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CORROSION BY MOLTEN FLUORIDES

OAK RIDGE NATIONAL LLABORATORY
OPERATED BY

CARBIDE AND CARBON CHEMICALS COMPANY

A DIVISION OF UNION CARBIDE AND CARBON CORPORATION

(<4

POST OFFICE BOX P
OAK RIDGE. TENNESSEE

;1'!
 

ORNL-1491

This document consists of 24 pages.

Copy 6 of 157 copies. Series A.

Contract No, W-7405-eng-26

METALLURGY DIVISION

CORROSION BY MOLTEN FLUORIDES

Interim Report — September 1952

L. S. Richardson D. C. Vreeland
W. D. Manly

Experimental work carried out by

A. deS. Brasunas L. S. Richardson
R. B. Day D. C. Vreeland
E- Eb Hflffm&n w- DI Ma‘fll‘y

DATE ISSUED

WAR 17 1988

 

OAK RIDGE NATIONAL LABORATORY
operated by
CARBIDE AND CARBON CHEMICALS COMPANY

A Division of Union Carbide and Carbon Corporation
Post Office Box P

0ak Ridge, Tennessee

aegumimndimran ||/ 111111]

3 445k D35322Y4 7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 
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This documenkesssRieine " e e iaiba Atomic
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in any manneéf 10 |

&
-
CORROSION BY MOLTEN FLUORIDES

L. S. Richardson

D. C. Vreeland

W. D. Manly

SUMMARY AND CONCLUSIONS

Both static and dynamic corrosion
tests of metals in molten mixtures of
alkali metal fluorides have been made
by the Metallurgy Division in the past
year. Evaluation of these tests has
led to some definite conclusions as
to the nature of corrosion by molten
fluorides under certain test conditions,

The major formof attackby fluorides
on Inconel results in the formation
of voids which are either scattered
uniformly throughout the area near
the surface or appear preferentially
at grain boundaries leading from the
surface inward. It is believed that
these voids are caused by depletion
of one constituent (usually chromium),
which leaves vacancies and precipi-
tation of these vacancies (cf.,
Appendix). A thin, adherent film of
U0, forms at temperatures above 1200°C
(and occasionally lower) and apparently
inhibits the depletion of chromium,.
The rate of attack (as measured by
depth of subsurface voids) decreases
with time and levels off to a very
slow rate after 500 hours. Prior
heat treatment and the presence of
small amounts of impurities in the
metal apparently do not change the
rate of attack appreciably. However,
in some cases in which a very large
grain size was induced by heat treat-
ment, void formation was found to be
localized at the grain boundaries and
caused deeper but less extensive
damage. Cold working the metal seems
to have no effect. Addition of
nickel and iron fluorides to the
corroding medium was found to increase
the corrosion, probably because of
the reduction of these fluorides
by chromium that entered the system
from the container walls. The ad-
dition of small amounts of certain

&

materials to the molten fluoride
mixture is, in some cases, effective
in reducing corrosion. In general,
metals high in the electromotive
series (such as calcium, beryllium,
and the alkali metals) reduce or
entirely stop void formation, whereas
metals that are relatively more noble
(such as iron and silver) have no
effect. The more active metals were
so effective in minimizing corrosion
that they are being used in the
larger-scale testing program at the
Y-12 site. Additions of zirconium,
sodium, and titanium have been found
to minimize corrosion in the thermal
convection loops used in these tests,
It is thought that the active metals
minimize corrosion by reducing the
availability of fluorine and/or oxygen
to the structural metals.

Mass transfer effects are not so
serious a problem in fluorides as in
liquid metals or molten hydroxides.
Metallic layers or crystals are noted
with pure metals, and nonmetallic
(sometimes partially UO,) films are
often (though not always) noted with
alloys. Metallic layers have been
noted in Inconel in both thermal
convection loops and seesaw tests,

INTRODUCTION

One of the better waysof introducing
fuel into a high-temperature reactor
is through the use of uranium-bearing
fused fluorides. The various metal
fluoride systems are being investigated
by the Materials Chemistry Division,
and mixtures of the fluoride salts
are being used in corrosion tests
by the Materials Chemistry Division,
the Experimental Engineering Group
of the ANP Division, and the Metallurgy
Division.

