ORNL-TM-0328.txt

- e

 

OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
for the

U.S. ATOMIC ENERGY COMMISSION

 

ORNL- TM- 328
R

 

MASTER

CORROSION BEHAVIOR OF REACTOR MATERIALS

IN FLUORIDE SALT MIXTURES

J. H. DeVan
R. B. Evans, Il

NOTICE

This document contains information of a preliminary nature and was prepared
primarily for internal use at the Oak Ridge National Laboratory. It is subject
to revision or correction and therefore does not represent a final report, The
information is not to be abstracted, reprinted or otherwise given public dis-
semination without the approval of the ORNL patent branch, Legal and Infor-
mation Control Department,
 

 

 

LEGAL NOTICE

This report was prepared os an account of Government sponsored work, Neither the Unitad Stctes,

nor the Commission, ner any person acting on behalf of the Commissicn:

A. Mckes any warronty or representation, expressed or implied, with respect to the accuracy,
completeness, or usefulness of the information contained in this report, or that the use of
any information, apparotus, methed, or process disclosed in this rapert may not infringe
privately owned rights; or

B

Assumes any liabilities with respect to the use of, or for damages resulting from the use of

any information, apparatus, method, or process disclosed in this report.
As used in the above, '""person acting on behalf of the Commission'' includes dany employee or
contractor of the Commission, or employee of such contracter, to the extent that such employee
or contractor of the Commissien, ¢r employse of such contracter prepares, disseminates, or
provides access to, any information pursuant to his employmant or contract with the Commission,

or his employment with such contractor,

 

 

 

 
¥

ORNL-TM-328
Copy

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

CORROSION BEHAVIOR OF REACTOR MATERIALS
IN FLUORIDE SALT MIXTURES

J. H, DeVan
Metals and Ceramics Division

and

R. B. Evans III
Reactor Chemistry Division

DATE ISSUED

SEP 191962

 

OAK RIDGE NATIONAL IABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U. 5. ATOMIC ENERGY COMMISSION
ABSTRACT -

Molten fluoride salts, because of their radiation stability and
ability to contain both thorium and uranium, offer important advantages
as high-temperature fuel solutions for nuclear reactors and as media
suitable for nuclear fuel processing. Both applications have stimulated
experimental and theoretical studies of the corrosion processes by which
molten-salt mixtures attack potential reactor materials. The subject
report discusses (1) corrosion experiments dealing with fluoride salts
which have been conducted in suppors of the Molten-Salt Reactor
Experiment at the Oak Ridge National Iaboratory (ORNL) and (2) analytical
methods employed to interpret corrosion and mass-transfer behavior in
this reactor system.

The products of corrosion of metals by fluoride melts are soluble
in the molten salt; accordingly passivation is precluded and corrosion
depends directly on the thermcdynamic driving force of the corrosion
reactions. Compatibility of the container metal and molten salt, there-
fore, demands the selection of salt constituents which are not
appreciably reduced by useful structural alloys and the development of
container materials whose components are in near thermodynamic equilibrium
with the salt medium.

Utilizing information gained in corrosion testing of commercial
alloys and in fundamental interpretations of the corrosion process, an
alloy development program was conducted at ORNL to provide a high- h
temperature container material that combined corrosion resistance with :
useful mechanical properties. The program culminated in the selection of
a high-strength nickel-base alloy containing 17% Mo, 7% Cr, and 5% Fe.

The results of several long-term corrosion loops and in-pile capsule tests
ccmpleted with this alloy are reviewed to demonstrate the excellent
corrosion resistance of this alloy composition to fluoride salt mixtures

at high temperatures. Methods based on thermodynamic properties of the
alloy container and fused salt are presented for predicting corrosion rates
in these systems. The results of radiotracer studies conducted to

demonstrate the proposed corrosion model also are discussed.
CORROSION BEHAVIOR OF REACTOR MATERTALS
IN FLUORTDE SALT MIXTURES

 

 

J. H. DeVan, Speaker, and R. B. Evans TIII

TNTRODUCTTON

Molten fluoride salts exhibit exceptional irradiation stability and
can be fused with fluorides of both thorium and uranium. Both properties
have led to their utilization as fuel-bearing heat-transfer fluids [1]
and as solvating agents for fuel reprocessing [2]. Intrinsic in these
applications, however, is the need for experimental and theoretical
studies of the corrosion processes by which molten salt mixtures attack
potential reactor materials,

Unlike the more conventional oxidizing media, the products of
oxidation of metals by fluoride melts tend to be completely soluble in
the corroding media [3]; hence passivation is precluded and corrosion
depends directly on the thermodynamic driving force of the corrosion
reactions, Design of a chemically stable system utilizing molten fluoride
salts, therefore, demands the selection of salt constituents that are not
appreciably reduced by available structural metals and the development of
containers whose components are in near thermodynamic equilibrium with
the salt medium.

Following the initiation of design studies of molten fluoride fuel
reactors, a corrosion program was begun at the Oak Ridge National
Laboratory to investigate the compatibility of experimental fluoride salt
mixtures [4,5] with commercially avallable high-temperature alloys [4,5].

As a result of these studies, the development of a preliminary reactor
-2 -

experiment was undertaken using a nickel-base alloy containing 15 Cr,

7 Fe,* and a fuel salt of the system NaF-ZrF,—UF,. This reactor experi-
ment, although of intentionally short duration, successfully demonstrated
the feasibility of the fluoride fuel concept [6,7].

The corrosive attack incurred by the Ni-Cr-Fe alloy was found to be
selective toward chromium and was initiated through chromium oxidation
at the metal surface by UF, and traces of impurities such as HF, NiF,,
and FeF, [3]. The overall rate of attack was governed primarily by the
diffusion rate of chromium within the alloy. Although suitable at low
temperatures, corrosion rates of the alloy above 700°C were excegsive
for leong-term use witnh most fluoride fuel systems.

Utilizing information gained in corrosion testing of commercial
alloys and in fTundamental Interpretations of the corrosion process, an
alloy develcpment program was carried out to provide an advanced container
material that combined corrcsion resistance with useful mechanical
properties. The alloy system used as the basis for this program was
composed of nickel with a primary strengthening addition of 15 to 20% Mo.
Experimental evaluations of the effectis of other solid-sclution alloying
additions to tnis basic composition culminated in the selection of a
high=strength nickel-base alloy containing 17 Mo, 7 Cr, and 5 Fe.

