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INTERIM REPORT ON CORROSION
BY ALKALI-METAL FLUORIDES:

WORK TO MAY 1, 1953

 

 

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This document consists of 44 pages.

Copy 7.6 of 222 copies. Series A,

Contract No. W-7405-eng-26

METALLURGY DIVISION

INTERIM REPORT ON CORROSION BY ALKALI-METAL FLUORIDES:

WORK TO MAY 1, 1953

G. M. Adamson
R. S. Crouse
W‘ D. Mdnly

DATE ISSUED

MAR £ 01333

 

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION

for the
U.5. ATOMIC ENERGY COMMISSION

 

SECRET” TR

3 4456 03548073 &
 
 
 

CONTENTS

Summary ..
Introduction ....

Experimental Methods...............
EQUIPMENt .uisisimissmonsessspnssvsonsy

Procedure.....cccceeeevrennnn.

Results and DiSCUSSION vuuviiivireeriverieeererressensenns
Screening Tests ....coceriiviennsnranns

Corrosion of Inconel ....
Standard Inconel Serles
Effect of Fluoride Composmon sipvisias
Oxide Removal Procedures....
Effect of Hot-Leg Temperature .....
Crevice Corrosion .... . ..
Mechanism of Corrosive Afluck

Corrosion of Type 316 Stainless Steel

Effect of Temperabure: ... uciiainamosohiiis s sieisiasiie

Plug Identification... = il et i
Effect of Fluoride Composmon ST

 

rerrmmEre

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11
11
11
21
22
22
24

28
28
28
28
 

FOREWORD

This report reflects the work done to May 1, 1953, on corrosion by alkali-metal
fluorides. |t is realized that some of the interpretations will appear to be naive when
compared with those which have been developed from subsequent investigations. The

purpose of issuing the report is to incorporate the data into a permanent record.

 
 

INTERIM REPORT ON CORROSION BY ALKALI-METAL FLUORIDES:
WORK TO MAY 1, 1953

G. M. Adamsen

SUMMARY

In connection with the search for container ma-
terials suitable for use in high-temperature molten-
fluoride reactor systems, the corrosion of several
metals by various fused fluoride mixtures was
studied in thermal-convection loops made from the
metal being tested. In these tests the temperature
drop was fairly large, but the velocity of the liquid
was low and not subject to control. The fluoride
mixture used in most of the study contained 43.5
mole % KF, 10.9 mole % NaF, 44.5 mole % LiF,
and 1.1 mole % UF4, and is referred to as "“fluoride
14, In a series of screening tests Inconel, or
similar high-nickel alloys, was shown to be the
most promising container material.

It is called to the reader’s attention that the data
reported are from loops that were operated for only
short times, 500 hr, and with impure

fluoride mixtures.
1

relatively
More recent data are reported
elsewhere,’ The lack of purity of the mixtures
was an important consideration in the tests re-
ported; however, the purification processes have
since been perfected by the Materials Chemistry
Division. The more recent work' shows that mass
transfer is of more importance than it was con-
sidered to be at the time of the reported tests and
also that the rate of mass transfer varies greatly
for different fluoride batches.

With fluoride 14 an average figure for the maxi-
mum microscopically visible attack on Inconel was
0.012 to 0.015 in. after 500 hr of circulation. The
attack was in the form of small voids which did not
connect with the surfoce or with each other. It is
shown that for the times used here the attack was
caused primarily by the reduction of fuel impurities
by chromium metal from the container and by the
leaching of sufficient chromium to reach equilibrium
concenfrations in reactions with the fluorides,
primarily with UF ;. In contrast to the effects of
impurities, the depth and type of attack varied only
slightly when changes were made in the major com-
ponents of the various fluoride mixtures studied.

 

16 M: Adamzom; R. 5: Crovse: ond We Doiddnly;
Interim Report on Corrosion by Zirconium-Based Fluo-
ride Mixtures, ORNL=2338 (to be issued).

R. S. Crouse

W. D. Manly

The only effect definitely established was that, in
every case, mixtures containing uronium fluoride
caused more attack than did similar mixtures con-
taining no uranium. When the loop was operated at
a fluid temperature of 1650°F, the attack was
slightly deeper thon it was ot 1300°F. The od-
dition of zirconium hydride to the fuel almost com-
pletely eliminated the attack.
acted as inhibitors.

It was found that type 316 stainless steel might
be considered for a container for molten fluoride
mixtures that do not include potassium fluoride.
With such stainless steels and zirconium fluoride—

Alkali metals alse

base mixtures, the maximum depth of attack was
about the same as that found with Inconel; how-
ever, more material was mass-transferred and cold-
leqg deposits were found. When olkali-metal-base
mixtures were circulated in type 316 stainless
steel, plugging occurred in a relatively short time;
the plugging was probably caused by the formation
of a compound similar to K NaCrF .

INTRODUCTION

A major problem connected with the development
of circulating-fluoride-fuel reactars for aircraft
propulsion at ORNL? has been that of corrosion,
since these reactors will involve high-temperature
operation and the use of corrosive liquids. There-
fore the Experimental Engineering Section of the
Aircraft Nuclear Propulsion Division, the ANP
Reactor Chemistry Section of the Materials Chem-
istry Division, and the ANP Materials Section of
the Metallurgy Division were assigned the joint
task of finding a material that could be fabricated
into thin-walled tubes and that could be used in a
molten-fluoride reactor system in which the maxi-
mum temperature of the fluid is 1500°F and the
temperature drop is 400°F. This interim report
deals with some of the metallurgical phases of the
problem,

The high temperature, considerable temperature
drop, and dynamic flow conditions under which the

 

ZA. F. Fraas and A. W. Savolainen, ORNL Aircrafi
Nuclear Power Plant Designs, ORNL=1721 (Nev. 10,
1954).

 
v

power reactors will operate all have an effect on
corrosion and therefore had to be included in the
study. A dynamic test is the only type of cor-
rosion test that will incorporate these variables
and permit a study of mass transfer. When this
work was started, pump loops were not feasible,
since reliable high-temperature fluoride pumps and
seals had not then been developed. A loop em-
ploying thermal convection for the driving force
that hod been developed by the General Electric
Company® and that had been modified by the Ex-
perimental Engineering Section of the ANP Di-
vision at ORNL for corrosion testing with sodium®
was simplified and odopted for the preliminary
stages of the investigation.

The thermal-convection loop could be operated
with a hot-zone temperature of 1500°F with o
reasonable temperature drop and was fairly simple
to fabricate. Its main disadvantages were the low
and fixed flow rates and the size of the system.
Standard loops comprise about 10 ft of pipe and
contain about 35 in.? of liquid, which made them
obviously unsuitable for conducting a large number
of screening tests. Thus the thermal loops were
used for an intermediate test. The preliminary
screening work was done with static and seesaw
tests,*r® whereas the final testing would utilize
high-velocity pump loops. Since the thermal loops
were an intermediate step, the data obtained from
them are not to be considered as design data.
Such data can be obtained only from systems that
more closely reproduce the proposed operating
conditions. However, thermal loops have proved
extremely useful in investigating corrosion mech-
anisms and in ascertaining relative effects of
system temperature ond temperature drop.

The program was divided into two parts: o
search, consisting in o series of screening tests
based upon data obtained in preliminary capsule
tests,” for the best material for containing the
fluorides and a search for a method to render

 

E'L. F. Epstein and C. E. Weber, in Use of Molten
Sodium as a Heat Trans fer Fluid, TID-70, p 59 (Jan. 1951).

4R.B. Day, Examination of Thermal Convection Loops,
memorandum to E. C. Miller, April 12, 1951,

L. §. Richardson, D. C. Vreeland, and W. D. Manly,
Fggac;sion by Molten Fluorides, ORNL-1491 (March 17,

6p. C. Vreeland, E. E. Hoffman, and W. D. Manly,
Nucleonics 11(11), 36=39 (1953).

 

Incone! acceptable as a container material. Ef-
forts were made to understand the chemical re-
actions and corrosion mechanisms
both portions of the program. All loops were
operated for a standard time of 500 hr.

The composition of the fluoride mixture pro-
posed for use as a reactor fuel was changed
several times during the period of this investi-
Because of economic reasons and limited
time, it was necessary to switch to o new fuel be-
fore work could be completed on the old one, which
resulted in incomplete corrosion information being
obtained for many of the proposed fuels. The
majority of the work reported herein was carried
out with fluoride 14 (composition given in Table 1),
the first composition proposed for a circulating-
fuel reactor. Considerable work was also done
with zirconium fluoride~base fuels, fluorides 27,
30, and 44. Details of this work are given ina
separate re.'porf.]

involved in

gation.

