ORNL-TM-3866.txt

 

 
  
 
 
  
  
   
   
   

 

RNL-TM-3866
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CORROSION AND MASS TRANSFER

CHARACTERISTICS OF NaBF,—NaF (22-8 mole %)
IN HASTELLOY N

J. W. Koger

THIS DOCUMENT CONFIRMED AS

UNCLASSIFIED
gl\yISION OF CLASSIFICATION

 

DATE__'_1e/iclHz

GISTRIBUTION OF THIS BOCUMEKT 15 UNLAETED

RIDGE NATIONAL LABORATORY

IO CARBIE R TS R T
 

 

This report was prepared as an account of work sponsored by the United
States Government. Neither the United States nor the United States Atomic
Energy Commission, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights.

 

 

 

 

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ORNL-TM~-3866

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

CORROSION AND MASS TRANSFER CHARACTERISTICS OF NaBF,—NaF (92-8 mole %)
IN HASTELLOY N

J. W. Koger -
. NOTICE

This report 'was prepared as an account of work
"} sponsored by the United States Government. Neither
the United States nor the United States Atomic Energy
Commission, nor any of their employees, nor any of
their contractors, subcontractors, or their employees,
makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, com-
pleteness or usefulness of any information, apparatus,
.| product or process disclosed, or represents that its use
would not infringe privately owned rights.

 

 

 

- -~ OCTOBER 1972

NOTICE This document contains information of s preliminary nature

and was prepared primarlw for internal use at the Oak Ridge National ;
Laboratory. It is subject to revision or correction and therefore does

not represent a final report, A

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
- operated by =
UNION CARBIDE CORPORATION
for the .
U.S. ATOMIC ENERGY COMMISSION

™
R7CES

BISTRIBUTION OF THIS DOCUMENT IS GNLIVHTED
 

 

 
 

 

<

iii

CONTENTS

 

Page

: Abstfact Gt e e e s e e e e e e s e e e e e e e e e e e e e s 1
Introduction « . ¢ ¢ &+ « ¢ ¢ ¢ ¢ 4 v e v e e e e e e e e e e e 1

Experimental Procedure . « « « « « o o o o o o o o ¢ o o & o o o . 6

Equipment . . « ¢ o ¢ ¢ v v o s 0 e e e e e e e e e e e e e e 6

Salt Preparation . . . « ¢ ¢« ¢ ¢ ¢ o 0 ¢ o o o s s s s s s e s 8

AN2lySEeS8 « 4 ¢ o s o o s s s s s s % s e s e s s e e e s s e e 11

Operation and Resulfs Gt e e e s s s e s e ee e e e e e e 11

Thermal Convection Loops NCL-13, NCL-13A, and NCL-14 . . . . . 11

LoOp NCL=17 & & &« ¢ o o s s o o o o o s o s s o o s o s s o o 20

LOOP NCL=20 &+ v v v v v v o o v o v v o oo e w e e e e 2]

FCL-1 Pump LOOD &+ « « « + o « o o o o o o o o o o o o o o » o+ o 28

RUNS 1 8Nd 2 « o v v o o v e e e e e e e e e e . 32

RUL 3 . v v o o o o o o o o o o o o o o s s e e e 0 e o 37

‘ Installation and Testing of Cold Finger During Run 3 . .. 38
) RUN 4 « 4 o o + o o o o o o o o s o s o s s s o oo o« 40
: PP .

RUD 6 &« v ¢ o o o o o o o o o o s o o o s o o o o o o« b5

RUN 7 « o « o o o o o o o o o o s- o o o s o o s o o+ o o 48

Salt ChemiStTY « « &« o & « o o o o o o o o o o o o o o o o o s s o= 33
Purification . . « v « ¢ ¢ « o ¢ o o o s o o o o o s o o = s s I3
Analytical Chemistry . . . . « « ¢« ¢« ¢ ¢ o ¢ ¢ o o o . . .. 54
Discussibn.. « e ..;‘. T . 7
Theory Q e e e e 0 e C e e e e e e e e ... « « o« o 54
Equations . . . . . . s e e e e e s e e e e e e e e 61
Kinetics . . e v e e e C et e s e s e s e s e e 62
‘Solid-State Diffusion Control . . . . . . . .‘.‘. .« +« + - 066

Solution Controlling . .';'. T Y
Experimental Data . . . . . Gt e e e e s e s s e v s e . . 68

- ; Diffusion Calculations . « « o + o o o « o o v o s oo o oo 10
SUmmAYY « o o« o o ; P e e e e e e e e e n e e e s .. 76

COI‘ICluSiOHS s 8 ® s o & 7 e s e s e s s s s 2 s .o . s 8 e | v e s » 78
g ACkDOW].edgment -® 8 8 & 8 & & ¥ 9 8 B2 8 & " » & 4 T v+ s e e = e 2 79

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CORROSION AND MASS TRANSFER CHARACTERISTICS OF NaBF,—NaF (92-8 mole %)
IN HASTELLOY N |

J. W. Koger

ABSTRACT

A series of corrosion experiments in thermal convection
and pump loops designed to test the compatibility of fused
NaBF,—8 mole % NaF with Hastelloy N has shown the extreme
effect of impurities on the system mass transfer.” The diffi-

" culty in keeping the melt sufficiently pure was also illus-
trated. Kinetic considerations showed that the mass transfer
process is controlled either by solid-state diffusion or by
the solution rate, depending on the amount of impurities
allowed to enter the salt. The mass transfer behavior is
very similar to that for the Cr-UF,4 corrosion reaction
(material removal in the hot section and deposition in the
cold section), the differences being that alloy constituents
other than chromium participate in the process and that the
reaction rates are different. Because of these differences,
the effective diffusion rate controlling the hot leg attack
is larger than that obtained in typical diffusion experi-

" ments for chromium in Hastelloy N.

Titanium-modified Hastelloy N with less chromium and
iron shows a greater resistance to attack in a fluoroborate
salt with 500 ppm oxide impurity than does standard
Hastelloy N. Its corrosion rate is doubled by increasing
the oxide concentration to 1500 ppm. Average corrosion
rates were low for all systems tested.

 

-~ : ~ INTRODUCTION.

‘The successful development and operation of the molten salt reactor

‘experiment (MSRE) have led to the present program for_ development of a

molten salt breeder reactor (MSBR) In the MSRE the heat transferred
from the fuel salt to the coolant salt was. rejected to an air-cooled
radiator. In the MSER a secondary‘coolant ‘will be required to remove

heat fromltne fuel in the primary heat exchanger and transport this

- heat to supercritical steam at quite low temperatures.

 
 

This secondary coolant should have low viscosity and density and
high heat capacity and thermal conductivity to permit use of écceptable
heat exchangers, coolant pumps, and steam generators. The melting point
must be low enough to meet the heat transfer temperature requirements,
and the vapor pressure must be low at the temperature of‘operation. The
salt must also be easily remelted, with‘no precipitation of high-melting

compounds during cboling. The coolant must be commercially available in

high purity, and the price should not be prohibitive. In nuclear systems

the coolant must be stable under the radiation that it encounters. We
_'also must consider the result of accidental mixing of the coolant fluid
and steam and select a codlant in which the effects of this mixing will
_bé minimiied. Last but not least,-the coolant must be compétible,fiith
the container materials .of the system. _ | _

It now appears that the best choice for the MSBR secondary coolant -

is the eutectic mixture of 8 mole % NaF in NaBF4, with a meltihg point! of
385°C (725°F) as shown in Fig. 1. The salt is quite inexpensive

 

lc. J. Barton, L. V. Gilpatrick, H. Insley, and T. N. McVay, MSR
Program Semiann. Progr. Rept. Feb. 29, 1968, ORNL-4254, p. 166.

 

1000
\
900 ~

ORNL-DWG 67-9423A

 

 

700 —— N ‘
600 - \

500 _ R

 

 

 

TEMPERATURE (°C)

 

400 - — —

 

 

300

 

 

 

 

 

 

 

 

 

 

 

 

 

200 —1—L —
NaF 20 40 60 80 Nt'JBF4
' NaBF, (mole %) :

Fig. 1. The System NaF-NaBF,, .

7]
 

 

 

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(< $0.50/1b). ‘At elevated temperatures the fluoroborates show an

appreciable equilibrium pressure of gaseous BF3; however, at. the

proposed maximum temperature of the MSBR secondary coolant (621 C) the

2

pressure is only 252 torr. ‘The equilibrium pressure above a melt of

NaBF,—8 mole 7 NaF is given as a function of temperature by-
= 0 _ . ' o o
log Ptorr'-'9°024 5920/T( K) .

Table 1 contains some of the pertinent physical properties for this

mixture at temperatures of interest.,‘Other possibilities'for secondary

coolants, sdEh'as'fluorides,_chlorides and liquid metais,'are discussed

3 l+ 5

elsewhere.® Before our compatibility tests with the‘sodium fluoro-
borate‘mixture, little was known or reported about its corrosive behavior
in the molten state. _

Because of its appreciable vapor.gressure at temperatures of
interest, BF 3 oorrosion is of importance'also. In corrosion experiments6
with gaseous BFj3, it was rapidly attacked by traces of moistnre to give
hydroxyfluoboricsacid (HBF 30H) and HF. Also BF3; and glass reacted at
an' appreciable rate just above 200°C. 'Underrthe conditions of those
experiments, BF3; did not appreciably attack a wide variety of metals or

alloys examined at temperatures up to 200°C.

 

2S Cantor, J. W. Cooke, A S. Dworkin, G D. Robblns, R. E. Thoma,
and G. M. Watson, Physical Properttes of'MbZten Salt Reactor Fuel,
0RNL-TM—2316 (August 1968). ol : , ;

W. R. Grimes, "Molten-Salt Reactor Chemistry," Nch Appl Pechnol.

8: 142 (1970) .

“J. W. Koger and A. P. Litman, Compattbtlzty of Hastelloy N and
Croloy 9M with NaBFy-NaF-KBF, (90-4-6 mole %) Fluoroborate S&Zt
ORNL-TH-2490 (April 1969).

‘53, W. Koger and A. P. Litman, Compatzbzltty of Fused Sodtum
Fluoroborates and BF3 Gas with HasteZZoy N Alloys, ORNL-TM-2978 -

"~ (June 1970).

®F. Hudswell, J. S. Nairn, and K. L. Wilkinson, "Corrosion Experi-
ments with Gaseous Boron Trifluoride,” J. Appl. Chem. 1: 33336 (1951).
 

Table 1. Some Pfoperties,of theAMixtfire NaBF,—8 moie % NaF

 

Approximate_meiting_point, °C
. Vapor pressure at 621°C, torr

Density,a g/cm3:
at t°C
‘at 621°C

at 538°C
at 455°C
Viscosifiy,b centipoise; 
at T°K
at 621°C
at 538°C
at 455°C
Heat Capacj.ty:c
., = 0.360 cal g~! °c"!

‘Thermal Conductivity:d

e
O & bl

384 -
252

p = 2.252 — 7.11 x 1071

1.82
1.87

1.93

0.0877 exp(2240/T)

at 621°C:K = 0.0039 W cm~! °c~!
at 538°C:K = 0.0041 W cm™}! °c~!?
at 455°C:K = 0.0043 W em™} °C™!

Latent Heat of Fusion = 31 cal/g

 

#s. Cantor, MSR Program Semtann. Progr. Rept. Aug. 31, 1969,

ORNL-4449, p. 14.

bS. Cantor, MSR Program Semiann. Progr Rept. Aug 31,

A. S. Dworkin, MSR Program Semiann. Progr. Rept. Feb 29 1968,

ORNL-4254, p. 168.
d
ORNL-4449, . 92.

J. W. Cooke, MSR Program Semiann. Progr. Rept. Aug 31, 1969,

1969,

1)

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All metal components of the MSRE in contact with molten salt were

made of Hastelloy’ N (formerly called INOR-8). Two decades of corrosion.

testingt’_15 and e)cperiencew""19 with-the MSRE have demonstrated the
excellent compatibility of‘Hastelloy N and graphite with fluoride salts
containing LiF, BeF,, UF,, and Tth. Hastelloy N, perhaps with some
modification of composition, is quite likely to be the primary contain-
ment material for MSBR. Thus, it was of great interest to the molten

salt program to determine ‘the compatibility of the fluoroborate salt

mixture with Hastelloy N and related alloys. Of Special interest is
Vtemperature-gradient mass transfer, which must always be considered

" where corrosion in a heat exchanger is possible.l Corrosion and deposi—

tion processes ‘in flowing nonisothermal systems are interdependent and

:each exerts considerable influence over the extent ‘and characteristics

of the other. Therefore,'a performance analysis of a nonisothermal
system must consider these processes as complementary and equal in sig—
nificance to the overall system behavior. This paper is an up-to-date,

open-ended report on these studies.

 

Hastelloy N is the trade name of Cabot Corporation for a nickel—

base alloy containing 16% Mo, 7% Cr, 5% Fe, and 0.057% C

L. S. Richardson, D. C. Vreeland, and W. D. Manly, Cbrrosron by
Molten Fluorides: Interim Report for prtember 1962, ORNL -1491
(April 20 1953) ,

G. M Adamson, R. S. Crouse, and W D ‘Manly, Interzm Report on
Corrosion by Alkali-Metal Fluorides: Wbrk_to Muy 1, _1953 0RNL-2337
(March 20 1959). . _

G. M. Adamson, R. S. Crouse, and W. -D. Manly, Interzm Report on
Corroszon by Ztrcontum-Base Fluorzdes 0RNL-2338 (Jan. 3, 1961)

. 'y, B. Cottrell, T. E. Crabtree, A. L. David, and W. G. Piper, -
Disassembly and Postoperative Examination of the Atrcraft Reactor
Emperzment 0RNL—1868 (April 2, 1958) . _

124, . Manly, G. M Adamson, Jr., J. H. Coobs, J. H. DeVan,
D. A. Douglas, E. E. Hoffman, and P. Patriarca, Aireraft Reactor
EwpertmenteMEtaZZurgtcaZ Aspects, ORNL-2349, pp. 2-24 (Dec. .20, 1957).

134. b. Manly, J. H. Coobs, J H. DeVan, D. A. Douglas, H. Inouye,
P. Patriarca, T. K. Roche, and J. L. Scott, '"Metallurgical Problems in
Molten Fluoride Systems, Progr. Nucl Energy Ser., IV 2: 164-79 (1960).
 

 

EXPERIMENTAL PROCEDURE

Equipment

In corrosion‘studies the thermal convection loop represents an
intermediate stage of sophistication and complexity between simple
capsule tests and a fulléscale engineering pump loop experiment. It
is particularly suited to small-scale tests that involve temperature_
gradient mass transfer. The flow of the liquid is caused by its varia-
tion in density with temperature. The development of a modified thermal
convection loop has permitted important strides in ohtaining basic'cOrro—
sion information; The thermal convection loop used in this work and
shown in Fig. 2 permits unrestricted access to specimens and salt at
any time without significantly disturbing 1oop operation or introducing'
air contamination. Access is provided by'twin ball valve arrangements
atop both the hot and cold legs of the loops. The molten salt sampling

r'device illustrated in Fig. 3 was used in our thermal loops and can also

 

W. D. Manly, J. W. Allen, W. H. Cook, J. H. DeVan, D. A. Douglas,
H. Inouye, D. H. Jansen, P. Patriarca, T. K. Roche, G. M. Slaughter,
A. Taboada, and G. M. Tolson, Fluid Fuel Reactors, pp. 595604,
James A. Lane, H. G. MacPherson and F. Maslan, eds., Addison Wesley,
Reading, Pa., 1958,

J. H. DeVan and R. B. Evans III, "Radiotracer Techniques in the
Study of Corrosion by Molten Fluorides,' pp. 557—79 in Conférence on
Corrosion of Reactor Materials, June 4-8, 1962, Proceedings Vol. II,
Internat10na1 Atomic Energy Agency, Vienna, 1962,

®H. E. McCoy, 4n Evaluation of the Molten-Salt Reactor Emperzment
HusteZZoy N Surveillance Specimens — First Group, ORNL-TM-1997 |
' (November 1967).