The purposc oi tThe present report 1s to summarize the corrosion
properties of alloys based on tihe nickel-molybdenum system and to then
discuss an analytical apvroach for precicting corrosion rates in these

systems based on thermodynanmic properties of the alloy and fluoride salt

 

*Compositions refer to percent by weight, except where otherwise
noted,
-3 o

mixture. The report is divided into three major sections: (1) a
presentation of experimental results showing the effects of alloying
additions of Cr, Fe, Nb, V, W, Al, and Ti on the corrosion properties

of nickel-molybdenum alloys; (2) a presentation of the experimentally
determined corrosion properties of the 17 Mo—7 Cr—5 Fe—bal Ni composition
(designated INOR-8); and (3) a discussion of the analytical model that
has been employed to interpret the corrosion and mass transfer properties
of these alloys.

FFFECTS OF ALLOYING ADDITIONS ON THE CORROSION
RESISTANCE OF NICKEL ALLOYS IN FLUCRIDE MIXTURES

Experimental

Several laboratory heats of experimental nickel-molybdenum alloy
compositions were prepared by vacuum- and air-induction melting to afford
a programmatic study of the effects of soclid-solution alloying additions
on corrosion behavior in fluoride salts. The compositions of the alloys
that were evaluated are shown in Table I, The cast alloys were formed
into tubing and were subsequently fabricated into a thermal-convection
loop for corrosion testing. Loops similar to the ones employed have
been described elsewhere {3]. Loops were exposed to the salt mixture
NaF-LiF-KF—UF, (11.2-45.3-41.0-2.5 mole %) for periods of 500 and 1000 hr
and were operated at a hot-zone temperature of 815°C and a cold-zone
temperature of 650°C. The corrosion susceptibility of alloying additions
was determined from analyses of the concentrations of corrosion products
in after-test salt samples and from metallographic examinations of the

loop walls.
Toble I. Compositions of Experimental Nickel-Mclybdenum Alloys Used for Corrosion Studies

 

Composition (wt %)

 

 

Heat No,*
Ni Mo Cr Fe Ti Al Nb W v
Series I:
OR 30-1 8C.12 16.93 2.83
OR 30-2 78.55 16.65 4.62
OR 30-4 73.65 16.37 9.21
OR 30-6 78.50 15.11 6.40
OR 37A-1 77.0 20.39 2.62
OR 43A-3 73.30 20.34 6.34
Series [:
OR 30-7 82.10 15.93 1.88
OR 30-8 80.30 17.80 1.89
OR 30-9 81.10 16.8 2.09
OR 30-10 81.10 16.60 2.23
OR 30-11% 79.80 16.53 3.68
OR 30-12 80.00 16.80 3.22
CR 30-19 79.00 16.90 4.10
OR 30-20 79.20 16.60 4.18
OR 30-21 78.90 16.40 4.71
ST 23012 82.00 17.42 0.53
OR 1491 86.58 11.23 2.19
Series HI:
OR 30-13 79.93 17.56 1.56 0.95
OR 30-14 79.53 16.50 1.52 2.45
OR 30-16 77.74 16.00 3.65 1.49 1.12
OR 30-22 77.65 15.90 5.69 1.16 0.60
OR 30-33 74,07 15.15 5.01 5.07 0.70
B 2897 76.13 20.50 1.25 1.32
B 2898 76.30 20.50 2.44 1.31
B 3276 69.19 2110 7.58 2.16
B 3277 66.95 21.60 7.82 1.31 2.32
ST 23011 71.50 15.06 3.84 0.53 4.17 4,90
ST 23013 74.42 15.20 0.58 4,57 5.23
ST 23014 80.86 16.70 2.10 0.57

 

**QR’’ denotes heats furnished by the ORNL Metals and Ceramics Division.
“ST"" denotes heats furnished by Superior Tube Company.

*'B’' denotes heats furnished by Battelle Memorial Institute.
-5 -

Salt mixtures utilized for this investigation were prepared from
reagent-grade materials and were purified to obtain a total impurity

content below 500 ppm.
Results

Chemical analyses of salts tested with alloys containing a single
alloying addition, i.e., with ternary alloys, are plotted as a Tunction
of alloy content in Fig. 1. It is seen that the corrosion susceptibility
of alloying elements, based on the concentration of the elements in the
after-test salt, tended to increase in the order: e, Nb, V, Cr, W, Ti,
and Al., Table IT indicates a similar trend in the corrosion-product
concentrations of salts tested with alloys containing multiple additions.
However, in comparing the values in Table II with Fig. 1, it is evident
that the corrosion-product concentrations assoclated with either iron,
niobium, or tungsten alloying additions were substantially lower when
these elements were present in multicomponent alloys than in simple
ternary alloys. In contrast, corrosion-product concentrations assoclated
with chromium, aluminum, or titanium were effectively unchanged by the
presence of other alloying constituents.

Because of beneficial effects on oxidation resistance and mechanical
properties imparted by chromium additions, a relatively large number of
alloy compositions containing this element were evaluated. The extent
of reaction between chromium and fluoride constituents, as indicated by
the chromium ion concentration of the salt, varied markedly with the
amount of chromium in the alloy (or the element). This variation is
illustrated graphically in Fig. 2, where the data are compared with data

for a 15 Cr—7 Fe—bal Ni alloy [4] and for pure chromium [8]. 1In all
UNCL ASSIFIED
ORNL — LR— DWG 46943

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

500
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400 l /,
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300 /
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ALLLOY CONTENT (at. %)
Fig. 1. Corrosion-Product Concentrations of Salts Tested with

Experimental Nickel-Molybdenum Alloys Containing Single Alloying Ad-
Salt mixture: NaF-TiFP-KF-UF, (11.2-45.341.0-2.5 mole %).

ditionse.
Table Il. Corrosion-Product Concentrations of Salts Tested with Experimental Nickel-Molybdenum Alloys
Containing Multiple Alloy Additions

Salt Mixture: NaF-LiF-K F—UF4 {11.2—45.3-41.0~2.5 mole %)

 

Concentration of Element in Salt (Mole %)