EXPERIMENTAL METHODS

Equipment

The major equipment used in this investigation
was the thermal-convection loop. The advantages
and disadvantages of this equipment are discussed
in the “Introduction.’”” Figure 1 shows the con-
figuration which has been used as a standard,
except for a few minor modifications to some of
the early loops used in the screening series.
These early loops were filled through a flanged
line into the side of the surge tank rather than
through a Swagelok fitting in the top. The loops
were assembled by Heliarc welding with an inert-
gas backing. Figure 2 shows a partly assembled
loop, its support, and the location and method of
attaching both the heaters and the thermocouples;
Fig. 3 shows some loops in operation and the
location and type of insulation used.

The loops were heated by six sets of Hevi-Duty
tubular-furnace heating elements. Each set of
elements was 6 in. in length and 11{‘ in. in inside
diameter. The heaters were centered on the pipe
and separated by means of ceramic spacers in-
serted in the ends. The heaters were connected
in two parallel circuits, and the power was supplied
from saturable-core reactors. This arrangement
provided for propertional control rather than on-off
control.

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Fig. 2. Partly Assembled Thermal-Convection Loop.
| UNCLASSIFIED
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Fig. 3. Thermal-Convection Loops in Operation.
During the filling of the loop, auxiliary heating
was needed. The loop was heated by passage of
an electric current through the pipe itself, and the
expansion pot was heated by a coiled 1500-w
Calrod heater. Once the loop was filled, these
auxiliary heat sources were turmed off and the
clamshell heaters were turned on.

Johns-Manville Superex insulation was used and
was applied as preformed semicylinders with a
wall thickness of 3 in. The two halves were wired
together, and the cracks were filled with Superex
cement. When the loops were operating ot 1500°F,
the surface temperature of the insulation was about
200°F,

The major experimental difficulties encountered
were concerned with temperature measurement, |t
was necessary to measure the maximum and the mini-
mum wall temperatures, and it was desirable to know
the temperature at several intermediate points, The
temperatures were measured with Chromel-Alumel
thermocouples and were recorded on 12-point
Brown Electronik instruments. Ina dynamic system
the question arose as to what temperatures should
be measured and used for control. It was realized
that saturation and reaction rates in the liquid
would depend upon the temperature of the fluorides,
that corrosion rates would depend upon the inner-
wall temperature, and that the outer-wall temper-
ature would be the easiest to measure. The di-
ameter of the loops was so small that an inserted
thermocouple well would change the flow con-
ditions; also, since the flow was laminar, it was
possible that the center fluid temperature would
be considerably lower than the outside fluid
Thermocouple wells could not be
drilled into the pipe walls, because they were too
thin. For these reasons, the only temperature that
could reasonably be measured was the outside
wall temperature. To obtain the outside wall
temperature, ond one that would check the inner
temperature reasonably well, the thermocouple
beads were welded to the pipe wall with a con-
denser discharge welder and then covered with a
layer of Sauereisen cement. The maximum temper-
ature was measured }’2 in. down the top horizontal
pipe, while the minimum temperature was meas-
ured 2 in. above the joint at the bottom of the cold
leg. Both these thermocouples were under the
layer of insulation. Other thermocouples, on ex-
posed sections, were covered by a layer of as-
bestos tape. Before these precautions were taken,

temperature,

temperature variations of 50 to 60°F were common,
Even with the above precautions the temperatures
should be regarded as being only relative and not
exact,

The power input to the loops was controlled by
Leeds & Northrup Micromax instruments operating
with Chromel-Alumel thermocouples, which were
located between the first and second heaters and
were welded on the vertical hot-leg section about
7 in. below the joint. A duplicate thermocouple
was always placed on the opposite side of the
pipe for use as a check and as a spare for control.

Procedure

One of the main advantages of the thermal-
convection loops was their simplicity of operation.
Since these loops had no seals or moving parts,
once filled they required very little attention, Dur-
ing filling, precautions were necessary to exclude
air and moisture, but those precautions would be
required with any corrosion test,

The following steps were necessary to place
the loop in operation, and some of them will be
discussed in detail:

1. machining parts,

2. preliminary degreasing of all parts with a
solvent degreaser,

3. Heliare welding of all
bottom plug,

4. final degreasing and rinsing,

5. welding in either bottom plug or Swagelok
fitting,

6. testing for leaks with an air pressure and soap
bubble technique and by pumping to @ 50-u
vacuum,

7. attaching heaters and thermocouples and
mounting the loop on a stand,

8. insulating,

9. connecting to operating line,

10. drying by heating to a minimum temperature
of 500°F and holding under a vacuum of at
least 50 p for 30 min,

11. tilling,

12. operating for 500 hr,

13. sectioning and sampling for examination.

The purity of the fluorides varied considerably,
since they had to be obtained from a single large
production facility rather than from o facility set
up especially for the corrosion test program. No
standard or fixed production procedure was used,
and since the production and handling of such

joints except the
mixtures was a new art, many changes were made
from batch to batch.

In general, fluoride 14 was made by the following
procedure. The sodium fluoride, lithium fluoride,
and petassium fluoride were mixed, melted under a
vacuum, and cooled to room temperature. The
uranium fluoride wos added; then the batch was
remelted and mixed by stirring with a stream of
argon.

The zirconium-base fuels were produced under
the supervision of W. R. Grimes of the Materials
Chemistry Division and were more highly purified
and more uniform. The dry ingredients were mixed
and then evocuated several times to remove all
the moisture possible. After the mixture was
melted, it was held at 1500°F and treated with
hydrogen for 1 hr to reduce all oxides and oxy-
fluorides of uranium to UO,; it was then treated
with hydrogen fluoride for 2 hr to convert the UO,
to UF ,. The last step was to purge the mixture of
the excess hydrogen fluoride. This procedure has
since been modified! for the fuels used in the main
investigation of zirconium fluoride—base mixtures.

Both types of fluoride mixtures were transferred
from the production vessel to storage cans. When
a loop was to be charged, the mixture was trans-
ferred by pressure from the storage can to the loop
through o B:t'in' nickel or Inconel line. All con-
nections were made with Swagelok fittings. A
spark plug probe was used to indicate when the
loop was full. A sompler was placed in the trans-
fer line but was not used for the majority of the
loops discussed in this report.

The distribution of impurities in the fluorides
after circulation had to be determined, which made
it necessary that the loops not be drained but that
the fluorides be allowed to freeze in place. After
cooling, the loop was cut up into approximately
6-in. lengths, with six 2-in. sections used for
chemical and metallographic study. The location
of these sections (x) is shown in Fig. 1. After the
loops were sectioned, the pieces were turned over
to J. P. Blakely, of the Materials Chemistry
Division, who was responsible for removing the
samples and obtaining the required analyses.
Since no solvent had been developed for re-
moving the frozen fluorides from the sections, they
were melted at around 1125°F in a helium atmos-
The sections were inspected visually, and
if any unusual phases were present, additional

phere,

sections were cut and submitted for a petrographic

examination. After the fluorides had been removed,
they were ground and submitted for chemical
analysis, and the small pipe sections were ex-
amined metallographically. Check samples of the
fluorides were obtained from the loop by drilling,
and they showed that no changes in fluoride com-
position or wall structure had token place during
the short melting period. If any layers were dis-
covered on the pipe wall during the metallographic
examination, a sample was submitted for an x-ray
diffraction study,

RESULTS AND DISCUSSION

The investigation consisted in a series of screen-
ing tests to determine the most promising container
materials for circuloting fluoride mixtures, a de-
tailed study of Inconel as a container material, and
a short, incomplete study of stainless steels.

Most of the observations made and conclusions
drawn in this report were based upon only one or
two runs. Many of them were checked in seesaw
tests, but this should not be regarded as com-
plete confirmation. As mentioned previously, the
loop tests were designed as an intermediate test
between static testing and pump loop testing. All
the conclusions were based upon loops operated
for the relatively short time of 500 hr. For these
reasons the data presented must be regarded as
tentative and not be used as design data for high-
velocity or high-temperature-gradient systems, |t
is possible that there will be an increase in cor-
rosion in going to high-velocity, high-temperature-
differential pump loops, as was the case in going
from the static tests to the thermal-convection
loops.

Various fluoride mixtures were used in this work.
Their nominal compositions’ and number desig-
nations are given in Table 1. Because of pro-
duction variables, actual compositions may vary
slightly from these figures.