This report contains the results
of tests made to date by the Metallurgy
CORROSION BY

Division on the corrosion of metals
by some of the fluoride mixtures.
The tests were performed at 816°C,
and two different types of tests were
used. The major portion of the tests
were run under static conditions in
evacuated metal capsules. Other
tests (seesaw tests) were run in
tilting furnaces in which the ligquid
is cycled continually from the hot
zone to the colder zone of evacuated
capsules; a complete cycle 1s made
every 13 seconds.(!) Al]l tests were
run with either fluoride mixture No. 2
(46.5 mole % NaF, 26.0 mole % KF,
27.5 mole % UF,) or No. 14 (43.5
mole% KF, 44.5 mole% LiF, 10.9 mole %
NaF, 1.1 mole % UF,).

EXPERIMENTAL PROCEDURE AND RESULTS

Static Test Loading Method. The
most recent technique evolved at the
Oak Ridge National Laboratory for the
preparation of static tests is shown
in Fig. 1. Tubing is loaded, with
specimen and corrodant, in a dry
box with a purified helium atmosphere
and sealed with vacuum tape, as 1s
seen in Part 3 of Fig. 1. After the
tube is removed from the dry beox,
a vacuum is applied, as in Part 4, to
break the vacuum tape; thus the
material in the tube is not exposed
to air. The tubing is then crimped
twice (also shownin Part 4 of Fig. 1),
and the bottom crimp is held in a
vise while the top of the tubing is
being welded, as shown in Part 5 of
Fig. 1. It was found that a vacuum
could not be held in the tubing
unless the crimped portion was gripped
in a vise during welding. Apparently
the crimps will relax enough, unless
gripped, to allow air to seep into
the tube.

If the specimen tube is connected
into a manifold when the vacuum tape
1s broken prior to welding, any type
of atmosphere can be placed in the

 

“)D. C. Vreeland, R. B, Day, E. E. Hoffman,
and L. D. Dyer, ANP Quar. Prog. Rep. March 10,
1952, ORNL-1227, p. 120.

tube by metering the proper gases
through the manifold system. Unless
otherwise mentioned in the text,
all the tests reported were run with
a vacuum in the capsule.

The specimen tubes used in the
seesaw test were prepared in the same
manner as those for the static tests.

Screening Tests., A series of
screening tests of various materials
in molten fluoride mixture No. 14
were made, These tests were made
with dehydrated, unpretreated fluoride
mixture for 100 hr at 816°C under
static vacuum. The results of these
tests are givenin Table 1. Molybdenum,
columbium, Monel, Stellite No. 25,
and nickel plus 1/4% zirconium alloy
were apparently unattacked. All of
the 300 series stainless steels tested
(304 ELC; 309, 310,317, 321, 347)
were attacked to a depth of 1 mil or
less, except types 304 ELC and 309,
which were attacked to a depth of
2 mils. Four-hundred series stainless
steels (only two types) were not
attacked over 1 mil. Z nickel was
the most severely attacked material
of those tested, being affected to a
depth of 5 mils. Figures 2 and 3
show typical samples of the types of
attack observed.

A seesaw test run with nickel-1/4%
zirconium alloy showed severe mass
transfer effects. Figure 4 shows
this test.

In Table 2 are shown the results
of a few screening tests with fluoride
mixture No. 2. A comparison of these
results with those in Table 1 reveals
that corrosive attack to approximately
the same extent can be expected with
both fluoride mixtures No. 2 and
No. 14 1in static tests,

Effect of Temperature om Corrosion.
Tests on the effect of temperature
have been runin both fluoride mixtures
No. 2 and No. 14. Tables 3, 4, and §
summarize the results of these tests.
At temperatures below 1100°C, no
significant effects can be noted that
could not be caused by normal test
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N FROTECTIVE CAPSULE WELDED - READY fOR TEST PIRE CUTTER ;;umw FOR ANALYSIS TUBE MOUNTED FOR
METALLOGRAPHIC EXAMINATION

 

 

Fig. 1. Loading and Testing Technigque Employed for Static Corrosion Tests.

SHATHONTd NALTOW
CORROSION BY

 

 

 

 

 

 

 

 

 