"H. E. McCoy, An Evaluation of the Molten-Salt Reactor Emperzment
Hastelloy N Surveillance Specimens —-Second Group, ORNL-TM-2359
(February 1969).

1%y, E. McCoy, An Evaluatton of the MbZten-SaZt Reactor Empertment
Hastelloy N Surveillance Specimens — Third Group, 0RNL-TM—2647
(January 1970) §

H. E. McCoy, An Evaluation of the Molten-Salt Reactor Empérzment'
Hastelloy N Surveillance Speczmens — Fourth Group, ORNL~TM-3063
(March 1971).

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*)
 

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ORNL-DWG 68-3987

STANDPIPE

=y}
s

BALL VALVES

!g! _ TWIN
I

   

-
©
3

/

  
  

| |

1N

' P coamsmere

| |~ HEATERS 1
‘ 30in. '

    
 

INSULATION

CORROSION
SPECIMENS-

 

 

 

 

———

SAMPLER

, . FREEZE :
N VALVES
" OFLUSH o
TANK - , DUMP
AN ' TANK

" Fig. 2. MSRP Natural Circulation Loop and Salt Sampler.

 

 

 
 

 

ORNL-DWG 68-2796

. SWAGE LOCK
FITTING

BUCKET -

v
CONNECTION

 

 

 

SAMPLE ' '
REMOVAL SECTION : O

 

 

 

 

 

VACUUM "
CONNECTION -

PERMANENT 7
LOOP ASSEMBLY~

 

 

 

 

 

BALL VALVE

LFB PUMP

 

MAXIMUM LIQUID LEVEL
MINIMUM LIQUID LEVEL

Fig. 3. Molten Salt Sampling Device.

be used in pumped 1oofis. Typical thermal convection loops in operation

are shown in Fig. 4.

A forced convection loop was also used to evaluate the fluoroborate
salt mixture and will be described later.

Salt Preparation

The salt for these tests was processed by the Fluoride Processing
Group of the Reactor Chemistry Division. Very pure (> 99.9%7) starting
materials were evacuated to about 380 torr, heated to 150°C in a vessel

-lined with nickel, and then held for about 15 hr under these qonditions.
 

 

PHOTO 75125A

 

 

¢

FLUSH TANK

 

A

Fig.

Thermal Convection"Ldops in Operation.
 

10

If the rise in pressure was not excessive (indicating no volatile impur-

ities), the salt was heated to 500°C while still'Under vacuum and agitated

- for a few hours with bubbling helium. It was then transferred_to the £fill

vessel and from it forced into the loops with‘helium pressure. In the

.case of the pumped loop the salt was transferred initially into a dump

tank and then into the loop tubing.-

The hot portion of each thermal convection loop was heated by sets

of clamshell heaters, with the input power controlled by silicon controlled

rectifiers (SCR units) and-the'temperature‘controlled by a Leeds and

Northrup Speedomax H series 60 type CAT controller.. The loop tempera-

'tures were measured by Chromel-P vs Alumel thermocouples spot welded to

the out31de of the tubing, covered first by quartz tape and then by stain—
less steel shim stock.

‘Each loop was degreased with ethyl alcohol heated to 150°C under

- vacuum to remove moisture, and leak checked before filllng with salt.

All lines from the fill tamk to the loop that were exposed to the fluo-
roborate salt were of the same material as the loop and were cleaned and
tested in the same manner as the loop. All temporary line connections
were made with stainless steel compression fittings. ‘

- Each loop was filled by heating it, the salt pot, and all connecting
lines to at least 530°C and applying helium pressure to the salt pot to

force the salt into the loop. Air was continuously blown on the freeze

valves leading to the dump and flush tanks to provide a positive salt

seal. Tubular electric heaters controlled by variable autotransformers

_heated the cold—leg portions. Once the loop was filled the heaters were

turned off, “and the proper temperature difference was obtained by remov1ng

some insulation to. :expose portions of the cold leg to ambient air.

The -first charge of salt was circulated under a small temperature

difference (20°C) for>24_hr and dumped. This flush removed surface

oxides and-other possible impurities. The loops were then refilled with
new salt and put into operation. A helium cover gas_under_slight posi-

tive pressure (about 5 psig) was maintained over the salt in the loops

~ during operation.

.
 

 

 

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"

)

ffand results ‘of all the tests are reviewed.

11
~ Analyses

o Severallmethodsican.be used;to;obtain;quantitative data froma
temperature-gradient mass transfer’experiment relating to the kinetics
of mass transfer, the . thermodynamics.of the process, or both. The most
obvious and easiest to obtain are weight change measurements on specimens
and liquid analyses. The weight changes allow the calculation of mass
transfer rates in both legs, We may also analyze specimens and loop
tubing with a microprobe to determine composition gradients, x-ray
fluorescence to determine the surface composition, and spectrochemistry
to determine overall composition. Standard metallographic examination
is also helpful to determine the extent of attack or void formation in

the hot leg or the amount of deposit in the cold leg.

" OPERATION AND RESULTS

. The results of tests of the fluoroborate salt mixture in two ther-

mal convection loops and in a ‘series of capsule tests have already been"

 reported.?%s2} In addition, four other thermal convection‘loops and one

pumped“loop‘have'been'operated. Thése latter tests are described below,

-4

Thermal Convection Loops NCL-13, NCL—13A and NCL-14

Thermal convection loops NCL—13 rand NCL—14 (constructed of standard
Hastelloy,N)_were started at the same time under the identical conditions

of 605°C maximum_temperature and a}temperature~difference of 145°C. .. .

| Theseioperatingrconditions,were,thosefipropoSed for the coolant circuit
~of the_MSRE.A,puring circulation each loop (0.75-in.-OD X 0.072-in.-wall

tubing) contained about 2.7 kg of salt that contacted 1740 cm’® (270, in.?)

 

2°J W, Koger and A. P. Litman, Compatzbzlzty of Hastelloy N and

iCroZoy M with NaBFu-NuF-KBFg (90-4 6 moZe %) Fluoroborate saZt

ORNL-TM-2490 (April 1969).

213, W. Koger'and A. P.: Litman, Compatzbtltty of‘Fused Sodium
Fluoroborates and BF3 Gas. with Hastelloy N Alloys, ORNL-TM-2978
(June 1970) ‘
 

12

of surface and traveled 254 cm around the harp. Typical flow under the

above conditions was 7 ft/min. Loop NCL-13 contained standard Hastelloy N

- specimens, while loop NCL-14 had titanium-modified Hastelloy N suspended

in the salt stream. The compositions of these alloys are given in

- Table 2. The modified alloy is being‘considered because of its superior

‘mechanical properties.under radiation’'at elevated temperature.

‘Table 2. Composition of Hastelloy N Alloys

 

Content, wt %

Allo , ‘ T
y Ni Mo Cr Fe  Si Mn  Ti

 

Standard Hastelloy N L 70 17.2 7.4 4,5 .0,6 0.54' 0.02

Titanium-modified Hastelloy N 78 13.6 7.3 < 0.1 < 0.01 0.14 0.5

 

The weight changes measured for the,specimens‘in NCL-13 and NCL—14
showed an increase in mass transfer rate between 3500 and 4300 hr of '
egpcsute to the salt (Fig. 5). This was accompanied by perturbations in
salt composition. Analyses of the circulating salts from these loops
showed that the oxide content increased to above 2000 ppm (from initially
less than 1000 ppm), and the nickel and molybdenum contents exceeded
100 ppm (from below 25 ppm). Also, the chromium and iron contents
increased "nbrmally"-with time, as shown in Fig. 6.

After 4700 hr of operation, the helium gas regulator that provided -
the ovérpressurerto NCL-13 failed and caused a surge of gas to the loop,
stopping the salt flow. Circulation of the salt could'notfbe resumed
until a vacuum was pulled on the loop, which we believe removed a gas
pocket. Shortly after circulation was restored, an electrical short,
which eventually burned out a heater, occurred and heated the bottom of
the hot 1eg to 870°C (1606°F). This disrupted the flow and caused a
loss of BF3; from the loop, which changed the salt COmposition and plugged
all.thelgas 1ines. The loop was drained of alllsqlt,'and plugged lines
were replaced or unplugged. Other necessary repairs to the loop were
made, and the loop was filled with new salt. The loop was then designafed
as NCL-13A. | I

P
 

 

~ ORNL-DWG 68-12031A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6
COLDEST SPECIMENS | - ) . - |
o * ORNL-DWG 68-13M7
4 /)/A 460 °C W - a0 —— “owe_
A . o Fe |
2 ] A : ] '/ _ . ‘
,/‘ T -
1 a _e—o—""" | :
§ 0G5, - T T | 1300
~N \A\ T~—o o , : Cr
o ; A e :
81 -2 < \'\ = g.
z RN -
- 4 ,. . — ' 200 ,
X : o
Ty | : ‘ A\ | é -
= -6 , - HOTTEST SPECIMENS £ |
N 607°C \\ o ////
-8 |— © TITANIUM MODIFIED HASTELLOY ; 100
N-NCL~-44 o
A STANDARD HASTELLOY = \
—40 (—  N-NCL-13 | : -
10 o
-2 ‘ 0 ' 3
o 1 2 3 4 5 6 (x10%) o 2 - 3 4 5 6 (x103)
- N “TIME (hr) ‘ | ' TIME (hr)
Fig. 5. Weight ChangerVérSus Time for Standard Fig. 6. Average Concentrations of Iron and
and Titanium-Modified Hastelloy N Specimens in Chromium in the Fluoroborate Salt in NCL-13 and
. NCL-13 and NCL-14 Exposed to Fluoroborate Salt at = NCL-14. -

Various Temperatures.

 

€T

 
 

 

 

14

After new fluoroborate salt was added to loop NCL-13A, circulation
could not be achieved. We coficluded that previous‘overheatiflggbf the
loop had caused BF3 to evolve and had made the salt much richer in NaF.
This in turn caused the higher melting NaF to segregate in the loop. To
bring the composition back to normal, we added BF; gas to the melt through
the surge tank. The initial amounts of gas were dissolved in the salt,
and additions continued until a slight overpressure built up over the

salt. (No devices were available to measure the amount of BF; gas added.)

- Circulation of the salt was then attemptéd and was successful. Only a

small temperature difference (55°C) was obtained at first. Further addi-
tions of BF3 as before lowered the temperature of the salt in the cold

leg and increased the temperaturefdifference to about 125°C. Since more

~BF3 did not cause immediate changes and did not appear to dissolve in

the salt, the additions of gés were stopped. . The loop has since operated

satisfactorily.

Loops NCL-13A and NCL-14 have now operated for two and three years,

fréspectively. Figure 7 gives the weight changes of specimens at various

 temperatures as functions of operating time. The overall corrosion rates

at the maximum temperature, 605°C, are 0.5 and 0,7 mil/year for loops
NCL-13A and NCL-14, respectively. The attack is general, and actual .

measurements of specimefls show changes in thickness agreeihg with those
_calculated from weight changes. We will now discuss the circumstances

‘that have affected the mass transfer conditions.

The air leaks in loops NCL-13A and NCL-14 occurred at various mechan-
ical joints in the system, such éé ball valves, level probes,'and pressure
gages, all located in the cover gas spaée above the circulating salt. We
had gaified extensive experience with similar mechanical joints before
testing fluoroborate salt mixtures and had encountered_no_leakage problems.
Therefore, the frequency of leaks in.theée two fluoroborate loops strongly
suggests that the BFg atmosphere in these.tests'has déteriorated our

mechanical seals. We are currently investigating the chemical nature of

‘the BF3 to determine ény posSible contaminants which might have caused

such deterioration. Despite the known broblems'offiair leakage, these
loops have opefated for over two years with the fluoroborate‘salt, and

the overall corrosion rate is within acceptable limits.

(\
 

N

NY)

)

-}

WEIGHT CHANGE (mg/cm?)

15

ORNL-DWG 69-4763B

502°C

~522
532
547

557

572

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0 2000 4000 ‘6000 8000 10,000 {2,000 14,000 16,000
TIME (hr)
: ORNL-DWG 69-12625RA
20
. 465°C
10 00 470
. 480
OO I
—- \ o—e-s 520
o
€ “v\‘\\mhi‘\\‘\ ' |
L , .
£ -10 N o ' No——0-0 540
W :
=
g
I - . B
e -2 —— g3 565
o \ '
w
Lz
=30
580
~40 —
_ \‘f-fi* 607
-50 : : ' |
O 5000 - 0,000 , 15,000 © 20,000 25,000 30,000

TIME OF OPERATION {hr)

18,000

Fig. 7. Weight Change Versus Time for Standard Hastelloy N Specimens'
in NCL-13A and Titanium-Modified Hastelloy N Specimens in NCL-14 Exposed
to Fluoroborate Salt at Various Temperatures.
 

 

 

 

16

We also used loop NCL-14 to determine the corrosion properties. of
pure Cr, Fe, Mo, and Nickel 280 (commercially pure nickel with Al,03
added as a grain refiner) at 600°C‘ifi”the‘fluoroborate mixture., As a
consequence of the air leaks discussed above, the oxide éontent of the
salt in loop NCL-14 was at an!abnormaliy high 1éve1 and indicated fairly
strongly oxidizing condifions. We.initialiyrexamined the pure metal
spécimens after a 234-hr exposure. On removal, we found thét'the chro-
mium and iron specimens had completely'disintegfated. 'The molybdenum
specimen showed no weight change, and the nickel specimen showed a
slight weight gain. The cHromium'and iron specimens’were-replaced, and
a second experiment was conducted for 23 hr. Again, the molybdenum
specimen did not change weight, and the nickel specimen showed a slight
weight gain. Very little of the chromium specimen remained, afid the
weight loss of the iron specimen was 40 mg/cm2 (70_fiils/year). Close
‘examination of the chromium épecimen showed it to be coated with~é black
reaction layer about 0.015 in. thick. Laser spectroscopy of the outer
surface of the black material showed iron and'bqron as major conStituents
and sodium and chromium as minor constituents. However, x-ray diffrac-
tion of the bulk coating showed it to be over 90% NaBF,. This blgck
coating has been observed on Hastelloy N specimens eprsed to impure
NaBF4,—8 mole % NaF in the past'22~but in these cases it was too thin or
too adherent to remove and identify. | ‘

Attack of Hastelloy N by impure fluoroborate salt tends to be non-
Selectlve, that is, all components of Hastelloy N suffer oxidation, and
surfaées of the alloy appear to recede uniformly. However, in this
experiment with pure metals, only the chromium and iron were noticeably
attacked, the chromium more than iron. We cbnclude, therefore, that
the pure chromium and iron specimens in this system inhibited the attack
of the nickel and molybdenum specimens., The inhibition apparently is
much less when chromium and iron are present as alloying additions in
Hastelloy N, presumably because their chemical activity in the alloy.is
too low or the rate at which. they can enter the salt is too slow,

being limited by solid-state,diffusion\in.the alloj. When the

 

- 22J W. Koger and A. P. Litman, MSR Program Semfdnn. Progr. Rept.
Feb. 29, 1968, ORNL-4254, p. 225. T
»

N

)

17

oxidation pbtential of the salt is reduced however,,as would be the
case in a purer fluoroborate salt attack of Hastelloy N becomes more
: selective, and the buffering effect of chromium may become more impor--
'tant. In loop tests of nickel-molybdenum alloys containing Fe, Nb, or

- W, DeVan observed23 that the. concentration of these elements in FLINAKZ“

salt was lowered by the presence of chromium in the alloy. These obser-

- vations suggest that the incorporation of an active metal such as chro-

‘mium would effectively inhibit attack in a Hastelloy N—fluoroborate

circuit. _ ‘

Figure 8 shows optical'and scanning electron micrographs of speci-
mens from the hot and cold leg of NCL-13. These specimens were exposed
to NaBF,—8 mole % NaF for 4180 hr at 580 and 476°C. The hot-leg specimen
lost -3.3 mg/cm?, and the cold-leg specimen_gained-3.2 mg/em?. The sur-
face of the hot-leg specimen, as seen in Fig. 8 (left) has been attacked

nonuniformly and is heavily contoured. Figure 8'(right) shows that the
;,cold-leg specimen is coated with a discontinuous depoSit which is essen-

5tia11y analyzed as Hastelloy N slightly enriched in nickel and molybdenum.