Screening Tests

The initial effort in this program was the oper-
ation of a series of loops to determine which ma-
terials were the best suited for use in a plumbing
system for high-temperature fluorides. The ma-
jority of materials tested were those that showed
promise in static corrosion tests carried out by

 

7C. J. Barton, Fused Salt Compositions, ORNL CF-
53"1']29 {Jdn- 15‘- TQSS).
Table 1. Molten Fluoride Compositions*

 

 

 

Flvoride UFs NaF KF ZrF, LiF BeF,
No. Weight % Mole % Weight % Mole % Weight %  Mole % Weight % Mole % Weight & Mole % Weight % Mole %
12 1.7 1.5 59.1 42,0 29.2 46,5
14 7.8 1.1 10.3 10.9 56.1 43.5 25.8 4.5
17 12,6 2.0 39,4 47.0 48.0 51.0
21 10.7 3.8 1.8 4.8 25.9 50.1 61.6 41,3
24 14.8 36,0 10.2 18.0 75.0 46.0
2 10.7 3.8 13.8 36,6 7.3 14.0 68.3 45.6
27 10.9 4.0 16.7 46.0 72.4 50.0
30 11.4 4.0 19.0 50,0 69.6 46.0
3] 20.1 50,0 79.9 50,0
32 21.4 52.0 78.6 48,0
35 54,2 57.0 45.8 43.0
44 18.6 6.5 20,5 63.5 60.9 40,0

 

*Sae C, J. Barton, Fused Salt Compositions, ORNL CF-53-1-129.

groups under the supervision of D. C. Vreeland in
the Metallurgy Division® and F. Kertesz in the
Materials Chemistry Division. Fluoride 14 was
the liquid circulated in these loops.

The data from the loops operated as part of these
screening tests are given in Table 2. A bar graph
showing the relative times in which plugging oc-
curred with the various materials is shown in Fig.
4. Typical photomicrographs from the top portion
of the hot legs from four of these loops are pre-
sented in Fig. 5.

It was apparent from these data that the material
selected must be one which would not cause plug-
ging in the loop. The depth of corrosion was
necessarily of secondary consideration. Nickel
and Monel, which had been rated as the most likely
containers in the static tests, had to be rejected
because the thermal loop tests revealed excessive
mass transfer of metal from the hot-leg surface to
the cold leg. Figure 6 is o photograph of a mass
of nickel crystals that had collected on a small
flat sample inserted in the bottom of the cold leg
of a nickel loop.

From the data obtained for the alloys tested, it
was apparent that the nickel-base alloys would
moke the best containers for the fluorides. All
other alloys tested either caused plugging in less
than 500 hr or showed excessive mass transfer,

Inconel, a commercial alloy, was deemed suffi-
ciently resistant to fluoride corrosion to warrant
its selection as a material to receive odditional
attention.

Since materials vorying widely in composition,
such as the various stainless steels, iron, and
nickel, all showed plugging or at least severe
mass transfer, it was obvious that more than one
metal or compound must be involved. In the iron
and nickel loops the deposits were metallic, where-
as in the stainless steel loops they were non-
metallic. With the iron and nickel loops the mass
transfer was possibly caused by very small changes
in solubility with temperature. Changes of only a
few ppm would be enough to account for the
amounts of metal transferred.

A material shown to be unsatisfactory in these
tests should not necessarily be excluded for use
with other fluoride systems.
force in a thermal-convection loop is very small,
the loop is quite easily plugged but may require
only

Since the driving

slight changes in loop design, operating
technique, or fuel composition to operate for
500 hr. In another part of this report it is shown
that type 316 stainless steel loops can be oper-
ated if the fuel composition or the minimum temper-
ature is changed. Also, large isothermal pump
loops of type 316 stainless steel were operated
Table 2. Results of Container Material Screening Tests

 

 

 

Loop Loop Migve. of Reason for Metallographic Examination
3 Circulation g Chemical Analysis
No, Material (hr) Termination Hot Leg Cold Leg
40 410 9 Loop plugged 0.010 in. even removal; ne pitting or Metallic deposit with oxide Fe decreased; Cr increased
intergranular ottack; tronsformed particles
surface
43 410 12 Loop plugged Surface rough, some grains removed, Metallic deposit with non- All materials varied
probably even removal; no inter= metallic erystals; visible
granular attack metallic crystals in hot
horizontal section
48 430 8 Loop plugged Rough and pitted; some removal Thin metallic layer with in-  Cr increased; all others
clusions varied
46 lzett iron 46 Loop plugged Surface rough with considerable even Many metal crystals on Large increase in Fe;
remova | surface decrease in Ni
104 Nickel 500 Scheduled Mo pitting or penetration; 0.009 in. Metallic erystals in all Fe and Cr unchanged; Ni
even removal sections increased in cold leg
107 Nickel 1000 Scheduled No pitting or intergranular attack; Heavy metallic crystal Fe decreased; Ni increased
0.010 in. even removal deposits slightly
341 Mone | 31 Leaked Approximately 0.010 in. even removal Metallic deposit
3465 Nimenic 75 500 Scheduled Intergranular pitting 0.008 o 0.013 in. Thin metallic deposit Cr increosed, Fe decreased,
Ni varied
210 Inconel 500 Scheduled Subsurfoce holes from 0.010 to 0.015 in., Intermittent deposited layer Cr increased, Fe decreased,
mainly in grain boundaries Ni and U varied
211 Inconel 524 Scheduled Subsurface holes mainly in grain Metallic-appearing deposit Cr increased; Fe and Ni

boundaries, 0.004 to 0.008 in.

0.0005 in,

decreased
0l

Table 2 (continued)

 

 

 

Loop Loop ¥ i 08 Reason for Metallographic Examination
Circulation Chemical Analysis
Ne. Material Termination Hot Leg Cold Leg
(he)

219 Inconel 480 Heater failed Heavy primarily intergranular voids Light metallic deposit with  Cr increased, Fe and U de-
0.005 to 0.013 in. o nonmetallic layer on top creased, Ni varied

229 Inconel 500 Scheduled Moderate to heavy primarily intergranular Deposit that was at least Cr increased; Fe and U de-
pitting to 0.018 in, partially nonmetallic creased

227 Inconel 500 Scheduled Moderate to heavy primarily intergranular Thin metallic layer Cr increased, Fe decreased,
pitting 0,006 to 0.016 in. Ni and U varied

112 Type 316 58 82 Loop plugged Rough surface with intergranular attack Rough uneven layer Cr increased, Fe decreased,
up to 0.008 in. Ni and U varied

120 Type 316 55 62 Loop plugged Rough and pitted surfoce with inter- Thin deposit with both Cr and U increased, Fe de-
granular attack 0.004 to 0.012 in. metallic and nonmetallic creased, Ni varied

crystals adhering
127 Type 43 Loop plugged Heavy intergranular attack up to Metallic deposit on wall, Cr increased, Fe decreased,
316 ELC sS 0.011 in.; grains spongy with some attached crystals Ni and U varied

251 Type 310 5S 75 Loop plugged Very heavy intergranular attack and Nonmetallic deposit with a Cr increased, Fe decreased,
general pitting 0.008 to 0.015 in.; thin metallic loyer Ni varied
large grain growth

252 Type 310 S5 368 Loop plugged Very heavy intergranular pitting 0.018 to Heavy deposit; at least a All analyses varied
0.025 in, partial plug of metal in

cold leg
275 Type 347 55 39 Loop lecked  Severe intergranular attack 0.008 to Metallic deposit 0.0005 to Cr increased, Fe decreased,
and plugged 0.013 in.; grain growth 0.001 in. thick Ni increosed
276 Type 347 SS 125 Loop plugged Considerable intergranular attack 0.002  Surface rough with thin de-  Cr increased, Fe decreased,

to 0,004 in.

posit layer

U varied, Ni constant
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TIME FOR PLUGGING (hr)
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INCONEL R\
INCONEL

   

 

 

 

 

 

 

 

 

 

 

 

 

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STAINLESS STEELS
CONTAINER MATERIALS

IRON
MNIMONIC

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Loops Made of Various Materials.

Times for Plugging of Thermal-Convection

with alkali-metal fluorides. The lack of time and
manpower made it necessary to concentrate efforts
on the most promising materials and leave the
others for future programs.