 

Heat No.* Alloy Composition {atomic %)
Cr Al Ti Nb Fe W

 

Test Duration: 500 hr

OR 30-13 2.18 Al, 2.02 Ti, 11.35 Mo, bal Ni 0.040

CR 30.-14 5.51 Al, 1.92 Ti, 10.42 mo, bal Ni 0.43 0.045

OR 30-16 2.54 Al, 1.90 Ti, 4.30 Cr, 0.023 0.33 0.038
10.20 Mo, bal Ni

OR 30-22 2.61 Al, 0.39 Nb, 6.64 Cr, 0.055 0.38 0.0018
10.05 Mo, bal Ni

B 2898 3.24 Ti, 0.90 Nb, 13.20 Mo, bal Ni 0.030 <0.0005

B 3276 1.48 Nb, 9.32 Cr, 14.00 Mo, bal Ni 0.061 0.0007

B 3277 3.06 Al, 1.57 Nb, 9.47 Cr, 0.067 0.50 <0.0005
14.20 Mo, bal Ni

ST 23011 1.27 Al, 291 Nb, 4.79 Cr, 1.72 W, 0.049 0.007
10.20 Mo, bal Ni

ST 23013 1.40 Al, 3.23 Nb, 1.85 W, 10,36 Mo, <0.001 <0.0005 0.002
bal Ni

ST 23014 1.30 Al, 2.71 Ti, 10.76 Mo, bal Ni 0.060 0.019

Test Duration: 1000 hr

OR 30-14 5.51 Al, 1.92 Ti, 10.42 Mo, bal Ni 0.25 0.043

OR 30-22 261 Al, 0.39 Nb, 6.64 Cr, 0.041 0.25 0.0010
10.05 Mo, bal Ni

OR 30-33 1.59 Al, 5.56 Fe, 5.90 Cr, 0.069 0.082 0.0051
Q.66 Mo, bal Ni

B 2897 1.68 Ti, 0.92 Nb, 13.77 Mo, bal Ni 0.038 0.0012

B 3277 3.06 Al, 1.57 Nb, 9.47 Cr, 0.073 0.47 0.0020

14.20 Mo, bal Ni

 

*"OR'" denotes heats furnished by the ORNL Metals and Ceramics Division
“*ST'' denotes heats furnished by Superior Tube Company.

**B'' denotes heats furnished by Battelle Memorial Institute.
UNCLASSIFIED
ORNL-LR-DWG 68376

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3000
T T T T TTPURE Cr=800°¢ | 1~ 1 17777

- 2000 — 200
& 1600 F = 130
&
= 1200 PURE Cr—600°C
< Y cue S chie s - . —— . P S G S G — e caver opre o jEn wEn GEn| . NS GG AL Gl mwy [ S SSE-w— 100
a 1000 M
> g0 @
& 800 - fi a‘é——i} Ni-Cr—Fe ALLOY ] <
& — Je0 &
© 600 § o © @
£ B — O
O £

_ 40
S 400 s ¥ N
S
o —l ] RANGE OF 3 SALT SAMPLES —
T 1 O SINGLE ALLOY ADDITION =500 hr TESTS -
O ® SINGLE ALLOY ADDITION —1000 hr TESTS

_5 § O MULTIPLE ALLOY ADDITIONS =500 hr TESTS _{ 29

200 H& ® MULTIPLE ALLOY ADDITIONS —1000 hr TESTS —
0.03 004 (006 008 040 0.2 0.4 06 0.8 10

ATOM FRACTION OF CHROMIUM IN ALLOY

Fig. 2. Chromium Concentration of Fluoride Salt Circulated in
Thermal-Convection Loops as a Function of Chromium Content of the Loop.
Salt mixture: NaP-LiF—KF-UF, (11.2—45.3=41.0~2.5 mole %). ILoop tem-
perature: hot leg, 815°C; cold leg, 650°C.
cases the corrosion-product concentrations in the experimental alloy
loops (which contained up to 11.0 at. % Cr) were less than corresponding
concentrations in nickel-chromium-iron loops under similar temperature
conditions or in pure chromium capsules exposed isothermally at 600 and
800°C., The latter observation indicates that the observed chromium ion
concentrations were below those required for the formation ol pure
chromium crystals in the cold-lecg region of the loops (650°C).

Metallographic examinations of alloys investigated under this
program showed little evidence of corrosion except for systems containing
combined additions of aluminum and titanium or aluminum and chromium,
Significant alloy depletion and attendant subsurface void formatlion in
the latter alloys occurred to depths of from 0,003 tc 0,004 in., In
all other systems, attack was manifested by shallow surface pits less
than 0,00l in. in depth., Figure 3 illustrates the typlcal appearance
of attack in alloy systems containing chromium at levels of 3,2 and
11.0 at. %, respectively. Although the depth of pitting was comparable
in both alloys, the intensity of pitting increased slightly with
chromium concentration.

In the case of the majority of alloys tested, the rate of attack
between O and 500 hr was substantially greater than the rate occurring
between 500 and 1000 hr. This finding is illustrated in Fig. 4, which
compares the surface appearance of a ternmary alloy containing 5.55 at. % Cr
after 500- and 1000-hr exposures. This result 1s in agreement with the
observed corrosion-product concentrations, which increased only slightly
between 500 and 1000 hr, and suggests that nearly steady-state conditions

were established within the first 500 hr of test opecration,
 

 

 

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Composition: 3.2 Cr—13.5 Mo-bal Ni (at. %)

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Composition: 11.0 Cr—10.6 Mo-bal Ni (at. %)

Fig. 3. Hot-Leg Sections of Nickel—Molybdenum-Chromilfin Thermal-
Convection Loops Following 500-hr Exposure to Fluoride Fuel. ©Salt
mixture: NaF-TiF-KF-UF, (11.2-45.3-41.0-2.5 mole %). 250X. Reduced

5.
 