TABLE 1. CORROSION OF VARIOUS MATERIALS TESTED IN FLUORIDE MIXTURE NO. 14
AT 1500°F FOR 100 hr IN VACUUM
DEPTH OF METAL
MATERIAL AFFECTED METALLOGRAPHIC NOTES
(mils)
Type 304 ELC stainless steel 2 Subsurface voids, some following grain bound-
aries, attack somewhat irregular
Type 309 stainless steel 2 Subsurface voids
Type 310 stainless steel 1 Subsurface voids
Type 317 stainless steel 1 Intergranular penetration
Type 321 stainless steel 0 to 1/2 Intergranular penetration
Type 347 stainless steel 1 Subsurface voids
Type 430 stainless steel 1 Slight intergranular penetration and de-
carburization
Type 446 stainless steel 1 Subsurface voids
Nickel + 1/4% zirconium 0 No visible attack
Hastelloy B 0 to 1/4 Subsurface voids
Hastelloy C 2 Subsurface voids
Z nickel 5 Voids along grain boundaries
Stellite No. 25 0 No visible attack
Nichrome V 3 Subsurface voids, some following grain
boundaries
Monel 0 No visible attack
Inconel Subsurface voids, some following grain
boundaries
Inconel X 1 Subsurface voids
Tantalum 1 Surface of specimen roughened
Columbium 0 Surface somewhat roughened
Globe iron 2 Surface of specimen very rough, thickness
decrease indicates solution type of attack
Vanadium 1 Voids and intergranular penetration
Molybdenum 0 Surface somewhat roughened
TABLE 2. CORROSION OF VARIOUS MATERIALS TESTED IN FLUORIDE MIXTURE NO. 2
AT 1500°F FOR 100 hr IN VACUUM
MATERIAL DEETHQF :T:?:;AFFECTED METALLOGRAPHIC NOTES
26% Mo-T74% Ni 1/2 Subsurface voids
Molybdenum 0 No attack
Inconel 2 Subsurface voids
Type 310 stainless steel L 2 Subsurface voids
Type 316 stainless steel 1 Subsurface voids

 

 

 
   
      
 

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TYPE 347 TYPE 430 TYPE 448&

Fig. 2. Static Corrosion Tests of Stainless Steels in Fluoride Mixture No. 14 for 100 hr at 1500°F.
Original magnification 250X, reduced 58%.

SAATHONTA NALTOW
 

INCONEL NICHROME Vv Z NICKEL

  
  

 

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NICKEL + 'fi"a ZIRCONIUM STELLITE NO. 25 MOLYBDENUM

Fig. 3. Static Corrosion Tests in Fluoride Mixture No. 14 for 100 hr at 1500°F. Original magnifi-
cation 250X, reduced 58%.

A9 NOISOHMOD
variations. However, at higher temper-
corrosion is appreciably
Analysis of the fluoride
and

atures,
diminished.
bath was made after these tests,

 

Fig. 4. Cold End of Nickel-1/4%
Zirconium Alloy Seesaw Test Specimen
Run for 216 hr at aHot-End Temperature
of 798°C and a Cold-End Temperature of
610°cC.

TABLE 3. MAXIMUM PENETRATION OF INCONEL
TESTED IN FLUORIDE MIXTURE NO. 14
AT ELEVATED TEMPERATURES .-
FOR 200 HOURS

 

 

 

 

 

MOLTEN FLUORIDES

the results are in good agreement
with the metallographic data. After
testing at 800°C, 1800 ppm'6f chromium
was detected in the bath; at 1300°C
only 65 ppm of chromium was found.

Figure 5 shows results obtained at
different temperatures. Figure 6
shows the type of layer formed at
temperatures of 1200°C and above.
It is believed that this film acts
as a barrier to the diffusion of
chromium and thus prevents the formation
of voids, X-ray-diffraction patterns
of this film show it to be UQ,.

This test was repeated but the
sample was first heated in fluoride
mixture No. 14 at 1250°C for a long
period of time and then tested at
816°C for 100 hr; at this temperature
also, the layer inhibited corrosion.
It might be mentioned that this is a
very unreliable method of minimizing
corrosion because the film might
rupture and localize the corrosion
in one region. The high-temperature
heat treatment is also undesirable
because it results in considerable
grain coarsening that is detrimental
from a corrosion standpoint and to
the ability to form the metal. The

 

 

 

TEMPERATURE MAXIMUM PENETRATION formation of this film was dependent
(“¢) (mils) on the fluoride mixture, and such a
film appeared only in the tests in
800 3 which the fluoride mixture contained
900 1 1/2 Na, Li, K, and U,
1000 1 Effect of Time on Corrosion. A
1100 1 series of seesaw tests with Inconel
1200 0.5 was made over time intervals from
1300 <0.5 66 to 3000 hours. The results are
summarized in Table 6. After about
TABLE 4. MAXIMUM PENETRATION IN STATIC TESTS OF MATERIALS IN FLUORIDE
MIXTURE NO. 2 AT ELEVATED TEMPERATURES FOR 100 HOURS
MAXIMUM PENETRATION (mils)
MATERIAL
At B816°C At B850°C At 900°C At 1000°C
Inconel 2 4 5 5
Type 310 stainless steel 1 172 2 4 3
Type 317 stainless steel 0 3 4 3

 

 

 

 

 
CORROSION BY

  

UNCLASSIFIED
Y-7183

—— S ———
e =

  

(o) . ©

  

UNCLASSIFIED

 
 

(56) _ .