Corrosion of Inconel

Standard Inconel Series. — As is pointed out in
the preceding section, Inconel was the material
that appeared most promising in the screening
tests. When the investigation was started, fluoride
14 was the mixture proposed for use as a reactor
fuel. It soon became apparent that fluoride 14
could not be used because isotopically separated
lithium was not available. The zirconium fluoride
fuels were then proposed but could not be ob-
tained immediately in sufficient quantity for test-
ing. Therefore the corrosion study was continued
with the Inconel-fluoride 14 system. The maxi-
mum fluid operating temperature was set at 1500°F,
as in the reactor designs then being considered.
The cold-leg temperature was about 1300°F. A
time of 500 hr was arbitrarily selected as the
standard operating period. These conditions were
established os standard and were used as a base
W the study of the variables reported; Table 3

gives the results obtained. Figures showing
samples taken from the hot legs of the loops are
presented in several sections of this report: loop
229 is shown in Fig. 7, loop 227 in Fig. 9, loop
219 in Fig. 12, and loop 210 in Fig. 5. The loops
were operated over a considerable time span with
fuel from various batches that showed considerable
variation in chemical composition.  Chemical
analyses of the fluorides after the loops had been
operated all showed an increase in chromium and
a decrease in iron content. The depth and in-
tensity of attack found in the hot legs of the loops
checked fairly well, except that the attack in
loop 211 was slightly less thon that in the other
loops.

The following detailed description of loop 229
is presented as typical of the standard loops.
Photomicrographs of various sections showing the
distribution of attack around the loop are pre-
sented in Fig. 7. Sections 3 and 4 show that once
the attack started it proceeded quite rapidly. The
maximum depth of penetration increased only
slightly in moving up the hot leg from section 3
to section 1, but the concentration of voids in-
creased considerably. In the upper hot-leg section
more general attack was found. |t can be noted
that the deepest penetration and the first sign of
attack always occurred in the grain boundaries but
that voids occurred frequently within the grains of
the metal.

The data obtained by analyzing the fluorides from
the sections shown in Fig. 7 are given in Table
4. As was true in about 75% of the loops, the
amount of both chromium and iren found in the cold
leg was slightly more than that found in the hot
leg. No systematic variation in uranium could be
found. In all loops the chromium concentration
increased during the run, while the iron decreased.
When nickel was present as an impurity, its con-
centration also decreased, but for these loops the
original nickel impurity was usually low.

Effect of Fluoride Composition. = Effect of Dif-
ferent Mixtures. — During the progress of the ANP
work a variety of fluoride mixtures have been pro-
posed as suitable fuels or coolants. With the ma-
jority of these fuels only exploratory corrosion
tests were carried out. In every case, the change
from one fuel or coolant to another was made for
reasons other than comrosion.

11
¢l

UHCLASMFIED
T 762

 

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INCONEL, 500 HOURS

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NICKEL, 500 HOURS

UNCLASSIFIED
T 682

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oo

kB

INCHES [&

8 8 B

FEE

 

 

 

Fig. 5, Typical Hot-Leg Sections from Thermal Loops Made of Various Materials After Circulating Alkali-Metal Fluorides. 250X, Reduced 39%.

-wifh caption)
UNCLASSIFIED
T-2824

 

INCHES

Fig. 6. Nickel Crystals Mass-Transferred in o Thermal Loop Circulating Fluoride 14 for 500 hr at 1500°F, -

with caption)

 

 

 

-
Table 3. Standard Inconel—Fluoride 14 Loops
Loop CTiml'! :’_f Reason for Metallographic Examination il Aot
irculation ST emica alysis
No. (hr) Termination Hot Leg Cold Leg %
210 500 Scheduled Heavy concentration of small  Very thin surface layer Cr increased, Fe decreased,
holes in band from 0.010 in. U and Ni varied in
to a maximum of 0,015 in, fluorides; Cr leached from
from surface; mainly inter- wall
granular but some in grains
211 524 Scheduled Moderate to heavy concentra- Metallic-appearing loyer Cr increosed, Fe decreased,
tion of subsurface voids 0.0005 in. thick Ni remained low in fluorides
0.004 to 0,008 in.; pri-
marily intergranular but
some in grains
219 480 Heater foiled Moderate to heavy concentra-  Thin metallic-oppearing Cr increased, Fe decreased,
tion subsurface voids 0.005 layer with nonmetallic U constant, Ni varied in
to 0.013 in.; primarily inter-  particles as layer on top fluorides; Cr leached from
granular wall
227 500 Scheduled Moderate to heavy subsurtace Thin metallic-appearing Cr increased, Fe decreased,
voids 0.006 to 0.016 in. layer Ni and U varied in fluorides
29 500 Scheduled Moderate to heavy subsurface Thin layer ot least partially  Cr increased, Fe decreased,

voids up te 0.018 in, and
averaging 0.008 in.

nonmetallic

Ni and U unchanged in
fluorides; Cr leached from
wall

 

13
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UGL ABILF ET
T Mani e
)

. o
: .'- I i 3
& ;
. & T i a I:
& g A'r el '.'
& " .
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STANDARD BEFORE CIRCULATION
o " -t =
. A'I - " : o
- B 1 - "‘".. .“
- b
, ~ .
b : ol
. x ..-
"
LOWER VERTICAL HOT LEG
SECTION 4

Fig. 7. Variation in Attack Around a Standard Loop. 250X.

»
e

 

 

W E§ wenk

i

(To=EFEEPERRER ~wENE

 

 

     
   

 

uatssuren ||
P _..‘{.. -.“fl' A L

  

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TOP OF VERTICAL HOT LEG
SECTION {

L aasriiD | |
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HOT HORIZONTAL LEG
SECTION 7

MIDDLE OF VERTICAL HOT LEG

SECTION 3

»
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LOWER COLD LEG

SECTION 8

Reduced 61%.-wirh caption)

 

 

iR FEEFENARD

R FEEFBLERD ~oiiE]

EE

i
i1

—

 

 

 

 

 
 

Table 4. Analysis of Fluorides from Loop 229

 

 

 

Section U (%) F (%) Fe (ppm) Cr (ppm) Ni (ppm)
1 4.18 43.7 40 2095 <20
2 4.16 42.3 140 2170 <30
7 4,18 42,2 130 2900 <20
9 4.13 43.0 65 3635 <20
10 4.20 42,2 210 3500 <30
14 4.19 42.1 200 3100 <30
Original botch 4,31 42.4 770 660 20
Table 5 lists the corrosive effects found in  The presence of these layers showed that even

Inconel loops after they had circulated various
fluoride mixtures (see Table 1 for compositions).
The variation in attack is shown by the typical
hot-leg sections presented in Figs. 8 and 9. These
loops were operated not necessarily to give a
minimum amount of corrosion but to be as nearly
comparable as possible. In the case of fluoride 30,
it was shown that lower corrosion rates were
possible with changes in the production procedure,
and it seemed likely that similer reductions could
be made with the other compositions.

From the data in Table 5 it is apparent that the
maximum depth of attack did not vary greatly with
the different fluoride mixtures. An average maxi-
mum penetration of about 0.010 in. was typical.
The variation in the intensity of attack and in the
size of the voids in the metal resulting from cor-
rosion found with different mixtures was thought
to be caused mainly by differences in batch purity.
The mechanism of attack appeared to be the same
for all these fluoride mixtures and is discussed
in detail in the section ‘‘Mechanism of Corrosive
Attack.” This single mechanism was evidenced
by both the similarity in the voids and the similor
variations in chemical analysis.

As listed in Table 5, thin altered layers were
often present on the cold-leg surfaces. Efforts
by x-ray diffraction and spectroscopy to identify
these layers failed. |t seemed that various metals
had been deposited on the cold-leg surface but
that their identities were lost through diffusion;
the resulting changes in the base metal were not
large enough to be picked up by the analytical
or diffraction techniques used. Any nonmetallic
particles found were usually identified as UO.,,.

with Inconel some mass transfer had taken place.

Effect of Uranium Fluoride. ~ Although variations
in depth of attack were fairly small, a close study
of the results showed one systematic trend: in all
cases the ottack was both deeper and more in-
tense in the loops which circulated uranium-bearing
fluoride mixtures than in loops which circulated
similar mixtures without uranium fluoride. The
data are summarized in Table 6, and photomicro-
graphs of hot-leg sections from some of the loops
are given in Figs. 8 and 9.

Effect of Reducing Agents. — In static tests,
the alkali metals and zirconium were found® to
inhibit corrasion by fluoride 14. A program was
then set up to check several reducing agents under
the dynamic conditions found in the thermal-
convection loops.

Zirconium hydride was substituted for zirconium
metal in the loops because it is more stable at
room, or slightly elevated, temperatures and does
not absorb as many gases. |t decomposes just be-
low the loop operating temperature to give very
fine particles of pure zirconium metal and nascent
hydrogen.
zirconium metal concentration of 0.5% was added
to the expansion pot of a loop, and the fluorides
were charged in over the zirconium hydride.

