
. —
/
=32 * / —
/ _
-a0 |- TOP OF TOP OF -
B HOT LEG 700°C COLD LEG 670°C _
o T T O T Y I
— . " w
o 4 8 12 46 20 40 44 48 52 56 60 80
DISTANCE FROM BOTTOM OF HOT LEG (in.)
Weight Changes in Molybdenum Samples Exposed to Bi—2.5% Li
in Thermal Convection Loop Test CPML-2,
Y=124386
Top
| 0 20 ";';‘,:88"-‘6‘-1’" 60 70
0.001 INCHES
Bottom
Hot Leg
Appearance of Molybdenum Samples From CPML-2.
32
Table 14. Room Temperature Mechanical Properties of Molybdenum Exposed
to Bi—2.5% Li for 8700 hr in Thermal-Convection Loop Test
‘ Ultimate 0.22 Offset -
Sample Locatlon oLt o S in.) Seressth Strensth ~
. , ' (psi) (psi)
1 Hot 700 15.1 100, 800 98,300
3 680 12.5 104,100 103,700
5 650 13.6 97,700 93,400
7 | 630 17.5 93,900 80,900
9 | 620 15.2 96,700 87,700
Average Hot 14.8 98,600 92,800
11 Cold 600 8.5 120,000 110,900
13 615 8.4 117,600 109,100
15 | 630 8.1 113,100 108,200
17 640 9.8 118,900 113,100
19 - | 650 . 10.4 111,100 106,100
Average Cold 9.0 - 116,100 109,500
SUMMARY AND CONCLUSIONS
Unalloyed molybdenum and two tantalum alloys (T-111 and Ta—10% W)
have excellent compatibility with bismuth-lithium solutions up to 700°C
even under temperature gradient conditions. The mass-transfer corrosion
rate of molybdenum at 600—700°C was quite low and decreased with time
(up to 10,000 hr). The mass-transfer rate increased with increasing
concentrations of lithium in bismuth, but the maximum depth of corrosion
that was observed was always less than 0.0005 in. and was confined to
the grain boundaries of fine-grain size surface grains. The room
temperature mechanical properties of molybdenum appeared to be unaffected
by exposure to bismuth or bismuth-lithium solutions. The tantalum alloy
T-111 also has excellent compatibility with bismuth-lithium solutions,
but was severely embrittled by 100—200 ppm O, when oxygen contamination
occurred in the 600—~700°C range. This embrittlement severely limits the
potential of T-111 as a MSBER chemical processing construction material.
The mechanical properties of the alloy Ta—10% W were not as sensitive
to oxygen contamination as T-111; also Ta—10Z W showed excellent compati- : ]
bility in tests with Bi—0.01%Z Li for 3000 hr. Unalloyed tantalum had a
corrosion rate about ten times that of the above materials when it was
exposed to bismuth or Bi—0.01Z Li. Unalloyed tantalum was not tested .
with Bi~2.57 Li. In addition, unalloyed tantalum is rapidly corroded?!
by lithium at these temperatures if the oxygen concentration of the
tantalum exceeds approximately 100 ppm; therefore, tantalum seems less
suitable for processing applications than the above materials.
33
Iron, niobium, and several of their alloys were found to be
unacceptable materials for containing bismuth under temperature gradient
conditions. Although the solubllity of these elements in bismuth is
low, rapid kinetics of dissolution and deposition led to plugging of
‘quartz thermal convection loops after only a few hundred hours of
operation. Molybdenum samples brazed with iron-based alloys (Fe-C-B, and
Fe-Mo-C-B) resisted mass transfer, but the braze alloys were attacked
by bismuth. An alloy of Fe—5% Mo completely dissolved by mass transfer
in bismuth after only a few hundred hours in a quartz thermal convection
loop.
ACKNOWLEDGMENTS
The work described in this report was supported by the Molten Salt
Reactor Program and was carried out over a period of several years.
Construction and operation of the quartz thermal convection loops was
carried out under the direction of L. R. Trotter. R. E. McDonald
provided considerable assistance during fabrication of the T-111 and
molybdenum thermal convection loop components. Joining these loop
components was the responsibility of A. J. Moorhead and J. D. Hudson.
B. W. McCullum, J. W. Hendricks, and J. L. Griffith were responsible
for construction and operation of the test loop facilities. George
Griffith edited and Susan Hanzelka prepared this manuscript for
reproduction.
These experiments were part of an overall. materials program that
was carried out under the direction of H. E. McCoy. Special thanks are
due him for the general and technical guidance that was provided.
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35
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