 

(d)

Fig. 5. Attack on Inconel by Fluoride Mixture No. 14 at (a) 900, (b) 1000,
(c) 1100, and (d) 1200°C in Static Test for 200 Hours. 250X.
TABLE 3.

MOLTEN FLUORIDES

MAXIMUM PENETRATION IN STATIC TESTS OF MATERIALS IN FLUORIDE
MIXTURE NO. 14 AT ELEVATED TEMPERATURES FOR 100 HOURS

 

 

 

 

 

MAXIMUM PENETRATION (wils)
MATERIAL At  538°cC At T04°C At 816°C At l000°C
Specimen Tube Spacimen Tube Specimen Tube Speacimen Tube
Inconel Slight Slight 4 1 1/2 1 1/2 3 3 3
roughening roughening
Type 430
stainless steal <1 <1 1/2 1/2 1/4 1/4 Ne attack
Type 304
stainless stesl < 1 <] 2 1 1/2 2 2 1 1/2 1
Type 321
stainless steel <1 <1 <1 1 1/2 /2 11/2 11/2

 

 

 

 

 

 

 

 

 

   

Fig. 6.
No. 14 in Static Test for 200 Hours,

500 hr, the amount of additional
attack is quite small. This is
possibly due to saturation of the
fluoride with chromium or the removal

UNCLASSIF IED
Y7074

Layer Formed on Inconel at 1200°C by Exposure to Fluoride Mixture
1000x%.

of impurities in the fluorides. The
test at 3000 hr had one small area
of dense corrosion, that was 13 mils
in depth.
CORROSION BY

Effect of Cold Work on Corrosion.
A series of cold-worked Inconel
specimens was prepared with from
0 to 70% reduction in thickness by
rolling, These specimens were tested
in fluoride mixture No. 14 for 100 hr
at 815°C. No significant differences
in behavior were noted. Another
series of Inconel specimens was
prepared with from 0 to 90% cold work.
No differences were noted in this

steel and Inconel were tested in
fluoride mixture No. 2 in both the
as-received and 20% cold-worked
condition. No appreciable differences
were noted in these tests. Data from
these tests are presented in Tables 7
and 8.

Addi tion of Inhibitors. A large
number of both static and seesaw tests
have been run with the addition of
small percentages of other materials

 

 

 

 

 

 

 

 

 

 

 

 

 

 

series. Types 316 and 310 stainless to the fluoride mixtures. In general,
TABLE 6. EFFECT OF TIME ON THE CORROSION OF INCONEL BY
FLUORIDE MIXTURE NO. 14 IN SEESAW TESTS
Frir op TEMPERATURE (°C) METALLOGRAPHIC OBSERVATIONS
Hot Zone
TEST (hr) Hot Zone Cold Zone (depth of voids) Cold Zone
66 810 620 1/2 mil, average No evidence of
1 mil, maximum reaction
115 780 600 1 mil, average No evidence of
2 mils, maximum reaction
210 780 630 2 mils, average No evidence of
5 mils, maximum reaction
500 750 610 3 mils, average Metallic deposit 1/2
7 mils, maximum mil thick
750 815 620 3 1/2 mils, average | Metallic deposit 1/2
6 mils, maximum mil thick
3000 800 560 4 mils, average Metallic deposit 1/2
13 mils, maximum mil thick
TABLE 7. EFFECT OF COLD WORK ON THE CORROSION OF INCONEL TESTED IN
FLUORIDE MIXTURE NO. 14 FOR 100 hr AT 815°C
REDUCTION BY COLD-ROLLING WEIGHT LOSS DEPTH OF SUBSURFACE THICKNESS
(%) (g) VOIDS (mils) CHANGE
0 0.0020 1 0
D 0.0043 2 0
11.5 0.0036 13,2 0
26.0 0.0036 1 1/2 0
50.0 0.0023 1 0
71.0 0.0028 1 0

 

 

 

 

10
MOLTEN FLUORIDES

TABLE 8. EFFECT OF COLD WORK ON CORROSION OF STAINLESS AND INCONEL
TESTED IN FLUORIDE MIXTURE NO. 2 FOR 100 hr AT 815°C

 