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- After 1000 hr

. Pig. 4. Appearance of Hot-Leg Surface of a Ternary Nickel-Molyb-
denum Alloy Containing 5.55 at. % Cr Following Exposure to Fluoride
Fuel. Heat No. OR30-2. Salt mixture: NaP-TiF-KF-UF; (11.2-45.3-41.0-
2.5 mole %). 250X. Reduced 4%. '

 
- 12 -

Discussion

In view of the uniform compositions of salt mixtures employed for
these alloy evaluations, it follows that the mixtures afforded comparable
oxidation potentials at the start of each test. Accordingly, if passi-
vation did not occur, one can readily show that the extent of reaction
resulting from equilibration of the salt mixture with given alloying
elements should be governed simply by the activity of the element in the
metal and by the stability (or standard free energy of formation) of the
fluoride compound involving the element. Consider, for example, the

component* chromium and the oxidation reaction

Cr + U, = CrFy + 2UF43 (1)
for which
2
Yorr, ¥ UF,
Q_I_'_' UF,

At the very dilute concentrations of CrF, and UF3, which are realized
under the test conditions, the activities of these products may be
approximated by thelr mole fractions in accordance with Henry's law.
Thus, for a salt system of fixed UF, concentrations, assuming the refer-

ence states for salt components to be the infinitely dilute solution,

2
- NCI‘Fg'N UF3

= — 3
Ky S (3)
Cre™ UF,
and
2 _ %’
NCrF2,N UF, " Koo, - (4)

 

*Solid-solution alloying elements are underlined.
- 13 -

If reaction products are initially absent, a mass balance exists between

h & -1 .
the products formed such that NCTF2 5 NfiF3 and Eq. (4) reduces to
- 1/3

Ny, = K 0191: , (5)

where
K y? - 1/3
[ K" - a'QfiFg J
4
Since
o o o
RT In K = AF° o+ 2(8F = 0F )

it follows that

and

O N
- 6(ag—l:-) AF (b)

NCng Cng) :

In Table IIT are listed the standard free energies of formation, per
gram-atom of fluorine, of Fluoride compounds at 8C0 and 600°C associated
with ecach of the alloying elements investigated [9]. Valucs are given for
the most stable compounds (i.e., those with most ncgative free energies)
and are listed in order of decreasing stabilities. Tne resultant order
suggests thatl corrosion-product concentrations associated with each
element (at a given activity) should have increased in the following
order: W, INb, Fe, Cr, V, Ti, and Al. Comparison with IFig. 1 shows that,
with the exception of niobium and tungsten, the corrosion-product
concentrations per atomic percent of alloy addition did increase in the
exact order predicted. Only tungsten noticeably deviates from the

predicted pattern, although the tests made on this addition were

statistically limited.
- 14 -

Table III. Relative Thermodynamic Stabilities of Fluoride
Compounds Formed by Elements Employed as Alloying Additions

 

Standard Free Energy of
Formation per Gram Atom
of Fluorine

kcal/g~-atom of F
Most Stable (keal/e )

 

 

Element Fluoride Compound at 800°C at 600°C
Al AlF, -87 92
Ti TiF, -85 -90
v VF -80 -84
Cr Cr¥F, -72 -'77
Fe FeF 5 -66 -69
Ni NiF, ~59 ~63
Nb NbF 5 -58 -60
Mo MoF 5 -57 -58

W WE 5 ~46 438

 
- 15 -

When tested in the presence of other alloylng elements, the corrosion-
product concentrations of iron, niobium,or tungsten were noticeably lower
than the values attained for ternary alloys. The reason for this behavior
undoubtedly relates to the presence of the more reactive alloying additions
in the multicomponent alloys. If one considers, for example, an alloy
containing comparable additions of chromium and iron, for which the corro-

sion reactions can be written

il
I
o

Cr + 2UF, = CrF, + 2UF;5: AF

Fe + 2UF, ~D

I
Il

FeF, + 2UF5: AF
where
a] > [v]

the equilibrium UF, concentration produced for the first reaction is
higher than that which would be produced by the second reaction. Accord-
ingly, in the presence of chromium, the FeF, concentration at equilibrium
will be reduced compared to the system containing iron only.

The results of these alloy evaluations provided further evidence
that corrosion in molten fluoride systems involved essentially the
attainment of thermodynamic equilibrium between the fluoride melt and
container metal. The results also implied that, over the alloy compositions
studled, the activities of the alloying additions could be reasonably
approximated by their atom fractions, i.e., that activity coefficients
were nearly the same for all of the alloying additions tested. The
favorable corrosion properties of the majority of alloys tested permitted
wide latitude in the selection of an optimum alloy composition for fluoride
fuel containment. Only titanium and aluminum were felt to afford potential

corrosion problems, particularly if used as combined additions or in
- 16 -

combination with chromium. Because chromium had proved an extremely
effective alloying agent in regard to both strength and oxidation
properties, this alloying addition was utilized in the selected alloy
composition., The level of chromium in this alloy was fixed at 7%, which
is the mimimum amount required to impart oxidation resistance to the
Wi-17% Mo system [10]. The addition of chromium as an iron-chromium alloy
also imparted approximately 5% Fe to the system. While this finalized
alloy composition, designated INOR-8, was not tested as part of the
initial alloy study, its corrosion properties can be considered equivalent

to those of the ternary chromium-containing alloys discussed above,

CORROSION PROPERTIES OF INOR-8

Experimental

The materialization of INOR-& as a container material for fluoride
fuels led to an extensive investigation of the corrosion properties of
this specific alloy composition under simulated reactor conditions.
Studies were conducted in forced-convection loops of the type shown in
Fig. 5. Tubular inserts contalned within the heated sections of the loops
provided an analysis of weight losses occurring during the tests, and
salt samplers located above the pump bowls provided a semicontinuous
indication of corrosion-product concentrations in the circulating salt.
All parts of the loops were constructed of commercially supplied INCR-8
material,

The salt compositions initially utilized for these studies were of
the type LiF-BeF UF, but, in later tests, the compositions were changed

to LiF-BeF ,—ThF,UF,, Corrosion rates were determined at a series of
 

Unclassip; ed
ORNL Photg 34693

LT -
- 18 -

hot-leg operating temperatures ranging from 700 to 815°C. At each tempera-
ture level, the mixtures experienced a temperature change of approximately
200°C between heater and cooler sections. Loop operating times were
generally in the range 15,000 to 20,000 hr, although loop operation was
interrupted at shorter time intervals to allow the removal of corrosion

inserts.
Results

The weight losses of corrosion inserts contained in the hot legs of
INOR-8 forced-convection loops at 700, 760, and 815°C are shown in
Table IV. 1In two test loops operated at 700°C, weight losses were in
the range 2 to 5 mg/cm2 and showed little change after 5000 hr of loop
operation., Weight losses at 760 and 815°C were slightly higher, being
in the range 8 to 10 mg/cm2, and agaln were essentially unchanged after
50C0 hr of operation., Comparisons of before- and after-test dimensions
of the inserts revealed no measurable changes in wall thickness. However,
if uniform attack is assumed, weight-loss measurements indicate wall
reductions on the order of 2 to 12 u (Table IV).

