ORNL-2338.txt

TR e essenon

3 445k D3505Lc H

ORNL-2338
UC-25 = Metallurgy and Ceramics

 

INTERIM REPORT ON CORROSION BY
ZIRCONIUM-BASE FLUORIDES

G. M. Adamson
R. S. Crouse
W. D. Manly

 

 

 

OAK RIDGE NATIONAL LABORATORY
operated by
UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION

   
ORNL-2338
UC-25 = Metallurgy and Ceramics

Contract No. W-7405-eng-26

METALLURGY DIVISION

INTERIM REPORT ON CORROSION BY ZIRCONIUM-BASE FLUORIDES

G. M. Adamson
R. S. Crouse
W. D. Manly

DATE ISSUED

JAN 3 1961

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

for the
MARTIN MARIETTA ENERGY SYSTEMS LIBRARE

e N

3 445L 0350512 y

 

 

 
 

 

FOREWORD

The corrosion data reported herein are an extension of the data given in the com-
panion report Interim Report on Corrosion by Alkali-Metal Fluorides: Work to May 1, 1953
(ORNL-2337) but concemn zirconium fluoride—base mixtures, principally fluoride 30. The
companion report should be used in conjunction with this one for background information,
which will not be repeated here. The period covered by this report is approximately from
July 1952 to June 1956.
 

CONTENTS

FOr@WOIT ...cccovisiossasssssssssssssssusassnasssiosess sinsunssisssssosnansnnasssissssssbsassss
A DS ACT oottt

ConClUSIonS .oy s R e T S

Expaiimantol Methoths  cicms i rimim i i s i P SR I T b v civR isean v bemab SR e s by

Equipment .....ocovoivniieiciiciienan,,

Procedure ....c.cooievnnnne

Results and Discussion......cuiiiamnwmmisnasms
Fluoride 30 in Inconel Loops......ccoivencrvnciinnicierire e

Standard Loops ...... R R
Dperating Time s

Effect of Temperature.........cccoiiieciiiesiesicssisssssssssesessssasesessssnesnns
NEMPOrature LD .. ovovsrres ramismsmssusssmesnsssssasssrmnssanssonsppaseenpassmiss st stass assiss

Additives .

OO0 WL v n N NN

B —
O —

Ratio of Surface Areu 10 Lnop Volume SR R A e BB bR

Oxide Removal Procedures ..............
Heating Methods ..................

Loop Size and Shape......c.cc.c...... e s e T A T e G SRR T TR A

Change in Analysis of inner Prpe Wail
Effect of Uranium Concenfration......c..........

Barren Fluoride Mixtures ..............

Screening Tests of Possible Container Materials..........

Staindess Staals . onininn s iR SR
BB VPO cmes e snansnesmas o msmontns sensos s HTeE e eSS FRS PR B WS S i

Nickel ..

Inconel X T S R S S PR G e A e TR

Mnlybdenum und Ntoblum
Hastallay B . .. ot i i e i Bt
Special Alloys ..............

Special Fuel Mixtures ................

Alkali-Metal-Base Mixtures Containing Trivalent Uranium......

Reactions and Mechanisms .....cccocovivvvevenvnnnnnn.

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INTERIM REPORT ON CORROSION BY ZIRCONIUM-BASE FLUORIDES

G. M. Adamson R. S. Crouse W. D. Manly

ABSTRACT

(rhe zirconium fluoride—base fluoride mixture NaF-ZrF -UF , (50-46-4 mole %), referred to as “‘fluoride 30,"
was circulated in Inconel thermal convection loops for periods varying from 500 to 5000 hr and ot a hot-leg tempera-
ture of 1500°F.

companying the circulation of zirconium fluoride—~base mixtures in Incenel,

The purpose of this program was to develop an understanding of the corrosion mechanism ac-

The attack in the Inconel loops was in the form of subsurface voids formed by selective leaching of chromium |
from the alloy. After 500 hr of operation the voids were found to depths of about 10 mils, with the depth continuing
to increase at a rate of abaut 4 mils per 1000 hr of operation. The effects of such variables as time, hot-leg temper-
ature, temperature drop, fluoride purity, loop size and shape, and inhibitors on the depth of carrosion were studied.
It wos found that the attack was reduced when a portion of the uranium wos present in the trivalent state, but the
results were not reproducible. ] —

Also, o few tests were carried out in loops constructed from nickel, stainless steels, iron, Hastelloy B,
molybdenum, and nicbium.

A limited amount of work was done on Inconel loops circulating alkali metal~bose mixtures (NaF, LiF, KF,

UF,) with portions of the uranium in the trivalent state. Reduced ottacks were found, but disproportienation and

production problems require additional study.

 

CONCLUSIONS

From the data presented in this report it may be
concluded that molten mixtures of NaF, ZrF ,, and
UF, may be circulated for several thousand hours
in low-velocity nonisothermal Inconel systems.
After 2000 hr of operation the corrosion depth in
such a system will be deeper than desirable but

will still not be excessive. A depth of about
0.010 in. will be reached in 500 hr; after this the
depth will increase at a rate of 0.004 in. per

additional 1000 hr of operation.

The corrosion of the Inconel takes place by
selective leaching of chromium metal from the
hotter surface of a nonisothermal system, The
removed chromium is reploced on the surface by
diffusion from the center, but since no inward
diffusion takes place, voids
surface of the metal.

The principal removal reaction is

QUF‘i + Cr— EUF:3 + CtFy

form under the

accompanied by the reverse reaction to form
chromium metal in the cold leg. With short
operating times a large portion of the attack
takes place by enough chromium being leached
from the wall to reach the equilibrium concen-
tration necessary for the mass transfer reactions

to take ploce, It is known that other mass
transfer reactions may occur when the uranium is
not present, and these may also be of secondary
importance when uranium is present, Neither
the nature nor the location of the rate-controlling
step has been determined. The diffusion of
chromium to the hot-leg surfoce has been shown
not to be controlling.

With present production techniques for the
fluoride mixtures, the nickel, iron, and hydrogen
fluoride impurities are reduced to levels where
they no longer play a vital part in the corrosion
mechanisms; however, if they are present in the
zirconium fluoride—base mixtures, they will
cause attack in the loop hot legs.

The most important variable controlling the
depth of corrosion is the maximum wall temper-
ature.  The wall temperature is more critical
than the maximum average bulk fluoride tempera-
ture. While wall temperature is important, it must
be a relative temperature; appreciable attack is
found only at the hottest point in a loop no matter
what the actual temperatures are, Variables of
secondary importance are uranium concentration,
loop size and shape, and temperature drop.

The replacement of a portion of the UF; content
of a batch with UF,, either in the production
procedure or by the addition of reducing agents,
will provide reduced depths of attack. The UF,
has only a limited solubility in zirconium fluoride—
base mixtures, so all the uranium cannot be
present in the trivalent form. These mixtures are
difficult to control during production, ond dis-
proportionation of the UF; may occur, causing the
formation of hot-leg deposits.

While Inconel is on acceptable material for
present reactors, it will not be adequate for
future larger reactors. With zirconium fluoride—
base fluoride mixtures at 1500°F, niobium, mo-
lybdenum, and Hastelloy B show promise as
future reactor materials. Some reduction in depth
of attack may be obtained in alloys similar to
Inconel but with reduced chromium concentration.
The stainless steels did not plug with these
fluoride mixtures, as they did in the case of the
alkali-metal-base fluoride mixtures, but they still
are not as good as Inconel.

The addition of UF, fo alkali-metal-base fluoride
mixtures also resulted in a reduction in depth of
attack in Inconel loops. The solubility of UF,
is higher in this system, but mixtures of UF,
and UF , were still necessary. While this mixture
shows promise for future use, production and
disproportionation problems remain to be solved,

EXPERIMENTAL METHODS
Equipment

Most of the loops used for this study were

slightly modified from those originally de-
veloped;! the modifications are discussed in
detail below (“‘Loop Size ond Shape’). The

expansion pot was reploced by a length of pipe of
the same size as that used for the loop itself.
The fill line and spark plug probe entered the
sides of this pipe through Swagelok fittings. A
typical loop is shown in Fig. 1. Most of the loops
were constructed from IPS ¥%-in. sched=10 pipe,
but, if this wos not available, IPS ]fz-in. sched-40
pipe was used as an altemate, No changes were
made in the auxiliary equipment.

Procedure

The operating procedure remained essentially
the same os that used for the alkali-metal-fluoride

 

6. M. Adamson, W. D. Manly, and R. S. Crouse,
Interim Report on Corrosion by Alkali-Metal Fluorides:

Work to May 1, 1953, ORNL-2337 (Mar. 20, 1959).

 

work,! except for the addition of a cleaning step,
which consisted in circulating another batch of
fluorides for 2 hr before the loop was filled with
the test mixture.

All the molten fluorides used in this study were
mixed and purified by the ANP Chemistry Section
of the Materiols Chemistry Division.?  The
purification procedure included treatment with
both hydrogen fluoride and hydrogen goses at
elevated temperature followed by o prolonged
stripping operation with hydrogen and helium,
During the latter part of this study the batch size
was increased, from 50 |b, to 250 |b; however, for
handling and use, these batches were subdivided
into 50-lb batches. The batching down was
carried out while the mixture was molten, and the
usual steps were taken to avoid contamination.
Once a pot was filled, it was held under a positive
helium pressure at all times. Helium gas was
allowed to leak to the atmosphere whenever a
connection was made or broken. During the
actual transfer to a loop, helium pressures were
maintained in both the loops and the fill pot.
The rate of transfer was controlled by adjusting
the difference between these pressures,

The liquid level in the loop was controlled by
the location of the outlet from the transfer line.
When o signal was obtained on the spark-plug
probe, which was located above this level, the
helium pressure in the loop was increased and
the excess liquid was transferred back to the fill
pot. After the loop reached operating temperature,
any excess liquid resulting from expansion was
blown back to the fill pot. A sample of the
original liquid was trapped in an enlarged section
of the transfer line for chemical analysis.

After the loop had operated for the desired
time, the power was turned off and the liguid
allowed to freeze in place. The loops were then
sectioned as shown in Fig. 2, The material was
melted out of the 2-in. sections in an inert-atmos-
phere fumace; the pipe sections were sent for
metallographic examination, and the fluorides for
chemical analysis, Sections 1A, 2A, and 4A
were stored in a dry box as reserve samples,
The ends of the other sections were covered with
tape., The loop sections were stored for three
months; then, if they had not been used, they
were sent to salvage.

 

*E. F. Joseph et al,, Aircraft Nuclear Propulsion
Fluoride Fuel Preparation Facility, ORNL CF-54-6-126
(June 1, 1954).
UNCLASSIFIED
PHOTO 21341

 

Fig. 1. Thermal Convection Loops.
 

 

 

 

——

 

ve

 

 

 

 

 

{-in. RESERVE METALLOGRAPHIC
SAMPLE
2-in. METALLOGRAPHIC SAMPLE

4-in. DRY-BOX SAMPLE (CHEMICAL)

2-in. METALLOGRAPHIC SAMPLE

6-in. CHEMICAL SAMPLE, SPECIAL
LOOPS ONLY

UNCLASSIFIED
ORNL-LR-DWG 48763

    
 

 

 

 

 

 

5| €-in. CHEMICAL SAMPLE, SPECIAL
LOOPS ONLY
4| 2-in. METALLOGRAPHIC SAMPLE
2| 5-in.DRY-BOX SAMPLE (CHEMICAL)
3| 2-in. METALLOGRAPHIC SAMPLE
f!f"e in,
st
N

 

 

 

 

Fig. 2. Location of Metallographic and Chemical Samples. Standard loops.
The amount of attack was determined by
metallographic exomination.  Uncertainties in
the original pipes, in cleaning the surfaces after
operation, and in determining losses from extesmol
oxidation prevented the use of weight loss data.
The depth of attack was determined by the depth
of the deepest void formation found metallographi-
cally in a traverse of an entire circumference of a
section. Other than classifying the intensity as
moderate, heavy, etc., no attempt was made to
determine the amount of the attack. Typical
sections of each type are shown in Fig. 3. Wall
thicknesses were measured microscopically, and
it was shown that no large, even surface remcval
type of attack was taking place; however, smoll
amounts less than the normal variation in pipe
wall thickness would not be measurable. For

UNCLASSIFIED
T 5183

 

 

 

Fig. 3.
Light; () moderate; (c) heavy, 250X. Reduced 29%.

 
   

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o . s B L . " Leot]
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il o | T'T';*'p g ..-"‘A:h
L B - r [ L& i, S —
e e R el
« : . bk e T e
i - = - 3 - ¢ N | :
. 1 - - _‘E_L..

 

Photomicrographs lllustrating Various Intensities of Attack as Reported for Fluoride Corrosion.

each loop, at least two sections from the hottest
area were examined to determine the maximum
attack.

The fluoride mixtures used in this study are
referred to by number, identified in
Table 1,

and are

RESULTS AND DISCUSSION

Fluoride 30 in Inconel Loops

Standard Loops. — A considerable number of
loops were operated under what were considered
to be standard conditions. These loops circu-

lated a zirconium fluoride mixture for 500 hr with
a hot-leg temperature of 1500°F and a tempera-
ture drop of about 225°F,

general groups,

They fall into two
The first group were operated

UNCLASSIFIED
T 5185

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{a)
Table 1. Composition of Molten Fluoride Mixtures

 

 

 

 

 

Fluecride UFai NaF ZrF4
Ne. Weight % Mole % Weight % Mole % Weight % Mole %
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
31 20.1 50.0 79.9 50.0
44 18.6 6.5 20.5 53.5 60.9 40.0

 

either as preliminary loops to outline the problems
to be encountered with this mixture or as control
loops for some of the early studies. These loops
were not precleaned, and close control of con-
ditions had not yet been established., The second
group were run as controls for later studies, and
all were precleaned. While some scatter in re-
sults still exists in this second group, the
reproducibility, especially during the fluoride
production, was much better than was found in
the early loops. The data for the first group are
tabulated in Table 2 and those for the second
group in Table 3,

It is apparent from Table 2 that a variation in
maximum depth of attack from 5 to 18 mils was
from supposedly duplicate loops. With
many of these loops the iron and nickel impurity
concentrations of the original fluorides were also
high, but the maximum depth of attack did not
vary in a systematic manner with these concen-
trations. Another cause of variation in attack
was an undetermined amount of hydrogen fluoride
left in the fluoride mixture from the purification
process. This hydrogen fluoride would react
with the loop wall, producing iron and nickel
fluorides which in turn would reduce the chromium,
When this group of loops were operated, no suit-
oble method waos available for measuring the

found

hydrogen fluoride concentration.

When a method became available for measuring
the hydrogen fluoride content of the mixtures, it
was found to be necessary to increase the
stripping time after purification to reduce the
hydrogen fluoride to acceptable levels. The
control loops tabulated in Table 3 were those in
which the controlled fluoride batches were circu-
lated., While considerable variation is still
found in the maximum depths of attack, it is not
as great as in Table 2,

The loops in Table 3 are tobulated approxi-
mately in the order in which they were filled or
in order of time. |f this table is divided into
quarters and average moximum ottacks calculated
for each quarter, it may be noted that the depths
of attack are higher at the end, that is, that the
attack is graduclly increasing with time, The
average depth for the entire table was 9.1 mils,
while the attack for the four quarters averaged
7.0, 9.2, 9.6, and 10.5 mils, respectively. As a
comparison, the average in Table 2 was 10.1 mils.
The poorer control of production with the large
batches was responsible for some of the increase
but not for all of it, since the trend was a gradual
one, Neither the other causes of this gradual
increase nor the cause of the occasional very
deep attacks has been determined. The increased
purging time and closer control in the second
group have made the ottack more consistent but
have not resulted in a large average reduction in
maximum depth of attack. Since the average
chromium content in the fluorides after operation
of the loops in Table 2 is higher than after
operation of those tabulated in Table 3, there
oppears also to be a reduction in intensity of
attack over the entire loop with the controlled
batches.