 

 

 

 

 

 

 

 

MATERIAL COLD WORK (%) PENETRATION (mils)
Type 316 stainless steel As-received 1
20 2 12
Type 310 stainless steel As-received 1 tol 1/2
21 /2 te 1 V)2
Inconel As-received 1/2 to 2
26 1/2
active metals are effective in reducing TABLE 9. RESULTS OF ADDITIONS TO
corrosion and in many tests they stopped FLUORIDE MIXTURE NO. 14 IN
corrosion completely; other metal CORROSION TESTS*
additions have no effect on the
corrosion; the addition of chemical ADDITION MAXIMUM DEPTH OF ATTACK
compounds often increases corrosion; (0.25 wet %) (mils)
however, many exceptions are noted.
Tables 9 and 10 summarize the results Na 0
of the seesaw and static tests. Be 0
In both types of tests, Na, Li, Ti, Mg 0
and Ca reduced corrosion. G. M. Al 0
Adamson has found that additions of Ti 0
zirconium and titanium are quite v 0
beneficial in minimizing corrosion Cr (2 tests) 0
by fluoride fuels in Inconel thermal Li (2 tests) 0
convection loops. Contradictory Cu 0
results were obtained on Zr, Mn, and NaH 0
Mg. The following tabulation classifies Ca 0
materials as those that have been Cc 1
beneficial and those that have not. Si 2
It is thought that the Zr and Mn used Mo (3 tests) 1/2
in the seesaw tests were not thoroughly Zn 4
dried. W 3
Fe 3
BENEFICIAL NOT BENEFICIAL Zr 4
Mn 4
Na Si Ag 4
Be Zn Nal 8
Al W KCl 10
¥ Fe KBr 10
Cr Ag KI 8
Li Nal NaCl 8
Cu KCl LilO 10
NaH KBr 2
Ca Mnoz *Seesaw tests run for 200 hr at BHBOC; material

C Nin tested was Inconel.

11
CORROSION BY

 

 

TABLE 10. EFFECT OF VARIOUS ADDITIONS TO FLUORIDE NO. 14 IN STATIC CORROSION
TESTS AT 816°C FOR 100 HOURS
DEPTH OF METAL
MATERIAL ADDITION AFFECTED METALLOGRAPHIC NOTES
(mils)
Inconel 2% Mg 5 Large voids in both specimen and tube
2% Zr 1 Surface layer 1 1/2 to 2 mils thick,
1/2 to 1 mil of attack beneath surface
layer
10% Zr 1 1 1{2-ni1 surface layer on specimen and
tube, subsurface voids to 1 mil under
surface layer.
10% Na 0 No attack
1% Na 1/2 Specimen and tube have a few subsurface
voids
10% U 1 l1- to 2-mil surface layer
5% Li 1/2 l-mil surface layer
5% K 0 No attack
5% Ca 0 1/2-mil surface layer, no attack be-
neath surface layer
5% Ti 0 1/2-mil surface layer, no attack be-
neath it
5% Mn 0 No attack
Type 321 stainless
steel 10% Zr 0 1/2-mil surface layer on specimen and
tube, no attack beneath layer
10% Na 0 No attack
1% Na 0 No attack
10% U 0 2 to 3-mil surface layer, no attack
under layer
5% Li 0 No attack
5% K 0 No attack
5% Ca 1/2 1/2-mil surface layer, attack to 1/2
mil beneath surface layer
5% Ti 0 No attack
5% Mn 1 Subsurface voids 1/2 to 1 mil
A-nickel 10% Zr 0 2 1/2-mil surface layer, no attack
under layer
10% Na 0 1/2-mil surface layer, no attack be-
neath layer
Type 309 stainless
steel 2% Mg 5 Subsurface voids, attack irregular
in occurrence
2% Ir 0 No attack on specimen or tube,
specimen has 1/2-mil surface layer
10% MnO, 4 Subsurface voids and intergranular
penetration to 4 mils
Inconel 10% MnO, 9 Subsurface voids and intergranular
penetration
10% CrF, 4 Subsurface voids
10% NiF, 7 Subsurface voids
10% FeF, 9 Subsurface voids
31/3% Cer. 2 Subsurface voids
3 1/3% NiFI.
3 1/3% Fer

 

 

 

 

12
Mo FelF,
Ti CrF,
Zr NaCl

LiIO,

It is thought possible that the
beneficial materials added may be
acting to reduce the available amount
of fluorine in the system, which would
cause the atoms of the structural
metals to go into solution. It is
also possible that the beneficial
additions act as oxygen getters.,