The zirconium hydride addition to fluoride 14
almost completely eliminated the corrosion of
The first photomicrograph in Fig. 10
shows the hot leg of loop 225. Some surface rough-
ness was found, but none of the usual subsurface
voids were present. An intermittent metallic-
appearing layer was found on the surface of the

Enough zirconium hydride for a final

Incenel.

15
91

Table 5. Corrosive Effect of Yarious Fluoride Mixtures on Inconel

 

 

 

Loop Fluoride Metallegraphic Examination Chemical Analysis
Comments
No. No. Hot Leg Cold Leg of Fluorides
78 12 Moderote subsurfoce voids to 0.013 in. No attack; nonmetallic layer 0.0002 in. Cr increased slightly; Fe Run for 1000 hr
ond Ni low and constant
217 17 Mederate subsurface voids to 0.013 in.; Heavy metallic-appearing layer, up to Cr high originally but still
mainly concentrated in grain 0.001 in. increased; lorge decrease
boundaries in Fe; smoll decrease in U
221 21 Maderate to light subsurface voids; Thin nonmetallic layer Some Cr increase; Ni and U
maximum penefration, 0.007 in. decreased
226 26 Widely scotterad voids, almost en- Deposit with both metallic and nen- U and Ni decreased slightly; Hydregen-cleaned;
tirely in grain boundaries; maximum metallic phases 0.001 in. thick Cr increased; Fe high, fuel from lob
penetration, 0.010 in. dropped to only half
227 14 Moderate amount of small subsurface Thin metallic layer Cr increased; Fe decreased;
voids from 0.006 to 0.016 in. Mi and U constant
230 24 Light, widely scattered voids usually Ne deposit Cr increased slightly; Fe
in grain boundaries; maximum pene- showed large decrease
tration, 0.009 in.
264 27 Moderate subsurface voids; penatra- No deposit Large Cr increase; Ni and Fe
tion, 0.005 te 0.009 in. decreased; U increosed
246 32 Light, scattered subsurface voids; No deposit Cr increased; Ni ond Fe
mainly in grain boundaries; 0.003 te decreased
0.008 in.
262 35 Moderate subsurfoce voids; maximum Scattered nonmetallic deposit Large Cr increase; Ni high
penetration, 0.009 in.; general eriginally but now low; Fe
attack, 0.004 in. decreased slightly
277 3 Light to moderate subsurfoce voids Thin metallic-oppearing layer Cr increased slightly; Ni and
0.0015 to 0.005 in.; mainly in grain Fe decreased
boundaries
283 30 Mederate to heovy subsurface voids to  Layer of metallic-appearing crystal Large increase in Cr; Ni and As melted
0.015 in.; deepest attack inter- Fe content high and de-
granular creased
282 30 Moderate subsurface voids to 0.009in.; MNo deposit Ni low and Fe moderate; Cr Prepared in grophite

general attack te 0.006 in.

increased; Fe decreased and hydrogen-treated
{1

 
   

Fig.

URCLASSIFIED
T 409

FLUCRIDE 42

FLUORIDE 7

8. Cotrosion of Inconel by Various Fluoride Mixtures.

250X,

FLUORIDE 14

FLUORICE 35

Reduced 43.5%._1« ith caption)

UNCLASSIFIED

 

 

 
8l

 

UMCLASSIFIED
T 1334

 

 

 

 

 

 

e o
UNCL .F\:'JIFILLT‘
T 1233

 

 

 

FLUCRIDE 32 FLUORIDE 24

Fig. 9. Corrosion of Inconel by Various Zirconium Fluoride —-Base Fluoride Mixtures, 250X, Reduced 40%. -with caption)
Table 6, Corrosive Effect of Uranium Flueride Addition

 

Loops Circulating Fluoride Mixtures

Containing Uranium

Loops Circulating Fluoride Mixtures

Containing Ne Uranium

 

 

 

luerid i
Loop Fluoride ot-Lag Aask Loop Fluoride HotoLag Abtask
No. No. Ne. No.
227 14 Moderate subsurface voids; 78* 12 Meoderate; maximum, 0.013 in.
maximum, 0.016 in.
217 17 Mederate, in grain boundaries; 262 35 Moderate; maximum, 0.009 in,
maximum, 0.013 in.
226 26 Widely scattered in grain 230 24 Light, widely scattered in
boundaries; maximum, grain boundaries; maximum,
0.010 in. 0.009 in.
264 27 Mederate; moximum, 0.009 in, 246 32 Light and scattered in grain
boundaries; maximum,
0.008 in.
282 30 Moderate subsurface voids; 277 31 Light to moederate; moximum,

maximum, 0.009 in.

 

 

0.004 in,

 

*QOperated for 1000 hr.

hot leg and in some of the grain boundaries near
the surface. The second photomicrograph in Fig.
10 shows the cold leg of a duplicate loop (242).
A well-bonded layer on the surface filled up what
fabrication cracks were present. Unsuccessful
attempts were made by both x-ray diffraction and
spectrographic means to identify these layers.
When the fluorides from these loops were analyzed,
it was found that iron, nickel, and chromium had
decreased in concentration during the circulation.

The zirconium inhibited the corrosion by reduc-
ing the impurities in the fuel before they reached
the chromium. It was thought that the surface
layers were products of the reduced impurities and
excess zirconium metal and that they could be
eliminated if the fluorides were treated with zir-
conium hydride in the transfer pot and filtered
into the loop. The fluoride 14 itself remained
essentially unchanged when low concentrations of
zirconium hydride were added. The fuel from these
loops was examined petrographically by Hoffman, &
of the Materials Chemistry Division, but she failed

 

Bp. & Hoffman, Examination of Thermal Convection
Loops with Added NaK or Zirconium Hydride, memo-
randum to W. R. Grimes, Oct. 10, 1952.

to find any reduced fuel compunents.? When
similar additions were made to fluoride 27, some
reduction of the fuel did take place.

Titanium hydride was similarly odded to loop
250. The addition of 0.5% titanium hydride de-
creased the corrosion to a maximum penetration of
0.0025 in. but produced a thin metallic layer,
identified by R. M. Steele by x-ray diffraction as
NijTi. Under a high magnification (750X) it was
revealed that the layer was actually made up of
several phases imposed one upon another, as can
be seen in Fig. 11, a photomicrograph of the hot
leg of loop 250. A similar layer was also found in
the cold leg.

On the basis of static tests, the alkali metals
also appeared to be effective corrosion inhibitors,
Therefore 0.5% of sodium-potassium alloy was
added to loop 224, in which fluoride 14 was circu-
lated. The maximum depth of attack measured
0.010 in., and although a reduction in intensity of

 

Mote recent work by improved techniques has shown
that the zirconium hydride works primarily by the forma-
tion of reduced uronium compounds, which change the
equilibrium conditions in the corrosion reactions. Since
metallic uranium will be formed, which alloys with the
container woll, the addition of zirconium hydride is not
a practical method for retardation of corrosion.

19
|

UNMCLASSIFIED
T 1764 g

|n E
o
]

1
INC‘;‘ES

T
1

 

s

4 ' - e oor

 

 

 

 

 

HOT LEG

UNCLASSIFIED | |
T 12333

 

 

1.

| 26
= k.

| ' .

COLD LEG

Fig. 10. Effect of Zirconium Hydride Additions on Fluoride Corrosion in Inconel. 250X. Reduced 25%. [SUoEr
with caption)

20
 

UNCLASSIFIED |
T-2488 .

 

Fig. 11, Hot Leg of Inconel Loop After Circulating Molten Fluorides with Small Addition of Titanium Hydride.
750X,

with caption)

attack was noted, it was less than had been ex-
pected. It is possible that a more efficient mixing
of the sodium-potassium alloy and the fuel would
result in a greater improvement, It is also possible
that some of the sodium-potassium alloy was
vaporized while the fuel was being charged. A
similar addition, but more carefully controlled,
definitely caused a reduction in depth of attack
with fluoride 30. When the fluorides from the loop
to which the sodium-potassium alloy had been
added were analyzed, the maximum chromium con-
centration found was 520 ppm, which was additional
evidence of decreased corrosion. No fuel reduc-
tion products® could be identified by spectro-
graphic studies.

Since corrosion occurs by the leaching of chro-
mium from the alloy, it was reasoned that adding
chromium metal to the fuel before it was placed in
the loop might reduce the attack in one of two
ways: the attack might be slowed down in accord-
ance with the law of mass action, or the corrosive
impurities in the fuel might be reduced before the
fuel was charged into the loop.