“The dumped salt from NCL-13 was chemically analyzed and the results
are given in Table 3. The Cr, Fe, Ni Mo, O, and H»0 contents of the

‘salt all showed large increases., Increases in the nickel and molybdenum

_content of the salt often indicate the onset of stronger oxidizing con-

ditions, usually due torincreased water or oxygen content. Thus.the

most obvious explanation of the-composition changes'and'increased weight

changes was contamination by water, which produced HF, which-in turn

‘ attacked all the constituents of the COntainer material. This was sub-

stantiated by the discovery of a faulty helium line common to both loops.

This line admitted air.

Loop NCL—14 initially operated about 3500 hr with the oxide content
in the salt between 500 and 1000 ppm. During that period the weight

, changes were small and- predictable as a function of time. Wet air then

»

237, R, DeVan, Efféat of.AZZaycng Addttzons on Corrosion Behavtor

 

- of Nickel-Molybdenum Alloys in, Fuaed FZuorzde thtures, 0RNL-TM—2021

Vol. I, pp. 3839 (May 1969)
2“Com.position' NaF-LiF-KF-UFu (11 2-45 3- 41 0—2 5 mole %)

 

 
 

 

 

 

 

 

 

 

 

18
Fig. 8. Hastelloy N Specimens from NCL-13 Exposed to NaBF,~8 mole % -
| NaF for 4180 hr. Left: in hot leg at 580°C; weight loss 3.3 mg/cm?.
. Right: in cold leg at 476°C; weight gain 3.2 mg/cm®. Top: optical
micrographs at 500X, etched with glyceria regia. Bottom: scanning
electron micrographs at 3000x (left) and 1000x. | o .
 

"

)

"N

»

)

19

‘Table 3. Chemical Analyses of Fluoroborate Salt in NCL-13

 

. Content, ppm | Content, wt %

 

 

Sample €t ~Fe N Mo Q0 Na B F
Salt before loop .. 19 223 28 <10 459 21.9 9.51 68.2
operation e : _ - . |

Dumped salt circulated 348 650 95 125 3000 21.7 9.04 67.0
for 4700 hr ' ’ : e .

 

camewin contact with the salt through the defective gas line, andrthe

water content and corrosion rate increased dramatically. During the

~next 5000 hr the oxide content of the salt decreased, and the corrosion

rate again decreased as a function of time. The maximum rate of weight
loss:during this 5000-hr period was about 1.5 X 1072 mg cm™ 2 hr™!

(0.65 mil/year). This corrosion rate is double that measured during the
earlier operating period. , : | '

. The concentration offiimpurities in the salt in loop NCL-14 is given
as a'fonction of.operating'time in Fig. 9. The.second.change_in corrosion
rate, at some tine between 11,000 and 13,000 hr, was due to a.leaking
standpipe ball valve. .Ihis last intake offimpnrities illustrates a con-

tinuing problem of detection (other than the obvious after-the-fact

increase of impurities and corrosion rate), since the loop overpressure
did not change significantly. Apparently moisture from the air effuses
into the relatively moisture-free athoSPhere,of'the'loop (equalization =~

of partialfpressure) without the need for a'gradient-in overall pressure.

In this last corrosion rate increase period the analyzed oxide content of

the salt increased from 1300 to 4500 ppm.l The_ball valve was repaired,

and the loop continued to Operate.‘

As we have shown, abrupt increases in the slope of the weight change

curves (increased rates of gain and loss) indicated changes in the oxida-

tion potential of the salt, which were caused by inanvertent air leaks

into the system.r These slope changes were accompanied by increases in

:Cr,rNi Fe, and Mo contents in the salt, which would indicate general
'attack of the alloy.' Under conditions where.external impurities did not
 

 

20

ORNL-DWG 68—11768AR

3000
2000

1000

 

450
400

350

250

CONCENTRATION (ppm)

200
150
100

50

 

0 2000 4000 6000 - 8000 _"10,000 12,000 14,000
' TIME (hr) ' ' -

Fig. 9. Variation of Impurity Contents with Time in NaBFw—B mole 7
NaF Thermal Convection Loop NCL-14.

enter the loops, chromium’ was the only element removed from the alloy,

and corrosion rates during these periods fell to as low as 0.08 mil/year,

- Loop NCL-17

Loop NCL-17, constructed of commercial Hasteiloy N with‘removable
specimens in each leg, is being operated to determine the efiect of steam
inleakage in a fluoroborate salt—Hastelloy N circuit The maximum tem-
perature is 607°C with a temperature difference of 107°C. ThlS experiment
has run for over 10 000 hr and is continuing in order to provide informa-
tion on the immediate and long—range corrosion of the system after steam
injection. The loop was operated for 1050 hr, the specimens were removed

and welghed and steam was forced into the flowing salt system through a
 

 

.

5

21

16-mil hole in a. closed 1/4—in.-Hastelloy N tnbe, simulating a leaking
heat exchanger.‘ Steam was forced.into the system until the pressure of
the cover gas began to increase, thus indicating that Nno more steam was
soluble in the salt. Figure 10 shows thelweight changes and temperatures
for the specimens in NCL-17. As usual, the changes depend on temperature,
and the rates decrease mith time after an initialrapid rate. The corro-
sion rate appears to have stabilized, and nd”plugging conditions have
been noted. Figure 11 shows the specimens from the loop 424 hr after
exposure to steam. The specimens with the etched‘appearance have lost
weight, and those that'are darkened'have gained;meight. |

For the last 3500 hr of operation, as shown in Table 4, ths concen-
trations of impurity ‘constituents in the salt (Fe, Cr, Ni, Mo, and 0)
remained relatively_constant. The present chromium level is abou:

320 ppm. The results of this test show that the Hastelloy N system

ORNL-DWG 70-4936A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

30 ‘ l
L STEAM .
20 lNJECTION : ‘
- " ¢ 527-493°C
: l .&."__-. .- :
10 - ‘ :
' ' fi’” ~ ~ | —tosaac|
g o 11 -;O-—_‘l . - ' | ‘
on e ——
E O e B
& -10 t‘\\.- - IR | ~~Jeseoc|
Z R T~ | Tesmec).
2—20 > -l \\ -
8. \\2:..\0 | \-"‘ . i
2 \.\ . \ |
-30 . . ' e 582°C
o ' o e ~ To593°C
- ~40 7 ‘ \\\\\\
| T : Ye607°C
- =50.

 

 

o '4ooo 2ooo 3000 4000 5000 6000 7000 8000 9000
' ‘ TIME OF opznmom (hr)

Fig. 10 Weight Change Versus Time for Hastelloy N Specimens in
NCL-17 Exposed to NaBFu—fi mole % NaF at Various Temperatures.

 
 

 

 

 

 

 

 

 

Fig. 11. Hastelloy N Specimens from NCL-17 Exposed to NaBF,—8 mole % :
, NaF. Specimens were exposed to salt for 1050 hr before steam injection
? - and for 424 hr after steam injection. The specimens on the left were in
the hot leg, and those on the right were in the cold leg. Specimens with
the etched appearance lost weight, and the darkened specimens gained , ;"
weight. _ .
 

 

n

»n

a)

+)

23

Iahle 4.A$Copgentration~of~lmpuritigg,in,NCL-l7 Salt

 

g id ik
Tt ;'.'.l.'i

rSalt Circulation "-Concentration”in'Salt,;ppm

 

 

Time f-:f L —————— ,
“(hr) Crr TFe = Ni - Mo 0
48 56 211 - 8 5

1179 82 204 <5 <5 630
11180 . Added steam

1182 109 222 51 71 1400
1468 243 303 7% 56 1900
1973 259 31 - 9 8

2880 320 375 11 12 3600
4324 310 356 15 3 3600

6239 317 362 11 < 2 3500

 

containing fluoroborate salt can tolerate inleakage of steam and, once
the leak is repaired can continue to operate without exten31ve damage
even if the salt is not repurified . '

Figure 12 shows micrographs of the specimens in the hottest and-
coldest positions. ‘The attack seen in the hot leg is general as
expected;for'impurityficontrolled mass transfer processes. Micrometer
measurenents showla loss“of'about'l'mil'from specimen surfaces at the
makimum’temperature position.a In Fig; 12(b) we see a large amount (about
2 mils) of deposited material. This material has been analyzed with
an electron beam miCroprobe, and Table 5 gives the results for the sub-

strate and the deposit. These results show that very little chromium

has depOsited.' The nickel and molybdenum-concentrations'reached maxima

-of 74 and 56 ppm, respectively, 288 hr after the steam addition, decreased

and quickly leveled off to about 10 .ppm each. From the microprobe evi-
dence much of the nickel and molybdenum apparently deposited in the cold
leg, although'not in the Same ratio'aslthey exist'in;Hastellofi N. . One
may conclude that under highly oxidizing conditions, during which large

‘weight changes occur (NCL~14 during ‘the air 1eak and NCL-17 after steam

inleakage), most of the changes are attributable to the movement of the
 

} Y-98296

 

 

Y-98297

 

 

 

Fig. 12. Microstructure of Standard Hastelloy. N Exposed to
NaBF,—8 mole % NaF in NCL-17 for 3942 hr. Steam was injected into salt
after 1054 hr. (a) 607°C, weight loss 26.8 mg/cm®, as polished;

) (b) 495°C, weight gain 14.8 mg/cm?, as polished. A relatively spongy
surface layer on this sample is about 2 mils thick. 500x, ' '

 
 

1

")

25

Table 5. Microprdhe Analysis of Specimen in Coldest Position. -
(493 ’C) in Loop -NCL-17 Exposed to Fluoroborate Salt for
fi:' 4000 hr* Steam Injected into Salt’ 1000 hr , .
AR after Beginning of - Run 1

 

L

Composition,afwt %

 

Elemeht S

 

E Substrate | Deposit
oML ST 73 34.8
Mo L 143 30.6
e 6.8 . <0.5
Fe 3.8 | 2.9
P - o a0P
Undetermined — N30

 

8Corrected for absorption, secondary fluorescence and
atomic number effects.

bThin layer close to sample surface.

'normally stable nickel and molybdenum. Any‘increased chromium removal

from the hot leg is noted only as an increase in the chromium concentra-

tion in the salt. The phosphorus found in the deposit was apparently
from a phosphate impurity in the steam._- ‘ |

Figure 13 ‘shows optical and scanning electron micrographs of speci-
mens from the hot and ¢old leg of NCL—17.- The specimens at 593 and 500°C
were exposed to the salt for 1050 hr, and then steam was injected
into the loop. The specimens then remained in the loop for an additional
9000 hr. The total weight loss of the hot—leg specimen was 40 9 mg/em?,

‘and the weight gain of the cold leg was 16.0 mg/cm . 1In Fig. 13 (top,

left) we see that the surface has ‘receded uniformly, although a Widmanstatten

" precipitate was left in relief [Fig..13 (lower 18ft)]. Figure 13 (top right)
__shows a dark deposit on the cold-leg Specimen, and the deposit exhibits a
—layered structure ‘when viewed from above [Fig. 13- (lower right)].

 
 

 

 

. - ) -

Fig. 13. Hastelloy N Specimens from NCL-17 Exposed to NaBF,—8 mole %

: NaF for 10,050 hr (1050 hr before Steam Addition). Left: in hot leg at |
| 593°C; wei§ht loss 40.9 mg/cm?. Right: in cold leg at 500°C; weight gain -
'16.0 mg/cm“. Top: optical micrographs at 500%, etched with glyceria regia

L Bottom: scanning electron micrographs at 1000x,

 

   

 
 

 

 

27
Loop NCL-20

In loop NCL-20, d%figiructed of standar&&figstelloy N with removable
specimens in each leg, the fluoroborate coolant salt has circulated for
12,000 hr. The selt used was especially pure (see Table 6). The loop
is being eperated at temperatufe conditions very near those proposed for
the meximum_(681°c) and minimum (387°C) ealt-metal temperature (primary
heat exchanger and steam generetor,.respectively) of the MSBR secondary
circuit. Fofced air eoolihg (as opposed to ambient air cooling used on
other thermal—convection'leops) is used on the lower half of the cold |
leg,,and the practical opefating temperatures obtained were 687°C maxi-
mum and about 437°C minimum: a temperature difference of 250°C. This
difference is thought to be the largest obtained at ORNL in a molten-salt
thermal-convection loop, and the maximum temperature is the highest for
fluoroborate salt in a loop. Table 6 gives the corrosion results for
this experiment. The corrosion rates are the lowest yet attained with a
fluoroborate mixture and confirm the importance of salt purity on com-
patibility with structurel_materials.‘ The low concentrations of chromium
and oxide in the salt are'eiso indicative'of low corrosion rates.
Figure 14 shows weight changes of the specimens as functions of time and

temperature.

Table 6. Weight‘CHenge of Specifien at 687°C and Impurity
~ Content of Salt of NCL-20

 

 

 

?;?; | _W%ig?zmggss .;,Cizzzzzfiztgige. Chemical Analysis, ppm
| P (mils/year)™ Cr . Fe 0

0 T S 58 . 227 - . 500
. 626 ,_0.3 - . _.0.19 90 198 . - 550
1460 0.7, ~ 0.18 99 110 590
2586 1.0 0.15 109 104 470
3784 1.7 017 - 131 91 670

 

aAssuming uniform loss.
 

 

28

ORNL-DWG 71-8059

 

S

 

WEIGHT CHANGE (mg /cm?)
O
L ,,,fi -
g v

 

460°C COLDEST
SPECIMENS _|

 

 

 

660°C
685°C HOTTEST
SPECIMENS

. | | | | |
O 2000 4000 6000 8000 10,000 12,000 14,000 16,000
TIME OF OPERATION (hr)

Fig.'14 Weight Changes of Removable Standard Hastelloy N Specimens
in Flowing Sodium Fluoroborate (NCL-20).