The metal fluorides were added with
the thought that perhaps a reaction
of the following type might be the
corrosion mechanism:

ALkF + Me T— MeF + Alk

where Alk could represent Na, Li, or
K and Me could represent Fe, Ni, or
Cr. If this were the case, then
additions of iron, nickel, or chromium
fluorides and also sodium, lithium, or
potassium should tend to drive the
reaction to the left and help reduce
corrosion. However, additions of iron,
nickel, and chromium fluorides had no
such effect in reducing corrosion.
The addition of iron and nickel
fluorides actually increased the
corrosion; this is probably due to the
reduction of the nickel and iron
fluorides by the chromium that was
removed from the container wall,
Metallic sodium, lithium, and potassium
did reduce corrosion; but, as mentioned
above, their action is probably that
of getters of oxygen and fluorine
rather than that of taking part in the
reaction.

The MnO additions were made to see
whether oxygen added to these tests in
this form would increase corrosion,
and 1t did. The alkali halides were
added to see whether the presence of
other negative ions than fluoride
would have an inhibiting effect, and
they did not.

In many of the tests that were run
with additions, surface layers were
apparent on the specimens after test.
The following tabulation gives the

MOLTEN FLUORIDES

combinations of containing metals and
additions that resulted in surface
layers. It is possible, of course,
that in some cases in which surface
layers were not observed, the layers
may have been inadvertently cracked
off during stripping.

ADDITION TO

FLUORIDE BATH CONTAINING METAL

Zr Inconel

Zr Type 309 stainless steel
Zr Type 321 stainless steel
Zr A Nickel

U Inconel

U Type 321 stainless steel
U A Nickel

Ca Type 321 stainless steel
Ca Inconel

Ti Inconel

Li Inconel

The i1dentity of the surface layers
is not definitely established. 1In
the case of zirconium additions in
type 309 stainless steel, the surface
layer was reported by x-ray to contain
UO, and ZrO,. In other checks on these
surface layers, UOzis.usually reported
and, occasionally, unknown constituents
are detected.

Effect of Heat Treatment and Carbon
Content. Five tests with different
heat treatments were made with Inconel,
and no significant differences were
noted. However, in tests in which
the Inconel had been hydrogen-fired
at temperatures up to 1200°C, which
created a large grain size, attack
has been observed to be more localized.
The depth of attack was greater, but
the total volume was less. Table 11
summarizes the tests run to determine
the effect of heat treatment,

Mass Transfer in Molten Fluorides.
Metallic deposits have been noted in
seesaw tests with fluoride mixture
No. 14 in Globe iron, A nickel, and in
a nickel alloy containing 1/4% zir-
conium. Yery slight amounts of metallic
deposits have been noted in a few
Inconel tests. Thin (less than 0.5
mil) layers have been seen in the cold

13
CORROSION BY

 

 

 

 

TABLE 11. EFFECT OF HEAT TREATMENT OF INCONEL ON CORROSION BY FLUORIDE
MIXTURE NO. 2 AT 815°C FOR 100 hr IN STATIC TEST
HEAT TREATMENT DEPTH OF SUBSURFACE
VOIDS (miis)
Water-quenched from 2100°F 1/2 to 2
Furnace-cooled from 2100°F 142 0] 1/2
Water-quenched from 2100°F, reheated to 1600°F
for 24 hr, and water-quenched 1/2 ta 3 1/2
Furnace-cooled from 2100 to 1600°F, held for
24 hr, and water-quenched 1/2 to 1
Heated to 1600°F, held for 24 hr, and water-
quenched 1/2 to 3 1/2

 

 

legs of thermal convection loops with
both Inconel and stainless steels,
Deposition of a nonmetallic layer
occurs in the cold zone of many seesaw
tests, thermal convection loops, and
in some static tests. This coating
is quite thin, being less than 1 mil
in all cases except with the additions
of zirconium and uranium. The film

14

apparently has an inhibiting effect
on mass transfer, and has been found
by x-ray-diffraction studies of several
tests to be composed of UO, and an
unknown constituent or constituents,

The small amount of mass transfer
occurring in these tests is not
believed to be a problem except in the
case of pure metals.
MOLTEN FLUORIDES

APPENDIX

SUBSURFACE VOID FORMATION IN METALS DURING HIGH-TEMPERATURE CORROSION TESTS

A. deS. Brasunas

Inconel and similar alloys have
been observed to be susceptible to
subsur face void formation during
processes involving chromium depletion
of the surface zone by any of the
following methods:

1. leaching with certain molten salts,
2. high-temperature vacuum treatment,
3. high-temperature oxidation.