In an effort to determine whether air contami-
nation during filling or operation could be re-
sponsible for the variation in ottack, o series of
loops were operated with fluoride batches which
had been deliberately contaminated. Loops were
operated under three conditions: (1) with helium
pressure only during filling, (2) with no protective
atmosphere combined with a small air leak, and
(3) with a l-liter volume of air bubbled through
the batch before transfer. The loop with no
protective atmosphere and a slow leck developed
an attack of 21 mils in 500 hr, which was double
Toble 2. Data from Preliminary Control Loops?

 

Fluoride Mixture

Anclysis of Flueride Mixture

 

 

 

 

i Eluoiide Barch Aftost Nickel (ppm) Iron (ppm) Chromium (ppm) Uranium (%)
o Ne. No. tnnty  Reptiei) e M DiEm - A0y B i See e
234 27 Lak Light 5 530 <30 4100 600 1460 1900 8.3 9.2
236° 27 Lab Moderate 10 610 <30 2900 1000 1070 2400 7.9 9.2
244 27 Moderate 9 140 <20 540 100 <20 1450 8.2 5.0
245 27 Moderate 12 340 <20 340 100 75 1500 8.4 9.3
2610 30 Lab Moderate 6 <20 150 950
273 30 Moderate 10 <20 60 860 175 160 1200 8.6 2.0
272 30 Moderate 16 720 30 520 200 45 1350 8.3 7.0°
283 30 d Heavy 15 1100 40 740 100 100 2800°¢ 4.6 5.1
280 30 EE-58  Heavy 8 85 <20 430 50 <20 1000 8.6 9.7
287 30 EE:50  Heavy 13 30 20 520 60 <20 1450 8.8 9.1¢
282 30 e Moderate 9 40 <20 520 30 <20 1100 8.9 8.9
289 30 EE-59  Heavy 8 95 20 620 60 230 900 8.8 9.5
295/ 30 EE-63  Heavy 6 145 <20 625 30 <20 1100 8.8 9.4
296 30 EE-63  Heavy 8 40 20 470 45 90 1050 8.9 9.1
284 30 d Heavy 12 500 <20 1000 40 1450 1400 9.0 9.2
310 30 R-110/  Heavy 9 40 <20 265 50 <20 900 8.6 9.0
307 30 e Heavy 5 90 <20 235 30 <20 1100 9.0 8.8
298 30 EE-68  Heavy 9.5 40 <20 550 25 100 1100 8.6 8.9
343/ 30 EE-106 Heavy 18 75 <20 210 50 890 1500¢ 9.0 8.9
324% 30 EE-92  Heavy 15 60 <20 c 45 250 1300 8.6 8.9
345/ 30 EE-113  Heavy 6 40 <20 65 45 120 600 9.0 9.2
326 44 EE-114  Heavy 750 <20 500 65 250 1100 14.4 14.0°
381 44 EE-140  Heavy 10 <20 <20 175 120 50 700 14.6 12.0°
387 44 EE-145  Haovy 16.5 <20 <20 395 90 135 700 15.3 15.5¢
10 (av) 1190 (av)

 

PAll loops circulated o zirconium fluoride mixture for 500 hr with a hot-leg temperature of 1500°F and a cold-leg temperature averaging 1300°F,

Loop treated with hydrogen.

“Considerable spread was found in the individual values.

4p: melted.

“Made in grophite.

Loop cleaned with flucride mixture 31,

 
Table 3. Data from Contrel Loops”

 

Fluoride Mixture Analysis af Fluoride Mixture

 

Lnnp EE Attack

 

 

Na. Fluaride e — e i Nickel (ppm) Chremium (ppm) Iren (ppm) Uranium (%) HF®
No Batch Intensity Depth (mils)
¥ b Before After Befare After Before After Before After
352 30 19 Moderate 5.5 <20 <20 50 450 110 60 8.8 9.4
353°¢ 30 e Heavy 3 <20 <20 50 600 130 75 8.8 9.1 3.6
3609 30 19 Heavy 4.5 <20 <20 100 500 100 70 8.7 9.0° 3.7
380/ 30 141 Heovy 7 <20 <20 95 700 185 90 8.7 8.9
382 30 155 Heavy 10 <10 <10 70 800° 65 90 8.6 8.6 2.2
383 30 150 Heavy 9 <10 <10 115 900 95 %0 9.0 B.5 2.1
384 30 150 Heavy 12 <20 <10 75 950 130 85 8.4 8.6 1.1
421 30 150 Heavy 5 <10 <10 100 800 110 300¢ B.8 8.6 3.0
435 30 160 Heavy 8 <10 <10 &5 400 40 70 8.6 8.8 2.5
440 30 158 Heavy g <10 <10 60 500 40 60 8.6 9.2
442 30 155 Heavy 10 <10 <10 80 800 150 60 8.5 8.6 4.6
4449 30 158 Heavy 8 120 <10 90 400 85 70 8.9 9.5 6.7
469 30 162 Heavy 8 25 15¢ 90 700% 55 70 8.5 8.8 0.9
462 44 203 Heavy 10 as <10 35 400 50 45 13.9 13.7¢ 2.8
463 44 203 Heavy 10 i5 <10 30 475 50 45 13.2 13.5° 2.5
540 30 188-1 Heavy 1 10 15 40 520 30 80 8.5 8.6 2.1
545 30 198 Heavy n 40 <10 70 400 65 70 9.5 8.8 1.7
571 30 232-8 Heavy 5 25 25 55 900¢ 165 e B.6 B.6
554 30 188-7 Heavy n 30 <10 80 600 90 60 8.5 B.6 2.7
570 30 232-8 Moderate 9 20 25 75 850 70 100 8.2 8.8 1.2
577 30 228-12 Heavy 9 40 40 45 850 85 80 B.9 9.2 0.5
586 44 239-3 Heavy 12 6 <10 50 800° 45 90° 14.0 13.7 0.5
607 44 241 Heavy 12 50 50 65 700 95 80 14,0 14.3 0.1
608 44 241 Heavy g 25 50 50 700 80 75 14.1 14.1 0.1
609 30 223-1 Heavy 7.5 <1 20 50 e 145 40 8.9 9.1 0.7
610 30 223-1 Heavy 1 <1 20 50 1000 145 50 8.9 8.7 0.7
651 30 513-4 Heavy 9 10 e 110 800 75 70 8.3 8.4 1.0
684 30 246-1 Heavy 11 15 10 55 200 255 50 8.9 9.1 1.2
685 30 246-1 Heavy 14 5 5 60 700 30 40 8.9 9.0 1.3
686 30 246-1 Heavy 9 15 10 80 775 60 15 8.8 9.0 1.5
698 30 434-R Moderate 10 9 10 40 800 105 55 B.6 8.9 1.5
700 30 246-4 Heavy 13 5 25 45 625 120 65 8.9 9.1 1.9
2.1 (av) 675 (av)

 

%A1l loaps circulated a zirconium flucride mixture for 500 he with a hot-leg temperature of 1500°F and a cold-leg temperature averaging 1300°F,

bRelative readings on Solubridge ofter bubbling 2 liters of helium through bath and then through boric acid.
“No cleaning.

dNu trap.

“Individual results vary,

ICurved loop.

 

 
—— e R e e e e e ——E

that found in the control loops. The other two
loops developed maximum depths of attack within
the usual spread. However, in loop operation
there is only a remote possibility that develop-
ment of a leak and failure of the protective gas
system would occur simultaneously.

When the gradual increase in attack became
apparent, loops 684, 685, and 686 were filled on
the same day from the same batch of fluorides.
The operators were watched and were extremely
careful to avoid any operation that could permit
contamination, The attack in 500 hr in these
loops was 11, 14, and 9 mils, respectively.
Since a variation was still found, these loops
indicated that the trouble was not from careless
operators or from the batch, Changes in the
batches could have caused the grodual increase
but not the wide and unpredictable variations.
As yet, no satisfactory explanation has been
offered for these variations.

To illustrate the distribution of attack around
the loop and to support the procedure of basing
conclusions primarily on the sample from the top

of the hot leg, loop 833 was sectioned in con-
siderable detail as shown by Fig. 4; the metallo-
graphic data from this loop are given in Table 4;
Fig. 5 shows typical photomicrographs from the
various loop sections. These data confirm the
fact that the maximum aottack was found at the
top of the hot leg. Very little attack was found
in the hot horizontal leg, and a gradual increase
in depth was found in moving up the vertical hot
leg. This increase was more gradual than was
found in o similar examination of a loop in which
alkali-metal-base fluorides were circulated, but
otherwise the two loops were similar.! The
attack depth decreased rapidly above the heated
area, and this will be discussed in more detail
below (‘“‘Effect of Temperature'’)., The three
samples from under the upper half of the top
heater checked each other very well,

Operating Time. — Since thermal loops have no
moving parts or mechanical seals, they are ideal
for long-time operation. The limiting feature in
such loops is the life of the heoting elements,
which can be replaced during operation only with

Table 4. Metallographic Examination of Sectioned Loop 833

 

Section Mo,

Metallegraphic Notes

 

1 Heavy surface pitting to 0.5 mil with heavy intergranular

subsurface veoids to 12 mils

1A Heavy surface pitting to 0.5 mil with heovy intergranular

subsurface void formation to a depth of 11 mils

2 Heavy surface pitting to 0.5 mil with heavy intergranular

subsurface voids to 9 mils

m N O ot B W

Light, shallow surface roughening
Moderate surface pits to 0.5 mil
Light, shallow surfoce roughening
Moderate surfoce pits ta 0.5 mil
Some as section 6

Moderate to heavy general subsurface void formation to a

maximum depth of 5 mils

9 Heavy surface pitting to 1 mil with intergronulor subsurfoce

voids to 6.5 mils

10 Some as section 9

11 Heavy surface pitting ta 1 mil with heavy general subsurface

voids to 3 mils

12 Heavy surface pitting te a depth of 1 mil

 
UNCLASSIFIED
ORNL-LR-DWG 48764

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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T-8870 (8] . ~
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T-8873|2] 2in.
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T-8874 (19| in,
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Fig. 4. Location of Metallographic Sections from Loop 833.

10
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some difficulty., In this program a considerable
number of loops were operated for 1000 hr or
longer,

To determine how the maximum depth of attack
varied with operating time, several series of
tests were made with various fluoride batches.
These data are tabulated in Table 5 and plotted
in Fig. 6. As shown on the plot, considerable

spread was apparent in the data; however, if they
are separated into the various fluoride production
batches, a series of nearly parallel curves is
found. In each case a few points do not fall on
the curve; these are thought to be another ex-
ample of the unexplained occasional wide varia-
tions found in the control loops.

With all these curves, after about 300 hr the

increase in depth of attack is linear and is at a

Table 5. Effect of Operating Time on Corrosion Attack

 

 

 

EE 0 ti
Locp PBTQ e Attack Average Cr Content .
Batch Time ) Fluoride Mixture

No. No. (hr) Intensity Depth (mils) (ppm) After Operation

263 30 100 Moderate 4 1000* 27 from 50-1b batch containing
720 ppm Ni

264 30 500 Modercote 9 1300 27 from 50-1b batch containing
720 ppm Ni

265 28 100 Moderate 5 1000 27 from 5-1b batch containing
1380 ppm Ni

266 31 500 Moderate 15 1800 27 from 5-1b batch containing
1070 ppm Ni

267 100 Moderate 3 200 27 pretreated

268 500 Moderate 13 1750 27 pretreated

274 35 1000 Heavy 11 1400 30

297 68 10 Light 1 230 30

300 68 50 Meoderate 2 575 30

305 68 250 Heavy 9 1200 30

298 68 500 Heavy 9.5 1100 30

306 68 1000 Heavy Q 1400 30

299 68 2850 Heavy 18 1500* 30

318 88 100 Heavy 4 900 30

317 88 350 Heavy 8 1200 30

345 113 500 Heavy 6 600 30

327 113 1000 Heavy 10 550 30

328 113 2000 Heavy 7 250 30

329 113 3000 Heavy 23 600 30

344 113 5000 Heavy 27 350"~ 30

381 140 525 Heavy 10 44

394 149 1000 Heavy 17 44

12

 

 
 

 

Taoble 5 (continued)

 

EE Operating

 

 

 

Attack A
Lo Batch Time verays Cr Conta‘nt Fluoride Mixture
Ne. b, (hr) Intensity  Depth (mils) {(ppm) After Operation
459 162 500 Heavy é 550 30 plus 0.2% ZrH,
413 150 1000 Light 7 550 30 plus 0.2% ZrH,
414 150 2000 Heavy 11 85 30 plus 0.2% Zer
340 113 3000 Heavy 13!5 300 30 plus 0.25% Zer
431 160 10 Light 2 300 30
432 160 50 Heavy 3 400 30
433 160 100 Moderate 4.5 425 30
434 160 250 Heavy 8 600 30
435 160 500 Heavy 8 400 30
436 160 1000 Heavy 14 700 30
445 160 1500 Heavy 13 525 30
437 160 2000 Heavy 16 600 30
450 160 2500 Heavy 11 600* 30
448 173 1000 Heavy 10 600 30
540 188-1 500 Heavy 11 520 30
535 188-1 1080 Moderate 10 475 30
501 188-7 100 Heavy 3 700 30
554 188-7 500 Heavy 11 600 30
463 203 500 Heavy 10 475 44
464 203 1000 Heavy 15 450* 44
570 232-8 500 Moderate 9 850 30
567 232-8 1000 Light 9 800 30
568 232-8 2000 Moderate 12 800 30
*Wide wvariation in individual values.
rate of about 4 mils per 1000 hr, A single loop rate of mass transfer is not strongly dependent

was operated for 5000 hr and the relationship
still held. This continuing increase in depth of
attack with operating time is attributed to mass
transfer. It is obvious that if this variable is to
be studied in a thermal loop, operating times of
from 1000 to 2000 hr are a necessity. Since the
curves for the different batches are parallel, the

upon the original batch. A series of photomicro-
graphs representing various test durations are
shown in Fig. 7. |t may be seen that the voids
grow in size with time and show a tendency to
concentrate in the grain boundaries,

The rapid attack during the initial stages was
sensitive to the nature of the batch. The attack
 

UNCLASSIFIED
ORNL=-LR-DWG 4B87E6E

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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X 160 (4mil=/1000hr ) TO 5000 hr
e |3 AT 27 mils
A 188-198 b s
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0 500 1000 1500 2000 2500 3000 3500 4000 4500
OPERATING TIME (hr)
Fig. 6. Variation in Depth of Attack with Operating Time.
during this time was caused by the system crystals found to explain all the attack. The

reaching equilibrium; it was affected by such
things as impurities from any source and by the
dissolving of sufficient chromium to reach equi-
librium concentrations.

An interesting point in the tests was the average
chromium content found in the fluorides after
operation of the loop. While the depth of attack
increased with time, the chromium concentration
in the fluorides did not. The concentration was
established during the first stage of the attack
and then remained constant. In some long-time
loops, metallic chromium crystals were found in
the trap at the bottom of the cold leg. This
deposit appeared first as a ring made up from
fine dendritic crystals sticking to the trap wall
just above the liquid-solid interface found in the
trap. It continued to grow until an entire disk
was formed, as shown in Fig. 8. In many 1000-
and 2000-hr loops, no crystals at all were found,
and in none of the loops were sufficient metallic

trap deposits were identified as metallic chromium
by x-ray diffraction and spectrographic studies,
With the depth of attack varying with time as
shown in Fig. 6, diffusion of chromium within the
pipe wall could possibly be the rate-controlling
step. |f this were true, the depth of attack would
be independent of the solutions and so should be
the same after circulating two fluoride batches
for 250 hr each as after circulating one batch for
500 hr. However, loop 281 developed a ftotal
depth of attack of 17 mils with two batches circu-
lated for 250 hr each, while loop 280 developed
an attack of 8 mils with a single 500-hr circulation
of another portion of the same batch, Additional
evidence that the controlling step is not in the
wall is provided in loops 333 and 334, Loop 334
circulated a batch that had been previously
circulated in loop 333. An attack to 5 mils was
found after 500 hr in loop 334, while in loop 333
an attack to 14 mils was found after only 240 hr.
Gl

    

 

 

 

 

 

  

 

 

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Fig. 7. Effect of Operating Time on Attack. (a) 10 hr; (k) 100 hr; (c) 1000 hr; (d) 3000 he. 250X. Reduced 17%.
UNCLASSIFIED
T-5211

 

Fig. 8. Metallic Deposit Found at Top of Cold Trap
from Loop 344,

The attack in loop 334 was higher than would be
predicted from the curves, but the original batch
was of poor quality, and the 240 hr in loop 333
does not appear to be sufficient time for it to
reach equilibrium. When the fluoride mixture was
transferred to loop 334, 225 ppm of iron was still
present.