 

/

/
[/ 1]

 

 

 

 

 

 

 

 

 

 

L
3

FCL-1 Pump Loop

The MSR-FCL-1 forced-circulation loop operated-for over 11,000 hr
(7 runs) to test the compatibility of standard Haételloy N with NaBF,—8 mole %
'NaF coolant salt at temperatures and flow rates similar to those that
‘existed in the MSRE coolant circuit. Salt velocity in the 1/2-in.-0D
X 0.042-in,-wall Hastelloy N loop waé nominally 10 fps and was limited by
the use of an available pump. The salt in the loop was heated by electri-
cal resistance and cooled by a finned, air-cooled heat exchanger. A
- schematic of the 100p is given in Fig. 15. Figure 16 is a typical tem-
perature profile of the system; this profile was based on a computer plot
of the temperature information from the test loop and on heat transfer
- calculations that estimaterthe'inner wall temperatures. The maximum and
minimum salf temperatures of the loop are 588 and 510°C. Hastelloy N
corrosion test specimens were exposed to the circuiating salt at three
temperatures, 510,.555, and 588°C. A typical set of specimens is shown
schematically in Fig. 17. One of the main drawbacks to this 1oop was
that the salt had to be drained from the loop and the loop tubing had to
 

 

-

.

"

")

GAS
LINES

TO
PUMP
OPERATING

PRESSURE 18 psio

LFB PUMP

Fig.
Corrosion

L | 950 °F

29

ORNL-DWG 6B-2T97R

SALT
SAMPLE

LINE AJUSTO SPEDE
5 hp MOTOR

  
 
 
   
  

 

et e ———— =
OIL LINES
TO PUMP

 

 

 

 

METALLURGICAL
SPECIMEN
‘ {030 *F

     
 

RESISTANCE HEATED SECTION.

METALLURGICAL
SPECIMEN

BLOWER

ICA
lsnggé:'figssc - HEATER LUG (TYPICAL)

   

 

—
/‘J RESISTANCE HEATED SECTION —

950 *F
- FREEZE i

VALVE — : ' '
})—.- AIR REYNOLD'S NUMBER

~3400
l SUMP I
C

VELOCITY ~7 ft/sec
Fig. 15. Schematic of MSR-FCL-1.

 

FLOW RATE - 3 gpm

‘ ORNL-DWG £%-253%
| FIRST

   

 
   
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

| - | SECOND | ' FINNED l
|HEATER | | HEATER |_ COOLING COIL 10
1 - - - i
| N P PUMP
) i ) METAL COUPON LOCATION
} ‘ :— LIQUID TEMPERATURE
1300 3 { === INNER WALL TEMPERATURE
L]
1200 +— ?": 1
. ; !
100 ! e
& b1l ™ .
= | l ] “‘-—_-h:‘fl-—
% 900 [— +—rt 4
BT
F 800 —t Lo i
0
700 |— }
o o
€00 i i |- .
.0 10 .20 0 % - &0 50 .60

~ LENGTH (f)

16.,vTemperature Profileiof.Molten-Salt Foréed—Coqyéction
Loop, MSR-FCL-1, at Typical Operating Conditions.
 

30

ORNL-DWG €8-4294

 

Fig. 17. Specimen in Molten Salt Corrosion Forded—ConVection

Loop.

be cut to remové the specimens. This, of course, necessitated welding to
replace the specimens and resulted in a fairly long turnaround time |
between runs. | | ‘ |

After 659 hr of shakedown operation under various conditions and
the initial 2082 hr of operation at design conditions a bearing failed
in the LFB centrifugal pump. The pump rotary element was removed, and
a new bearing was installed. To take advafitage'of this unscheduled
“downtime, portions of the loop piping and the corrosion specimens were:
removed for metallographic ekamination and weight change measurefients.-
This first operating period was designated run 1. It was then decided:
that the loop would be shut down each 2000 hr for pump-bearing replace-
ment and specimen.examination. , |

The 6perating times and corrosion rates'fo:_the first five runs are
summarized in Table 7, and Fig. 18 shofis the weight changes of specimens
as measured after each run. The details of each run willlbe discussed
in the following pages.‘ It is nbted‘that the mass transfer rates were
higher in the pump loop than in thermal-convection loops circulating | -

salt of éqfial purity. Thus,_a‘velocity effect is suspected., o | \ )
 

 

wh

"

31

Table 7. Average Weight Change of Specimens in MSR-FCL-1

 

 

 

 

Total Salt | " Average Specimen’weigfit Corrosion Rate at
R Exposure Time, hr Change at Indicated - 588°C Assuming
un Tem erature mg/cm? Uniform Loss,? mils/year
Design Isothermal P &/ ch 2 Y
Conditions Conditions 510°C 555°C 588°C Overall Incremental
1 2082 - 659  +1.1  -2.7  -11.1 1.6 1.6
2 4088 667 +2.5  —4,2  -17.3 1.4 1.2
3 6097 . - 667 +3.1 5.2 —21.0 1.2 0.7
4 8573 1018 +2.1  -5.8  -23.1 0.9 0.3
5 8995 1068 - +2.5 —7.9 —31.9 1.2 . 7.2

 

aBased‘on total salt exposure time.

ORNL-DWG 7{-4935R

20

10

    
   

950°F
I

-10 1030°F

~-20

WEIGHT CHANGES (mg/cm?)-

-30

1090°F

o . 200 4ooo . 6000 8000 10,000 12,000
L - "SALT EXPOSURE TIME (hr) : B

'Fig;'18 Welght Changes of Removable Standard Hastelloy N Specimens
in Flowing Sodium Fluoroborate in Pumped LoopiMSR-FCL-l. A 1-mil/year
corrosion rate’ is included for comparison.,' o : o
 

 

 

32
" Runs 1 and 2.
Figure 19 shows micrographs of specimens from each location after
_2082 and 4088 hr (run 2) of design operation. In (a) and (b) it is clear
~ that the-attaek at 588°C, the highest'salt temperature,foccurred primarily
'at'the intersections of the grain boundaries with the surface. At 555°C
as the weight changes indicated, much less attack has occurred [(e) and
(d)]1. . For 510°C, the lowest salt temperature, a metallic deposit is
visible [(e) and (f)] along the surface in the as-polished condition.
This deposit occurs under steady-state conditions and was shown by elec-
tron microprobe analysis to have. the approximate composition of Hastelloy N;
no salt-like deposit was observed. Figure 20 shows the surface of the
specimens at 588 and 510°C. Note the smoothness of the surface of-the
specimen that has been attacked and the deposit on the specimen in the
cold region. | .
Samples of loop tubing were periodically removed and exanined metal-
lographically for comparison with the weight-change specimens. The
behavior was similar, as shown in Fig. 21. The attaekdconcentrated at
'the-interseotions of the grain bonndariesiwith-the surface, and deposits
nere-seen in the cold region,' The tuBing had.a much_Smaller grain size . =
than the specimens; from this one might.anticipate‘a_greater corrosion
rate for the tubing. No difference could be seen between tubing exposed
under the heaters'and tubing at the same temperature but not under the
heaters.
| Examination of the pump (Fig. 22) and the pump bowl (Fig. 23) did
not disclose any gross contamination; however, small amounts of a green
materiai were found on the pump bowl at the sait level, After the bowl
was washed with water to remove the salt, very fine, whisker-like, green
crystals were also found in the bottom of the pump bowl (Fig;.Qé).
large piece of mixed green material and white salt (< 1 g total weight)
was analyzed and identified; Table 8 shows the composition.' These ,
results are equivalent to 55 wt %Z NaBFy,, 30 wt % Naachs, and smaller o
amounts of iron, nickeI; and molybdenum fluorides. The presence of the |
green corrosion product and the level of chromium concentration in the | -

‘salt indicate that we reached the saturation concentration of chromium kusj
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

&

"

 

 

Y-95551

   

 

 

Fig. 19, StandardiHastelloy'N SpeCimehs from MSR~-FCL-1, Exposed to

. NaBF ,—8 mole % NaF for the Indicated Times and Temperatures and for an .

Additional 659 to 667 hr at Varying Conditions. (a) 588°C, 2082 hr,
weight loss 12.5 mg/cm?, etched With glyceria regia, 500%x. (b) 588°C,
4088 hr, weight loss 18 mg/cm?.  (¢) 555°C, 2082 hr, weight loss i

3.0 mglcm (d) 555°C, 4088 hr, weight loss 4 mg/cm?. (e) 510°C, 2082 hr,
weight §ain 2. 2 mg/cm , as polished (f) 510°C, 4088 hr, weight gain

4 mg/cm . ‘

 

 

 

 

 
 

91

 

Surface of Standard Hastelloy Spécimens‘Eqused in Loop

Condi-

st 12,5 mg/cm? at
°C.

2

(b) Deposit on specimen that gained 2.2 mg/cm® at 510

ions (and 659 hr at Varying

(a) Smooth surface on specimen that lo

45%.

Fig. 20.
MSR-FCL-1 for 2082 hr at Design Condit

tions).
588°C.

 

 

1 Exposéd to
(a) Tub

ing exposed at

from MSR-FCL-

-

ing

Standard Hastelloy N Loop Tub
% NaF for 2082 hr at Design Cond

exposed at 588°

510°C.

21.

NaBF,—8 mole

Fig

ing

itions. 500x,
egia. (b) Tub

lar

Etched w

c

ith glycer

As polished.
 

 

 

PHOTO 96311

 

at

 

 

 

.Fig. 22, Pump'Impéller from Loop MSR-FCL-1. This impellerloperated
in NaBF,—8 mole % NaF for 659 hr under variable conditions and 4088 hr of
‘normal operation at 510°C.

PHOTO 96313

"

 

 

 

 

 

»)

 

5

; ' ' Fig. 23. Pump Bowl from Loop MSR~FCL-l. The pump operated in
' NaBF4—8 mole Z NaF for 659 hr under variable conditions and 4088 hr of
normal operation at 510°C.

 

 
 

36

 

 

 

Fig. 24. Inside of Pump Bowl from Loop MSR~FCL-1 After Being Washed
with Water to Remove Salt, Showing Crystalline Residue. The pump operated
in NaBF4~8 mole % NaF for 659 hr under variable conditions and 4088 hr of
normal operation at 510°C. B -

Table 8. Chemical Composition of Green Deposit Removéd
from the Pump Bowl After Run 2 '

 

 

Element - Concentration, wt %
Cr | 6.14
Fe - 0.26
Ni j 0.30
Mo - 0.022
Na o 21.6
B - . 4.56

-~ F . ' 57.0

 
 

 

 

o

&

-

37

corrosion product in the pump bowl during the test. No indication of

fluoride deposit or plugging in any other part of the system has been

 

seen.

Run 3

'Figure 25 shows micrographs'of specimens in MSR-FCL-1 after more
‘than 6000 hr salt exposure (3 runs). Uniform attack is seen on the
specimens exposed to salt at 588 and 555°C, with the rougher surface
seen on thé higher temperature samplo. In (c) the mounting material
remqved some of the deposit on the'cold-leg specimen, but portiomns can
still be seen. |

Visualrexamination of the pump and pump bowl after run 3 showed no
obvious corrosion. None'of the green Na3CrF5 oorrosion product that

had been observed previously was present.

Y-97618

[Y-97616

 

(a) - " . (b) (c)
Fig. 25. Standard Hastelloy N Specimens from MSR-FCL-1, Exposed to
NaBFy—8 mole % NaF for 667 hr under Variable Conditions and 6097 hr at
Design Conditions. As polished, 500x. (a) Exposed at 588°C, weight
loss 21.4 mg/em?. (b) Exposed at 555°C, weight loss 4.7 mg/cm
‘(c) Exposed at 510°C weight gain 5.0 mg/cm - The light material away
from the surface of the sample is part of the surface deposit that
became separated from the sample during mounting.
 

 

38

Installation and Testing of Cold Finger During Run 3

During run 3 a cold-finger corrosion product trap, shown in Fig. 26

d2% in sodium fluoroborate

and similar in design to one previously use
test loop PKP-1, was inserted into thefisalt in the pump bowl in an attempt
to induce preferential deposition of corrosion products from the salt |
circulating in the loop. Such deposition had been observed on the cold
finger in loop PKP-1. Gross.deposition'is always possible in a temperature-
gradient system, and we feel that this eold—fingefrdevice is a likely com?
bonent to prevent this problem. o

The cold finger-ueed in MSR-FCL-1 is a closed-end nickel cylinder,

1 3/4 in. long X 3/8 in. OD, with a 0.070-in.-thick wall. It is cooled

by an argon—water mixture injected into it and then discharged to the

atmosphere. The metal wall temperature is measured and recorded by twe
0.020-in.-0D sheathed, ungrounded Chromel vs Alumel thermoeouples inserted
in two 1-1n.—deep axial holes (0.023 in. diam) in the 0. 070—1n.-thick wall
of the cold finger.

 

 25), N. Smith, P. G. Smith, and R. B. Gallaher, MSR Program Semiann.
Progr. Rept. Feb. 28, 1969, 0RNL—4396, p. 102,

 

Fig. 26. Cold-Finger Corrosion Product Trap for MSR-FCL-1.

d
 

 

 

*h

had been foun

39

" Eight tests were ‘made in fihich'the-cold fingerlwas inserted into
the salt in the pump bowl of the loop and cooled to temperatures ranging
from 493 to '140°C, = The duration of the tests ranged from 1 5 to 5.3 hr.
The temperature of the salt in the pump bowl was 510° C.

In contrast to the deposits of material containing Nagchs, which
d?® on a cold'finger'in°PKP-1'loop"at metal wall tempera-
tures of 400, 460, and 477°C no significant deposit of any kind was
seen on the cold finger tests. in loop MSR-FCL-l Even in the three

tests where indicated wall temperatures were below the salt liquidus

‘temperature (385°C), the surface of the cold finger was essentially

clean, as visually obser#ed'fihen‘it7Was withdrawn into a sight glass.

0ccasionally, small patches (1/8 to 1/4 in. across) of white material

‘estimated to be a few mils thick were seen on the surface.

The cold finger was then installed in the PKP- 1 fluoroborate loop
-n an attempt to duplicate the previous cold—finger test results of

that loop. The chromium concentration of the fluoroborate salt in loop

PKP-1 1is about 500 ppm,'while that in MSR-FCL-1 is about 250 ppm. Two
“tests were run in which the indicated cold-finger wall temperature was

" about 150°C (salt'temperature'in"the'pump bowl was 548°C). Test times

were 1-and 4.5 hr. No deposition on the cold'fingerewas seen after

withdrawal into a sight glass. Tests at higher temperatures were not

made because of temperature control difficulties with the cold finger.\

Since the cold finger"preVious1y used in the PKP-1 loop.contained

. grooves on the outside Surface, the lower half of the surface of the

MSR-FCL-1 cold finger was scored with file marks about 0 010 in. deep,
and a third test run ‘at about 150°C lasting 671/2 hr was made in the

 PKP-1 loop.  ‘In this run a deposit was obtained. Generally, the entire

:surface'of the'éold”finger was covered'With‘a'white deposit,NWith‘an

overlay of bright green’ ‘material on the lower half of the cold finger.

The cold finger was allowed to ‘stand overnight under a helium atmosphere,

and by this time the deposit,had begun to separate from the cold-finger
surface. While the cold finger was moved for photographing, the entire

 

+26p, B. Gallaher and A. N. Smith, MSR Program Semiann. Progr. -Rept.