Chemical analyses of salt samples that were periodically witndrawn
from the loops showed a slight upward shift in chromium concentration,
while concentrations of other metallic components remained unchanged.
During a typical run, as pictured in Fig. &, the chromium concentration
reached an asymptotic limit after about 5000 hr of operation. This limit
was between 300 and 500 ppm at 700°C and between 600 and 800 ppm at
760 and 815°C.

Metallographic examinations of loops operated at 700°C for time

periods up to 5000 hr showed no evidence of surface attack. When test
- 19 -

Table IV. Corrosion Rates of Inserts Located in the
Hot Legs of INOR-8 Forced-Convection Loops as a
Function of Operating Temperature

Loop temperature gradient: 200°C
Flow rate: approximately 2.0 gal/min
Reynolds number: approximately 3000

 

 

Insert Weight Loss  Equivalent Loss
Loop Salt a Temperature Time per Unit Ares in Wall Thickness

Number Mixture (°c)P (hr) (mg/cm?) (p)
9354-4 130 700 5,000 1.8 2.0
10, GO0 2.1 2.3

15,140 1.8 2.0

MSRP-14  Bu-l4 700 2,200 0.7 0.8
8,460 3.8 4,3

10,570 5.1 5.8

MSRP-15  Bu-l4 760 8,770 11.2 12.7
10,880  10.0° 11.2

MSRP-16  Bu-1l4 815 5,250 9.6 10.9
7,240 9.0° 9.1

 

%521t Compositiocns:
130 LiF-BeF2-UF,; (62-37-1 mole %)
Bu-14 LiF-BeF ~ThF,—UF, (67-18.5-14-0.5 mole %).

b .
Same as maximum wall temperature.

C .
Average of two inserts.
UNCLASSIFIED
ORNL-LR-DWG 58961R

 

 

 

 

 

 

CCRROSION PRODUCT CONCENTRATION (ppm)

 

 

 

 

 

T T ! ; i
| | f | ! | |
SALT COMPOSITION: Lif -BeFp-ThF4-UF4 (67-18.5-14-0.5 mole %) |
- MAXIMUM METAL INTERFACE TEMPERATURE: 760°C e
~ LOOP AT: 200°F ‘ ! |
o | | | | i |
| | | ! : o Ni | F l I
1 l 2310 hr,LOOP e e . . 8770 hr,LOOP ———
290 hr,LOOP ISOTHERMAL . 4 Cr | DOWN 195 hr O LOOP TERMINATED |
ISOTHERMAL FOR 29 hr ' m Fe | REMOVE INSERT ~ AFTER 10,878 hr
FOR 14 hr I | | | | OPERATION
800 I i e o ot e b Il ‘L iy | P - ,.‘,_.‘.H..‘..._.___.._____JI._.... .o + II e = e ]
| 12400 hr,LOOP ; s ! | 4 A
| l|s THERMAL | B |
| 1l 3650n,L00P f |
| 1 ISOTHERMAL , i :
A 1L FORItthr S S | o
400 lug i " | | |
" I | L | | |
A | ‘ | | | | | '
200 _Jf, ‘ 1 || _ | | ~ . ‘ B } A — || ]
i | f I . | " i " | |
| | | I | l
ot s | |l 4 e | o P ?
0 1000 2000 3ooo 4000 5000 6000 7000 8000 9000 10,000 11,000 12,000

TIME (hr)

Fig. 6. Concentration of Corrosion Products in Fluoride Salt Circulated in an INOR-8 Forced-
Convection Loop.

_OZ—
- 21 -

times exceeded 5000 hr, however, an extremely thin continuous surface
layer was evident along exposed surfaces, as shown in Fig, 7, Similar
results were observed in 760°C tests. No transition or diffusion zone
was apparent between the layer and the base metal, and analyses of the
layer showed it to be composed predominantly of nickel with smaller
amounts of iron, chromium, and molybdenumn.,

At 815°C surface attack was manifested by the appearance of sub-

surface voids, shown in Fig. 8, and the surface layer was not observed.
Discussion

The corrosion rates of INOR-8 measured in pumped loops cperating
with maximum temperatures ranging from 700 toc 815°C indicate that corro-
sion reactions with fluoride salts are essentially completed within the
first 5000 hr of loop operation and that weight losses thereafter remain
essentially constant. When Jjudged from the standpoint of total weight
loss, the temperature dependence of the corrosion rates was relatively
small over the ranges studied, However, the metallographic appearance
of specimen surfaces was noticeably influenced by the test temperature.
At the highest test temperature (815°C), exposed surfaces underwent a
noticeable depletion of chromium, as indicated by the appearance of sub-
surface voids, At lower temperatures, surfaces were slightly pitted and
were lined with a thin surface film which became apparent only after
5000 hr of operation. The composition of this layer indicates a higher
nickel-to-molybdenum ratio than originally present in the base metal and
suggests that the layer may constitute an intermetallic transformation

product of the nickel-molybdenum system.
 