Other evidence that chromium diffusion is not
the limiting factor during the longer operating
times is presented below (‘‘Change in Analysis
of Inner Pipe Wall*).

These tests show that while the chromium is
continually removed from the inner surface of the
pipe wall, the wall is not being continually
depleted.  Chromium is removed fairly rapidly
during the early stages of rapid attack, but with
the decrease in removal rate the chromium diffuses
to the surface as fast as it is removed and
possibly even at a slightly higher rate. If the
chromium diffusion rate was the limiting factor,
the surfoce would be continually depleted, or at
least remain constant, instead of the concentration
evening out.

Effect of Temperature. — For all this work, the
loop operating temperature was measured from
thermocouples spot-welded to the pipe wall
several inches above the top heater. As long as
the temperature is measured in the same manner
and at the same spot in all loops, the measure-
ments are comparable, but they may be only
relative. The development of the methods of
installing the thermocouples and the choice of
locations were discussed in the previous prGrt.1
With the techniques used on the loops several
errors are known to be present. The temperatures
were measured on the outer pipe wall surface
above the heaters. While the bulk fluoride temper-

16

ature would change little, if at all, in this dis-
tance, a difference in wall temperature would be
found. Under the heaters the driving force is
inward, and therefore the walls are hotter than
the fluoride; above the heaters, the reverse is
true. The temperature of interest is the maximum
temperature at the interface between the liquid
and the pipe wall, but for practical reasons it is
the temperature of the outer wall that is measured,
and this at only a few spots. Thus there is no
assurance that the maximum interface temperature
is being measured.

Two attempts were made to determine the
maximum wall temperature under the heaters,
Thermocouples were attached to the walls of
these loops by various methods, and several
different radiation shields were provided for the
thermocouple beads., In addition, several thermo-
couples were buried in the wall, As was ex-
pected, a spread in temperatures was obtained,
but for both loops the maximum wall temperature
appeared to be between 150 and 175°F above the
measured standard loop operating temperature.
This temperature difference was larger than was
expected, but since so much work hod already
been done with the temperature above the last
heater used as the so-called maximum, it was
decided to complete the work without making a
change in the technique. The thermal loop data
are suitable only for direct comparison to give
trends and for studies of mechanisms and are not
intended as reactor or loop design data; therefore
such o decision was reasonable,

When this work was started, considerable
discussion had occurred as to what temperatures
should be measured and how. Considerable
difficulty had been encountered with failure, by
breaking loose, of thermocouples under the
heaters; installation there was more difficult,
and such failure could easily produce high values.
For these reasons, it was decided to measure
the temperature just above the heaters. |In
retrospect this appears to have been a mistake,
since wall temperature has now been shown to be
the most important variable aond the error in
measuring with this method is larger than was
thought; so it would now appear desirable with
each loop to actually measure the temperature
under the heaters.

The data showing the variation in depth of
attack with variations in hot-leg temperature are
tabulated in Table 6 and summarized in Table 7.
Table 6. Dota from Loops Operated with Various Hot-Leg Temperatures

 

 

 

Loop EE Batch Hot-Leg Temperature Temperature Drop Operoting Time Attack
No. No,? (°F) (°F) (hr) Intensity Depth (mils)
275 35 1650 125 500 Moderate 11
273 22 1500 500 Moderate 10
286 59 1300 175 500 Heavy 9
287 59 1500 500 Heavy 13
289 59 1500 500 Heavy 8
288 59 1650 500 Moderate 12
314 73 1250 500 Heavy 3
318 88 1500 100 Heavy 4
Ky} 88 1650 150 100 Heavy 7
350 113 1200 150 500 Heavy 3
345 113 1500 500 Heavy 6
390 141 1200 140 500 Heavy 5
380° 141 1500 500 Heavy 7
439 158 1300 160 500 Heavy 4‘6
440 158 1500 200 500 Heavy 9
441 158 1650 150 500 Heavy 8
537 188-1 1200 170 500 Heavy 6
538 188-1 1300 170 500 Heavy 4
539 188-1 1400 200 500 Heavy 5%
540 188-1 1500 200 500 Heavy 1
562 232-8 1200 150 500 Heavy 7
575 232-8 1200 150 500 Heavy 8
563 232-8 1350 185 500 Heavy 6
570° 232-8 1500 500 Moderate 9
5719 2328 1500 500 Heavy 5
586 239-3° 1500 200 500 Heavy 12
581 239-3° 1600 215 714 Heavy 14
351 119 1650 120 1000 Moderate 8
391 141 1200 150 1310 Heavy 7
392 141 1400 180 2000 Heavy 8
393 141 1600 185 1900 Heavy 13
449 158 1250 160 2000 Heavy 6
533 188-1 1250 165 1060 Moderate 4
534 188-1 1350 175 1125 Heavy 5
535 188-1 1500 215 1080 Moderate 10
536 188-1 1600 220 1150 Heavy 9
578 239-3€ 1200 170 1500 Heavy 8
5772 239-3° 1350 190 1500 Heavy 5
583 239-3° 1350 200 1500 Heavy 5
584 239.3° 1500 220 1500 Heavy 15
580 239-3° 1500 1500 Heavy 12
585 239-3° 1600 250 1500 Heavy 18

 

Elyoride 30 except where indicated.

bCurvad locp.
c

].5 in. sched 40.

dy
.fi in. sched 10,
®Fluoride 44.
Table 7. Summary of Effect of Hot<Leg Temperature on Depth of Attack

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Operating Time  EE Batch Depth of Attack (mils) ot
(hr) No. 1200°F 1250°F 1300°F 1350°F 1400°F 1500°F 1600°F  1650°F
100 88 4 7
500 59 and 73 3 2 13,8 11,12
113 3 6
141 5 7
158 A 9 8
188-1 6 4 5% 1
232-8 7,8 6 59
Av 5.8 5.8 8.5 10.3
1500 239.3 8 5,5 15, 12 18
2000 141 8 13
Table 7 is divided into horizontal sections ouu{fifiifléfgm‘r
corresponding to various operating times, with 1600 .
the batches tabulated separately in each section.
An increase in depth of attack with increasing =
|
temperature is apparent both at 500 hr and in the S 1500 =4
long-time loops. At 500 hr the individual batches 'E'r;
show considerable spread and the effect is not =
definite; however, it is shown up by averaging §14(}D . X el
the results for each temperature, Since the =
amount of this increase varies with the batch, UZJ
no qualitative conclusions may be drawn, and the = 1300
increase must be regarded only as a trend. 2
It may be noted that o deeper attack was found
in both the short- and long-time loops operated at 1200 . l
around 1200°F than in those at 1350°F. A typical 0 S 0 W, B8 25
plot of the variation in depth of attack with P TH G RV toithy
» . . o '
temperature showing this increase at 1200°F is Fig. 9. Change in Depth of Attack with Increasing

shown in Fig. 9. The curve in this plot is from
the 1500-hr loops with mixture 44, but the shape
is typical, With other mixtures and times the
slopes of the upper curves change, but in all
cases a nose is apparent, While brecks have
also been noted in some physical properties in
this temperature range, no definite reasons have
been advanced for the increase in depth of attack.

As shown by the photomicrographs in Fig. 10,
the nature of the voids also changes with temper-

ature. At low temperatures, even with the long
times, the voids are small and fairly evenly
distributed. As the temperature increases, the

voids grow in size and concentrate in the grain
boundaries.

18

Temperature in Loops Operated for 1500 hr.

While a variation in attack occurs with temper-
ature, this variation is relative. |t has been
shown that considerable attack can be found in a
loop whose hot leg was at 1500°F, but none is
found in the 1300°F cold leg. When the loop
temperature was increased until the cold leg was
at 1500°F, no change was found in the distribution
of the attack. This was also true when the hot-
leg temperature was dropped to 1350°F,

Additional evidence that the actual temperature
is not as important as the relative temperature
was found in the data in Table 4. As discussed
previously in this section, very little change
6l

    

 

 

 

     

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Fig. 10. Changes in Appearance of the Attack with Increasing Hot-Leg Temperature. (a) 1300°F; (b) 1400°F; (c) 1500°F; (d) 1650°F. Note how
voids grow in size. 250X. Reduced 17%.
would be found in bulk fluoride temperature
between samples B and 1A; however, some differ-
ence would be found in wall temperature. The
depth of attack has been reduced by one-half in
this 1-in, distance. This rapid decrease in attack
was confirmed with loop 268. The top of the hot
leg of this loop was sectioned longitudinally, and
the attack decreased from a heavy 12 mils to a
light 4 mils in a distonce of 0.75 in.

In another attempt to show the correlation
between depth of attack and heater location,
loop 456 was operated with an unheated, but
well-insulated, area near the top of the hot leg.
The maximum depth of attack in the unheated
area was 3 mils, while attacks to 9 mils were
found in the heated areas on both ends of this
zone. The change in depth was quite rapid at
both ends. The effects of the unheated area must
be complex, since the 9-mil attack below the gap
was deeper than would be found in a comparable
area in a control loop. It is possible that the
gap in the heaters had an effect upon the forced
convection currents found by Hamilton,® These
currents could affect both the laminar layer and
the wall temperature.

The importance of location is also shown
clearly in another loop. Loop 259 was part of the
temperature study and had an Inconel thermocouple
well suspended in the middle of the hot leg.
Fluoride 27 was circulated in this loop, and
after 500 hr a typical attack to 8 mils was found
on the loop wall. In a section directly opposite
this area, the thermocouple well showed only a
light attack to @ maximum depth of 1 mil. The
distance between the thermocouple well and the
loop wall was only 0.2 in., but all the attack had
been concentrated on the hotter surface,

All these tests illustrate the fact that the
maximum wall temperature is a more critical
variable than average bulk fluoride temperature
is, When a temperature difference is present
between two surfaces, the attack will be concen-
trated on the hotter surface, even if the colder
one is at a level where attack is normally
encountered,

Temperature Drop. — The thermal loop is not
ideal for studying the effects of temperature drop
in @ dynamic corrosion test, Any change in

 

3I:J'. C. Hamilton, F. E. Lynch, and L. D. Palmer,
The Nature of Flow of Ordinary Fluids in a Thermal
Convection Harp, ORNL-1624 (Feb. 23, 1954).

20

temperature difference between the two legs is
also a change in flow driving force, and so must
be reflected as a change in velocity. With a
standard loop design and heater location, any
change in temperature drop requires a change in
heat input and therefore a change in maximum
wall temperature, Since in a thermal loop the
Reynolds number is already very low, it was
speculated that the changes in flow would not
have a big effect on the laminor layer and there-
fore that changes in temperature drop could be
studied, ot least in an approximate manner. |t
has now been shown in the pump loops that
velocity is not a critical varioble, The varia-
tions in wall temperature would, however, still
cause errors,

For this series of tests the temperature drops
in the loops were varied by placing various
amounts of insulation on the cold leg. An attempt
was made to increase the drop by air-cocling the
cold leg, but with the standard loop design,
sufficient heating was not available to maintain
the hot-leg temperature at 1500°F. The long-time
loops in this series were terminated by a building
power failure after they had been circulated for
varying periods. For comparative purposes, small
corrections were made in the depths of attack to
compensate for the variations in time. In all
cases the corrections, and therefore any errors,
were smaller than the normal spread in results.

As shown by the data tabuloted in Table 8 and
summarized in Table 9, a decrease in depth of
attack was found with smaller temperature drops.
This was found with both the short- and the long-
time loops. While these dota seem to show an
effect, they cannot be considered as proof, since
with the lower temperature drops, lower wall
temperature would be found. This would also
reduce the attack, and the two variables cannot
be separated from these data, |t was shown
previously that while wall temperature was critical
it was relative in that a temperature drop was
also necessary,
mation that increased temperature drop will cause
increased attack.

A conflict is apparent in the data in Table 9.
An increase in attack was found for each time
series with an increase in temperature drop;
however, the increases were independent of time,
Thus these data indicate that the depth of attack
does not increase with time and therefore conflict
with the results presented above (‘‘Operating

This would be indirect confir-
 

 

Table 8. Data from Loops Operated with Various Temperature Drops

 

Average Chromium

 

 

_ EE Batch Goldilsg Temperature Dperating Sy S Attack

Ne. Ne. Condition thop it Operation : Depth

(°F) (hr) Intensity :
(ppm) (mils)

422 155 Wrapped 130 500 750* Heavy 5

442 155 Control 200 500 800 Heavy 10

546 198 Insulated 70 500 400 Heovy 4
547 198 Wrapped 140 500 400 Heavy 6%
545 198 Control 200 500 400 Heavy i

423 155 Wrapped 140 2000 650 Heavy 6

447 173 Insulated 65 1000 350 Moderate 4

448 173 Control 200 1000 600 Heavy 10

446 173 Cocled 220 1000 500 Heavy 12

510 198 Insulated 85 1820 450 Heavy 6 (5))**
530 198 Wrapped 130 1640 1700* Moderate 7 (7)**
531 198 Control 220 1400 350 Moderate 9% (10})**
532 198 Cooled 235 1410 475 Heavy 10 (1)**
677 246-1 Insulated 70 14 675 Light 3

678 2461 Insulated 95 50 700 Mederate  2J

679 246-1 Insulated 100 100 750 Moderate 4

680 246-1 Insulated 70 100 725 Moderate 4

682 246-1 Insuloted 80 500 800 Moderate 5%

684 246-1 Control 220 500 900 Heavy n

 

*Wide variation in individual values.

**The attacks in the loops in this series were adjusted to 1600 hr; the corrected values are shown in parentheses.

Time''), This conflict cannot be blamed on batch
variation, since portions of both EE batches
155 and 198 were operated for different times
with the same temperature drop and in both cases
this independence with time was noted. A series
of loops with insulated cold legs were therefore
operated for varying times, The data for this
series are also tabulated in Table 8 and do show
an increase in depth with time. The attack in the
500-hr insulated loop is only half that found in

the control loop, confirming the data reported
above. The fact that 3 mils attack was found
after only 14 hr with a low temperature drop shows
that prolonged cleaning times should be avoided.