- Aug. 31, 1969, 0RNL—&449, PpP. - 74—75.,;
 

 

40

depositrspalled off, leaving_a'clean metalrsurface. The total deposit
weight was 1.63 g, of which_l;26 g was mostly green material (complete
separation was not possible). Chemical_analysis_of'the green material
disclosed 2.93 wt % Cr, 1.03 wt % Fe, 370 ppm Ni, and < 500 ppm Mo, with
the remainder:Na, B ~and F, Stoichiometric calculations indicated

11 mole % Na3CrF5, 4 mole Z NagFeFe, and the remainder a mixture of.
NaBF, and NaF

Subsequent operation of the cold finger at the same condltions in |

'loop MSR-FCL-1 did not produce a deposit. On the basis of these tests

we concluded that a wetting problem exists with the salt-and that a

- grooved or roughened surface is required to obtain a depositl -We then

tried various designs to improve adherence of the deposit to the .cold.

finger and to'lessen the_prohability of accidental removal during with-

drawal from the pump bowl.

Run 4

.After run 4 when the pump had been removed, we found approx1mately

50 g of intermixed white and green material on the pump liner above the

liquid level  Analysis of'a small portion of this material showed about

equal amounts of NaF and NaBFq, with 4300 ppm Cr, which we assume to be
in the form of Na3CrF5. We do not know the conditions that caused the
dep091t10n of the corrosion product in the pump liner. |

Speclmens in the coldest part of the system gained weight during

the first three runs‘and then lost a small amount of weight during run‘4.

Thickness measurements of these specimens disclosed that the upstream

end had reduced in thickness compared with the middle and downstream
end. Specimens at other locations in the loop (555 or 588 C) showed
similar but smaller thickness variations. Figure 27 shows a specimen
at 588°C after run 4 and clearly illustrates the‘heavier attack at the |
upstream end. Changes in specimen thickness‘are‘Summarized,in Table 9,

and theylare-quite consistent with the changes calculatedffrom_weight

'measurements.

Figure 28 shows micrographs of specimens from MSR-FCL-l after run 4
(9600 hr total exposure to the fluorohorate mixture). The left micro-

graph shows the surface of the specimen~at~the'hottest'position (5889C),d

o
 

»

41

 

 

Fig. 27. Hastelloy N Specimen 4 from MSR-FCL-1 Exposed to
NaBF,—8 mole % NaF at 588°C for 9600 hr (Through Run 4). Flow is to
the left. ‘

Table 9. Average Thickness Change of Specimens After 9600 hr Total
Exposure to NaBF,—8 mole 7% NaF in MSR-FCL-1

 

 

Specimen Average Thickness
Temperature Change
(°c) (mils)
588 ‘ ' ' —-1.25
555 . : -0.75
510 R +1.0%

 

aDiscounting upstream end of sample, thickness increase is
+2.5 (see text).

Y-99704

 

 

 

 

Temperature, *C 588 . _ : 555 - 510 .
Weight Change, mg/cm? -21.3 i : —4.6 ~0.1

Fig. 28. Hastelloy N Specimens from MSR-FCL-1 Exposed to
NaBF,—8 mole % NaF for 9600 hr (4 Runs).
 

 

42

Average weight losses of the specimens are 21.3 mg/cmz. ‘Microprobe
analysis_disglosed.arconcentrafion gradient of chromium and iron for a
‘distance of 0.6 mil. ‘There is less than 1.0 wt % Cr and only 1.0 wt Z Fe
0.4 mil into the Specifien. This'depletéd area is represented by;thé
darkened area_extending'from the'surface for approximafely'O.S mil. The
center micrograph shows the surface of a specimén at 555°C. The average
weight loss of'ffieserépecimenswas 4.6@g/cm2. Microprobe analysis of
these specimensvdisclosed only a small concentration gradient for a dis-
tance of O.i mil. This follows from the small amount of attack seen on
the.specimen;"The micrograph to the right shows the deposit on the sur-
face of the spéciméns at 510°C. The average weight loss of the specimen
is.O.i mg/cm?, but thiS“overall'loss stems from a great deal of corrosion
on the leading edge'and is not indicative of the whole speéimen. Micro-
probe analysis of.this surface showed a large amount of iron and a little
more nickel than usual. This situation existed for a distance of 0.5 mil,
after which the conéentrations of both elements approached that of the
matrix. The average deposit is only 0.2 mil thick; thus, it appears that
portions of the deposit have diffused in. Prior evidence in deposit

study had disclosed similar fifidings.

Run. 5

Run 5 was terminated after 422 hr of operation at design conditions
‘when the pump bearings failed. This failurg allowed the pump impeller
to contact the impeller houéing, abrading the impeller. Metal particles
from the impeller were seen in the salt left in the pump bowl after
dumping.' Acéordingly, the pump was removed from the loop, andlat the
same time we removed'the corrosion test specimen assemblies. This
allowed fis to clean the loop piping by flushing withideminerélized water.
A total of 3.31 g of Hastglloy N particles.was recovered from thesloop.v
The loop_Was dried bj air purging, and the corrosion specimen assemblies
and pump were reinstalled. A ni¢ke1 filter with 40-um pores was'also
installéd in the dump tank line to filter any metal particles from the
salt being returned to the loop. | | | _

During the scheduled loop shutdown at the end of run 4, we had
enlarged the access port in the pump bowl from 1/2- t6.3/4-in._diam
 

 

43

(maximum size possible’in'the pump bowl) to provide more flexibility in
the design and operation of cold-finger devices. With the enlarged
access port we were able to insert a 5/8-in.-0D.cold finger with circum-
ferential grooves that had been previously used in another circulating
fluoroborate loop (PKP-1) and on which significant deposits of salt con-
taining Na3;CrF¢ had ‘been obtained.

Several tests were made during run 5 with this cold finger at tem-
peratures between 150 and 420°C. The cold.finger was inserted below the
liquid surface in the pump bowl'for:periods of 5 to 6 hr for each test.
The salt temperature_in the'pump bowl was approximately 516°C. Small
deposits with a light-green'color'(indication of corrosion product
Na3CrFg) were-occasionally seen on the cold finger when it was withdrawn
into the sight glass above therpump'bowl. Howeuer, such deposits usually
dropped off immediately. Apparently any slight vibration is enough to

knock most of the deposited material loose before the cold finger can be

 .pulled all the way out of the pump bowl.

The weight changes of Hastelloy N specimens in loop MSR-FCL~1l are
compared in Table 7 for each of five test runs. The corrosion rate |
decreased steadily over the first four runs and, at the conclusion of
run 4 “had dropped to 0.3 mil/hr at the point of maximum temperature
(588°C). However, the corrosion rate during run 5 was higher than in
.any of the preceding runs.

Although the increased rate of attack during run 5 is undoubtedly
assoclated with operating difficulties encountered during the run, we
have not pinpointed the specific causes. Increased corrosion is normally
accompanied by an increase in impurities in the salt; however, we did
not see any significant impurity changes during run 5. Because of the
nature of the last pump-bearing failure, we were not able to take a salt
' sample from the loop'before shutdown=in.our normal manner.‘ Thus a sample
'was_taken from the dump tank after the loop had been shut down. However,
because of dilution-there'was no assurance that.the'sample was represen-
tative of the salt that had circulated in the loop. '

'Figure 29 shows specimens 2 (555°C) 4 (588°C), and 8 (510°C) at
the end of rum 5. Note the change in appearance of specimen 4 between run

4 (Fig. 27) and run 5 (Fig. 29). The etched appearance of specimen 4 after

 

 
 

" 'PHOTO 77947

  

 

 

 

Fig. 29. Hastelloy N Specimens from MSR-FCL~1 Exposed to NaBFy;—8 mole % NaF for 10,000 hr Total
(Through Run 5). From top to bottom the temperatures were 555, 588, and 510°C.

9%

 
 

 

by

45

run 5 1s typical of the appearance of Hastelloy N specimens in thermal
convection loop NCL-17 (see previous section, Fig. 13) after extreme
oxidizing conditions had been created by intentional steam inleakage and

large amounts of mass~transfer had occurred.

Run 6

The sixth run was abruptly terminated after 1241 hr, when oil from

‘a broken pump cooling oil line ignited on contact with the pump bowl and
‘adjacent piping. We'extinguished the fire and turned off all loop
‘heaters, which allowed the salt to freeze in the loop piping. Figure 30

shows the pump bowl and pump with no apparent damage after the fire.
The material in the beaker is salt and corrosion products left in the
pump bowl. |

We replaced the electrical wiring, thermocouples, and service

piping necessary to melt the salt in the loop piping and transfer it to
- the drain tank. This was the first time that we had attempted to melt

salt in the entire loop circuit,:although salt had been melted in the
cooling'cdil without difficulty on several occasions. Normally when

the loop is placed in standby condition, the salt circulation is stopped,

 

| Fig. 30. Standard Hastelloy N Pump from MSR-FCL-1. Impeller
exposed to NaBF,—8 mole % NaF 1319 hr; baffles exposed 11,371 hr.
Outside of the pump was exposed to oil fire.

 

 
 

46

the main loop heaters are turned off, and sufficient heat is supplied
to the loop to maintain the loop circuit above the salt liquidus tem-
perature, 385°C. During the attempted melting, the main loop piping
(1/2~in.-0D x 0.042-in.~wall Hastelloy N) ruptured at a point_near the
U-bend in the loop and adjacent to one of the metallurgical specimen
holders, as shown in Fig. 31. After the rupture, we froze the salt in
the loop as quickly as possible by turning off all loop heaters. How-
ever,,approximately 77 g salt was lost from the loop. Before the rupture
the salt'in the bowl and cooling coil section of the main loop had been
melted without difficulty,.the main loop piping had‘reached'temperatfires
varying between 371 and 482°C around the loop circuit, and the drain

line temperatures were above 426°C.

ORNL-LR-DWG 64T40R

METALLUREGY

: SAMPL
HOkva POWER SUPPLY A
(MAIN POWER} _ (DETAIL

1600-amp BREAKER

    
     

%

=) METALLURGY
74 SAMPLE

1 LFB PUMP

    

 

 

 

 

il 10 kva POWER SUPPLY
{COOLER PREHEAT)

Fig. 31. Molten-Salt Corrosion Testing Loop MSR-FCL-l and Power
Supplies Showing Points of Rupture in Runs 6 and 7.

(.
e
 

 

 

”»

47

In reviewing the loop melting procedure to determine the cause of
the rupture, we concluded that uneven thicknesses of thermal insulation
caused considerable variation in the temperatures. In partiéular, the
temperatures under the clamshell emergency heaters (where‘the thermal
insulation was about 1 in. thick, compared to about 2 in. on the remainder
of the tubing) were about '80°C below temperatures in the more heavily
insulated sections of tubing. Thus, the sélt in the ruptured area melted
but was not able-toiexpand‘properly because of frozen salt plugs under
the emergency'heaters. Another problem was the difficulty in controlling
‘the rate .of heatup of the main loop by resistance heating. Even though
'thé lowest setting on the‘ioop heater was used, we still had to turn off
the heater supply power for short periods to maintain temperature control.

We removed -the metallurgical Specimen'éssembly and ruptured section

of tube and replaced it with new tubing. Figure 32 shows the ruptured

section of tubing. Measurements of the outside diameter of the remaining
loop tubing were made énd'compared with the original outside diameter to

determine if excessive permanent strain had occurred at other locatioms.

 

 

| Fig. 32. Tube Ruptured During Salt Melting after Run 6,ALdop

 
 

 

 

48

Generally all measurements were in the range of 0.500 to 0.505 in., as
compared to the original diameter of 0.500 in.

The tubing that surrounded the specimens was removed after run 6
(11,371 hr) and examined by both optical metallography and scanning
electron microscopy. Figure 33 shows the tubing exposed to'nalt at 588 _
‘and 555°C. 1In the nptical micrograph of Fig. 33(a) the surface roughening
(some salt is still in place) can be seen, and the attack at the grain‘
.boundaries is noted. In the scanning electron micrograph we can see the
~delineation of the grains. Figure 33(b) shows much less attack at 555°C.
Figure 34 shows the tubing ddwnstream of the cooler in the coldest posi-
‘tion (510°C) of therloop. The deposit is easily seen in the optical
-m1crograph and is seen in the scanning mlcrograph to be in the form of
nodules. The upper portion of Fig. 34(c) shows a side view of the nodu-
'-lar deposits. ‘

After the oil fire that ended run 6 and the subsequent rupture of
the tubing during the attempt to melt the salt from the loop, the corro-
sion specimens-were‘removed, weighed, and examined. Surprisingly, weight
was lost by all specimens. The weight loss rates found for specimens
exposed to the salt at 588 and 555°C were (assnming uniform losses)labout
1.9 and 0.5 mils/year, respectively, for the 1240 hr of run 6. The corro-
sion rate ratio for these specimens (severnl timeS'greater>1oss at 588°C
‘than at 555°C) was fairly consistent with past findings."However, in
earlier runs, specimens at 510°C had gained weight. .For tnis time period
specimens at this-temperatnre 1evelnlost-ma£efia1 at the rate of 1.9_mils/year,
the same as for specimens at 588°C. This weight loss anomaly and the
overall higher weight losses are attributed to the problems encountered
during this run and probably atteét to a local high temperature in the
coldet_section alongrwith oxidizing impurities in the salt. A salt
analysis showed small increases in chromium, iron, and nickel contents,

all of which reflect the increased corrosion rates.

"Run 7

After repairs we returned the loop to design conditions for the
start of run 7, circulating the same NaBF,—8 mole % NaF coolant salt

used since the start of loop operation.

C.
 

 

49

— e 105076

 

-}

 

 

 

 

 

 

Hastelloy N Tubing from.MSR—FCL~1 Exposed to NaBF,—8 mole %

Left: optical micrographs, 500x. Right: scanning
Top: at 588°C and bottom at 555°C during

Fig. 33.
NaF for 11,371 hr.
‘electron micrographs, 1000X
design operation.

¥

 

 
 

1
|
i
1
j
i

    
  

 

50

Y-105978

 

(c) ‘
Fig. 34. Hastelloy N Tubing from MSR-FCL-1 Exposed to NaBF,—8 mole %
NaF at 510°C for 11,371 hr. (a) Optical micrograph, 500x. (b) Scanning

electron micrograph, 1000%. (c) Scanning electron micrograph, 3000X, taken
with the specimen tilted to give a side view.
 

 

1

51

During run 7 while repairs were .under way on the loop high-temperature

\protective instrumentation, an operating error resulted in a portion of

the loop tubing- being heated above 1100°C.‘ This caused one failure of
the loop tubing adjacent to one of the electrical power input lugs
(see Fig. 31) and also about 12 in. downstream. Part of the salt in the

‘loop'(about 2 1iters) was discharged into the 1oop secondary containment.

‘ Temperatures were immediately reduced and the facility was shut down.

At about 1100°C the strength of Hastelloy N is very low, and the salt
vapor pressure approaches 1000 psi. The salt boiled locally, and over-

heating could have reduced'the tube strength until rupture occurred.

Local melting of the tubing was also efidenced. At the time of failure

the loop had accumulated 10,335 hr of operation at design conditions
(101 br during run 7). The tube failure is shown in Fig. 35.