Several attempts were made to add chromium
metal to the fluorides both before and after they
were charged into the loop. It proved surprisingly
difficult to add chromium metal to these mixtures,

and the results obtained were erratic and not re-
producible.

Oxide Removal Procedures. — In any corrosion
test the method of cleaning the metal surface
prior to testing is one of the most sensitive vari-
ables. It is desirable to clean the surface without
changing the basic corrosion mechanism or corro-
sion rate. Many of the early failures with the
sodium loops were caused, or at least influenced,
by the cleaning cycle.*

The standard cleaning cycle chosen for use on
the loops consisted in degreasing and inspecting;
making all welds with a helium backup to reduce
oxidation; degreasing; and inspecting with both
swab and borescope. |t was realized that oxides,
whether initially present or formed during welding,
would not be removed by this procedure. The work
discussed in this section was an investigation of
several possible methods for removing these
oxides without etching or otherwise changing the
surface. None of these methods were adopted as
a standard procedure,

A series of loops was cleaned by passing dried
hydrogen through them while they were heated to
above 1800°F. The loops were held at this tem-
perature for 1 hr or until the dew point of the exit
hydrogen was —60°F, In most cases the dew point

21
was about —100°F at the end of 1 hr. The corro-
sion results from loops cleaned by this methaod
were quite erratic, and considerable grain growth
usually occurred, The size of the loops, varia-
tions in wall thickness, and variations in type and
thickness of insulation made it extremely difficult
to heat a loop uniformly; therefore temperature
gradients of less than several hundred degrees
were difficult to obtain. With these temperature
gradients it was necessary to heat most sections
of the loops into the range of rapid grain growth
(above 1850°F) to get the colder sections to the
temperature (1800°F). The resulting
large grains caused variation in attack and made
comparisons difficult. Because of the difficulties
in control, this method was not considered as
being adaptable either to the loops or to the large
engineering components of a reactor,

The results obtained from the operation of loops
with sodium and sodium-potassium alloy, as well
as cleaning tests carried out by the group under
D. C. Vreeland, indicated that these metals would
remove the oxides without attacking the tube
wall.4'®  Vreeland showed that, with small tem-
perature drops, temperatures in excess of 1450°F
were required for cleaning but that the time was
not critical. Several loops were filled with sodium-
potassium alloy, which was allowed to circulate
for 4 hr with the hot leg at 1600°F and the cold
leg at 1500°F. The sodium-potassium alloy was
drained while the locap was still hot, The erratic
variations in corrosion were traced to variations in
the oxides left in the loop after cleaning, The
oxides were left as deposits in cold sections of
the loop and as a solid deposit when the loop was
drained.  The oxides from the weld scale had
been converted to alkali-metal oxides with low
solubilities.  This cleaning method is feasible
where a cold bypass or filter may be used to re-
move the oxides, but not for the thermal loops.

Effect of Hot-Leg Temperature. — Two loops
were circulated with fluoride 14 at temperatures
other than the standard temperature of 1500°F,
Loop 218 was circulated at a hot-fluid temperature
of 1300°F and loop 222 at a hot-fluid temperature
of 1650°F, Photomicrographs of typical hot-leg
sections from both loops and from standard loop

cleaning

 

mD. C. Vreeland, Experiments on Removing Welding
Scale from Inconel, memorandum to W, D. Manly, Aug. 7,
1952.

22

229 are presented in Fig. 12. Some increase in
depth of attack was noted with increasing tempera-
ture: from a maximum of 0.010 in. at 1300°F to
0.015 in. at 1500°F and to 0.018 in. at 1650°F.
At the higher temperature fewer voids were found,
but they were much larger and showed more
tendency to migrate to the grain boundaries. Con-
siderable grain growth also was noted.

Crevice Corrosion. — Although the accepted
mechanism of corrosion in the fused fluorides
would not be expected fo lead to crevice corrosion,
a direct test was desired, inasmuch as crevices
may be present in the hot portion of a reactor
plumbing system where tubes join to headers. In
order to maoke such a test, two crevices were
built into loop 223. The pipe was sawed through
in two places near the center of the hot leg, and
the cut ends ot each place were secured by a
sleeve welded over the ocutside of the pipe. Thus,
at each end of the section which had been cut out,
a crevice opening into the fluorides existed be-
tween the sleeve and the outside of the pipe wall,
The maximum attack found in the upper crevice
was from 0.006 to 0.008 in., while that on. the in-
side of the pipe wall, from the same area, was
from 0.004 to 0.007 in. Although the increase in
maximum penetration was small, the attack was
much more intense inside the crevice. While the
attack was not so severe in the lower crevice, the
same relationships to that on the inside of the
pipe wall were found.

As an additional check on crevice corrosion,
the top welds from standard loops 219 and 229
were examined. The welding techniques used
had resulted in an incomplete weld penetration,
and hence crevices were present on the inside of
the loops. The increase in corrosion in such a
crevice is apparent from Fig, 13, a photomicro-
graph of the top weld of loop 219. The maximum
penetration of the wall in this area was 0.003 in.,
while in the crevice the attack was 0.006 in. deep
and several times as intense, Similar results were
obtained with loop 229.

These results indicated that crevice corrosion by
the fused fluorides was not a serious problem.
The most likely explanation for the small increase
in corrosion found in the crevices is that the
cleaning was inadequate. Some oxidation had
taken place during welding, and the oxides were
not removed during cleaning. Helium was used
as a backup gas during welding, but purging of
i~

 

 

 

LOOP 229, {500°F

£C

 

 

 

EEE]

]
IM’."PIIES

EERBE .

EREEF

| 2

 

 

UNCLASSIFIED
T 12330

 

LOOP 222, 1650°F

Fig. 12, Effect of Hot-Leg Temperature on Fluoride Corrosion in Inconel. 250X. Reduced 39%. ' with caption)

E [asox |

EEE]

T
|NCJHE5

FEEFPERERER,

 

 
e air was not complete, The oxides in the
crevice reacted with the fuel and caused the in-
crease in attack in the crevice. In the loop itself,
sufficient fuel and agitation were present to dilute
any effect caused by the weld scale on the pipe
surfaces,

Mechanism of Corrosive Attack. — The theory
that was developed during this work to explain the
mechanism of corrosive attack by fluoride salts
must be regarded as tentative, even though much
evidence is available to support it. Considerable
work has been carried out, principally with zir-
conium fluoride—base fuels, to confirm the theory;
the results will be presented in another report.

To determine what changes took place in the
pipe material, concentric samples were drilled
from the inside of both the hot and cold legs of
several loops. Chemical analyses of the drillings
from loop 229 are presented in Table 7. These
data show that chromium was leached from the
hot-leg wall to @ depth beyond the maximum depth
at which the attack was visible under the micro-
In the cut taken between 0.025 and 0.030
in, deep, the chromium content was still not so
high as in the external, or standard, sample.

scope.

24

 

UNCLASSIFIED |
T-1802

 

42 CH

Fig. 13. Increase in Attack in a Crevice of an Inconel Loop. 250X,

Figure 14 is a plot of the change in chromium con-
centration with depth. With one exception all the
points lie quite close to a straight line. From
other evidence it appears that the solubility of
chromium in the fuel in these temperature ranges
is between 3000 and 3500 ppm. The chromium
concentrations for loop 229 were in this range,
but for most loops they were approximately 2000
ppm.

Any chaonges in the iron and nickel content of
the drilled hot-leg samples were small. The nickel
percentage did change, but mainly because the
chromium was depleted. Except in the first two
samples, both the iron percentage and the iron-
In other loops
no variation was found even in the surface samples.
In the surface samples from the cold leg the iron

to-nickel ratio remained constant.

content was higher, as was expected, since iron
and nickel were deposited out of the solution.