Additives. — |n the discussion on the control
loops it was pointed out that the depth of attack
seemed to increase with the presence of iron or
nickel in the fluorides, but because of other
variables a definite correlation could not be

21
Table 9. Summory of Changes in Depth of Attack with Variations in Temperature Drops

 

Operating Time

Depth (mils) of Attack at Temperature Drops of

 

 

(hr) 65 to B5°F 130 to 140°F 200 to 220°F
1
500 4%, 5% 5,6, 6% 10, 11, 1
1000 4 12, 10
1 1
1600 54 7 10%, 11
2000 &

 

made, The wvariation of attack with iron and
nickel concentrations was also noted in the
original alkali metol work.! To establish a

better correlation, several loops were therefore
operated with controlled odditions of nickel
fluoride. The nickel fluoride was added to the
batch in the tronsfer pot in order to produce a
more uniform distribution, While the batch was
being agitated with helium, some of the nickel
fluoride reacted with the pot walls, resulting in
smaller additions than had been planned. A
Micro Metallic filter was used in the transfer line
to prevent transfer of any metallic nickel. The
data from these loops are tabulated in Table 10,
From these data, on increase in depth of attack
over that of a control loop was found in each of
the 500-hr loops to which nickel fluoride had
been added. Since the amount of nickel actually
added is not known, an exact comparison cannot
be made, but the depth of attack does increase
with increasing additions. The increases ore not
as large as was expected, with an addition of
1000 ppm of nickel fluoride causing a doubling in
the depth of attack, Since some of the chromium
in these loops was picked up in the charge pot,
the fluorides were partially saturated before
being circulated. To compare these loops with
control loops the hot-leg attack would have to be
increased sufficiently to balance the chromium
picked up in the pot. With present production
techniques for the fluoride mixtures, the varia-
tions in nickel and iron found in the original
batches are not large, and fram this work it does
not appear that they are the cause of the erratic
attack.  Typical hot-leg sections from one of
these loops and from the 500-hr loops with the
other additions discussed below are presented in
Fig. 11, Portions of the same fluoride batch

22

were used in loops 454 and 455 to determine the
effect of nickel fluoride on the rate of mass
transfer,  The longer-time loop developed an
attack of 16 mils, which is an increase of only
1 mil in 1335 hr; however, some increase was
found in the number and size of the voids, While
it is possible that nickel fluoride will not increase
the rate of mass transfer, it should react to
produce chromium fluoride, which is shown below
to increase this rote, No satisfactory explanation
has been found for this inconsistency. Typical
hot-leg sections showing the effect of this addi-
tive and others on mass transfer are shown in
Fige. 12,

Additions of both chromic and chromous fluoride
were made in a manner similar to that described
for the nickel fluoride. The data for these loops
are also tabulated in Table 10, 1t has been
shown by work in the Materials Chemistry Division
that when CrF, is odded to fluoride mixtures
with a zirconium fluoride base it is reduced to
CrF,, the latter being the only valence state that
has been found in such mixtures, The addition of
CrF, should reduce the attack found in the first
stage by providing some of the chromium required
to reach equilibrium, With CrF, this effect would
also be found, but it would be offset by an in-
crease in attack caused by the extra fluorine atom,

The data in Table 10 do show a reduction in
depth of attack when CrF, was added to a loop
operated for 500 hr, However, in a loop operated
for 1710 hr the depth was slightly deeper than
was found in control loops operated for similar
times; thus, since the depth at 500 hr wos re-
duced, the rate of mass transfer increased. For
these loops the data would indicate a rate of
1 mil per 100 hr rather than the 0.4 mil found
with the control loops.
Table 10. Variation in Depth of Attack with Impurity Additions

 

 

 

Loop aih o Metal in Additive (%) Opsrating Time  Depth of Attock
No. No. Added By Analysis (hr) (mils)
289 59 Control 500 8
290 59 NiF, 0.05 0.03 500 10
291 59 NiF, 0.2 0.05 500 11
442 155 Control 500 10
418 155 NiF2 0.5 0.18 500 17
419 155 NiF, 0.5 0.18 500 17
466 173 Control 500 8
448 173 Control 1000 10
454 173 NiF, 0.4 0.11 500 15
455 173 NiF, 0.4 0.13 1835 16
452 173 CrF4 0.3 0.14 500 9
453 173 CrF, 0.3 0.14 2000 22
545 198 Control 500 11
512 198 CrF, 0.5 0.26 500 6
513 198 CrF, 0.5 0.27 1710 17
296 63 Control 500 8
294 63 Graphite rod 500 13
674 246-4 Graphite tube 1000 23
675 246-4 Graphite tube 1000 28
312 73 Cr powder 1.0 0.20 500 7
323 92 Cr floke 0.5 0.21 500 4
324 92 Control 500 15
335 92 Inconel turmings 500 11

 

After 500 hr, batches to which CrF, had been
added developed the same depth of attack found
in the control loops. It is possible that the
presence of CrF, aond the accompanying free
fluorine atom caonceled each other. A definite
increase in amount and depth of attack was
noted for these loops at 2000 hr; the rate during
the period from 500 to 2000 hr was 0.9 mil per
100 hr,

In both cases the addition of chromium fluoride
resulted in an increase in the rate of mass
transfer, This appears to conflict with the
statement above (see '‘Operating Time'') that the

rate of mass transfer seems to be independent of
batch purity; however, the additions made here
are much larger than would be found in normal
batch variations.

In some of the early loops, chromium metal
powder and Inconel turnings were added to the
fluorides in the charge pot. The chromium did
result in a small decrease in attack, but metallic
layers were found on the cold-leg surface. A
small improvement appears to have been obtained
from the Inconel turnings, but the results are not
definite, From the time data obtained with the

23
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T4

   
       

 

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UNCLASSIFIED
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Fig. 12. Effect of Additives on Mass Transfer Reaction in 2000 hr Operation. () CrFT‘ (b) CrFa; (c) Nin; (d) standard loop. 100X.
Reduced 17.5%.
loops, it was shown that about 200 hr was re-
quired for the impurity and saturation reactions
to be completed, so it appears that fairly long
treatment times would be necessary for equi-
librium. For these batches 24 hr was the longest
treatment time used.

A group of loops was operated with graphite
present in the hot legs, The purpose of these
loops was to determine whether graphite could be
used in an Inconel-fused-salt system. With
loop 294 a h-in. graphite rod was suspended
in the center of the upper portion of the hot leg,
while with loops 674 and 675 tubular inserts with
internal diameters the same as the diameter of
the Inconel pipe were used, The tubular inserts
were about 6 in. long ond were located just below
the hottest portion of the loop. With each of
these loops a definite increase in the depth of
attack in the Inconel was noted. A deposit was
present on the surface of the graphite and pene-
trated it for a considerable depth, The photo-
micrograph in Fig. 13 shows the surface of the
insert from loop 294 and shows the deposit
extending for a depth of 17 mils, On the surface
the deposit is in the form of a thin film of metal
surrounding nonmetallic particles,

 

Another material that was added to the fluorides
was zirconium hydride, It is now known that the
function of this material was to convert a portion
of the UF, to UF,; therefore this material will be
discussed in the section on the reduced uranium
fluoride mixtures (“'Special Fuel Mixtures'’).

Ratio of Surface Area to Loop Volume., — One
difficulty in comparing results from various
corrosion tests is the wide variation between the
tests in the surface area to volume ratios. In
isothermal static tests it is easy to define and
figure such ratios; however, with dynamic sys-
tems it is difficult to determine what the signifi-
cant parameters are, let alone measure them.
The significant surface area is the one where
the rate-controlling step takes place and could be
the cold-leg surface as well as the hot-leg
surface,

The thermal loop is not an ideal rig in which to
study variations in these ratios, since they
cannot be varied independently. Any variation in
the ratio of surfoce aorea to volume also results
in variations in rate of heating and in temperature
distribution, which in tum affect the wall temper-
atures and velocities,

 

 

JUNCLASSIFIED |
¥ 79681
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Fig. 13. Metallic Deposit on Surface of Graphite Rod Suspended in Hot Leg

of Loop 294. 100X.

26
For this discussion the ratio of the heated
surface to the total loop volume was used, Since
each loop was made from a single size of pipe
and all loops are the same length, such ratios are
dependent only upon the pipe diameter. The
ratios would vary in value depending upon what
is considered as the critical surface, but such
variations would cancel out in comparing various
loops with the same length of critical surfaces
ond would depend only upon differences in the
diameters.

A series of otherwise identical loops were
constructed from tubing ond pipe of various
diameters, The data from these loops are tabu-
lated in Table 11, After 500 hr the loops with the
low ratios of surface area to volume show the
most attack. This is as would be expected, since
the chromium required by the batch was obtained

 

 

from a smaller surface., With the longer-time
loops, no definite conclusion may be drawn, as
shown by the data summarized in Table 12,
After 1000 hr, less attack is found in the loops
with the low ratios than in those with high ratios;
while this is still true after 2000 hr, the differ-
ences are smaller rather than having continued to
increase, |f the time from 500 to 2000 hr is con-
sidered, the differences in rate are within the
vsual spread; for the loops from ]é-in. tubing the
rate is about the same as those obtained from the
time curves in Fig. 6.

Oxide Removal Procedures. — While much of
the available commercial tubing contains internal
surface scale and dirt, the pipe used in this study
was supposed to have been annealed and pickled.
In most cases this specification was met, but
some loops were found in which heavy surface

Table 11. Effect of Ratio of Heated Surfoce Area to Total Loop Volume on Depth of Attack

 

Ratio of Heated Surface

 

 

Loop EE Batch Pipe Size* e late Vb Operating Time Attack

No. No. (in.2/in.3) (hr) Intensity Depth (mils)
349 19 lin. T 1.7 500 Moderate 9
359 119 Y%in. T 3.9 500 Heavy 4
352 19 Y in. P 2.3 500 Moderate 5%
362 19 %oin T 3.9 500 Heavy 5
549 232-8 Yin, T 3.9 2000 Heavy 147
550 232-8 % in. P 2.6 500 Moderate 9
551 232-8 % in. T 3.9 1000 Heavy 14
565 232-8 iy T 1.7 1000 Heavy 7
566 232-8 Vs T 1.7 2000 Heavy 12
567 232-8 % in. P 2.6 1000 Light 9
568 232-8 % in. P 2.6 2000 Moderate 12
569 232.8 % in. P 2.3 2000 Heavy 1Y,
570 232-8 % in. P 2.6 500 Moderate 9
160 217-5 Yin. T 3.9 3000 Moderate 18
529 217-5 Yin T 3.9 1000 Heavy 12

 

*P = pipe (IPS); T = tubing (outside diameter).

27
Table 12. Summary of Corrosion Results from Loops with Various Ratios

of Heoted Surface Area to Loop Volume

 

Attack Rate (mils per 100 hr of operation) for

 

 

 

i i geya) BE Baiseh Surface to Volume Ratio (in.2/in.’) of
(hr) Ne.
3.9 2.6 2.3 1.7
0-500 119 0.9 1.1 1.8
0-500 232-8 1.9 1.8
500-1000 232-8 1 0
10002000 232-8 0.05 0,3 0.5
500-2000 232-8 0.3 0.2
10003000 217-5 0.3
layers were present on the walls before operation. opparent but they are not systematic. |f aony con-

Since these loops had been previously degreased
with a solvent degreaser, all loose dirt and grease
had been washed out. The layer was identified
as being primarily chromium oxide. Several
procedures were tried in an effort to remove these
oxide |oyers before the loops were filled and
operated,

The loops designated “'First Ser.es’’ in Table 13
were operated in connection with the original
cleaning or oxide removal study made shortly
after this work had started, As shown by the
data, no large differences in depth of attack
were found with any of the methods studied.
Although none of these loops showed a reduction
in attack from the usual control loops, a cleaning
step was still thought to be desirable to elimi-
nate erratic results caused by the occasional
heavily contaminated loops. The surest and
simplest method of oxide removal appeared to be
the prior circulation for short times of another
batch of fluorides. This method was then adopted
as standard procedure,

When the gradual increase and poor reproduci-
bility in the depths of attack were noted, cleaning
variables were one of the suspected causes and
were checked again. The results obtained in this
second study are reported as ‘‘Second Series’ in
Table 13. Since cleoning variables represent
attack due primarily to the reduction of impuri-
ties, they should show up more in short-time
loops; therefore the loops in this series were
operated for only 250 hr. These loops again show
no apparent variation in depth of attack with the

various oxide removal steps. Some variations are

28

clusion were to be drawn, it would have to be
that the lowest attack was in the loops that
received only a careful degrease and visual
check with no oxide removal operation.

Heating Methods. — Both to reduce the number
of variables and for ease in operation, it was
desirable to use only a single heating method.
From previous work it was known that clamshell
resistance-wound heaters and saturable-core
reactors for control were a satisfactory combi-
nation. Consequently, these heaters were adopted
for this study, but, since it was shown previously
that the attack was heavier and deeper under the
heaters, a study of the effect of heat flux on
corrosion was desirable. In addition, some
corrosion theories were proposed in which the
heating method was a variable. To answer these
questions it was necessary to operate some loops
with a different method of heating,

A method of heating by means of the electrical
resistance of the pipe wall was used during the
cleaning operation and was then modified for
use during operation. To concentrate the heating
in the hot leg, so that the heating area and dis-
tribution would remain constant, one electric
terminal was connected to the middle of the hot
leg and the other one connected in parallel to
both ends of this leg. Because of the short path,
the resistance was low and very high currents
were required to give the required power. The
data from the loops heated in this manner are
tabulated in Taoble 14, and a typical hot-leg
section is shown in Fig. 14. The depths of
attack obtained with this second heating method
 

 

 

 

Table 13. Variations in Depth of Attack with Oxide Removal Procedures
Loo EE Batch Attack
H: o Oxide Removal Method ey TR
First Series
237 R-56 Hot sodium 7
234 R-43% None Light 5
236 R-44¢ Dry hydrogen Muderate' 10
231 R-514 Hot NaK Moderate 9
244 144 None Moderate 9
293° 63 None Haivy 7
295 63 Molten fluorides Heavy 6
296 63 None Heavy 8
353 119 None Heavy 3
352 N9 Molten flucrides Moderate 5%
Second Series®
722 249-1 HNO, + HF Heavy 8
737 249-4 HNO, + HF Heavy 8
723 249-1 Dry hydrogen Heavy 9
724 249-1 Dry hydrogen Heavy 8
725 249-1 Molten fluorides Heavy 8
726 249-1 Melten flucrides Heavy 8
727 249-1 None Heavy 7
728 249-1 Neone Heavy 7
732 514-1 Machined surface Heavy 7
733 514-1 Machined surface Heavy 4
739 514-1 HNO, + HF Heavy 9
740 514-1 HNOB +HF Heavy 6
742 514-1 Dry hydrogen Moderate 8
743 514-1 Molten fluorides Moderate 6
744 514-1 Molten fluorides Heavy 6
745 514-1 None Heavy 6
746 514-1 None Moderate 7
“Fluoride 27.

bOx'rdizad hot leg.
“Operated for 250

hr.

29
 

 

 

Table 14. Depth of Attack in Loops Heated by Electrical Resistance of Hot-Leg Wall

Loo EE Batch Time Attack

NO-P No. (hr) S o Intensity Depth (mils)
685 246-1 500 Clamshell Heavy 14

684 246-1 500 Clamshell Heavy 11

686 246-1 500 Clamshell Heavy 9

618 246-1 500 Resistance Heavy 10

619 246-1 1000 Resistance Heavy 13

703 246-1 1000 Resistance Heawvy 15

736 249-1 1500 Resistance Heavy 18

 

 
  
  

 

 

 

 

 

 

 

 

 

 

 

 

 

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Fig. 14. Appearonce of Attack in Loop Heated by Electrical Resistance of
Wall. 250X.

are within the spread of depths from the control

loops.

Some effort was spent by the Experimental
in developing a gas-fired

Engineering Section

and the maximum fluoride temperature occur in
the same vicinity and in which no hot spots are
present,  While direct comparisons cannot be
made at this time, no larger differences in attack

This

furnace capable of heating the loops.
method would be especially advontageous for
higher temperature loops, since at 1500°F the
heating capacity of the electrical equipment is
being approached. As yet, a gas furnace has not
been built in which the maximum wall temperature

30

have been found with the preliminary gas-heated
loops.

While it has been shown that deeper attack may
be found under a heater, the attack does not
depend upon the method of heating. The differ-
ences are caused by the higher temperatures,
either in the wall or, more likely, in the fluid
boundary layer adjacent to the walls,

When a reactor system is compared with a
thermal loop, two differences in heating are
found. In the reactor the heat is generated within
the liquid, and so the wall is at a slightly lower
temperature than the liquid. In addition, tempera-
ture cycling may be present and may be quite
ropid. As yet, no method has been devised by
which a loop may be internally heated, so that
the first difference will have to be checked in
the radiation tests. A series of tests were set
up to determine whether the second difference —
temperature cycling — increased the rate of mass
transfer,

The temperature cycle used for these loops was
periodic and, to avoid excessive wall temper-
atures, of fairly long duration. Figure 15 shows
the hot-leg temperature pattern of a typical loop.
The corrosion data for this series are tabulated
in Table 15, Within the accuracy of the data
this slow cycling did not affect the depth of
corrosion. |t appears that the cycling averages
the attack over the entire temperature range rather
than effectively increasing the differences, and
it has already been shown that the depth of
attack is not affected greatly by changes in hot-
leg temperatures, It is possible that a more
rapid cycling could increase the attack by
effectively increasing the temperature drop, but
this cannot be confirmed in the present equipment.