     

Unclassified [,
W
T-19555  }¥-
S z
: 006
007
. A ':* - ""' \\,\
. . el
- -~ . * o -
- . - : TN ! Rt . L.Q02
- ~ ‘o . - " . ;,._—\‘h"‘ ) . T- Vl:' :- *
.- ST ‘ T BT Y 010
Lo s -, o . -
-t -' T ; - ~ \ "‘ * . .\. ¥ .- D a Ot 1
- ot Lt “ LRt i tain
. h - 7 ‘. -

-
t
.
/
2
Y
o
*
s
’
* *_,-...l
¥
L
1

 

 

 

- *
.. .
o .
. Ol4
* . - * * 018
) - - - .‘V x
. - o
Toe o e . - o
LU L |
- ' s LT TR

Fig. 7. Appearance of Metallographic Specimen from Point of
Maximum Wall Temperature (700°C) of INOR-8 Forced-Convection Loop
9354-4. Operating time: 15,140 hr. Salt mixture: LiF-BeFo—~UF,
(62371 mole %). 250X. Reduced 7%. o -

 

 

 

 

 

 

 

 

 

 

 

e Dy e R LT
B - . . \ : W -

S XNRS

 

Fig. 8. Appearance of metallographic Specimen from Point of
Maximum Wall Temperature (815°C) of INOR-8 Forced-Convection Loop
MSRP-16. Operating time: 7,240 hr. Salt mixture: ILiF-BeF,~ThF,~UF,
(6718.5-14-0.5 mole %). 250X. Reduced 7%. ,
- 23 .

ANATLYSTS OF CORROSION PROCESSES

The above-mentioned tests of nickel-molybdenum alloys in fluoride
mixtures have given considerable insight into the mechanisms of corrosion
in these systems, In turn, the conclusions drawn have enabled the develop-
ment of procedures for predicting corrosion rates on the basis of the
operating conditions and chemical properties of the system. The method
of approach and assumptions used for these calculations are discussed
below. For purposes of illustration, the discussion deals specifically
with the NaF-ZrF,—UF, salt system contained in INOR-&.

The corrosion resistance of metals to fluoride fuels has been found
to vary directly with the "nobility" of the metal — that is, inversely
with the magnitude of free energy of formation of fluorides involving the
metal. Accordingly, corrosion of multicomponent alloys tends to be
manifested by the selective oxidation and removal of the least noble
component., In the case of INOR-8, corrosion is selective with respect
to chromium (Figs. 2 and 8). If pure salt containing UF, (and no
corrosion products) is added to an INOR-8 loop operating polythermally, all
points of the loop initially experience a loss of chromium in accordance
with the Cr-UF, reaction,Eq. (1), and by reaction with impurities in the
salt {such as HF, NiF,, or FeF,). Impurity reactions go rapidly to
completion at all temperature points and are important only in terms of
short-range corrosion effects.

The UF, reaction, however, which is temperature-sensitive, provides
a mechanism by which the alloy at high temperature is continuously depleted
and the alloy at low temperature is continuously enriched in chromium. As

the corrosion-product concentration of salt is increased by the impurity
- 24 -

and UF, reactions, the lowest temperature point of the loop eventually
achieves egquilibrium with respect to the UF, reaction. At regions of
higher temperature, because of the temperature depencence for this reaction,
a driving force still exists for chromium to react with UF,. Thus, the
corrosion-product concentratiorn will continue to increase and the tempera-
ture points at equilibrium will begin to move away from the coldest tem-
perature point., At this stage, chromium is returned toc the walls of the
coldest point of the system. The rise in corrosion-product concentration
in the circulating salt continues until the amount of chromium returning

to the walls exactly balances the amount of chromium entering the system

in the hot-leg regions. Under these conditions, the two positions of the
lecop at equilitrium with the galt that are termed the "balance points" do
not shi’lt meastrably with time. Thus, a quasi-steady~state situation is
eventually acnieved whereby chromium 1s transported at very low rates

and under concditions of a [ixed chromium surface concentration at any

given loop positior., This Zdea is supported by the fact that concentrations
of CrF,, UF,, and UFs; achieve steady-state values even though attack

cslowly increases with time,

During the initial or unsieacy-state period of corrosion, the
mathematical interpretatior of chromium migration rates in polythermal
systems 1s extremely comnlex, because of rapid compositional changes which
occur continuously in both the salt mixture and at the surface of the
container wall. A manifestation of this complexity is apparent in cold-
leg regions, where chromium concentration gradients are reversed &as
corrosicn products build up within the salt. Unfortunately, the effects

of unsteady-siate operation serve also to complicate the interpretation
- 25 -

of steady-state operation., However, an idealized approach to the study

of chromium migration under steady-state conditions i1s afforded by assuming
the salt mixture to be preequilibrated so as to contaln amounts of CrF,

and UF5; which establish a steady-state condition at the beginning of loop
Operation.* Under this assumption the following conditions may be
specified for the resulting migration of chromium:

1. The overall rate of transfer is controlled by the diffusion rate
of chromium in the container material, since this diffusiocon rate can be
shown to be considerably lower than the rates of reaction and cf mixing
which are involved in the transfer process.

2. Chemical equilibrium with respect to chromium and the salt
mixture exists at every surface point within the loop system.

3. At any time the cumulative amount of chromium removed from the
hot zone equals the cumulative amount deposited in the cold zone. Accord-
ingly, the concentrations of UF;, UF,, and CrF,, assuming the salt to be
rapidly circulated, do not vary with loop position or time.

The "equilibrium" concentrations of UF,, UF;, and CrF, can be
estimated for steady~state loop operation by applying these three condi-
tions to a given set of loop parameters and operating conditions., As a
means for demonstrating these calculations, it is convenient to apply them

to a preequilibrated polythermal INOR-8 loop in which the salt system

 

*It is evident that preequilibration of the salt mixture would afford
an expedient means for reducing corrosive attack in INOR-8 systems, since
it would eliminate the initial or unsteady-state period of corrosion
during which relatively high corrosion rates are sustained. However, to
achieve criticality in reactor systems which are presently under study, it
is planned to add UF, incrementally to the sall mixture until a desired
reactivity is achieved. Under these conditions, the program required to
maintain a preequilibrated salt with each UF, level becomes extremely
complex.
- 26 -

NaF-ZrF ,—UF,; (50-46—4 mole %) is circulated between the temperature limits
of 600 and 800°C. It is assumed that the balance point of the system is
at 600°C. Under this condition, the amount of chromium depletion and
hence the corrosion rate at 800°C are the highest attainable under steady-
state conditions.

In zccordance with the second condition given above, the surface
concentration, <#9£>s’ at the 800°C temperature point can be expressed
in terms of the equilibrium constant, Ka, determined for the UE4-Cr

reaction in accordance with Eq. (3).