The depth to which the alkali metals were found
in the drillings was surprising. While the maximum
attack extended to 0,018 in., traces of alkali
metals were found in the 0.020- to 0.025-in. cut.
Enough sample was not available from these
drillings for fluoride analysis, so the form in which
Table 7. Analyses of Pipe Wall —~ Loop 229

 

Total Depth

Others®

 

Layer Fe Ni Cr Ratio Na® K2 Li® u? Total
No. “::_‘;’ % (% (%) Fe/Ni (%) (%) %) (%) (%) (%)
Hot Leg — Section 3
] 0.003 6.73  81.7 8.2  0.083 0.15 0.5 0.3 c 1.34 98.9
2 0.006 6.83  80.9 9.4  0.085 0.15 0.3 0.15 c 1.19 98.9
3 0.010 6.99 797 110  0.088 0.08 0.2 0.1 c 1.23 99.4
4 0.015 6.98 78.1 13.1  0.089 0.06 0.1 0.08 c 1.40 100.0
5 0.020 6.85 77.8 142  0.088 0.04 0.08 0.06 c 1.42 100.4
6 0.025 6.91 76.8 154 0.090  Trace Trace  Trace c 1.47 100.6
7 0.030 6.87 76.4 15.6  0.089 ¢ 1.66 100.5
8 External 6.90 76.5 16.0  0.090 c 1.36 100.8
Cold Leg ~ Section 11
1 0.003 8.46 73.8 166 0.114 ¢ c c c 1.45 100.3
2 0.006 8.5 746 165  0.110 c c c c 1.64 100.9
3 0.010 7.82 744 167 0,105 c c c c 1.75 100,7
4 0.015 763 745 16.6 0,102 c c c c 1.79 100.5
5 0.020 7.72 744 16,5  0.104 c c c c 1.64 100.3
6 External 7.62 746 167  0.102 c c c c 1.65 100.6

 

A5pectrographic determination.

b1otal of Ce, Cu, 5i, Al, Mg, Mn, and Ti (spectrographic analyses).

Not present.

UNCLASSIFIED
ORML-LR-0WG 35693

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

w 20

3

[

&

Ly iy

1S - -

o

=

o

=

ol —~
o e | O 2EO |
u—e—see L OOP 273

D 5 — LOOP 229 |

3

£, |

o 10 20 a0 40

DEPTH FROM SURFACE (mils)

Fig. 14. Change in Chromium Cencentration in Wall of

an Inconel Thermal-Convection Loop.

the metals are present is not known. The ratio of
the alkali metals, as shown by these analyses,
varied greatly both from sample to sample and
from that in the fuel (Na, 1; K, 10; Li, 4). Further-
more, the total amount of the alkali metals, on
either a weight or a molecular basis, was much
less than the amount of chromium removed. It is
possible that the presence of alkali metals in
the deeper samples resulted from poor sampling,
either through nonconcentric drilling or from runout.

During metallographic examination of the hot-leg
sections, an area was noted near the surface that
etched differently from the remainder of the sample.
In this area carbides were no longer visible. This
layer extended slightly deeper than the visible

25
attack and followed the line of the attack. A hot-
leg somple from a standard loop was submitted to
the International Nickel Company for study of this
layer, It was found that apparently the carbon had
not been leached out with the chromium but had
gone back into solution in the metal, The carbon
content of the surface layer and that of the re-
mainder of the pipe were the same, !’

While examining some sections from a loop,
W. C. Tunnell noted that the inner pipe surface
had become magnetic. Some chemical samples
drilled at about this same time were then tested,
and it was found that the magnetism decreased
with depth but that a few magnetic particles were
still present in the last cut. The surface layer
in the cold leg was also magnetic. The presence
of these magnetic loyers was also confirmed
metallographically; when the sample was covered
with a colloidal dispersion of iron and made the
core of an electromagnet, the iron collected on
the magnetic portions of the alloy. Figure 15
shows the magnetic areas of the hot and cold
legs of loop 229. The heavy black line is the
area where the magnetism begins; it is heavy be-
cause the particles from the unaffected surface
moved to this areq,

The presence of magnetic layers on the hot-leg
surface was additional confirmation that chromium
had been leached out., When chromium is added to
nickel, the Curie point is lowered quite markedly,
An alloy with 8% chromium has a Curie point at
about rocom temperature, while that of pure nickel
is at 665°F.'2 The addition of iron to nickel raises
the Curie point, and presumably it would do the
same in these alloys. The Curie temperature for
commercial lnconel is approximotely —40°F, Since
the Curie point of the specimens which had been
exposed to fluorides was above room temperature,
the change must have been caused by the removal
of chromium,

In the examination of hot-leg sections one strik-
ing fact is opparent. In spite of the depth to
which the attack extends, there is wvery little
elongation of the voids in this direction; all voids
are nearly spherical. With normal intergranular
attack, the wvoids are shaped like worm holes,

 

”E. N. Skinner, International Nickel Co., Inc., per-
sonal communication to W. D. Maenly,

12M£*ra!s Handbook, 1948 ed., p 1046, American Society,
for Metals, Clevelond.

26

with definite elongation in one direction. In an
effort to follow the course of separate holes, a
sample was examined metallographically, reground,
and then re-examined at depths increasing by
0.00025 to 0.0005 in. From this study it was
apparent that at least the majority of the holes did
not connect to the surface or to each other. This
conclusion was substantiated by a vacuum leak
test on a section from which the outer, unaffected
area had been removed by machining. Tests with
dye penetrants produced the same results.

In view of the above facts, W. D. Manly proposed
a mechanism to explain the formation of the holes.
It had been established that chromium was selec-
tively leached from the alloys by the fluorides
and thot the chromium at the surface of the pipe
was reploced by a diffusion process. In a dif-
fusion process that is unidirectional, there will be
a change in the density ond/or shape of that
portion of the specimen in which the composition
is changed. Since diffusion in metals is believed
fo occur by migration of vacancies from the surface,
it may produce a concentration of vacancies in the
lattice that exceeds the ‘‘solubility limit"" for
vacancies at the test temperoture, If this limit is
exceeded, the vacancies will ‘‘precipitate’’ from
the lattice, collect at discontinuities, and grow
to visible voids whose size will be a function of
time and temperature, Such formation of voids
has been observed in many bimetallic diffusion
couples, and it has been demonstrated that identi-
cal effects can be obtained in metal-liquid and
metal-gas systems in which similar diffusion
phenomena occur,”

The removal of the chromium from the Inconel
surface takes place by several chemical reactions.
Since these are primarily chemical problems and
are still under study by W, R, Grimes and F, F,
Blankenship in the Materials Chemistry Division,
they will be discussed only very briefly, From the
dota developed, the reactions listed below appear
to be the most probable ones for removing the
chromium:

(1) 3Ni ™ + 2C0 =22Cr**" 4 3Ni0
(2) 3Fe*? + 2CI9==2Cr"** 4 3Fe0
(3) 2HF + NiF==NiF, + H,
(4) 3UF, + Cr==CF, + 3UF,

The metallic iron and nickel formed in these re-
actions are deposited on the cold-leg surface and
UNCLASSIFIED
T 3151

EEE

T
IHB?ES

T
1

EEFEERER

2
>

50X |

| 2

 

 

 

 

b
=

HOT LEG

h
j

UNCLASSIFIED
T 12332

: B B

:

T
INCHES
1

 

 

o 2a

 

 

 

COLD LEG

Fig. 15. Magnetic Areas on Sections from the Hot and Cold Legs of an Inconel Loop. 250X. Reduced 19%.
th caption)

27
diffuse into the base metal. Reaction 4 becomes
more important with loops operated for longer
times than those in this study, since it is thought
to be the principal reaction for the mass transfer
of chromium. It does, however, account for some
of the attack, since equilibrium is established in
a relatively short time ond requires the addition
to the fluorides of between 1000 and 2500 ppm of

chromium, depending upon the temperature,

Corrosion of Type 316 Stainless Steel

It was pointed out in the section on "‘Screening
Tests” that all tests in which fluoride 14 was cir-
culated in a stainless steel loop were terminated
because the loop became plugged. Since stainless
steels are such well-known engineering materials,
a few tests were carried out to see whether the
corrosion could be prevented by slight modifi-
cations in operating conditions or fluoride compo-
sition,

Effect of Temperature. — A series of type 316
stainless steel loops were operated at different
temperatures to ascertain whether plugging could
be prevented by a slight increase in the cold-leg
temperature. The data from these loops (presented
in Table 8) showed that the plugging was tempera-
ture sensitive. Loop 123, which was operated
with a minimum cold-leg temperature of 1500°F,
was the only one to be operated for 500 hr, A
similar loop, with the same hot-leg temperature
but with a slightly lower cold-leg temperature,
became plugged in 91 hr. The longer time for the
loop with a hot-leg temperature of 1300°F to be-
come plugged was probably caused by a lower
corrosion rate,

In the loop that was operated for 500 hr a layer
up to 0,001 in, thick was transferred to the cold-
leg surface. This layer was identified by dif-
fraction and spectrographic methods as being
iron with some chromium in solution. Such a layer
would not cause plugging but could interfere with
proper heat transfer,

Plug ldentification. — The nature of the plugs in
the type 316 stainless steel loops is not definitely
known. Although the standard temperature used
for melting out the fluorides for examination was
about 100°F below the minimum cold-leg tempera-
ture, all the material in the loop flowed out. Loops
were also sectioned longitudinally without dis-
closing an apparent plug. D. C. Hoffman, of the
Materials Chemistry Division, made a petrographic

28

examination of several of these loops before the
fluorides were melted out and found'® that the
only substance likely to form o plug was K ,NoCrF ,
which has a physical appearance very similar to
the fuel and would be difficult to detect visually.
This compound melts at around 1850°F, but it
would probably separate from fluoride 14 ot about
1500°F under the conditions existing in the loops.
Crystals of this compound might form as dendrites,
which could collect together in o tangled structure
similar to a log jam. Without being o complete
plug, such a mass could furnish enough resistance
to stop circulation in the loop. This is known to
be the mechanism operating with metallic plugs.
The few crystals holding such a mass to the side
of the loop could be broken during freezing and
remelting, and thus might not have interfered when
the fluorides were melted out for examination.