Following the rupture'of the_loop-tubing that ended run 7, the
corrosion test specimens were again removed for weighing and metallurgi-
cal examination. The specimens adjacent to the ruptured area (hottest
section under normal conditions, 588°C) were badly warped and partially
fusedzto the tube wall. Thus, only metallographic analysis was possible.
The specimens'in the intermediate zone (555°C) appeared to be unharmed.

The average weight loss for these specimens was 1.3 mg/cm? in the 101 hr

“of run 7. Expected 1oss for these specimens in 100 hr would be approxi-
,mately 0.1 mg/cm . Thus, if we attribute the balance of the corrosion

to just the period of the temperature excursion (to about 1000° C), the loss

rate was 1.2 mg/cm® in 2.5 min or 10,000 mils/year (10 in. /year).

Weight losses were also found for the specimens downstream of the

. cooler. Although the specimens were adjacent, one specimen lost

0.3 mg/cm2 and the other 3.0 mg/cm®. Again the severity-of the over-
heating was'evident. Certainly a large part of these losses may be
attributed to moisture contamination of the salt and air oxidation
after the loop rupture. Nonetheless, the corrosion rate was quite high.

The loop is now being modernized and prepared for additional service.

 
          
 

 

       

PHOTO 78452

r VR 2 i

HEATER LUG A

cs

 

Fig. 35. Tube Rupture Caused By Overheating During Run 7, Lobp MSR-FCL-1.

 

 

 
 

ah

53
" SALT CHEMISTRY -

| " "Purification

With the realization that the fluoroborate salt is easily contami-
nated with oxygen and water-type impurities and that corrosion increases

with the amount of these'impurities;‘we“have attempted, in close coopera~-

~tion with R. F. Apple and A. S. Meyer, of the Analytical Chemistry
-'Division, to purify the fluoroborate in a manner that could be adapted

to a‘large system.

| In ‘the method that gave favorable résdlts,-a‘mixture of He, BF;,,
and HF (purified with fluorine) was pASSed through the fluoroborate at
454°C. A schematic of the process is given in Fig. 36. The gas mixture
was passed for 15 hr into a Hastelléy N vessel containing the impure
(about~2000'ppm H,0) fluoroborate salt (2.7 kg) at 480°C. 'The'exit gas
entered a solutionfof'lo_v61,% pyridine in methyl alcohol. At the com-

pletion of the purification process titration of this solution with
" Karl:Fischer reagent to the dead-stop end point indicated that about

600 -ppm-H,0 fidsfremoved;'“Chémicalfanalysis;discIosed'that the oxygen
content Hécreaéed from-1700 to 1400 ppm"and‘that.the H,0 content changed

ORNL-DWG €9-42547R

    

- . 3 h." "
BF, 35 CIT\- J/min |

  
  
 

KARL FISCHER

60 cm>/min . ‘REAGENT

        
         
           
    
   

125 crns/min

 
 

A

 

END POINT
INDICATOR

000 -

90% METHYL ALCOHOL

NoBF, — NoF (92-8 mole %) 10% PYRIDINE

450 ~ 510° C
‘ORIGINAL = 2000 ppm H0 .
AFTER 15 hr SPARGING = 300 ppm -H,0

Fig;;36. System for Removing Water from Coolant Salt.

 
 

 

 

sh

from 2000 to 300 ppm. Changes in the BFj; content of the salt appeared
to be minimal. Further work is planned using a nickel vessel to elimi-

nate corrosion products resulting from the reaction of HF with Hastelloy N.

Analytical Chemistry

Over the last few years, we have been-concerned‘dver.the behavior
of water in molten fluoride salts. Water in these salts has implications
for both corrosion and tritium removal. In corrosion work, we have long
- recognized the need for analyses that would allow us to distinguish B
between water and (1) compounds that contain H ions, (2) compounds
which contain 0%~ ions, and (3) HF (highly corrosive). In the past, a
complementary indication-of mass transfer (besides weight changes and
metal analysis of the salt) was the analysis for'HZO,ahd 0, in the salt.
On the basis of our tritium injection experiment?’ and work in the
Analytical Chemistry and Reactor Chemistry Divisions, we now feel that
the results of the water determinations included many other constituents
~of the salts, including oxides. Thus, a resulfi_indicating a large amount
of water really only showed a corresponding amount of oxide. The lack
~of watervifi the salt is completely reasonable because all the hydfogen
in impurity cofipcunds that enter the salt would eventually cause oxida-
tion of the metal wall and would in turn be reduced to hydrogen gas,
. which would diffuse out of the loop (the corrosion equations will be
discussed later). However, increased mass transfer (weight losses and
‘gains by specimens) by virtue of water impurity‘is still accompanied b&
an increase in the oxide content of the salt;' Thus; the oxide content

is still somewhat indicative of the mass transfer of the loop system.
DISCUSSION

“Theory

Metal corrosion by salt mixtures consists of oxidation of metal

constituents to their fluorides, which are soluble in the melt and hence

 

273. W. Koger, MSR Program Semiann. Progr. Rept Feb. 28, 1971,
ORNL-4676, pp. 210-11.

1 .
r
 

55

do not form a protective film.. This electrochemical-type attack is

»

therefore limited only by the’thermodynamic potential for the oxidation
reaction and is selective, removing the least noble constituent, which
- _ in the case of Hastelloy N is chromium. Possible corrosion reactions

involving impurities in the melt may be written as

2HF + Cr = H, + CrF,,.
 NiF, + Cr < Ni + CrF,,
Fer + Cr = Ni + CrF,. .

Although the reaction product is written as Cer, the actual species in
_the melt is unknown. Chromium(II) is possibly unstable in the fluoroborate
melt, since the only corrosion product isolated and identified is Naachs.
Table 10 illustrates the thermodynamic stability of the fluoride compounds

in question.

Table 10. Relative Thermodynamic Stabilities of Fluoride Compounds

 

S S | cL - Standard Free Energy
. - L o , Most Stable of Formation

 

 

Element Fluoride " (kcal/g-atom F)
- Compound at 800°C  at 600°C
Structurel metals Cr N ‘Cer ’ =12 :—77d
| | Fe  FeFy ~66 69
N ~ NiF» - =59 —63
| Mo MoFs - -s7.. -58
Diluent sales  ~  Li - .. LiF - ~120 125
Na ' NaF -110 =115
‘XK . KF. - =108 . -113
Be. " BeF2. —103 - . -100
.zr . .- .2ZrFy. . 92 .. 96
: "B BFg . —86 . 88
Active salts U T URy =92 94

Th " "ThFy —99 e -102

 
 

 

 

 

56

- In an isothermalmsystem, immediately after immersion,io the fused
salt system and before thermodjnamic;equilibrium-has been fully estab- - : ”
lished, the electrode potential of the metal has a much higher electro-
negative value than the redox potential of the surrounding medium. : As ' ~
a result, reduction of the oxidizer commences.which takes place along .
with the corrosion of the metal. As the corrosion products accumulate
the metal's electrode'potential shifts in the positive direction, whereas
the redox potential moves toward the'negative.side; Equilibrium occurs.
when therpotentials are eqoel. Since we are dealing with corrosion in
a nonisothermal system (i. e.,-a heat exchanger), the mechanism of
interest to us is temperature-gradient mass transfer. A schematic ofi
this process is glven in Fig. 37. In a loop ‘that contains salt with
UF, and no corrosion products, ‘all points of the loop initially expe-

" rience a loss of chromium by the reaction

2UF, + Cr = CrF2 + 2UFj3.

As the corrosion—product concentration in the salt is”increeseo, eqoi--‘-
librium with respect to the corrosion reaction is eventually reaehed at
the point of lowest temperature of the system. At regions of higher
temperature, a driving force for the corrosion reaction stiil exists.
Thus, the corrosion-product concentration will continue to increase and
the equilibrium temperature will begin to increase from the coldest
temperature. At this stage, chromium is returned to the walls of the
coldest point of the system. The rise in oorrosion-product conoentra-
tion in the circuleting salt continues until the amount of chromium
returning to the malls exactly balances the amount of chromium entering
the system in the hot-leg regions in the same interval.‘ Under these
conditions, the two positions of the loop at equilibrium with the salt,
‘which are termed the "balance points," do not shift measurably with time.
Thus, a quasi—steadf—state situatiom is eventually achieved, whereby
chromium is transported at very low rates and under conditions of a ?
fixed chromium surface concentration at any given loop position. At

this point the chromium removal.is controlled by the solid-stete diffu~ s
sion rate of chromium in Hastelloy N. Table 11 illustrates the small . | \N-j
 

 

 

57

. .. ORNL-DWG 67-6800R

 

 

 

 

 

HOT SECTION

DIFFUSION TO SURFACE:

 

 

' SOLUTE ESCAPE THROUGH
NEAR-SURFACE LIQUID LAYER

 

 

, '_—--— ) o DIFFUSION INTO BULK LIQUID

| —  TRANSPORT T
TO COLD PORTION OF SYSTEM

 

 

 

 

 

COLD SECTION e
— — [ X SUPERSATURATION —_ =

      

 

— NUCLEATION —
o GROWTH TO STABLE CRYSTAL SIZE

.. _0OR o =

'SUPERSATURATION AND- DIFFUSION _ -~
" THROUGH LIQUD |
NUCLEATION ANd GROWTH - -
ON METALLIC WALL — —— - —
OR DIFFUSION INTO WALL :

 

 

 

Fig. 37. Temperature-Gradient Mass Transfer.
 

 

58

Table 11. Chromium Transfer from Hastelloy N with Surface
Concentration Reduced to Zero

 

 

 

| Diffusion Coefficienta‘ Depth of
Temperature : D Time Chromium Gradient
- (°c) 2o (hr) |
. (cm”“/sec) . (cm) (mils)
| | | x 1073
650 2 x 1071 4,0000 1.5 0.6
| | 16,000 3 - 1.2
| | . 256,000 (30 years) 12 ; 4.7
850 2 x 10712 4,000 12 4.7
' - 16,000 - 18 7.1
256,000 130,years) 70 27

 

By surface activity measurements and microprobe traces after 250~
and 500-hr exposures. i :

bChromium concentration within 95% of initial concentration in
 Hastelloy N (about 7 wt %). ,

amount of chromium that will be removed if oxidizing conditions are mini-
mal and solid-state diffusion is controlling. - |

There should'be-Veryrlittie corrosion of Hastelloy N by pure molten
NaBF, or NaF, such as'is seen.fOr LiF or Ber; thus we.attribute the |
mass transfer to other sources. We have shown several examples'of'
inadvertent air (moisture) inleakage?and~thesconcomitant increase in
mass transfer. This was also iilustratedkiniNCL-l7; where_steam was
added. The corrosion reactions in the sodium fluoroborate salt mixture
have not been well established but can be expected to be reasonably

represented by the following equations:'

"H20 + NaBF, = NaBF30H + HF,
NaBF 30H == NaBOF, + HF,

6HF 4+ 2Cr + 6NaF = 2Na3;CrFg + 3H2 (and similar reactions with
nickel and iron),

¥

30; + 4Cr = Zgr203,
Cr,0; + 3NaBF, + 6NaF <= 2Na3CrFs + 3NaBOF,

"FeF, + Cr= CrF, + Fe (and similar reactions with nickel and

molybdenum fluorides),

N
 

 

 

59

"LTheinext‘result'is'thatfwater reacts with sodium fluoroboraterto
produce hydrogen fluoride and exide ions or an oxygen—containing complex
" 'in the salt. The hydrogen fluoride reacts with the constituents of
_Hastelloy N to bring'metal ions into solution in the 'salt and to release
hydrogen. It is not known if the oxide in solution can. corrode Hastelloy N,
but oxygen in air can react*w1th Hastelloy N to produce metal oxides.
' These metal oxides dissolve in the melt to form metal ions and oxide ions
in solution. Metal dions in solution in the salt react with and diSplace
less noble constituents of the Hastelloy N. Highly oxidizing conditions
(presence of HF) can bring all the constituents into, solution. As the
salt becomes less oxidizing the more noble'metals in solution react with
1ess noble alloy constituents until only chromium ions are present in
the salt in important. quantlties. This movement of chromium is governed
primarily by the rate of diffusion of chromium in the Hastelloy N.
Assuming that HgO (or HF) causes the mass transfer in sodium fluoro-
“borate and on the basis that 1 mole of H20 is required to corrode 1 mole
of metal we have calculated the amount of water required to cause known
amounts of corrosion on several of our systems (Table 12) For NCL~17
in the 1054-hr operation before steam addition, 41 mg Hgo would have
been required to cause the 137 mg metal removed leaving 75 pPpm ox1de
in the salt. At the end of 10,000 hr- after steam addition 23 g of metal
had:been'removed " This would have required-6.3 g H20 and would have
resulted in 13,800 ppm oxide in the salt. Since we have never measured
| this much oxide in the salt, this material must deposit. The amount
of water required to produce the corrosion seen in NCL~20 is small but
has continued to increase with time. This can mean several things. One,
there is a constant inleakage in our test systems or, two, there are
impurities present in the salt which continually cause corrosion.f‘The
first possibllity is quite plausible because we have seen faulty gas B
regulators, which can leak undetected for quite a long time.' As for the
fiseCOnd possibllity, the Fe ¥ concentration in the salt has decreased over
120 ppm, which would result in the removal of only 350 mg of chromium
from the Hastelloy N. This still leaves .about 600‘mg.that=we might

"attribute to water-caused corrosion.

 
 

 

 

.60

Table 12, Calculated Amount of Water Necessary to Remove Experimentally
Determined Amount of Metal in Different Loop Systems

 

Total Material Corrosion Rate H20 Required Oxide

 

Time HfiimgzgdAgizz At E:Z;::zn of To Remove Total Resulting From
(hr) - ‘A Given Time  Temperature Material . . React%on
(mg) . (mils/year) _ (me) - (mg)
NCL-17
1,0542 137 0.22 41 200
239 3,320 19.4 996 . . 5,000
424 4,150 13.6 1,245 6,000
663 4,700 9.9 1,410 - 7,000
1,474 6,090 5.8 - 1,827 9,000
2,888 7,200 3.5 2,160 - 12,800
4,756 - 9,960 S 2.9 2,988 ~ 15,000
6,030 12,700 - 2.9 3,810 19,000
9,006 14,400 2.2 4,320 - 21,600
10,178 . 23,000 3.1 6,900 35,000
| | | NCL-20
3,898 349 | 0.17 o 105 | 525
6,173 615 0.19 ~ 185 925
8,501 923 - 0.20 o211 1,400
NCL-13A |
958 1,380 1.6 414 - 2,070
2,225 1,890 0.95 - 567 | 2,850
3,545 2,340 0.74 672 | 3,360
5,339 3,030 0.64 909 4,550
11,680 5,300 0.51 1,590 . 8,000
14,495 7,030 0.54 2,109 — 10,550
16,843 11,500 0.76 3,450 17,250
| FCL-1 |
2,741 11,790 1.7 3,540 ' 17,700
4,747 17,690 1.5 5,310 | 26,500
6,756 21,620 1.3 . 6,486 . 32,000
9,600 23,580 ~0.96 7,074 " 35,000
10,000 35,370 1.4 ' 10,611 53,000
11,472 42,250 1.4 12,675 63,500

 

7 ‘aBefore steam addition, all other times of loop NCL-17 after steam
addition. : ' ' | ' B
 

 

 

6l

_NKEquetions

The experiméntalJWeight?changeslof specimens in the hot and cold
portions of the polythermal systems are functions of time at a given

temperature and are expressed by the equation

W= at? , | . D

where _ , o S L L : ,
W= weignt‘chsnge_in milligrams-ner‘square centimeter of surface‘
. area, -
a‘= eonstant,
t = time,in'hours,
b = kinetic time constant. N y _
The actual weight loss per unit. time is a function of temperature

and is represented by the Arrhenius relation

3‘;’ Aema o
where
'%%fi='w918ht change in miiligrams per square centimeter of surface

area per unit time, -
‘= constant, '

o

A
R = gas constant,
T = absolute temperature in °K,

Q = activation energy.