Loop Size ond Shape. — As discussed in the
companion report,' the size ond shape of the

 

loops were arrived at primarily from physical
considerations such as sizes of availoble heaters
and ease of operation rather than from flow and
considerations. As

corrosion the corrosion

UNCLASSIFIED
ORNL-LR-DWG 48768

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TIME (hr}

 

Fig. 15.
from Thermal-Cycled Loop.

Hot- ond Cold-Leg Temperoture Patterns

Table 15, Effect of Temperature Cycling on Hot<Leg Attock

 

Hot- Leg Tampuru ture

Cald-Lag Temperature

 

 

 

 

Loop EE Batch Time (°F) (°F) Attack

o v il Maximum  Minimum Moximom | Minigwm | nesasity.  Depthi{mils)
564 232-8 500 1600 1400 1385 1200 Heavy 11

570 232-8 500 1500 1500 Mis devats 9

576 228-12 500 1600 1400 1360 1200 Haavy 9

577 228-12 500 1500 1500 Heavy 9

752 514-1 430 1600 1400 1395 1200 Moderate 10

538 248-4 788 1600 1400 1350 1180 Heavy 8

692 246-1 1500 1400 1400 1360 1125 Heawy 14

779 513-4 2000 1600 1400 Heavy 18

 

31
studies progressed, some effort was made to
improve the design of the loops.

A major simplification in fabrication was the
elimination of the expansion pot, which required
several different sizes of material aond con-
siderable welding. @ This pot was originally
provided on the loops to take care of expansion
of the liquid and to allow more leeway in filling.
With experience in loop handling, it was found
possible to fill to between rather narrow limits;
consequently, the pots were replaced by a straight
length of pipe of the same size as the loop, which
was welded into the top of the loop. The gas
and vacuum connections as well os the fill line
and spark-plug probe were connected into the
side of this pipe by meons of Swagelok fittings
as shown in Fig, 1.

In maoking some calculations on loop design,
J. F. Bailey showed that a major portion of the
flow resistance was in the sharp bends where the
legs joined. The loop was therefore redesigned
with smooth curves at both these positions. The
riser leg and trap were saddle-welded on the high
ond low points of the loop. In making this change,
it was necessary to relocate the thermocouples;
so it was not definitely established whether an
increase in velocity was obtained, No measur-
able change in depth of attack was found in the
control loops of this design compared with those
with the sharp joints.

When work was started with liquid metals it
was noted that the deposits of metallic crystals
found in loops with the trap in line with the cold
leg were larger than those in loops with a smooth
bend and the trap offset several inches, In both
cases the temperature pattern in the trap would
be the same, but it is speculated that with the
sharp bend the crystals could settle out, while
with an offset trap they were easily carried
across the small opening. This does not neces-
sarily mean that more crystals were formed, only
that they were concentrated and easier to find,
Since it was desirable to use a standard loop for
all work, the in-line trap and sharp bend for the
lower joint were made standard. This change
caused an increase in temperature drop of about
10°F, showing that some decrease in velocity
had taken place. The configuration of the stand-
ard loop after incorporating these changes, shown
in Fig. 1, is satisfactory for normal loop materials
and coolants but is objectionable for special
materials in that it requires considerable amounts

32

 

of materials both for the loops and for filling
them. Such materials as niobium and molybdenum
not only are expensive in the required quantities
but are hard to obtain in the required lengths.

A series of loops were fabricated with various
ratios of height to width. Unfortunately, as with
so many other variables in the loops, any variation
in size offects other variables, such as tempera-
ture drop, so that only the over-all effect can be
observed, With the change in temperature drop,
it is likely that a change in wall temperature was
also occurring. The corrosion data for this
series of loops are tabulated in Table 16 and
indicate that it is possible to reduce the loop
size by one-half with only a small reduction in
depth of attack.

Chaonge in Analysis of Inner Pipe Wall. - As
shown by all the photomicrogrophs in this report,
visually the attack found after circulating zir-
conium fluoride—base fluorides appears to be the
same as that reported for alkali-metal-base
mixtures,!  Thin layers were removed from the
inner surfaces of both the hot and cold legs to
determine whether the changes were also similar
chemically, Obtaining thin, uniform layers with
enough material for analyses is a difficult pro-
cedure, After operation at 1500°F the pipes are
no longer perfectly round, and in addition, lengths
of at least 6 in, are required to get enough sample
with 5-mil layers being removed; both these
facts make it difficult for the machinist to main-
tain the required depth during the drilling. For
this reason the depths reported in the work must
be considered as being only relative.

The analyses of the various layers removed
from the hot legs of three 500-hr loops are given
in Table 17. The samples were cut 0s concentric
layers, and the deepest cut in each sample is
listed in the '‘depth’’ column. The tabulated
analyses give the average concentration between
the depth listed for the line in which it is tabu-
lated and the depth listed for the line directly
above. These analyses show that the mechanism
has not changed and that chromium is still the
major constituent removed from the wall by the
attack, The chromium is removed from a depth
greater than that of the microscopic voids, con-
firming the obvious assumption that a certain
contentration of vacancies is necessary for the
appearance of voids, As shown by Fig. 16, the
change of chromium concentration with depth
is a straight line on a logarithmic plot. Any
Table 16. Variation in Depth of Attack with Variations of Loop Height and Width

 

 

 

 

Leoi  EE Bawch  Tima Loop Size (in.) Temperature Drop Attack

No. No. (hr) Vertical Horizontal (°F) Intensity Depth (mils)
496 198 500 15 15 Moderate 7
545* 198 500 26 15 Heavy 1
502 188-7 1000 15 15 Heavy 7
535 188-1 1080 26 15 Moderate 10
597 248-4 1000 15 15 208 Heavy 10
598 248-4 1000 15 8 172 Heavy 1
599 248-4 1000 15 26 195 Heavy 15
600 248-4 1000 26 8 220 Heavy 16Y,
601 248-4 1000 8 8 143 Heavy 10
602 248-4 1000 15 15 195 Heavy 13
603 248-4 1000 15 8 188 Heavy 14
604 248-4 1000 15 26 205 Heavy 12
605 248-4 1000 26 8 218 Heavy 16
606 248-4 1000 8 8 145 Heavy 10
614* 248-4 1000 26 15 245 Heavy 15

 

*Control loop (standord size).

UNCLASSIFIED
ORNL-LR-DWG 48769

PER CENT CHROMIUM

 

l 2 5 10 20
DEPTH FROM SURFACE (mils)

Fig. 16.
Depth Below Surface in Hot Legs of Thermal Convec-

Chonge in Chromium Concentration with

tion Loops.

change in iron or nickel is difficult to determine,
and if it occurs must be small. With two of the
loops the Fe/Ni ratio decreased slightly, while
with the third no change was noted.

With these same samples, an attempt was made
to determine whether any components of the
fluoride mixture were present below the surface,
although satisfactory analyses were not possible
because sufficient sample did not remain after

Sodium,
zirconium, uranium, and fluoride ions were re-
ported as being present in all samples and in
aomounts decreasing with depth; however, the
ratio of their concentrations varied from sample
to sample and in all cases was different from that
in the original mixture. In no case were the
amounts reported sufficient to account for the
volume of holes seen under the microscope.
This work does not prove that the fluoride mixture
is not present below the surface; however, it
indicates that fluorides are not present in
sufficient quantities to have filled all the voids,

Analyses from similor chemical samples re-
moved from loop 765 after it had circulated for
2000 hr are shown in Table 18, These values do
not show any significant decrease compared with
those obtained from the 500-hr loops. This would
indicate that chromium is replaced in the surface
layers by diffusion as fast as it is removed by
mass transfer.

Additional evidence that the chromium concen-
tration of the surfoce does not continue to de-
crease with operating time is shown in Fig. 17,

the above onalyses had been made.

33
Table 17. Change in Wall Composition with Depth from Surface in Thermal Loop Hot Legs

 

Constituents of Pipe

 

Constituents of Fluoride Mixture

 

 

Maximum Depth of Cut (%) Fe/Ni (%)
{mils}
Fe Ni Cr Na Zr U F

Loop 273, Section 1

2 7.31 76.8 8.9 0.095 <0.6 1.2 1.99

6 7.65 79.3 10.7 0.097 <0.6 1.2 0.69

10 Tudl 78.3 1.9 0.094 <0.6 1.2 0.56

15 7.00 77.9 13.6 0.090 <0.6 0.3 0.21

20 7.25 77.0 14.6 0.094 <0.6 0.3 0.17

Base metal 6.93 76.8 15.4 0.090 <0.6 <0.15 0.02
Loop 280, Section 2

3 8.10 Lo 10.7 0.108 0.14 1.3 0.145 0.67

6 8.08 75.1 12.0 0.108 0.65 1.3 0.130 0.37

10 7.58 74.9 12.9 0.101 0.11 1.0 0.123 0.32

15 7.32 73.4 15.1 0.100 0.13 0.3 0.012 0.06

20 7.06 72.9 15.5 0.097 0.14 <0.15 0.004 0,03

25 7.26 72.5 155 0.100 0.10 <0.15 0.002 0.00

Base metal 7.30 73.8 15.5 0.099 0.09 <0.15 0.002 0.02
Loop 287, Section 2

i 7.89 78.3 10.2 0.101 0.36 13 0.130 0.64

6 7.50 74.8 13.3 0.100 0.16 1.3 0.070 0.26

11 7.28 74.1 15.0 0.098 0.08 0.15 0.014 0.04

16 7.33 74.4 15.6 0.099 0.07 <0.15 0.003 0.05

 

Table 18. Change in Chromium Concentration
in Loop 765 with Depth Below Surface in 2000 hr

 

Depth Below Surface

Chromium Concentration

 

{in.) (%)
0.003
0.006 11.6
0.009 1.7
0.013 12,3
0.018 5.1
0.023 14.2
0.028 14.1
Outside 14.0

 

When chromium is removed from Inconel, a mag-
netic alloy results when the chromium concen-
tration is reduced below a value which lies be-
tween 8 and 10% chromium.® The magnetic areas
in the samples shown in Fig. 17 are outlined by
the use of an electromagnet and a fine colloidal

34

suspension of iron on the sample, The thickness
of the magnetic layer gradually increases for the
first 100 hr, then after about 350 hr, during which
time it remains about the same, it starts to
decrease, After 500 hr, only scattered spots are
present and they gradually disappear with longer
times until at 3000 and 5000 hr no traces of
magnetism were found, These tests show that the
minimum chromium concentration in the wall
occurred between 100 and 350 hr, which is the
same time at which the break occurs in the time
curve, and that with the longer times the chromium
diffused to the surface faster than it was removed.

Chemical analyses were also made of layers
from the cold legs of several loops, but no varia-
tions large enough to show with these techniques
were found. |f chromium is depositing on the
surface, it is either present as an extremely
thin layer or is diffusing inward rapidly.

 

4Merai’s Handbook, 1948 ed., p 1046, American
Society for Metals, Cleveland.
   

UMCLASSIFIED
T-9198

"k b E

T
1

 

EEEERERESE

(&)

(a)

UNCLASSIFIED
T-9202

EEE |

T
INCHES
1

 

FEREERED

(¢) . ) (d)

o}
-

 

 

l

UNCLASSIFIED

  

% i - ‘;'.‘ g -
£ y T . o L m "
g .. .' " -
- W
' ¥
. : & ‘ L ‘
ol 0 :
P 0™ e
= .
}' N e Q. @
- g .I =
® & . ‘ - Nie ‘g
o) o . -~ .. ¢

Fig. 17. Change in Chromium Content with Time as Shown by Magnetic Etching Technique.

100 hr; (¢) 250 hr; (d) 500 hr; (e) 5000 hr. 250X. Reduced 29.5%.

 

 

quf 3|

T
IN
1

EEEERES

 

 

a
-

SIS
Loia ]
UMCLASSIFIED ™
T.9206
1901 |
DO% |

 

 

(a) 50 hr; (&)

35
Effect of Uranium Concentration. — The uranium
concentrations used in this study were based
upon nuclear requirements and were periodically
revised as reactor design was developed. |t
became obvious that to study corrosion funda-
mentals it would be necessary to adopt one
composition as the standard and check the others
only sufficiently to determine that the data could
be extrapolated, Fluoride mixture 30 was the one
under consideration at the fime and so was
adopted as the standard. A mixture close to
No. 44 was used for the Aircraft Reactor Experi-
ment and received some study. During the pre-
liminary testing of reactor systems, it will be
necessary to operate for considerable periods of
times with mixtures that do not contain uranium,
and during the enriching operation mixtures
having a range of uranium concentrations will be
used.,

To provide data on the effect of uranium concen-
tration upon depth of attack several series of

loops were operated. Since in the fluoride pro-
duction a considerable portion of the impurities
are added with the uranium, it was difficult to
make comparable batches which differ only in
amounts of uranium. From data presented above
(*‘Additives’’), it appears that slight variations
in impurity are not as critical as was feared and
that comparisons of these data should be
permissible,

Data from loops operated for both 500 and
2000 hr with batches containing various uranium
concentrations are presented in Table 19. The
changes in depth of attack after 500 hr with
variations in uranium concentration are plotted in
Fig. 18. These data indicate that an increase in
depth of attack was found with increasing uranium
content, This increase was at a rate of about
0.5 mil for each per cent of uranium added. A
more rapid increase is possible with low addi-
tions, but these batches were of a higher purity,
and so the results need confirming. On the same

Table 19. Loops with Various Uranium Concentrations

 

 

 

Operating Attack Uranium (%) Gl Bosrn
Loop EE Batch B AT ranium romium (ppm
No. No. Vi Intensity Da-pth Added Before After Before After Sstman
(hr) (mils)
474 191 500 5 0 65 Layer
475 192 500 5 0.5 50 250 Layer
476 193 500 b 1 65 275
477 194 500 7 2 130 465
479 196 500 12 10 100 1225
481 191 2000 14 0 70 70
482 192 2000 7 0.5 35 230 Layer
483 193 2000 7 1 65 275 Layer
484 194 1805 12 2 70 550 Crystals
486 1926 1800 12 10 a5 1075
356 124 500 M 7% 4 100 200  Possible layer
357 123 500 H 9 10 20 1500
358 121 500 H 13 15 70 2000
354 129 500 H 3 0.5 20 250 Thin deposit
355 126 500 H 3 | 5. 325 400 Thin deposit
574 262-R 500 H 10 46 46,3 45,3-51,0 55 225
572 500 H 15 46 45.7 43.6-49.9 50 65-330

 

36
 

 

UMCLASSIFIED
DRNL-LR-DWG 487 7D

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

16 . T
O AVERAGE OF CONTROL BATCHES
& U ADDITIONS Lo
= —N0.3Q — =" NO,44
E /_....--CH""
* 8 ot — L
= NO.31 $—— = |
L e
0 Q a | . |
4 - o —
s im | I
0 ' |
o 2 4 & B8 o 12 14 16
PER CENT URANIUM
Fig. 18. Chonge in Depth of Attack with Uranium

Concentration.

figure the average depths obtained with the
control batches of fluorides 30, 31, aond 44 are
shown. For these averages a straight line was
found with o rate only slightly lower than the
other one. Considerable variation was found in
the depths of attack after 2000 hr, and no definite
conclusions may be drawn. Additional loops will
be necessary to show the effect of uranium concen-
tration on depth of attack resulting strictly from
mass transfer,

Two loops were operated with batches con-
taining high uranium concentrations, and the data
from these loops, also tabulated in Table 19, do
not lie on the curves shown in Fig. 18, In both
cases, the depth of attack was less than would
be predicted. Another difference in these two
loops was that the chromium concentration did
not build up to the usual levels, A comparison
with the data presented below (“‘Special Fuel
Mixtures'') points out the possibility thot o
portion of the high UF, concentration has dis-
proportionated into UF,. The UF; would lower
the equilibrium chromium concentration ond
reduce the depth of attack.