Analysis of the fluorides melted from these
loops showed that the chromium content was con-
siderably higher in the cold leg, where the plugging
presumably occurred. This result would be ex-
pected if the plugs were formed by KzNuCrFfi.

Effect of Fluoride Composition. — Many of the
early large-scale engineering tests were carried
out with type 316 stainless steel loops containing
fluoride 12. Inconel parts for the loops were not
available at that time, and the use of fluoride 12,
which contains no uranium, required no accounta-
bility. Later, several loops were operated with
zirconium fluoride~base fuels in order to test the
proposed theory of plugging, Data obtained from
these loops are presented in Table 9, and repre-
sentative hot-leg sections are shown in Fig. 16.
The compositions of the fluoride mixtures are given

in Table 1.

Of the four mixtures included in Table 9, fluoride
14 was the only one that caused plugging. The
proposed theories of plugging are substantiated
by the fact that the zirconium-base fuels, which
contained no potassium, did not form plugs. The
corrosion rate in the two loops with fluoride 12
was probably too low for enough material to be
furnished to form a plug.

Figure 17 shows a section from the hot leg of
the type 316 stainless steel loop in which NaK
was added to fluoride 12. Comparison with the

 

Bp, e Hoffman, ANP Quar, Prog. Rep.’Mnch 10,
1953, ORNL-1515, p 136.
6

Table 8. Effect of Temperature on Corrosion of Type 316 Stainless Steel by Fluoride 14

 

Loop

Hot=Leg
Temperature

(°F)

Initial
Cold-Leg

Temperature

(°F)

Time of
Circulation

(hr)

Reason for

Termination

Metallographic Examination

 

Hot Leg

Cold Leg

Chemical Analysis

 

121

120

125

123

117-a*

117<b

1300

1500

1650

1650

1650

1500

1150

1300

1465

1510

1525

1300

153

62

21

500

500

366

Loop plugged

Loop plugged

Loop plugged

Scheduled

Scheduled

Loop plugged

Very rough surface with o
heavy intergranular
attack to 0.010 in,

Very rough surface with in-
tergranular attack to
0.012 in.; material near

grain boundaries altered

Very rough surface; inter-
granular attack to 0.011
in. with very large voids

Very rough surface; inter-
granular attack to 0.013

in. with very large voids

Very rough surface with
severe intergranular

attack to 0.006 in.

Adherent metallic deposit

in all sections

Thin deposit with both
metallic and nonmetallic
crystals adhering

Metallic deposits with
attached metallic
crystals

Adherent metallic layer up

to 0.001 in. thick; some
attached erystals

Fairly thick metallic layer

Cr increased, Fe decreased,
Ni remained low in fluo-
rides; Cr and Fe both re-

moved from loop wall

All impurities in fuel varied;
Cr increased and Fe de-
creased in other similar
loops

Impurities in fluorides

var jed

Impurities in fluorides
varied, but Cr increased
while Fe decreased; Ni

remained low

Impurities in fluorides
varied; Cr increased
slightly

 

*After 500 hr, hot-leg temperatures

were dropped and asbestos tape was removed from the cold leg; the loop then continued to circulate as a standard loop.
0

Table 9. Corrosive Effect of Various Fluoride Mixtures on Type 316 Stainless Steel

 

 

 

Loop Fluoride s Reason for Metallographic Examination
Circulation Y Chemical Analysis

Ne. No. thr) Termination Hot Leg Cold Leg

112 14 82 Loop plugged Rough surface; primarily on inter- Metallic crystal layer on surfoce Cr increased, Fe decreased,
granular attack to 0.008 in. with some crystals in fuel while Ni remained low in

fuel

116 12 500 Scheduled Some roughness; light subsurface  Thin, continuous metallic layer, Fe decreased, Cr and Ni
voids in grain boundaries to some nonmetallic inclusion remained low in fluorides
0.004 in.

119 12 with 0.5% 500 Scheduled Some roughening of surface; some No depos it Fe decreased, Cr and Ni

NaK pits 0.002 in., scattered occlu- remained low in fluorides
sions to 0.004 in,

128 27 500 Scheduled Surface very rough; heavy inter- Heavy nonmetallic layer; em- Cr increased, Ni and Fe
granular attack and subsurface bedded metallic crystals; some decreased in fluorides
voids to 0.008 in. nonmetallic crystols enclosed

in metallic skin

133 30 500 Scheduled Rough surface; moderate to heavy Heavy adherent metallic deposit; Cold leg deposit mainly Fe
subsurface voids to 0.011 in. metal crystals in fuel with some Cr and Zr;
mainly in grain boundaries Cr increased while Fe and

MNi decreased in fluorides

134 30 500 Scheduled Rough surface; moderate to heavy Heavy adherent nonmetallic de- Cr increased, Ni and Fe
subsurface voids to 0.007 in. posit with metallic deposit decreased in fluorides
mainly in grain boundaries underneath; metallic crystals

in fuel
126 14, hydrogen- 309 Loop plugged Heavy intergranular attack te Metallic deposit with non- Cr increased, Fe decreased;

treated

0.014 in.

metallic inclusions

Ni and U constant in
fluorides

 
T 950

v h B E ]

iahfi

 

 

 

FLUQRIDE 12

 

 

 

 

 

 

 

UNCLASSIFIED UNCLASSIFIED
T 768 Lo T 2708

i
Lsos
-
LY.
x
D08 |
LooT )

| fssef
Loas)
5o
r-m
Loia |

; Loia | o .';:l_.?_ - e g
ow ' Bio. L34 | ; '_h=-=:|
o h - | =
L2 ko TN . L8
- o } oy

FLUORIDE 14 FLUORIDE 30

Fig. 16. Hot-Leg Sections from Type 316 Stainless Steel Loops After Circulating Various Fluoride Mixtures. 250X. Reduced 39%, -wiih caption)
photomicrograph in Fig. 16 which shows the cor-
rosion by fluoride 12 indicates that NaK reduced
the attack on type 316 stainless steel, as well as
on Inconel.

The depth of attack was no greater in the type
3L6 stainless steel loops that were operated for
500 hr than it was in Inconel. In stainless steel
the attack was concentrated at the grain boundaries
to a greater extent than in Inconel. However,
more mass transfer was found with all fuels tested,
a serious disadvantage of type 316 stainless steel
as compared with Inconel, Figure 18 shows a
typical layer on the cold leg of loop 134, in which
fluoride 30 was circulated for 500 hr,

Spectro-

   

graphic analysis showed the layer to be mainly
iron, with some chromium present,

Loop 126, also listed in Table 9, circulated
fluoride 14 through which hydrogen had been
bubbled, The loop was operated with a hydrogen

overpressure. Although this loop became plugged,
it was operated longer than the standard loops.
The hydrogen reduced the nickel and part of the
iron present as fluorides. Therefore chromium
was
rate

not taken into solution so rapidly, and the
of plug formation was reduced. (It now seems
likely that this effect was actually caused by re-
duction of some of the uranium, although such a
reduction was not found with the techniques avail-
able at the time.)

 

UNCLASSIFIED
T-12328

 

 

 

 

 

 

 

 

 

Fig. 17. Hot-Leg Section from a Type 316 Stainless Steel Loop After Circulating Fluoride 12 with a Small NaK

Addition. 250X. —irh caption)

3
 

    

UNCLASSIFIED

 

T-2995 002
;jflgp:_'g- -
i R _| |.003
Wt Samde T X E
& <. H ST
NP N2

 

 

 

 

 

 

-

Fig. 18, Cold-Leg Section from o Type 316 Stainless Steel Loop After Circulating Fluoride 30. 250X. -

with caption)

33
 

 

 
 

s

ORNL.-2337
C-84 — Reactors-Special Features
of Aircraft Reactors

M-3679 (22nd ed.)

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