From Fick's second law of diffusion, one can develop the’ equation

e, -

_AM = weight of material transported,

- where

Az = internal peripheral area of loop tubing,
p, = density of original alloy,

 

 
 

 

62

initial chromium concentration,

equilibrium constant at the balance point,

equilibrium constant at the absolute temperature T,

diffusion coefficient of chromium in alloy, and

o o ™
n

time.

This equation describes the mass transfer that can occur in a temperature-
gradient system. The use of equations of this type is given in a paper

by Evans, Koger, and DeVan.?2®

Of importance is the fact that at the
balance point no material is transported at temperatures above the |
balance point material is transported away, and at temperatures below '
the balance point material is brought in. The equation was originally

considered only for_diffusion—controlled processes such as the corrosion

reaction between chromium and UFy, but we see in our plots of weight change'

versus position that balance points exist even when solid-state diffusion
is not controlling (Figs. 38, 39, 40, and 41). Thus the same mass
transfer theory holds for our impurity-controlled reactions. _
The material deposited in the cold leg during the normalfoperationv
of the loop is metallic, generally nickel and molybdenum.- Plugging-
type deposits, usually resulting when the solubility of a metal fluoride
in the salt is exceeded, have been found to be mixed or complex fluorides
such as NajCrFg, NaFeF3, NaNiF;. .This report does not include discussions
of condition of saturation of any components. Most of the material
retained in the salt is chromium with some iron, both in the form of
fluorides. ' | |

Kinetics

According to prevailing concepts of reaction mechanisms, a reaction
such as the corrosion of a solid material by a fused salt occurs as a
sequence of steps. The combined effect of these steps determines the

net rate of the reaction. However 1if, as is common, one step is

 

\ 28R. B. Evans III, J. W. Koger, ‘and J. H. DeVan, Cbrroszon in Pbe-
thermal Loop Systems. II. A Solid-State thfuscon Mechanism With and
thhout quuzd lem Effects, 0RNL-4575 Vol 2, (June 1971).

C,
 

»

 

63

ML-m--NTT?. .
LOOP ¥4~ MODIFIED SPECIMENS oo
065 02 W0 seress eS| T s

o HEATED AND INSULATED

   
 

o _

o

L

" 600 w

:

-2 8
5502

| g
3 800

WEIGHT CHANGE (mg/om?)
3

 

 

-4
|
-‘ v
' e Sl
o .o 82T
- W22
. 4 HHW
-8 . 2044 W
. s 3603w
. 4448 N
-9 . s SOIGw
* 2w
¢ 7600Mm
-.o. . . . m" .
-H
-2 -
13
-4 -
sl S
Q L 2 0 40 50 & -0 L 0 -
DISTANCE FROM MOTTEST PORTION OF THE UPPER CROSSOVER {in)
ok vhoohe e o
UPPER COLD LEG BEND LOWER BEND HOT LEG
CROSSOVER {VERTICAL} CROSSOVER (VERTICAL)

Fig. 38. Weight Change as Function of Position (Temperature) for
‘Titanium-Modified Hastelloy N Exposed to Sodium Fluoroborate Salt in -
. Loop NCL-14, ' o . - . - :

O

 

 
-8

-10

WEIGHT CHANGE (mg /cm?)
!
@

-2

-18

-20

 

 

ORNL-DWG 71-8062

 

J ! t B

l'-*HEATED AND INSULATED —i

 

 

 

?\x
Y
3

 

X
‘3¥‘» \\
4

 

 

 

 

N/ L

.y

S
=A
/
/
(

 

 

 

     

 

 

 

THERMAL CONVECTION LOOP NCL-13A

 

A
f

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

550

500

-
450

|
[ a \\
_ _ [ - °
. ! \
: | v 958 br \
{ T w2225 hr
‘ : [ -® 3545 hr | \
{ | . 45339bh |
, T S AaYWe80h | T T T T
{014,495 hr |
L essa3nr L
| |
” * ' ! -t -
i J | !
i |- ! ] |
0 10 20 30 40 50 60 70 80 90 100
DISTANCE FROM HOTTEST PORTION OF THE UPPER CROSSOVER (in.)
t“ I - .
i | _|..+_mfi,,{..*|.___ ..|
UPPER COLD LEG BEND LOWER BEND HOT LEG

CROSSOVER . {(VERTICAL} - CROSSOVER (VERTICAL)

Fig. 39. Weight Change as Function of Position

(Temperature) for Hastelloy N Exposed to Sodium
Fluoborate Salt in Loop NCL-13A.

) .

TEMPERATURE (°C)

ORNL-DWG T1-8060

|=——HEATED AND INSULATED

30

 

 

600
o
- <
e 550 §
S e
o
£ &
w 500 W
Q =
z2 L
[ - -
o ‘ 450
; THERMAL CONVECTION LOOP NCL-17
@
W
=
. ® 239 hr
o 424 hr
4 663 hr
41474 hr | 1)ME AFTER
v 2888 hr )
v 4756 hr STEAM INJECTION ‘
e 6030 hr
e 9006 hr
¢ 10,478 hr
¢ 1054 hr BEFORE STEAM
O 0 20 30 40 50 60 70 80 20 100
DISTANCE FROM HOTTEST PORTION OF THE UPPER CROSSOVER (in.)
L ol [ [ |
I _ I P | | i
UPPER COLD LEG BEND LOWER BEND HOT LEG '
CROSSOVER (VERTICAL) CROSSOVER (VERTICAL)

‘Fig. 40. Weight Change as Function of Position
 (Temperature) for Hastelloy N Exposed to Sodium
Fluoborate Salt in Loop NCL-17.

%9

 
 

 

 

65

ORNL-DWG 71-8061

AIR-. :
COOLED-I-iHEATED AND INSULATED

 

 

SECTION |
450
& 500
5 &
< :
£ 550
- e
g T - 600 Ul
< HERMAL CONVECTION LOOP NCL-20 &
o = | | Z
I- 650
9
o |
= o 3784 hr |
® 6059 hr 700
a 8387 hr
A 11,883 hr
-20 :
0 10 20 30 40 50 60 70 80 = 90 100
DISTANCE FROM HOTTEST PORTION OF THE UPPER CROSSOVER (in.)
L L - b | el 1
~ T T =T . i
UPPER COLD LEG BEND LOWER BEND - HOT LEG

CROSSOVER {VERTICAL) CROSSOVER (VERTICAL)

Fig. 41. Weight Change as Function of Position (Tenperature)
for Hastelloy N Exposed to Sodium Fluoborate Salt in Loop NCL-~20.

significantly slower than the others, the—:ate of the entire reaction
is governed by the slowest step.

It is convenient to break up the dissolution process in the hot leg

_ 1nto four separate steps:

1. diffusion of the attacked elements through the matrix of the alloy
to the surface, |

2, migration of the oxidizing species from the salt to the surface,

3. oxidation of the metal to a dissolved fluoride,‘ o

4. migratlon of the oxidized metal to ‘the flowing salt.

In cases where the slow step involves awdiffusion;proceSS‘(volume
diffusinn‘in a selid), the reecfion is tefmed &iffusionecontrolled and
is govefned by the laws of diffusion kinetics.‘ Alternately; if a
boundary-region process‘(solutiqnj constitutes the slow step, .the rate

ofithe,overallsreactien,is_determined-by the kinetics of that process.

 
 

 

66

The rate of formation of the corrosion product fluoride should be included
in this analysis, but experimental data'relating to its formation and
the effect of impurities are lacking. |

_ If. the diffusion step is rate controlling, then no velocity effect
" would be seen. However, if step 2 was slowest and was controlling, the
process would be velocity dependent. At a certain critical velocity
(possibly even that of the salt inm natural'circulatien loops),.the pathv
for step 2 would become short and step 3 would contrel. Above this
critical velocity, no velocityrdependence would again be seen. Step 4,
1ike step 2, would be‘velocity dependent. Thus, experifients in systems
with different salt velocities could help to determine the rate-controlling
mechanism.‘ |

In this work, we will consider the possibilities of solid-state

diffusion and solution control.

"80lid-State Diffusion Control

A form of Fick's second law can be used for the weight loss in this

case
= AC /Dt , (4)

where

W = weight change,

C = concentration change of diffusing substance,

D = solid state diffusion coefficient,

t = time,

A = constant.

1f Eq. (&) applies, b of Eq.‘(l) is theoretically equal to 1/2.
The diffusion coefficient follows the Arrhenius relationship

.D = D, exp(-Dg/RT) , | | 6y

where
Do is constent and

Dg is the effective activation energy for diffusion in the alloy.

O
 

4

67
If we differentiate Eq. (4) and substitute Eq. (5) for D, we obtain

aw

AC BTt ext ,
dt -'-'i— ¥ fl/t ’EXP(“'DSIZRT) »

shoWing‘thét‘the theoretical activation energy for the corrosion reaction

is half that for diffusion:

Q. = Dg/2

theo . o ®

Solution Controlling -

 

d?? to describe solution attack

The following type of equation is use
controlled’by,1iquid<diffusion 1#:3 thermal_gradient system

dW

" 0.023d °*%y°" o0t s, (D
where |

'V = velocity,

u = kinematic viscosity,
DL‘= diffusion-coefficient'intheliquid,

d = diameter of pipe,

p = density of liquid,

S = solubility of container materiél in 1iquid.

The diffusion coefficient €an be expressed by ‘the Stokes-Einstein

relation

 

Di'éf6;§gn-éngfqv/RT) . | | »‘f (8)

w
-

= Boltzmann constant,
n, = constant, |

- radius of diffuslng ion,

II

: Qv:é activation energy for~viscosity.

 

29G, W. Horsely, “Maés'Trénspott and'Corrosionibf Trbn;Based Alloys
in Liquid Metals," J. Nucl. Energy Part B: Reactor Technol. 1l: 8491
{1959).

 
 

 

 

68
The kinematic viscosity can be expressed as

W =T, exp(Qv/RT)/p , @

and the solubility as

S = S, exp(—AH/RT) , | (10)
where S, is a constant and AH is the heat of solution. From the tempera-
ture dependence on dW/dt in Eq. (7) after substitution using Eqs. (8), (9),

and (10), we can obtain for the theoretical heat of activation of
corrosion

Q = Q, + RT(0.6 + 1;4%' %.%) + AH . . (11)

theo

ComparingiEq. (11) with experiment?® shows that at leést 80% of cheo

is the value of AH, the heat of solution of the corrosion product in
the fluid. Thus we take

Q = 1.25 AH .

theo

For this system btheo = 1.

Experimental Data

 

The heats of solution of FeFé and NiF2 in molten fluoride salt are

30

about 12 and 8 kcal/mole, respectively. The average value 10 kcal/mole

. will be used for AH. A value of 60 kcal/mole will be used for the

effective activation energy, D., for volume self-diffusion for iron and

S’

chromium. Table 13 compares the cheo and bt ~ for the two proposed

corrosion mechanisms with experimental valueshsg Q and b for the specimens
in the systems that lost material. Values are given for the coolént
salt at two different impurity levels. - o

One might imagine that at the beginning of a mass transfer experiment

diffusion of the elements through the matrix would not play a large part

 

- %%. M, Blood, F. F. Blankenship, G. M. Watson, W. R. Grimes, Reactor
Chem. Div. Annual Progr. Rept. Jan. 31, 1960, ORNL-2931, pp. 39—43.
 

 

L3

 

69

Table 13. Rate Expressions for Material Removed in Coolant Salt
' NaBF,—8 mole % NaF

 

Oxide: l Weight Change Temperature

 

 

Material Content Ti“:};era Constants?. | Dependence
Of Salt (°c)
| (ppm) a b A Qs cal/mole
Ti-modified 500 604 0.0036 0.7 8 x 10* 32,200

2 ,
Hastelloy N 579 0.0025 0.7

555 0.00032 0.9

Standard 500 604  0.13 0.37 3 x10* 29,900
Hastelloy N 579 0.16 0.30

555 0.063 0.35
Ti-modified 1500 604 0.0085  0.75 1 x 10° 32,200
Hastelloy N 579  0.007 0.75

555  0.00015 0.1
524  0.000055 1.35
Theoretical Values
Diffusion control 0.5 30,000
Solution rate control. - | 1 12,500
b

 

aConstants for equation W=at", where W = weight change, mg/cm

= time, hr.

bConstants for equation g;-— A exp(—Q/RT) .

in the process since the material which is to be removed is already at

fthe surface. Thus, step 2, 3, or 4 might be controlling. After the

initial fast portion of the reaction the corrosion rate generally levels

out. At this time, the diffusion of the elements to the surface would _

fcontrol. Thus, the calculated overall rate would appear to be of

mixed control. When a velocity effect 'is noted, .as 1is thought to occur
in MSR-FCL-1, then it can be assumed that enough impurities are controlling
the corrosion reactions to cause solution rate control of the entire
process. | | ',

Although the proposed corrosion mechanisms are usually first considered

for material removal, they may also be used for the deposition in the
 

 

 

70

- cold leg of the system. The temperature dependence is reversed, the
largest deposition occurs at the,highest temperature, which causes a

| change in sign of the activation energy. Table 14 gives the experimental
values of Q and b for the systems which gained weight. The kinetic
constants of’one group,‘titanium-modified alloy and fluoroborate salt with
1500 ppm oxide, corresponds to solid~state diffusion control but the -

value of the others are in mixed agreement.

:Diffusion Calculations

Weight change rate constants based on diffusion control were cal~
culated for loops NCL-13 and NCL-14 after 2500 hr with the following

equation
Aw, = C J&Dt/mw, T > 520 * 15°C,
oss
where |
C = concentration of Fe + Cr in Hastelloy N, g/cm?,
D = diffusion coefficient, cm?/sec,
t = time, sec.,

By using the weight changes and the combined iron and chromium
concentration for the loop specimens, almost identical D values were
-obtained for the standard and modified Hastelloy N. This is quite
significant, since the iron content of titaniumemodified Hastelloy N
is negligible. Thus D, or in reality a rate constant Kl’ is independent
of the concentration of chromium, iron, and other diffusing species.

The weight change, however, is quite sensitive to the total concentration
of the migrating elements. Confirmation of this hypothesis is evident

when K1 is compared with D, 1in Hastelloy N 31»32 ~ For loops NCL~13 and

C -

 

31y. R. Grimes, G. M., Watson, J. H. DeVan, and R. B. Evans, "Radio-
tracer Techniques in the Study of Corrosion by Molten: Fluorides,
pp. 559-74 in Conference on the Use of Radioisotopes in the Physical
- Setences and Industry, September 6-—17, 1960, Proceedings, Vol. III,
International Atomic Energy Agency, Vienna, 1962.

- 32R. B. Evans III, J. H. DeVan, and G. M. Watson, SerBthfuston of
. Chromzum in Ntckel Base AZZoys, 0RNL—2982 (January 1961).
 