Barren Fluoride Mixtures

During the cleaning and preliminary testing
stages of reactor operation, the material circu-
lated is similar to that used during operation,
except for the absence of uranium. The operating
time during such operations can be a considerable
fraction of the total operating time, and therefore
corrosion during this period must also be studied.

An examination of the data tabulated in Table 20
reveals that the corrosion mechanism is not the
same when the uranium is present as when it is

not. The average depth of attack in the standard
loops after 500 hr was 5.7 mils or about two-
thirds of the depth in similar loops containing
UF, mixtures. The appearance of the attack, as
shown in Fig, 19, is similar to that previously
discussed. The iron and nickel in the fluorides
after operation decreased in the usual manner,
but the chromium concentration is lower,

An increase in depth of attack was found with
increases in operating time., While the values
vary with the different fluoride batches, an
average rate of increase, as shown by Fig. 20,
would be about 2 mils per 1000 hr, or about half
that with the UF, addition. The voids also grow
in size with increasing operating times, as shown
in Fig. 19. The increase in depth of attack or
mass transfer takes place in this mixture with
very low chromium concentrations, In the cold
legs of both the 2000- and 3000-hr loops, operated
with a portion of the high-purity materials used in
the ARE, dendritic crystals of chromium metal
were found.

Another difference with the barren mixtures
was revealed by loops 337 and 338, Both these
loops circulated for 500 hr, but loop 337 operated
with two batches of high-purity ARE fluorides,
and the first batch drained from 337 was then
circulated for on additional 500 hr in 338, The
depth of attack with both these loops was about
the same as in the control loops, rather than
being double and half, respectively. Hot-leg
sections from these loops are shown in Fig. 21.
These data indicate that this attack is coused
primarily by mass transfer, with impurities and
saturation playing only a minor part.

The actual mass transfer mechanism with the
barren solutions is not known, since insufficient
work has been done on concentrations and
equilibrium constants in systems which do not
contain wuranium.  The most likely reactions
would result in a change in solubility with temper-
ature, for example, 3CrF, == 2CrF, + Cr or
possibly 2Na + CrF, ==Cr + 2NoF. Reactions
with zirconium are also a possibility, but no
evidence for them has been found.

Screening Tests of Possible Container Materials

The screening tests' on which the original
material selection wos made were carried out
with alkali-metal-base mixtures. In that study all
stainless steel and ferritic-base alloy loops

37
g€

Table 20. Corrosion Data from Inconel Loops with Barren Fluorides

 

Analysis of Fluoride Mixture (ppm)

 

 

 

L;:.F EENifm:h Dpera;::}g Time quirll['h:i'l:s.;nfluck Nickel o mium — = P
Before After Before After Before After

276 44 500 8 1300 <20 25 2000 520 130
278 1000 5 130 <20 30 650 850 75
277 49 500 4 75 <20 180 350 800 30
336  ARE-2 500 6 <20 s 110 - 50
341  ARE-2 500 sk <20 <20 310 65
342 ARE-2 500 6 <20 200 35 No cleaning
348 ARE-2 500 5 <20 130 80
346  ARE-2 2000 9 <20 300 110 Cr in trop
347  ARE-2 3000 1 <20 110 65 Cr in trap
400 134 50 3 <10 . 135 200 365 100
410 134 100 4 <10 <20 135 240 365 70 2.5 Loyer
411 134 250 4% <10 <10 135 200 365 90 2.4 Layer
399 134 1000 10 <10 <10 320 365 80
516  210-5 500 6 55 20 80 100 115 120 3.0 Crystals
517 210-5 822 5% 70 60* 75 160 200 60 3.5
518 210-5 3000 1 70 30 60 110 100 110 3.5 Deposit
519 210-5 2000 12} 70 100* 15 125 135 75 3.0 Depasit
337 2 botches 575 8 1-25+% 1-250 1-150

ARE-2 2-20* 2-200 2-50
338 Batch from 500 6 25 20 250 470 150 50

337
Av for 500-hr loops 57

 

*Individual results varied.

 
6€

      

UNMCLASSIFIED UNCLASSIFIED
T-4134 i T-10373 ool
002 002
s

.‘ 003 .003

3 ¥ S - ’ . - s 0 o
o bl w
% - 3 * o . : &R

_ z o z

W
5
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2
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A
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r
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y
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.007 | .
Q ¢ 9
3 ] g ; R T T~ - 2o
\ ; 2 : @ r -4 -
i gL S 1t laml e 009
- - L¥ . s
4 _*\ & v . . /° Lew - 010
%, : ’ p TSR “ o1 | ’, 3 ou |
i ' - / ¢ Loz @
2 2 ] ¥ g Loz 012
; ¥ °_,— & . ¢ 4 P ¢ 212
- 5 . . -~ . OIS | . \ 013 |
(@) . St as| (5) Lot |
UNCI:I_ZSSiFIED UNCLASSIFIED
558 y T-5187 901
L.002 | | .002 |
. 003
® y ] .003
b ¢ . e w o
.-- ' . % - .‘ —5— - L —
e . _il= z
@ Fo a - - T -
== 4 008 008
a fi s
¥ l.‘-'- - 00T | 007
, g * 008 >
- By i ‘ .‘ e L ma
. e ; s E . ‘ L.00%)
. 9 - ;
210 . & Q9 ‘ " 010
., -
-
P |ane | \ 0l
o : . e NG
. O8] J \\\' s =8 |ois
- " 4 013 ' . P 3

 

 

 

 

il

2
-
Q
e
!
<

(¢)

Fig. 19. Growth of Voids Formed by Barren Fluorides with Increasing Time of Operation. (a) 500 hr; (b) 1000 hr; (¢) 2000 hr; (&) 3000 hr. 250X.
Reduced 17%.
plugged in relatively short times, Nickel loops
operated for 500 hr, but metallic crystals were
found, and Hastelloy B loops failed by cata-
strophic oxidation. Only the high-nickel alloys
similar to Inconel looked promising. Since the
properties of the zirconium fluoride—base mixtures
are considerably different from those of the
alkali-metal-base mixtures, many of these tests
were repeated with the zirconium mixtures. In

UNCLASSIFIED
DRNL-LR-OWG 48771

 

 

 

 

 

|4 — ' — —I—
2 + 1 - | J
| v
¢ ‘T_ /
3
“ ! /
o E - —t 4 S e - 4
: |
g:} ; A-—A LAB BATCH
E o6 4 L | ¥ AREBATCH —|
o ! » X © EE 134
I | i « EE 210-5
e
q -0 2 t
o
al 1= __:, 4 b
| |
| |
Q | - L i el —r L
o 500 1000 1500 2000 2500 3000
OPERATING TIME (hr)
Fig. 20. Increase in Depth of Attack with Operating

Time for Barren Flueride Mixtures.

  

 

(@)

Fig. 21.

UNCLASSIFIED
T-4045

 

  

l“"r .
.

general, these tests are much more favorable than
the previous ones were,

Stainless Steels. — All stainless steel loops
operated satisfactorily for 500 hr when zirconium
fluoride—base mixtures were circulated in them,
as shown by the data tabulated in Table 21. The
hot-leg attack in type 316 stainless steel loops
was similar in appearance (Fig. 22) and to about
the same depth as was found in Inconel loops.
As would be expected from solubility studies,
the iron and nickel concentrations of the fluorides
remained low and the chromium increased during
operation. The disadvantage of this alloy is the
metallic deposits that were always found in the
cold legs (Fig. 22). Attempts to identify the
deposit by spectrographic and diffraction studies
were not successful, but indications are that iron
and chromium are both present,

When the loops were fabricated from a 400
series stainless steel, the nature of the attack
changed. The hot-leg surfaces were rough, with
entire crystals being removed, and no subsurface
voids were present, Large deposits of dendritic
metallic crystals were found in the cold legs.
Typical cold-leg sections with types 410 and
446 stainless steel are shown in Fig. 23. The
cold-leg deposit in a type 410 loop analyzed
53.4% Fe-4,3% Cr=1% Ni-12,9% Zr., In a type
446 loop the crystals were 35.7% Fe-7.6% Cr-1%
Ni-13.8% Zr. The zirconium is not part of the
crystals but is from unseparated fluorides. These
data indicate that both iron and chromium can be

UNCLASSIFIED
T-4130

VL . 4 -) ppR- e |oos)
\ v 1
. > B L.008
% | ".

 

 

 

Comparison of Attack in Loops Operated with Two Barren Batches and with a Precirculated Batch.

(a) Loop 337, operated with two batches; (b) loop 338, operated with a precirculated batch. 250X. Reduced 29%.

40
Ly

Tflblfl 21.

Data from Stainless Steel Loops

 

Analysis of Fluoride Mixture

 

 

 

 

Loop Alloy Batch Time Metallographic Examination Nickel Chromium Uranium
No.  Type  No. (hr) Hot Leg Cold Leg (ppm) (ppm) (ppm)
Before  After Before After Before After Before  After
128 316 R-50% 500 8 mils, intergranular  Crystals 320 <20 <20 1600° 665 110 7.50 9.3
133°€ 316 500 11 mils Deposit 200 20 35 1300 2100 150 8.80 9.0
134 316 52 500 7 mils, heavy; rough Deposit 30 <20 110 Bflflb 260 60 8.81 9.1
surface

403 430 1199 500  Even Deposit <20 <20 75 825 230 b 8.99 8.8
138 410 141 500 Very rough Layer and crystals® <20 <10 95 800 185 120 9.14 8.8
137 446 141 500 10 mils, voids; rough Layer and crystuls'f <20 <20 90 800 155 80 8.53 8.8
142 430 158 500 7 mils, light® Some deposit and <20 <10 30 SGUb 55 b 8.80 9.0

crystals

 

“Fluoride 27.

Individual results varied.
“Cleaned only by degreasing.
d3.0 ppm HF.

®Crystals 53.4% Fe, 12.9% Zr, 4.3% Cr.
ICrystals 35.7% Fe, 13.8% Zr, 7.6% Cr.
&L arge grains and no carbides in hot leg.

 

 
42

   

L v
8

-
- A l; - ‘\ | T
- - * - ¥ . x,
= .
. '\::—.’"" i » - » " o ;
k NN o P - 1
- - - & - - L
/" " .. \\- " p -
o %
: b .- 2o N\ .
J - b % i E
+ ¥ S ¥ s
¢ \ . - - e -
- -
1)4 - = * - wg . "
- ."‘. . .
= . @ o F A
« . i
r T
- 4 -
)
- .

(a) -, : - - - e

  

Fig. 22,
(note thin metal deposit). 250X. Reduced 15%.

Corrosion of Type 316 Stoinless Steel by Fluoride 30.

\
)
S

UNCLASSIFIED
T-2992

 

 

.
b
le
o

!
250X

 

UNCLASSIFIED [ ]
T-2995

 

o
o
o

!
T
1flcrE5

3
J .
B

 

o
o
o

 

 

4,
250%

 

LY
o
=

(a) Hot leg; (k) cold leg
UNCLASSIFIED
INCHES gy

   

@0 LOOP 138 - SECT. 10

() LOOP

UNCLASSIFIED

INCHES T-4435
2 3

   

137 - SECT. 10

Fig. 23. Deposits Found in Cold Legs of 400 Series Stainless Steel Loops. (2) Type 410; (b) type 446.

mass-transferred in these systems, and while the
loops operated without plugging for 500 hr, the
rate of mass transfer is much too high for use of
these alloys.

Izett lron. — The single instance of a loop
plugging with zirconium fluoride—~base mixtures
was found with lzett iron (low-carbon high-purity
iron) loop No. 47. This loop plugged in 39 hr by
the formation of balls of dendritic metal crystals
(Fig. 24). The hot-leg attock was an even re-
moval, but the depth could not be determined,
since considerable oxidation of the outer surface
also occurred.

Nickel. — Two *“‘A’" nickel loops operated for
500 hr with very little hot-leg attack. The wall
surfaces were polished, but the thicknesses were
still within commercial tolerance, so at least no
large amount of metal was removed. Neither
intergronular nor void type of attack was found,
as shown in Fig. 25. A few metallic crystals
were found in the cold leg of loop 108, but none
were found in loop 110, In both loops, the nickel
content of the fluorides remained very low.

Inconel X. — Attacks of 22 and 19 mils were
found after circulating flvoride 44 for 1000 hr in
loops constructed from Inconel X (15% Cr, 9% Fe,
2.5% Ti, 0.6% Al, 11,0% Mn, 1.0% Cb, bal Ni),
This is slightly deeper than would be found in
comparable loops of Inconel, but the attack is
similar in nature, as shown in Fig. 26. No
definite deposit was found in the cold leg of
either loop; however, with loop 170, an area on
the cold-leg surface |1 mil deep etched differently

UNCLASSIFIED
T-11282

ONE INCH

 

Fig. 24, Metal Deposit Token from Cold Leg of on
lzett Iron Loop.

from the remaining material. The fluorides in-
creased in aluminum and chromium concentration
but not in titanium,

Molybdenum and Niobium. — Although, because
of fabrication difficulties, it does not seem likely
that they can be used immediately for reactors,

melybdenum and niobium show considerable

43
UNCLASSIFIED
T-4233
B
L 7 LOO% |

 

 

 

. " \...&.

Fig. 25. Hot Leg of a Nickel Loop. 250X. Reduced
33%.

promise of future use as containers for molten
fluorides, Low corrosion rates have been found
with both metals. Both these metals are difficult
to weld and must be protected against oxidation,
In attempting to operate these loops, many
failures occurred from cracks in the original
tubing and from welding stresses.

The molybdenum loops were about half size,
being 15 in, in each leg, and were made from
3af‘-in. tubing. The niobium loops were full-size
loops but were constructed from ‘fz-in. tubing,
The corrosion data from these loops are tabulated

 
 

 

 

 

UNCLASSIFIED[ |
T-T655 m

- 5 : s e - - o las . -.' 5 .‘.
IAMSS e T Y (e

.{'. i ‘oW t -'hl', .: r_’-'_'. o ‘-. _I-:I=-I

u ‘, " " & S . o8 " i - * = -;‘l‘ g-.
=y . l s - u'.“ - C"t“- ¥ 3 /=]

t W f LT E _\. . ™A
v & . W - "‘ 008 |
: . o =1 3 -JI '\.‘ wot
] Pl ! « b
i - - v L- k L Loos|

- e

s » ‘- ‘ 4 - 010 |
. ¢ * lau

. -
‘ ¥, loia

8 k'l

. - 215
= -, & "-. . \, o
Fig. 26. Hot Leg of an Inconel X Loop. 250X. Re-

duced 37%.

in Table 22, and typical photomicrographs of the
hot legs are shown in Figs. 27 and 28. No
evidence of attack or of any deposit was found
in the molybdenum loops. The surface was rough
ond pitted, but as shown in Fig. 27, the same
condition was present in the original tubing.
Difficulty was encountered with the analytical
determination of molybdenum in fluorides, but it
is obvious that the solubility is very low. These
loops were not operated with in-line traps, so a
final judgment on mass transfer should be re-
served until this has been done,

Table 22. Corrosion in Molybdenum and Niobium Loops

 

 

 

Loop Alloy Time  Maximum Attack Metallographic Examination Mo or Nb After
No. (hr) (mils) Hot Leg Cold Leg (ppm)

180 Molybdenum 250* None Few pits No ottack or deposit <1

184 Molybdenum B43* None MNe attack or deposit 15

185 Molybdenum 1000 Mone Mo attack or deposit 11-40
1003 Moly bdenum 424* MNone No attack or deposit 250
1002 Molybdenum 2000 None No attack or deposit 12-65
760 Miobium 1000 ]5 Light intergranular No attack or deposit

761  Niobium 695+ % Light infergianulor  No atfack or deposit 15

762  Hishivi 1000 Y Light intergranuler  Niobium erystals; no <1-270

deposit

 

*Operation terminated becouse of leak.