- N\ a? O
KN@/J T [ s (7)

1

 

800° ¢ & 800°¢ UF,
but
a2 Q _
UF3' CrF, KN )J : <Ka> ; (8)
2 - ‘Cr o
a UF, Z=/8 600°C 600°C
so that
[/ } - K eooec g ‘)
o)) o Ve | (
— 5 goce0 a’ 800°¢C 600°C

The valance points ol the systerm, wnich are at 600°C, have been defined

BZNCKJ J - (FQ; in INOR-a) ' (10)

— 57600°C

such tnat

Therefore, since the concentration of fluorides involved do not change

with position or time,

[ N‘_M - ((I;a)jeomc <Nc in TIOR 8) ‘ (11)
T/ J o gooee N == -
- 27 -

and
(CS)SOOOC - (KQ:)600°C (12)
CO EKOEESOO"C
where
CS = gurface concentration of chromium (g/cm3)
C = original chromium concentration of INOR-8 (g/cmB).

The first of the assumed boundary conditions established that the over-
all chromium migration rate was diffusion controlled. The applicable
diffusion equation at any given temperature position under the boundary

condition of constant surface concentration is given as

X

2\/ot

(13)

 

C (X,t) = C_ - AC exf

where

i}

C(X,t) chromium concentration at depth X after time t

NC = CS - CO.

This equation together with Eq. (12) enables the determination of
the concentration gradient of chromium and hence the mass of metal removed
at selected temperature points. DBasic material data needed are
(1) diffusion coefficients for chromium in INOR-8 as a function of tempera-
ture and (2) equilibrium ratios for the redox equation (Eg., 2) as a
function of temperature. Values of these variasbles are given in Fig. 9.
The diffusion coefficients are based on determinations of the self-
diffusion of chromium in INOR-8 using the tracer Cr’?Y, [11, 12] and the

equilibrium ratios are based on an interpolation of experimental values

for NaF-ZrF,~UF, mixtures in contact with chromium at 600 and 800°C.
1080 1040 1000 960 920
°0 | | I
50 P
™
S
o 4.0
3
” \
o
X 30 <
I_
— \
n 2.5
= \ \
O
O \04/2 \
& 20
|._._
Q
<
L
aC
ES 1.5
N
o
e
-
O
O
o U | | |
9.2 9.6 10.0 10.4 10.8
10%7 (oK)
Fig. 9. Effect of Temperature on Parameters Which Control

Chromium Migration in INOR-8 Salt Systems.

- 28 -

TEMPERATURE (°K)

UNCLASSIFIED

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ORNL- LR-DWG 68377

(x10~1)

5.0

40

3.0

2.5

2.0

1.5

1.0

SQUARE ROOT OF DIFFUSION COEFFICIENT OF

CHROMIUM IN INOR-8 (em?2/sec)2
- 20 .

Calculations of the corrosion rate and total attack at 800°C are
plotted in Fig. 10 for time periods up to 18 months. The corrosion rate,
measured in units of 10 % g/month'cme, 1s seen to decline to a relatively
low value after the first 3 months of operation. After 18 months the
specimen has lost only about 1 mg/cm2 cf chromium,

This calculated weight loss, based on a preeguilibrated system
contalining the salt NaF-ZrF,-UF,, 1s an order of magnitude lower than
the weight loss experimentally determined in an INOR-8 forced-convection
loop containing the mixture LiF—BeF »UF,—ThF, at 815°C (Table IV). Al-
though the Cr-UF, equilibrium constants for the latter salt system are
not completely determined, indirect measurements indicate that values of
these constants are not significantly different from those of the
NabF—zrr,—UF, system. Thus differences between the experimental and the
calculated steady-state losses essentially reflect the effects of initial
nonequilibrium conditions and, as discussed below, quite probably of
traces of impurities in the experimental system., The very low rates of
attack in the experimental systems after steady-state was achieved, i.e.,
after 5000 hr, are in good agreement with the predictions based on
preequilibrated salt.

Evidence of impurity effects in the pumped-loop systems 1s provided
by the following consideration., Calculations based on complete chromium
depletion at the alloy surface yield mass transfer rates which are still
somewhat lower than the observed rates; thus, it must be concluded that
components other than chromium per se have been removed by the fuel salt.
Oxidation of these more noble components requires the presence of

relatively strong cxidants, such as HF, H,0, or NiF,,
, CORROSION RATE (10%g OF Cr/cm?2/month)

d{AM)
ar

 

800°C Section of INOR-8 Loop Containing Pre-equilibrated NaF-ZrF,—UF,
(50—46—4 mole %) Salt. Assumed test conditions:

800°C; cold-leg temperature, 600°C.

- 30 -

UNCLASSIFIED
ORNL—LR— DWG 50117

 

-~

 

 

<

f /

 

 

10‘&

 

 

 

 

 

 

 

 

\/

 

 

 

[

 

 

 

 

 

 

 

 

 

at 600°C.

——g— A
! —— CORROSION RATE ———
|
0 3 9 12 15
EXPOSURE TIME (months)
Fig. 10. Calculated Corrosion Rate and Cumulative Attack for

H

&3

AM, TOTAL ATTACK (10~ % g OF Cr/cm?)

hot-leg temperature,
Salt in equilibrium with INOR-8
- 31 -

The validity of the proposed corrosion model for chremium removal
was experimentally evaluated by implanting the radiocisctope cr®! in an
INOR-8 thermal-convection loop and following the movement of this tracer
under controlled-corrosion conditions. These experiments, which have been
reported elsewhere [12], utilized the corrosive agent FeF, contained in
an equimolar mixture of NaF-ZrF,. The presence of FeF, caused the total
chromium concentration at the surface to be reduced practically to zero but
it did not affect other components of INOR-8. The observed rates of tracer
migration closely corresponded to rates predicted by the use of self-
diffusion coefficients for chromium in INCR-8, and chromium diffusivities
measured under the strongly corrosive conditions of these experiments

closely approximated the self-diffusion measurements.
SUMMARY AND CONCLUSIONS

1. The corrosion properties of solid-solution alloying elements in
the Ni—-17% Mo system have been investigated in molten mixtures of
NaF-Li¥F-KF-4F,. The corrcsion susceptibility of alloying additions was
found to increase in the order: Fe, Nb, V, Cr, W, Ti, and Al. With the
exception of tungsten and niobium, the susceptibility of these elements
increased in the same order as the stabilities of fluoride compounds of
the elements.