Porosity inmetals has been observed
previously in diffusion experi-
ments(1+2+3) and is attributed to the
uneven diffusion rates of different
metal atoms through a metal lattice.
Although similar observations have not
previously been reported in corrosion
studies, their occurrence should not
be too surprising. (Voids have been
reported(** %) in the oxide layers of
metals and alloys undergoing oxidation
in air. Under these conditions, metal
ions diffuse outward more rapidly than
oxygen ions can diffuse inward through
the oxide layer.) Alloys are known to
change in composition in the surface
regions because of the higher reactivity
of certain alloy constituents at the
metal inter face. Such depletion
without adequate replacement of removed
atoms causes vacancies, which may
“precipitate’ to form visible voids.
Al though undesirable, this form of
corrosion is not as damaging as some
and could be tolerated in certain
instances.

 

(1)), Smigelskas and E. Kirkeandall, Tranas.
Am, Inst. Mining Wet. Engrs. 171, 130 (1947).

U w6 daBlive snd B B Mokl i Wetala
191, 153 (1951).

3)H. W. Balluff and B. H. Alexander, Sylvania
Electrie Produets, Inc., SEP-83 (Feb. 1952).

6D L Preil, J. Insts Watdla, 119 561
(1929).

ISIA. deS. Brasunas and N. J. Grant, Trans.
An. Soc. Metals 44, 1117 (1952).

Corrosion in Molten Fluoride Salts.
Corrosion tests in which fused alkala
metal fluoride salts (fluoride mixture
No. 14) come into contact with chromium-
bearing alloys such as Inconel at
temperatures above 1300°F have resulted
in the formation of a porous surface
layer, as illustrated in Figs. Al and
A2. Although it is fully recognized
that in many cases voids have been
observed metallographically because
(1) particles have been physically
removed during polishing, (2) certain
phases may react with water and dissolve,
or (3) etch pits can result, it seemed
nevertheless probable that these may
be true voids. Careful polishing by
using nonaqueous techniques has failed
to show any difference in the appearance
of the porous surface region of the
metal specimens in the etched or un-
etched conditions. Chemical analyses
of the metal surface before and after
testing have indicated only one
significant change; there was an
appreciable drop in the chromium con-
tent from about 15% to about 5%. The
analysis of the fluoride salt bath
showed correspondingly higher chromium-
to-iron and chromium-to-nickel ratios
than would result from uniform attack
of the alloy. This clearly establishes
the selective removal of chromium.

Supplementary Data. The saturation
of the fluoride salt with chromium
prior to testing was effective in
suppressing void formation in tests
that would otherwise have resulted in
voids to a depth of several mils.
Furthermore, the measured area of
voids observed agrees very closely
with the amount calculated on the
basis of the change in chemistry of
the metal surface mentioned above.

15
CORROSION BY

UNCLASSIFIED
Y-7014

 

Fig. Al. Subsurface Voids Usually Encountered After Inconel or Stainless
Steels Contact Molten Alkali Fluoride at Temperatures of About 816°C for 100
Hours. Inconel, unetched, 250X.

      

UNCLASSIFIED

: - Y6794
S e

Fig. AZ. Appearance of Subsurface Voids at High Magnification. Etched, 2000x.

16
This computation assumes no over-all
volume change of the specimen,

If voids, are, caused by chromium

depletion, as these experiments indi-
cate, then voids should also be observed
when loss of chromium is effected by
other means. Therefore two methods
were tried; the first technique in-
volved high-temperature vacuum treat-
ment and the second involved high-
temperature oxidation. Both attempts
were successful and will be described
briefly.

High-Temperature Vacuum Treatment.
The high vapor pressure of chromium
relative to that of iron or nickel
suggests that high-temperature vacuum
treatment would also be a suitable
means of producing a chromium gradient,
Specimens of Inconel and an 80% Ni-20%
Cr alloy were exposed to a vacuum of
0.1 mm Hg for 42 hr at 1375°C and then
furnace-cooled. Both samples showed
many subsurface voids; the voids in
Inconel appeared to be spherical,
whereas those in the 80% Ni-20% Cr
alloy were angular. These are shown
in Fig. A3.

High-Temperature Oxidation. It 1is
well known that chromium-bearing
alloys have good high-temperature
oxidation resistance that can be
attributed to the formation of a
chromium-rich, diffusion-resistant
oxide phase on the surface of the
alloy. The chromium in the oxide
phase is supplied by the surface and
the underlying regions of the speci-
men, which would tend to be partially
depleted of chromium.