44
Sy

 

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UNMCLASSIFIED
T-8238

 

 

 

Fig. 27. Effect of Operating Time on Corrosion of Molybdenum by Fluoride 30. (a) As received; (b) 250 hr; (c) 1000 hr; (4) 2000 hr. 250X.

Reduced 17%.
UNCLASSIFIED
T-8358

 
 

 

R N e & .
. A
| | Jes R i
fi* | ( i = » 'i
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> . ; . A = .I,J
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{ p {--,_,
a5 A B 4 :
Vel R S
R \%IJ [ i I :. .,_"‘q-J i .
¢ TS~
] 23 C AN
4
Fig. 28. Hot Leg of Niobium Loop After 1000 hr

with Fluoride 30. 250X. Reduced 33%.

No evidence of attack was found in the hot legs
of the niocbium loops; however, possible crystals
were found in one of the cold legs. Further work
will be required to resolve this discrepancy.

Hastelloy B. — Another material that was
investigated was Hastelloy B (28% Mo, 5% Fe,
0.12% max C, bal Ni). In the original screening
studies,' two such loops failed in short times by
catastrophic oxidation. The first two loops in
this series failed in a similar manner, but the
difficulty was remedied by replacing the Sauer-
eisen cement normally used to protect thermo-
couples with metallic shields.

Hastelloy B is another material showing promise
for use with the fluorides, as shown by the data
in Table 23, A few voids and cracks to shallow
depths were found in both legs, with the cracks
also being found in the as-received pipe. Very
little, if any, increase in attack was found with
an increase in hot-leg temperature from 1500 to
1650°F or with an increase in operating time to
2000 hr (Fig., 29). While the rate of attack was
low, some occurred, since in several loops a few
dendritic crystals of nickel were found in the

cold legs.
When Hastelloy B is fabricated, some mo-
lybdenum is vaporized; therefore there was o

possibility that a nickel-enriched layer on the
surface was causing the mass transfer. The
internal surfaces of several loops were machined
to eliminate any such layer. The attack found in
these loops was essentially the same as in the
normal loops.

46

 

The major disadvantage with Hastelloy B is
the large amount of mass transfer found when
molten sodium or sodium-potassium alloy is
circulated in it. With the presently proposed
reactors, it would be highly desirable for o metal
to have corrosion resistance to both fluorides and
alkali metals,

Special Alloys. — |t has been shown that the
only metal leached from Inconel was the chromium;
therefore, in an effort to reduce the attack, alloys
were made with decreasing chromium concen-
trations. These alloys were vacuum-melted and
extruded by the ORNL Metallurgy Division and
then drawn into 'I:-'z-in. tubing by the Superior Tube
Company.  Fluoride corrosion data from these
loops are tabulated in Table 24, and representa-
tive hot-leg sections are shown in Fig. 30,

In each case when the chromium in these alloys
was reduced from the original 15% to about 5%,
the depth of attack was reduced by a factor of
3. The reduction was found when the chromium
was replaced with either iron or molybdenum,
When the chromium concentration was reduced to
10%, erratic results were found,

Results from another series of special loops
ore tabulated in the lower portion of Table 24,
These loops were operated to determine whether
variations in carbon content had an effect on
corrosion. Loops 366 and 367 were fabricated
from vacuum-cast metal and contained only 0.01%
carbon. The attack in both these loops was
essentially the same as that found in control
loop 359 made from commercial alloy containing

0.07% carbon (Fig. 31).

Special Fuel Mixtures

In the last report,’ it was pointed out on the
basis of only a few tests that the addition of
zirconium hydride to alkali-metal-base mixtures
showed promise of reduced depths of attack, but
that unidentified layers had been found on the
hot-leg surfaces. This type of work has con-
tinved, with similor additions being made to
zirconium fluoride~base mixtures, A large in-
vestigation into the basic mechanisms in such a
system has also been carried out by the ANP
Chemistry Section.

In two series of tests, various amounts of
zirconium hydride were added to portions of the
some large batch of fluoride 30. The zirconium
hydride was added to the fluorides in small
Table 23. Corrosion Data from Hastelloy B Loops

 

 

 

Loop Time Heat Metallographic Examination Nickel (ppm) Molybdenum

No. (hr) Treatment Hot Leg Cold Leg Before  After (‘:;:; Comments

378 94* Rough surface No deposit <20 No attack on welds

140 500 Aged 1950°F 2 mils, light; rough 20 10 1 No attack on welds

153 394* 1 mil, heavy; rough 1.5 mils, intergranular cracks 20 20

154 1000 1 mil, moderate; rough 1.5 mils, cracks; rough <20 <10

157 736** Aged 1.5 mils, moderate Rough <10 <10 1

156 0 1.5 mils; rough 1.5 mils; rough

158 2000 2 mils, heavy; rough 2 mils, heavy <20 1

548 2000 1 mil, moderate 1.5 mils, intergranular 60 <20

162 1000 3 mils, moderate; rough 2 mils, intergranular 15 25 15 1650°F

161 1000 2 mils, moderate; rough 2 mils, intergranular 25 25 10 1650°F

163 1000 Aged 2 mils, moderate 2 mils, moderate 35 3-135 10

167 1000 2 mils, heavy 1.5 mils, moderate 40 <20 <10 Flueride 44

164 1000 Aged 2 mils, moderate 2 mils, moderate 35 <20 <10

174 1000 3 mils, heavy; rough 1.5 mils; rough 35 20-60 Metal crystals

159 2000 2 mils Rough 20 20 <]

186 1000 3.5 mils, heavy; rough 2.5 mils; rough; crystals 15 <1=-70 <5 1650°F

187 1000 4 mils, heavy 1.5 mils; rough; few crystals <20 <10 <20 1650°F

769 500 1.5 mils; large voids 1.5 mils, voids; metal in trap 5 10 15 Machined internal surface
770 1000 1 mil, heavy; rough No deposit 10 70 Machined internal surface
771 1500 2 mils, maderate 2 mils, moderate 10 <10 Machined internal surface

 

Ly

*Operation terminated because of leak.

**Operation ferminated because of pawer failure.

 

 
   

UNCLASSIFIED
6

 

 

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UNCLASSIFIED
T-6085

 

 

     
   
   
     
 
   
   

 

       

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ked 3 ud

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UMEL ASSIFIED
T-6084

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o
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T T
INCHES
1 1

&

 

 

 

 

Fig. 29. Effect of Time and Temperature of Operation on Attack in Hastelloy B. (a2) As received; (&) 500 hr at

1500°F; (c) 1000 hr at 1500°F; () 2000 hr ot 1500°F; (&) 1000 hr at 1650°F. 250X. Reduced 30%.

43
 

 

 

 

Table 24. Corrosion Data from Modified Inconel-Type Alloys
D Alloy Composition (%) Time Attack Chrom.iurn Con::entrariun
No. Cr Fe Ni Other (hr) Intensity Depth (mils) 3 Tiuondes
ppm)
520 176 68 759 1000  Heavy 13, 1000
522 9.8  14.5 75.6 8239  Heavy 8 580
523 9.8  14.5 75.6 1000 Heavy 12 500
524 4.8  19.8 76.1 647°  Moderate 3 400
525 4.8 19.8 76.1 460  Heavy 3 300
526 9.8 6.7 83.4 1000 Moderate 13 650
527 9.8 6.7 83.4 1000 Heavy 15 650
521 6.0  10.1 74.4 Mo, 9.8 1000 Light 2 450
472 6.0 10.1 744 Mo, 9.8 500 Light 3% 300
529€¢  ~15 V75 76 1000 Heavy 12 550
366 15.0 7.3 78.1  C, 0.01 500 Moderate 3% 950
367 147 7.2 78.1  C, 0.0 500  Heavy 3% 850
359¢  ~15 ~TS T8 C, 0.07 500 Heavy 4 850

 

aDperution terminated because of leck.

bOpercfion terminated because of power failure.

CCommercial Inconel.

  

(a)

Fig. 30. Typical Hot-Leg Sections from Loops of Modified Inconel-Type Alloys.
(b) 4.8Cr-19.8Fe-76.1Ni. 250X.

Reduced 21%.

UNCL ASSIFIED
T-5455

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T
]

 

 

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EREE

 

 

 

 

(a) 6Cr-10.1Fe-74.4Ni-9.8Mo;

49
UMCLASSIFIED
T-4517

 
 

 

 

  

 

 

Fig. 31.
(k) low-carbon Inconel, 0.01% C. 250X. Reduced 30%.

charge pots, and after being agitated at 1300°F
by helium, the mixture was transferred to the
loops through grade G Micro Metallic filters, The
metallographic data from these loops are tabu-

lated in Table 25, with the chemical data in
Table 26. These data show that a gradual re-
duction in depth of attack was found as the

zirconium hydride concentration was increased to
about 0.7%, With larger hydride additions, layers
of increasing thickness were found on the surfaces
of the hot and cold legs, as shown in Fig. 32,
The deposits attained greater thickness in the
hot legs. With the use of a special microspark
spectrographic method, it was found that the
thicker layers were predominantly zirconium.
This
so this metal may have also been present and not
detected, The shallow attack found in many of
these loops is not the usual subsurface void
type of attack, These areas are holes which are
open to the surface and may have originated as
scratches or pits in the original pipe.

After operation, the chromium
fluorides to which zirconium hydride had been
added was always much lower than in similar
tests without the addition. The chromium gradu-
ally decreased with hydride additions up to about

technique was not sensitive to uranium;

content of

50

Comparisen of Attack in Commercial and Low-Carbon Inconel.

UNCLASSIFIED
T-4575

 

 

 

 

(5) sid

(a) Commercial Inconel, 0.07% C;

0.5% but remained low and constant with further
additions.
affected by the zirconium hydride additions.
As the hydride was increased above 0.5%, notice-
able amounts of uranium were reduced in the
charge pots and left on the filters during transfer.
With a 2% hydride addition, half the uranium was
lost. With the higher additions, small amounts of
uranium were also lost during operation of the

The uranium concentration was also

loop.

It has been found by the ANP Chemistry Section
that the action of the zirconium hydride in re-
ducing the attack is not from purifying the so-
lutions, as was first thought, but is by the
reduction of a portion of the UF, to UF,.
Attempts were made to determine the UF, content
of the series both before and after
operation; however, the results are in such doubt
that they are not being reported. From these
results it does appear, however, that in every

second

case the UF, content was reduced to values
below 1% in 500 hr of operation.

A comparison of the depths of attack from
loops 413 and 414 shows that the addition of
small amounts of hydride does not stop mass
transfer. Between 1000 and 2000 hr the rate

was the same as that found from the time curve
Table 25. Metallographic Results from Loops in Which Mixtures Containing ZrH, Were Circulated

 

Loop ZrH, Added EE Batch

Operating Time

 

No. (%) No. (hr) Hot Leg Cold Leg

248 0.6 19 500 Moderate attack to 4 mils Deposit

308 0.5 68 500 Light attack to 2‘5 mils Deposit

413 0.2 150 1000 Light attack to 7 mils

414 0.2 150 2000 Heavy attack to 11 mils

469 0 162 500 Heavy attack to 8 mils No deposit

459 0.2 162 500 Moderate to heavy attack to 6 mils Deposit

470 0.5 162 500 Light to moderate attack to 3 mils Deposit

460 0.9 162 500 Thin deposit Deposit

471 2.0 162 500 Deposit to 1 mil Deposit

800 0 513-4 500 Heavy attack to 10 mils No deposit

787 0.4 513-4 500 Moderate attack to 5 mils No deposit

788 0.5 513-4 500 Scattered light deposit; pitting Scattered deposit
2 mils

789 0.6 513-4 500 Pitting 1 mil Scattered deposit

790 0.7 513-4 500 Intermittent deposit; pitting Deposit
1],-5 mils

791 0.8 513-4 500 Deposit; pitting 1% mils Deposit

792 0.9 513-4 500 Deposit 1 mil Thin layer

 

  

UNCLASSIFIED

1 T L.
INCHES IS E I
1 1 = =

E

 

 

 

(a)

UNCLASSIFIED
T-11064

 
   

(&)

 

 

Fig. 32, Deposits in Hot Legs of Loops with Zirconium Hydride Additions. (a) 0.8% Zer; (&) 2% ZrHQ (note

slight increase in thickness), 250X. Reduced 29.5%.

51
 

Table 26, Chemical Results from Loops in Which Mixtures Containing Zer Were Circuloted

 

 

 

 

 

e ZiH, Added Uranium (%) Nickel (ppm) Chromium (ppm) Iron (ppm)
No. (%) Before ~ After  Before  After  Before  After  Before  After
248 0.6 8.0 2.0 1590 <20 40 350 710 100
308 0.5 8.9 8.7 <20 <20 30 100 55 40
413 0.2 8.6 8.9 <10 <10 170 200 150 45
414 0.2 8.5 8.7 <10 <10 130 250 85 50
469 0 8.5 8.9 25 10 90 700 55 70
459 0.2 8.6 8.6 40 <10 75 200 %0 70
470 0.5 7.5 7.0 10 40 30 20 50 70
460 0.9 54 5.1 35 <10 20 35 65 60
471 2.0 4.0 4.1 <10 15 60 45 55 45
800 0 8.6 8.5 20 40 105 850 30 20
787 0.4 8.8 8.5 15 30 85 110 95 45
788 0.5 9.3 7.5 3 270 15 65 50 85
789 0.6 8.3 6.6 15 <1 35 60 75 65
790 0.7 6.6 6.2 40 15 25 90 30 85
791 0.8 6.5 6.3 70 <] 35 65 75 90
792 0.9 6.7 6.3 30 <1 35 40 45 85

(Fig. 6). The actual depths are less in both Difficulty was encountered in making batches

cases, showing that some reduction in attack had
occurred in the early stages. While the rate of
mass transfer is the same, the chromium concen-
trations of the fluorides are much less with the
additions. This low value did not increase in the
1000 hr of operation even with the mass transfer,

While zirconium hydride will reduce the depth
of attack, it does not reduce the rate of mass
tran sfer. Unless the compositions are con-
trolled very closely, uranium will be lost from the
system both before and during operation. |f such
a system were to be used, the deposits in both
legs would have to be studied in detail.

After it was found that the formation of UF,
was responsible for the reduction of attack with
zirconium hydride additions, work was started
on batches with a portion of the uranium present
as UF,. The program consisted of both corresion
loops and a study of solubilities and equilibriums
in such systems,

The most striking feature of this study was the
inability to obtain reproducibility in the results.