2. The corrosion~product concentrations produced by either iron,
nicbium, or tungsten alloying additiocons in the above salt mixture were
lower in combination with chromium, aluminum, or titanium alloying

additions than when present as single alloying additions.
- 32 -

3. Depths of attack of experimental alloys based on the Ni-17% Mo
system were negligible except in the case of alloys containing combined
additions of aluminum and chromium or aluminum and titanium.

4. Pumped loops composed of the alloy 17 Mo—7 Cr—5 Fe—bal Ni (INOR-8)
were investigated with salt mixtures of the type LiF-Bel ,—ThF,UF, over
a series of operating temperatures. Welght losses of corrosion specimens
in these systems achieved essentially constant values after 5000 hr.
Corrosicon products detected in the circulating salts consisted primarily
of chromium. The level of chromium in the salt remained constant after
5000 hr.

5. At hot-leg temperatures of 700 and 760°C, corrosion specimens in
pumped loops underwent little visible attack, although maximum weight
losses of from 2 to 1l mg/cm® were recorded. At 760°C, weight losses
were not substantially nigher, but specimens exhibited a zone of sub-
surface volds caused by chromium depletion,

6. The chromium corrosion rates of INOR-8& in fluoride mixtures can
be calculated under the assumptions that these rates are controlled by
the diffusion rates of chromium in INOR-8 and that the surface concen-

tration of chromium is in equilibrium with the salt mixture.
10.

11.

12.

- 33 -

REFERENCES

R. C. Briant and A. M. Weinberg, '"Molten Fluorides as Power Reactor

Fuels, " Nuclear Science and Engineering 2, 797-803 (1957).

 

G. I. Cathers, "Fluoride Volatility Process for High-Alloy Fuels,"
Symposium on Reprocessing of Irradiated Fuels, Brussels, Belgium,
TID-7534, Book 2, p. 560 (1957).

W. D. Manly et al., "Metallurgical Problems in Molten Fluoride Systems,'

Progress in Nuclear Energy, Series IV 2, 164-179 (1960).

 

G. M, Adamson, R. S. Crouse, and W. D. Manly, Interim Report on

 

Corrosion by Alkali-Metal Fluorides: Work to May 1, 1953, ORNL-2337
(Mar. 20, 1957).

G. M. Adamson, R. S. Crouse, and W. D. Manly, Interim Report on

 

Corrosion by Zirconium-Base Fluorides, ORNL-2338 (Jan. 3, 1961).

 

E. S. Bettis et al., "The Aircraft Reactor Experiment — Design and

Construction,” Nuclear Science and Engineering 2, 804~825 (1957).

 

W. D. Manly et al., Aircraft and Reactor Experiment — Metallurgical
Aspects, ORNL-2349 (1957).

J. D. Redman, Oak Ridge National Laboratory, personal communication.
A. Glassner, The Thermodynamic Properties of the Oxides, Fluorides,

and Chlorides tc 2500°K, ANL-5750.,

 

H. Inouye, Oak Ridge National Laboratory, private communication.
R. B. Evans, J. H. DeVan, and G. M. Watson, Self-Diffusion of

Chromium in Nickel-Base Alloys, ORNL-2982 (Jan. 20, 1961).

 

W. R. Grimes et al., Radio Isotopes in Physical Science and Industry

 

Vol. 3, pp. 559-574, TAEA, Vienna, Austria, 1962.

f
- 35 -

DISTRIBUTION
1-2. Central Research Library 76. H. W. Hoffman
3. Reactor Division Library 77. P. P. Holz
4. ORNL — Y-12 Technical ILibrary 78. L. N. Howell
Document Reference Section 79. R. G. Jordan
5-14. Iaboratory Records 80. P. R. Kasten
15. ILaboratory Records, ORNL RC  81. R. J. Kedl
16. ORNL Patent Office 82. G. W. Keilholtz
17-18. D. F. Cope, AEC-ORO 83. M. T. Kelley
19. Research and Development 84. B. W. Kinyon
Division AEC-CRO g5. R. W. Knight
20—34. Division of Technical 86. J. A. lane
Information Extension (DTIE) &7. R. B. Lindauer
35. G. M. Adamson 88. M. I. Lundin
36, L. G. Alexander 89. H. G. MacPherson
37. S. E. Beall 90. W. D. Manly
38. C. E. Bettis 91, K., R. Mann
39. E. S. Bettis 92. W. B. McDonald
40, F. ¥. Blankenship 93. C. K. McGlothlan
41. E. P. Blizard 9. E. C. Miller
42. A. L. Boch 95. R. L. Moore
43. S. E. Bolt 96. C. W. Nestor
44. R. B. Briggs o7. T. E. Northup
45, 0O, W. Burke 98. W. R. Osborn
46. D. 0. Campbell 99, L. F. Parsly
47. W. G. Cobb 100. P. Patriarca
48, J. A. Conlin 101. H, R. Payne
49, W. H, Cook 102. W. B. Pike
50. J. E. Cunningham 103. M. Richardson
51. G. A, Cristy 104. R. C. Robertson
52. dJ. L. Crowley 105. T. K. Roche
53. F. L. Culler 106. H. W. Savage
54~58, J. H. DeVan 107. D. Scott
59. R. G. Donnelly 108. 0. Sisman
60. D. A. Douglas 109, G. M. Slaughter
61. J. L. English 110. A. N. Smith
62. E. P. Epler 111, P. G. Smith
63. W. K. Ergen 112. I. Spiewak
64. A. P. Fraas 113. A, Taboada
65. J. H Frye, Jr. 1314. J. R. Tallackson
66. C. H. Gabbard 115. E. H. Taylor
57. W. R. Gall 116. R. E. Thoma
68. R. B. Gallaher 117. D. B. Trauger
69. W. R. Grimes 118, W. C. Ulrich
70. A. G. Grindell 119, D. C. Watkin
71. C. S. Harrill 120. J. H. Westsik
72-74. M., R. Hill 121. L. V. Wilson
75. E. C. Hise 122. C, H. Wodtke

123. J. F. Kaufmann, AEC, Washington

124. R. W. McNamee, Manager, Research Administration,
UCC, New York

125, F. P. Self, AEC-CROC