 

71

Table 14. Rate Expressions for Material Deposited from Coolant Salt
~ NaBFy—S8 mole 7 NaF

 

 

 

 

Oxide Weight Change | ' Tempereture
Content Iempera- Constants? . ' Dependence
Material ture
of Salt °c) _
(ppm) a b A Q, cal/mole
Ti-modified 500 460  0.0085 0.6 9 x 1077 8,240
Hastelloy N 482 0.0069 0.6 |
527  0.0056 0.6
Standard 500 460  0.17 0.33 4x10°% 14,400
Hastelloy N 482 0.13  0.33
527 0.088  0.33 |
Ti-modified 1500 460  0.007 0.65 - 2.9 x 10 12 28,800
Hastelloy N 482 0.0052  0.65
527  0.0016  0.65
qConstants for equation W atb; where W = weight change, mg/cmz;
= time, hr, |

bconstants for equation gt = A eXp(+Q/RT),.
NCL-14 at 607°C Ky =5 X 10—_12 cm?/sec, while the literature value for
D, at 607°C is 5 x 10 '* cm®?/sec. The appropriate Arrhenius relation
constants are Do = 6.068 x 10 ° cm?/sec and Q = 41.48 kcal/mole.

The effect of impurity oxide content in fluoroborate salt on weight
losses of the Hastelloy N alloys at 555 and 604°C is given in Fig. 42,
The data show that titanium—modified alloys lose less weight than the .
standard Hastelloy N for exposure in fluoroborate salt containing
500 ppm sz-impurity‘at temperatures between 555 and’ 604°C. However, at
the same tempetetufes the titaniumrmodified alloy exposed to fluoroborate
salt containing 1500 ppm oxide shows a greater weight 1oss than either
..alloy exposed to the 500 ppm oxide salt. B

The actual -average weight loss rate,wes_calcnlateq.for several
alloy=-salt combinations and_plntted in Fig. 43 against the inverse of
~ the absolute temperature. . Also included in Fig. 43 are weight loss

rates calculated from
 

 

72

ORNL —DWG €9-763R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

g T T TN
' : 'Sl4n1fl
|
605°C
5 ,‘/ ‘
7
7
£ ¥ 05 °C
7 605 °
o 7 ”‘y,—’ly’
E 7 555°c .
L 2l o= / o4 mil ————
S “’,av’ 1 A mi
g ‘ 5°C 4
S
. e //
I N
O ©5655°C i _
J - ‘ i
z o
- i . : -
* 05 L2 0.025 mil
/’s85°C
/1 T
| b |
7 © Ti-MODIFED HASTELLOY N
. © STANDARD HASTELLOY N
0.2 —— {500 ppm OXIDE IN SALT |
500 ppm oxnm»: IN SALT -
| ] .
i ‘ r ye0r '
0.1 L | _* _
10° 2 5 10% 2

TIME (hr)

Fig. 42. Weight Loss for Haételloy N Alloys in Sodium Fluoroborate
Coolant Salts. — ' S '

= (Coécs)_/ADt/fl ,

where.

W= weight loss in milligrams per square centimeter of surface
area,

‘D = the diffusion coefficient in square centimeters'per'Second,
t = time in seconds,
Cg and C, = the concentration maintained at surface and the initial uni-

form concentration, respectively, in milligrams per cubic

centimeter.
 

73

ORNL-DWG €9-762R2

¢ TYPE 304L STAINLESS STEEL

4 4 Ti-MODIFIED HASTELLOY N

0 ¢ STANDARD HASTELLOY N
x CALCULATED-ALLOY CONTAINS 7% Cr
+ .CALCULATED-ALLOY CONTAINS 8% Cr

® 4 ¢ FUEL SALT
4 ¢ FLUOROBORATE COOLANT SALT

TEMPERATURE (°C)

 

o2 700 650 - 600 550
5
2 ‘
' 4073 | D (cmPsec)
.cqu . 3;10‘“‘4 ‘ppm -OXIDE
Pl 23:10“4 +1500
E : 4 [84x10715— — <4500
s 2 .' 45 —
%\ __45 + 5.0*‘0-‘ N 4500
0 e ox0 P =
5
2
107> _ ‘ -
098 102 106 440 444 148 122 126
- 1000y | |

Fig 43. -Arrhenius-Type'Plotffor Corrosion .of Materials in Fluoride
Salts. ' - ' . -

For 'this calculation we assumed C = 0, ‘that all'weight loss was the removal

of chromium, and that this removal was controlled by ‘the diffusion of
chromium to-the surface. The chromium dlffusion coefficient32 in
Hastelloy N Was:usedain?the calculation. | -
_~An.extrapolation-of thc.graphs of?Fig} 43 shows that both fluorobo-
rate.salts (500 ahd 1500‘ppmrokidc impurity)oarevmore‘aggreSSiVe than the
fuel :salt to ‘the Hastelloy N- alloys. ‘Also, it appéarsfithatrideotical rates
would ‘be found for the type 304L stainless. steelrfuel salt combination '
and ‘the titanium-modified Hastelloy N fluoroborate . salt (500 ppm oxide

impurity).
 

 

 

 

74

It would be expected, if the solid-state diffusion mechanism
controlled, that the calculated rates of Fig. 43 based on diffusion
- coefficients would be identical to the-experimental rates. However,
results of only the titanium-modified Hastelloy N —fuel salt system
coincided with thercalculated values. Incidentally, fuel salts were
used in the earlier experiments'that showed_that the rate was controlled
by the diffusion coefficient of chromium in the alloy.’® Thus, we have
shown that another mechanism besides chromium solid-state diffusion is
controlling the process for several of the metal-salt systems investigated
~here.

‘The weight losses of the specimens exposed to the fluoroborate salt
_ and the value calculated for 600°C and 4000 hr are compared in Table 15.
It is noted that the experimental losses are greater than those calculated.
As has been discussed earlier, the impurities in the fluoroborate salt
appear to be very important because of the formation_of‘HF, which is
strongly oxidizing and will.attack all the constituents of the Hastelloy N
alloys. However, in the salt'containing 500 ppm oxide impurity the
titanium-modified alloy showed smaller weight losses than the standard
alloy, again because of its lack of iron. 1In the salt with 1500 PpPM oxide
impurity more HF was available for attack and the weight losses
were higher, since measurable amounts of nickel and molybdenum were
removed from the alloy. An increase in nickel and molybdenum concentrations
r-is often noted in the salt analyses under these conditions. The amount
of attack is approximately doubled. Because of the large amounts of
nickel and molybdenum in-the alloys very little differences in the weight
losses of the standard and titaniumrmodified.alloys will be noted when
‘large amounts of HF are available for attack. It is also believed that
this HF attack is responsible for the mixed kinetics noted earlier.

 

%34, R. Grimes, G. M. Watson, J. H. DeVan, and R. B. Evans, "Radio-
tracer Techniques in the Study of Corrosion by Molten Fluorides,"
PP. 55974 in Conference on the Use of Radioisotopes in the Physical -
Setences and Industry, September 6—17, 1960, Proceedings, Vol 111,
International Atomic Energy Agency, Vienna, 1962
 

L4

 

75

'fiTable”IS.' Effect of Impurities on Hastelloy N Corrosion by Fluoroborate

Coolant Salt for 4000 hr at 600°C

 

a ight Loss entration ¢
Material Weig Concentration Oxide

 

(mg/em®)  (ppm)
Ti-modified Hastelloy N - f5.7 ) 1500
B Standard Hastelloy N 2.9 | . 500
- Ti—modified Hastelloy N - | - 1.9 . 500
Calculated ‘ "0,2

Using Doy = 2.0 X 10 15 szlsec
and Cg = 0,07 - oo

 

With reSpect-to'the deposition process, it is obvious that the

calculated rate includes only the weight changes that occur from the

‘deposition on the surface. The amount of the diffusion into the metal

is not measured. However, during the straight-line portion of deposition
with ‘time, on which the kinetic calculations are based it was theorized
that the rate at which the metal deposited is the same as the rate at
which it diffuses into the container wall. In time the weight gains

would continue, the amount of deposited material on the surface would

hremain essentially constant, and a chromium gradient would appear in
- the colder sections. All experimental findings do support these

'.suppositions.

' The kinetic calculations for the weight—gain portion of the mass -

transfer show a mixed control between solid-state diffusion and. solution

rate. Analogous to the behavior in the hot leg, in the cold leg the .
- weight gains could be first solution-rate controlled and later, as the

'process slows down, be controlled by the diffusion. Thus, the overall

"effect would be mixed control As theorized earlier ‘it still appears
fi‘that the action in the cold leg controls the mass transfer process for

‘,‘the entire system.

In liquid metal systems, the simple approach to the mass transfer
problem 15 the statement that the weight loss rate is a function of the
difference in the solubility of the dissolved metal at the temperatures

 
 

 

76

of interest.“_The'proportionality constant may be relafed_to the diffusion
coefficient of the metal in the.liquid if diffusion is the controlling
.step or may be related to another factor concerning,Some process:occurring

at the_inteffaee if the rate of solution is controlling. However, many

additional ‘models have been required to explain all the experimental
findings.v~The most ticklish problems involve the possible effect of
oxygen on.the solubility of certain metals in”liqmid metals, the velocity
effect; end the downstream effect. ‘Similar problems eXist‘in'the fused
fluoride'selt systems. Inrthe'salt systems, we can make the.general
statement that the corrosion rate in fuel salts is a functiom of the

chromium and iron contents of the alloy. In the fluoroborate salts, the

 rate is a function of the alloy composition and the impurity level of the

salt. The proportionality constant for_bbth cases is a function of

temperature amd can be found experimentally. As in liquid metal systems,

the rate controlling mechanisms are not entirely clear cut but are-

functions of many variables.

- Although the'corrosion ef these systems has been widely diseussed,

ié should be pointed out that the weight loss ‘rates are quite low.
SUMMARY
Our data on mass transfer in the fluoroborate sait mixture points
out that impurities such as water and.oxygen strongly affect theffluoride

oxidation fiotential of this salt. However, we do not yet completely

know the chemical forms takem by these impurities upon entering the salt.

Corrosion rates show a marked increase following an air or steam leak

into the fluoroborate salt. Once the leak has been detected and corrected,
the'corrosien rete dfbps shafply. Analyses of tfie salt shows afi increase
in oxide level as the corrosion fate'incfeases; the explanation for this
behavior is that moisture reacts with the salt to produce HF and comprnds
(oxides and hydroxides) that analyze as exides_by thexpresent techniques.
The HF causes rapid oxidation of the containment material and is used up
in the.proeess. The compounds that ensiyze as water and oiygen remain

but have only a minor effect on cott6sioh rate. Kinetic eomsiderations

 showed that the mass transfer process is between solid-state diffusion
 

 

 

0

77

control and solution control, probably depending on the amount of

impurities allowed to enter the salt. The mass transfer behavior is very

similar to that seen for the chromium-UF, corrosion reaction, the

difference being that other alloy constituents (beside chromium)
participate in the process and.the,reaction rates are different. We have

also shown the difficulty.in keeping the fluoroborate salt mixture free-

. of moisture-type impurities but feel that this can be done by exercising

sufficient care.

Except for certain periods of the fluoroborate tests, the overall

‘corrosion rates have been relatively small, .Thermal-convection loop

. NCL-14 has operated successfully for over two years and the pump loop

MSR-FCL-1 nearly one_year with the fluoroborate mixture. Comparison of
the rates experienced by NCL-13A, NCL-14, and MSR-FCL-1, all circulating
the sodium fluoroborate-mixture,'indicates a velocity effect on the mass
transfer (Table 16). We believe that the effect of velocity on corrosion
in this system is a function of declining importance as the purity level
of the salt improves. The impurity effect per se has been discussed in
detail previously. Because of the’importance of the impurities on the
corrosion of Hastelloy N by fluoroborate, efforts have begun on methods
of purifying the salt. Also to be considered are better methods for
analyzing ‘and identifying the impurities. At some purity level, perhaps
200 to 400 PP oxide as analyzed solid-state diffusion of chromium in
the Hastelloy N will likely control corrosion, as it does in the fuel
salts. This is most important, since it suggests that an entire MSBR

of either the one~ or two-fluid variety can be operated with none of the

| main circulating channels suffering more than a few tenths of a mil per

year corrosion attack ‘Experimentalpproof of this is one of;our major

near—term goals.
 

 

T

78

able 16. Comparison of Corrosion Rates for Standard Hastelloy N in
MSR Systems After More Than 5000 hr Operation

 

 

s o Teosiey o, M
o | | (°c) (°c) (mils/year)
. NCL-13A Coolant® 605 145 0.1 0.6

NCL-14 - Coolantd 605 145 0.1 0.55

MSR-FCL-1 ~ Coolant® 588 78 10 1.2

NCL-16 .~  Fuell 705 170 0.1 0.04

NCL-15A Blanket© 675 55 0.1 0.03

NCL-18 - Fertile- 705 170 0.1 0.05
Fissile -

 

2NaBF ,—NaF (92-8 mole %), 1000 ppm oxide.
bLiF~Ber—UF4 (65.5-34, 0-0.5 mole %), < 200 ppm oxide.
CLiF—BeF2—ThF, (73-2-25 mole Z), < 200 ppm oxide.

d1,1FBeF,~ThF,—UF;, (68-20-11.7-0.3 mole %), < 200 ppm oxide.

CONCLUSIONS

The corrosion rate is a function of the. alloy composition and

impurity level of the salt.

Because of the corrosive nature of the BF3 in an impure state and

because it is present in appreciable quantities over the melt, leaks

in mechanical portions of the systems can essily'occur.

Kinetic considerations disclose that the weight change'rates are

controlled by a mixture of solid-state diffusion and solution rate.

The effective diffusion rate that controls the hot-leg attack is

larger than the volume diffusion coefficients obtained in typical

diffusion experiments.

The mass transfer behavior (weight losses in hot leg, weight gains

in cold leg, steady balance points) is very similar to that seen for

the chromium-UF, corrosion reaction.
 

 

79

6. Titanium-modified Hastelloy N with less chromium and iron shows a
greater resistance to attack in a fluoroborate salt with 500 ppm oxide
impurity than standard Hastelloy N.

7. The corrosion rate of titanium-modified Hastelloy N in fluoroborate
salt with 1500 ppm oxide impurity is double that found in salt with

500 ppm oxide.

8. Corrosion rates were relatively low for all systems tested.

ACKNOWLEDGMENT

It is a pleaSuré to acknowledge that E. J. Lawrence supervised

‘construction and operation of the thermal convection loops and was

responsible for the weight change measurements of all the corrosion
specimens. _We'also_recbgnize P. A, Gnadt and W. R. Huntley of the
Reactor Division for operation and design of‘the pump loop. We thank
A. P, Litman for his invaluable assistance through part of this work.
We are also indebted to H. E. McCoy, Jr., J. H. Devan, T. S. Lundy,

"and J. V. Cathcart for constructive review of the manuscript.

Special thanks are extended to the Metallography group, especially

-H. R. Gaddis, C. E. Zachery, H. V. Mateer, T. J. Henson, and R. S. Crouse
.and_to the Analytical Chemistry Division, particularly Cyrus Feldman

and H. W. Dunn, the Graphic Arts Department, the Metals and Ceramics.
Division Reports Office, and particularly S. Peterson, for invaluable

assistance.'

 
 

 

 
 

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