An Inconel specimen was therefore
exposed toair at 1250°C for 200 hours.
After oxidation, metallographic exami-
nation revealed the usual oxide layer
on the surface and in the adjacent
grain-boundary areas. In addition,
subsurface voids were observed as
anticipated; these are shown in
Fig. A4,

Theoretical Comnsiderations. The
tests just described strongly indicate
that void formation is caused by

MOLTEN FLUORIDES

chromium depletion. The formation of
cavities large enough to be resolved

" by an ordinary microscope is visualized

to occur as illustrated schematically
in Fig. AS5. As chromium is leached
from the surface of the metal, a con-
centration (activity) gradient results
that causes chromium atoms from the
underlying region to diffuse toward
the surface and leave behind a zone
enriched with vacancies, as shown in
the center sketch of Fig. A5. These
vacancies can agglomerate at suitable
sites and become visible as voids.

A certain number of vacancies can
be tolerated in crystal lattices of
solid metals, and such imperfections
are believed to exist in practically

UMCLASSIFIED
¥-4825

 

Fig. A3. Section of 80% Ni-20% Cr
Alloy After 42-hr Exposure to Vacuum
at 1375°C. Void shape bears some
relationship to twin lines; note simi-
larity to Fig. A2, Etched, Original
magnification 250X, reduced 32%.

17
CORROSION BY

UNCLASSIFIED
7072

 

Fig. A4. Section of Inconel Specimen After 200-hr Exposure to Air at 1250°C.
Note void formation beneath oxidized region. Unetched, 250X.

C CHROMIUM ATOM UNCLASSIFIED
F IRON ATOM DWG. 16322
N NICKEL ATOM

 

BEFORE LEACHING VACANCIES CAUSED BY LEACHING VACANCIES PRECIPITATED

Fig. A5. Mechanism of Subsurface Void Formation.

18
all solid metals, the amount increasing
with increasing temperature. Further-
more, they can also be generated at
sur faces, grain boundaries, inclusions,
dislocations, and other possible
crystal defect areas. One of the
mechanisms of diffusion is therefore
visualized as taking place by the
movement of these vacancies; that 1is,
neighboring atoms move to occupy vacant
sites which give the 1llusion of
vacancy movement in the opposite
direction,

For the sake of clarity none of the
existing “tolerated’ vacancies are
shown in Fig. A5, although they must
be assumed to be present at all times.
The left sketch shows the Fe, Ni, and
Cr atoms arranged at random as one may
expect to find them on the 111 plane
of an Inconel specimen. After selective
diffusion of chromium has occurred by
any of the test conditions described
(from about 15 to 5%), vacancies must
result if these vacant lattice sites
are not filled by diffusion of nickel
or iron atoms from the surface proper.
If the lattice had been “saturated”
with vacancies, then these additional
ones are not ‘“‘tolerated’ and must
“precipitate” as shown in the sketch
at the right in Fig. A5. For ease of
illustration, only six atoms are shown
to have precipitated to form a void;
the number of missing atoms in voids
actually observed in these experiments
is of the order of 102,

e S - — — — —
5

MOLTEN FLUORIDES

\\

The voids shown in Fig, A3 are of
two sizes, The larger ones are believed
to have occurred at temperature in the
usual manner, whereas in the case of
the smaller ones, it is interesting to
visualize them as having occurred on
cooling. Rough approximations of
vacancy densities at several tempera-
tures based on the formula of Mott and
Gurney‘®’ indicate that such a theory
is not improbable. Hence, it may be
assumed that there is a temperature
coefficient of “solubility” for vacant
atoms such as there i1s for foreign
atoms.

Conclusions. In reactions in which
the neteffect of the diffusion involved
is essentially monodirectional, there
is a movement of mass that results in
a change in density and/or shape of
that portion of the specimen. If the
concentration of vacancies left behind
is greater than some critical value,
they will tend to collect at suitable
locations and appear as visible voids.
Such diffusion phenomena have been
observed on a number of occasions when
bimetallic (i.e., metal-metal) dif-
fusion couples were studied,(1:2.3)
It has been demonstrated that identical
effects can be obtained in metal-
liquid and metal-gas systems in which
similar diffusion phenomena occur.

 

{511'!. F. Mott and R. W, Gurney, Electronic
Processes in Jonie Crystals, p. 31 Oxferd
Clarendon Press, 1940.

19
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