52

containing known and predetermined amounts of
UF, ond in developing a suitable method of
analyzing such mixtures,

The hot-leg attack was replaced by thin de-
posits in the first three loops, which circulated
batches made under carefully controlled con-
ditions in the laboratory (Table 27). Two of
these loops operated for 500 hr and the other for
2000, These loops all operated with low total
uranium concentrations ond therefore high UF,
to UF, ratios. The hot-leg layer in the 2000-hr
loop was identified as zirconium by the micro-
spark spectrographic procedure, To show the
layers were formed during operation and not
during cooling, a loop was drained aofter 500 hr
of operation and while still at temperature. This
loop also showed no attack and the presence of a
zirconium layer,

The solubility of UF, in the zirconium fluoride—
base mixtures is not sufficient to provide the
concentration necessary for reactor operation;
therefore it would be necessary to use mixtures
£S

Table 27. Corrosion Data from Loops with Zirconium Fluoride—Base Mixtures Containing UF,

 

 

 

 

 

 

EE Analysis of Fluoride Mixture
Loop Time Metallogrophic Examination ThE
Batch h Mickel (ppm) Chromium (ppm) Iron (ppm) Uranium (%) u (%)
No. No. (hr) Hot Leg Cold Leg
Before  After Before Afrer Before After Before After Before  After
438 Lab 500 Deposit 10 10 10 30 95 60 2.79 3.0 a
443 159 500 0.4-mil deposit Intermittent deposit 20 10 20 40 75 60 5.8 5.3 1.5
457  F-2 2000 Layer,? 0.5 10 5 10 80 120 75 2.8 2.6 1.6
mil; rough; no
attack
473 F-4 500 Madrrme, 5 Occasional crystals 10 20 30 125 40 45 B.7 8.3 1.2
mils
488 B-85 2000 Modplerafe, 7 Thin deposit 15 10 60 90°¢ 40 60 B.7 8.8 2.0
mils
491 F-5 500 Heavy, 7 mils Deposit 30 20 70 180 30 50 12.8 13.2 2.5
492 F-5 2000 Heavy, 15 mils 1-mil deposit 10 10 35 250 40 40 14.2 13.2 1.8 0.7-2.8
5{13‘f B-89 500 annsitb No deposit 40 15 30 35 8O 60 2.4 2.5 2ud 1.0
508 207 500 No attack 0.5-mil deposit 10 c 260 60 50 100 10.4 9.7 4.3 0.6-3.3
627 426RF 1000 Heavy, 8 mils  1-mil deposit 15 20 50 200 40 40 9.0 8.9 1.2 0.4
628 427RF 1000 Heavy, 10 mils Thin Cr deposit 10 20 45 225 35 100 8.7 8.7 0.8 0.2
633 420RF 500 Heavy, 10 mils Intermittent deposit 25 20 100 575 65 35 11.8 11.4 1.4 0.7
634 422RF 500 Heavy, 8 mils Deposit 35 20 65 300 40 60 B.8 B.7 1.3 0.5-1.4
635 424RF 1000 Moc!?rntm 7 Thin deposit 60 20 100 200 BO 50 8.9 9.7 2.0 0.4-1.2
mils
636 430RF 1371° Mndrrula, 9 Mo deposit 15 20 40 250 BO 50 8.4 9.1 0.6 0.3
mils
683 423RF 500 Mndrram, 7 Rough; no deposit 25 1-45 140 225 25 40 9.0 8.4 1.4 0.6-1.0
mils
690  432RF 1000 Heavy, 10 mils T-mil attack; no 40 20 190 275 40 40 7.7 8.3 1.3 0.4
deposit
781 371 500 Mo:_lgerute, 9 Deposit 7 40 75 700 40 55 9.5 9.6 0.1 0.1
mils
782 ar2 500 Heavy, 9 mils No deposit 10 25 75 390-525 50 85 9.8 10.2 0.3 0.1
784 373 500 Heavy, 7 mils No depesit 20 25-185 60 200 45 70 10.7 11.2 1.5 0.4=1.0
785 372 1500 Mnt_ifmte, 12 Deposit 10 10 75 305-485 50 85 9.9 11.2 0.1 0.1-0.4
mils
786 373 1500 Heovy, 9 mils  No deposit 35 20 60  200-600 40 95 10.6 2.6=-11.6 Ll 0.4
a
All UF3.

bZir:unium layer.

“Individual results varied.

dLnt:p drained while hot.

Operation terminated because of power failure,

 

 
 

of UF, and UF,. The next group of loops (473-
508 and 781-786 in Table 27) operated for
varying times with mixtures containing various
proportions of UF; to UF, and produced erratic
results, The attack was reduced, but not elimi-
nated, and the results were not consistent. Cold-
leg deposits were found in almost all of these
loops. The attack in the 2000-hr loop is much
lower than would be found in an all-UF, loop,
but drawing any conclusion concerning mass
transfer from these data would be dangerous. An
actual reduction in mass transfer rate would,
however, be partially confirmed by loop 457,
which operated with an all-UF ; mixture.

The loops (627690 in Table 27) operated with
batches made in the production facility, ond
very little, if any, reduction in attack was found
in these loops. Analyses of these batches have
shown them to contain much less UF, than had
been predicted. With both these loops and those
discussed above, the attack in most cases does
large increase in the chromium
concentration of the fluorides. Even small ad-
ditions of UF, must markedly lower the chromium
solubility level. The attack must be predomi-
nantly from mass transfer,

No additional loops are being operated at this
time with zirconium fluoride—base mixtures con-
taining UF,. What is causing the production
difficulties and why it has not been possible to

not cause @

obtain corrosion results comparable with those
obtained with the hydride addition, has not been
explained, Both the production ond the dis-
proportionation problems would have to be over-
come to allow the use of such mixtures,

Alkali-Metal<Base Mixtures Containing
Trivalent Uranium

While the solubility of UF, in zirconium
fluoride—base mixtures is limited, it is much
higher in fluoride mixtures with an alkali-metal
base. Such mixtures are also to be preferred
from a physical property viewpoint, and so some
work was done on the mechaonism and rate of
corrosion of Inconel by alkali-metal-base mixtures
containing trivalent uranium.

Again, difficulty was encountered in obtaining
reproducibility in the results and also with the
analytical In the first series of

supposedly Nos. 499 to 528

procedures.

identical loops,

54

 

(see Table 28), essentially no attack was found
in two loops, while it was deep ond heavy in the
others, The reported UF, values are well below
the predicted values, but do not vary in a system-
atic manner with the attack,

After the production techniques hod been im-
proved, the remaining loops in Table 28 were
operated and gave more consistent results. The
maximum attack in these loops was to 2 mils and
was from a batch with o low UF3 proportion,
Hot-leg deposits to 1 mil in thickness were found
in the loops with the slightly higher UF; concen-
trations.  Typical hot-leg sections from these
loops are shown in Figs. 33 and 34. Definite
identification of the layer has not been made, but
metallographically it appears to be uranium, No
hot-leg attack was found in any loops with a
layer., Shollow attacks with no visible hot-leg
deposits were found in the loops with UT**
concentrations of about 2%, Cold-leg deposits,
which in one case were identified as chromium,

were found in all these loops. Thus mass
transfer is not eliminated by U*** additions.
In the second group of loops, there is a

tendency for large decreases in total uranium to
accompany the high UF, concentrations, but
exceptions may be noted. The variations are
larger than would be found with a mixture with
all the uranium present as UF ;. With the batches
having high concentrations of UF,, the chromium
content of the fluorides was low; however, with
low UF3 concentrations high and erratic chromium
values were found. The higher chromium levels
were found in the cold legs. These mixtures
also differed from the zirconium-base mixtures
in that the nickel content of the fluorides after
operation was higher,

From these data, it would appear that a thermal
loop may be operated without a hot-leg deposit
and with only shallow depths of attack if the
U**" content could be controlled at a concen-
tration just above 2%, To operate in this range
will require closer production and analytical
control than is now available. In such a system,
additional study is necessary on dispropor-
tionation during long operating times aond also on
the cold-leg deposits. The attractiveness of this
system would moke such further studies appear to
be worth while if production problems can be
overcome,
 

99

Table 28. Corrosion Data from Alkali-Metal-Base Fluorides Containing UF,

 

Analysis of Fluoride Mixture

 

 

 

 

Leoy Bf:h Dp:::ng Mgwi Sirehic Reweta s Nickel {(ppm) Chromium (ppm) Iron (ppm) Uranium (ppm) u*** (=)
Mo. Hot Leg Cold Leg
No. (hr) Before After Before After Before Afrer Befora After Befare Afrer
439 27 500 Heavy attack, 13 mils 1 mil deposit BO 1-100 35 650 60 100 11.0 9.9 1.8 0.9-2.8
500 274 1000 Heavy attack, 13.5 mils 0.5 mil deposit 45 45 25 75-1175 70 75 11.0 10.5 1.5 0.7
504 302Cs 500 Rough; intergranular 0.1 mil deposit 190 75 25 50-280 70 120 11.3 10.2 3.9 0.7
attack, 2 mils
505 276RF 1000 Heavy attack, 13 mils 2.5 mils deposit 50 40 45 400 1 70 10.7 10.6 2.6 0.6
506 277 1000 Heavy attack, B mils 1 mil deposit 15 20 30 25-1880 65 100 1.6 11.3 0.7 0.5-2.6
507 272 500 Heavy ottack, 11 mils 0.5 mil deposit 215 40 60 200 135 50 1.9 11.2* 2.4 0.6
509 208 500 Heavy attack, 2 mils 0.2 mil deposit 35 20 35 60 150 175* 6.9 7.1 0.7 0.5-3.5
511 284RF 500 Heavy attack, 13.5mils 0.5 mil deposit 860 10 65 890-1100 115 50 11.4 11.1 1.2 0.7
528 189RF 2000 Very heovy attack, 25 0.5 mil deposit 25-90 816-1450 90-265 11.0 0.9-1.7
mils
589  405RF 500 Layer to 0.5 mil Deposit 30 20 40 50 95 125 7.9 6.3-7.7 4.7 0.7
590  407RF 500 Heavy deposit Deposit 75 40 55 55 125 50-160 10.8 9.8 7.7 1.1
591 225Cs 500 Light attack, 2 mils Thin deposit 65 100 60 25 140 75 11.5 11.5 1.7 0.7
592 INcs 1000 1 mil deposit 0.5 mil deposit 110 90 58 50 165 70 10.9 10.5 5.2 1.0
593 322C5 1000 1 mil deposit 0.5 mil deposit 45 BS 75 15 B0 80 13.8 10.0 5.6 0.9
594  412RF 1000 2 mils attack 0.3 mil deposit 60 100 115 70 70 12.6 10.7 2.0 0.7
595  31%Cs 500 1 mil deposit 0.5 mil deposit 30 65 35 20 75 130 11.0 10.0 5.3 1.3
596 320Cs 500 1 mil deposit 0.5 mil deposit 110 115 40 15 130 130 10.7 10.6 5.8 1.4
625 323Cs 500 0.5 mil deposit 0.3 mil deposit 60 5-105 40 35 140 80-195 13.2 12.6 2.9 0.9
626 328Cs 500 Light attack, 'I.lzé mils 0.3 mil deposit 10 100 40 35 120 B5 11.2 11.2 1.5 0.9
(Cr)
708 461 1000 Moderate attack to Deposit 1 120 25 50-160 60 B0-665 12.0 11.7 0.7
2 mils; intermittent
layer
793 496 500 Heavy attack, 2 mils Thin deposit 155 20-165 20 20-200 150 70-885 12.9 12.5 0.9 0.2

 

*Average value;

results varied.
 
   

T-7484

-
-
»
i
t-l
-
e

hY
I

8

 

{a) ' %

Fig. 33.

  

UMCLASSIFIED

e

{

T
INCHES

1

 

 

a
®
I

(2)

UNCLASSIFIED
75

 

 

 

 

Typical Hot-Leg Sections from Loops Circulating Alkali-Metal Fluorides with Small Amounts of

Trivalent Uranivm. () 1.7% UTFF, 500 hr ot 1500°F; (5) 2% U***, 1000 hr ot 1500°F. 250X. Reduced 29.5%.

 
  
 
 
    

  

  
  
  

  

 

UNCLASSIFIED
T-11045
- =
— . e ] " o "'.# e
TR P A R PO T
R g -
' 008 |
o
(a) g1 |
Fig. 34.

Trivolent Uranium. (a) 5.3% U*T™, 500 hr at 1500°F; (b) 5.6% U

REACTIONS AND MECHANISMS

The mechonism for formation of the holes
below the surface of the Inconel hot-leg wall and
evidence that they are voids were presented in
the previous report and so will only be summarized
here,

1. Chromium is leached from the hot-leg surface
by chemical action.

2. Since the metal surface is depleted in chro-
mium, a gradient is established and chromium
diffuses to the surface from the areas under-
neath the surface,

36

  

 

 

(&)

UNCLASSIFIED[™ ]
T-7542

EEE

r
-
o
T
INC;-IES

T
1

EEEEREER

 

 

Typical Hot-Leg Sections from Loops Circulating Alkali-Metal Fluorides with Larger Amounts of

*** 1000 hr at 1500°F. 250X. Reduced 30%

3. Since nothing diffuses inward to replace the
chromium, vacancies occur in the area below
the surface.

4, The vaconcies collect in areas of discon-
tinuity and weakness, such as the grain
boundaries, and form the voids.

5. The voids concentrate and grow in size with
increasing time or temperature.

It was shown that with the zirconium fluoride—
bose mixtures, chromium is still the major
ingredient leached from the wall. The holes have
the same appearance and change in a similar
manner, and the some mechanism of hole formation
is thought to apply. The fact that the holes grew
and concentrate with time is additional evidence
that they are voids. |f they contained fluorides
they would have to connect to the surface and
have appreciable size; it does not appear possible
for the fluorides to move through the metal and
collect at a few larger sites; vaconcies can move
but not fluorides,

The study of the chemical reactions by which
the chromium is removed from the hot-leg surface
is still the object of o large effort by the ANP
Chemistry Section. This Section studied the data
from the thermal loops and also worked with
laboratory experiments, A detailed and complete
report on these reactions will be the responsibility
of that Section. In this report, these reactions
will be discussed only briefly and where they
are indicated by the loop data. Some of the
reactions discussed here were worked out in the
chemical experiments.

With the relatively impure alkali-metal-base
mixtures previously re|;:f::nr‘r<=.-f.l_.1 a large portion of
the attack found in the 500-hr tests was caused
by the impurities. Analyses of the batches used
for the loops described in this report, combined
with the corresion data when known additions
were made, show that this is no longer true; only
a small part of the attack may now be attributed
to the known impurities,

In spite of the purity of the fluorides, the
attack is still rapid at first and then continues
at a slower rate. The chromium in solution also
increases rapidly at first but then remains
constant with increasing operating times. The
second stage of the process, in which the attack
continues but the chromium concentration in the
fluorides remains constant, is the mass transfer
phase of the attack. The first, rapid stage is
caused by the system reaching equilibrium, Any
reactions from impurities and the dissolving of
enough chromium to reach equilibrium concen-
trations are thought to be the mojor contributors.
The unexplained variations in depth of attack in
a period of 500 hr point to additional, as yet

unknown, reactions also occurring during this
period,

The main reaction contributing to the mass
transfer is

2UF, + CrF, ==2UF, + Cr

The equilibrium concentrations for this reaction

are of the proper order and direction to cause
mass transfer to occur. No evidence of a similar
mass transfer reaction in fluoride mixtures with
zirconium and uranium has been found. The
equilibrium concentration of CrF. in these mix-
tures is too low to be determined analytically;
therefore mass transfer reactions involving this
compound must be of very minor importance.

With the reaction shown above, the rate-con-
trolling step in the process has not been de-
termined, That it is not the diffusion of chro-
mium through the wall to the loop surface is
shown by the magnetic etching studies. The
chromium gradient is steeper during the initial
stages than it is during the mass transfer stages,
After the rapid attack, the chromium diffuses to
the surface faster than it is removed and thereby
evens out the gradient somewhat. This would not
be the case if the diffusion step was the rate-
controlling step. The fact that the depth of
attack was doubled when two batches were circu-
lated in the same loop for 250 hr each is evidence
that the chromium diffusion rate is not controlling
even during the rapid initial attack. For mass
transfer to continue, the chromium must diffuse
into the cold-leg wall., The suggestion has been
made that this step is controlling; however, no
evidence of any chromium-rich layer on the cold-
leg surfaces has been found in any long-time
loops.  Any chromium metal had always been
found in the cold-leg trap, where it is essentially
removed from the system. Tracer tests are now
in progress fo provide a better answer to this
question.

The mass transfer reactions with the barren
mixtures are not as well understood., They are
the same type of reactions, since the hotsleg
attack is similar but not as heavy, and chromium
metal has also been found in the cold-leg traps
of these loops. The most likely reactions are

CrF, + 2Na ~— 2NaF + Cr

or
3CrF, <—2CrF, + Cr

In either case, it would be an equilibrium

reaction.

The addition of UF, to these mixtures com-
pletely chonges their reducing power aond equi-
librium concentrations.,  The chromium is not
taken into solution as rapidly, and smaller
amounts are left in the cold leg on each cycle,

57
 

 
0NN

10-29.
30.
31.

33.

35.
36.
37.
38.
39.
40.
41.
42.
43.
44,
45.
46.
47.
48.
49.
50.
51.
52.
53.

55.
56.

57.

58.
9.
60-64.
65.

67.
68.

ORNL-2338
UC-25 - Metallurgy and Ceramics
TID-4500 (15th ed.)

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59
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118.
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127.
128.
129.
130.
131.
132.
133.
134.

135.
136.
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M- -0Z0

—q“rraon-—0=