ORNL-TM-4221.txt

 

ORNL-TM-4221

 

3 4

A FORCED-CIRCULATION LOOP FOR

CORROSION STUDIES:
HASTELLOY N COMPATIBILITY WITH

NaBF,-NaF (92-8 mole %)

J. W. Koger

 

 

HOPER'ATED*_ vi

 
 

 

 

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-4221

Contract No. W-7405-eng-26

 

METALS AND CERAMICS DIVISION

A FORCED-CIRCULATION LOOP FOR CORROSION STUDIES: HASTELLOY N
COMPATIBILITY WITH NaBF,-NaF (92-8 mole %)

J. W. Koger

DECEMBER 1972

 

— NOTICE——
This “seport’ 'was prepsred as an account of wi
T . - m
:gonéoud_z» by thie United States Government. Neithe‘: |1
c: United States nor the United States Atomic Energy |
& t_nm]ssiqn.;'nor sny of their émployees, nor sny of
_ e:-e contractors, subcontractors, or their employees
ll:ll 33 any warranty, express or implied, or assumes an);
- lgal Alability or responsibility for the accuracy, com-
: ,g_;t::::sd:r ““fulfl;? of any information, spparstus, |
. r process losed, or represents th .
would not infringe privately owned rights, ot fhs use

 

 

 

 

 

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION

for the o s e '
U.S. ATOMIC ENERGY COMMISSION MASTER

PISTRIBUTION OF THIS DOCUMENT 1S UNLIMITED
 

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CONTENTS
ADSIIACt . .ottt i ittt e ettt e 1
Introduction . ... e 1
Design AppProach .. ...... it i i i it a i 1
0T 1411 121 g 6
OPeratiON . ..ttt ittt e i et it a it e ae e 9
Preliminary- OPeration ... ......uuiniieeiitnneereereennsseseseaniinaeeeeessnannnnn 9
. Design Operation .. ... ...ttt iiiiieininrernentnrrrarosatnsasosacnensnsas 11
Corrosion Results .. .....it ittt i ittt tiiiitisea et feeeas 11
= CONCIISIONS 4. e vttt ettt e ettt et ettt et e ettt e e aae e e e aaneeeesannneneeaniansens 16

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A FORCED-CIRCULATION LOOP FOR CORROSION STUDIES:
HASTELLOY N COMPATIBILITY WITH NaBF,-NaF (92-8 mole %)

J. W. Koger

ABSTRACT

A sophisticated pumped loop system was designed and operated to study the compatibility of
Hastelloy N to flowing NaBF 4-NaF (92-8 mole %) at a maximum temperature of 620° C and a minimum
~of 454°C. One unique feature of the design of this loop was the ability to remove or add corrosion test
specimens without draining the fluid from the loop. Principal test variables included temperature,
velocity, time, and impurities in the fluid.

Measurements disclosed weight losses in hot-zone specimens, weight gains in cold-zone specimens,
with a balance point (ro gain or loss) at a temperature halfway between the maximum and minimum.
Under certain conditions, larger weight losses and gains were found in specimens exposed to .
higher-velocity salt. In time, mass transfer rates became much lower, and no velocity effect was noted.
Specimen examination early in the life of the loop disclosed a definite downstream effect. The first
specimen of three exposed to the salt (same temperature, same velocity for each specimen) showed the
largest gain or loss, the next specimen showed the next largest change, and the last specimen showed
the smallest change of the three. Qverall average corrosion rate of the hottest specimen (620°) was
0.94 mil/year at 20.8 fps and 0.74 mil/year at 10.9 fps. An average corrosion rate of 0.05 mil/year for
the hottest specimen was found in one 1256-hr increment.

INTRODUCTION

A molten-salt forced-circulation loop, MSR-FCL-2, was designed and operated by W. R. Huntley of the
Reactor Division and H. C. Savage of the Reactor Chemistry Division to determine the corrosion resistance
and mass transfer characteristics of Hastelloy N in a fluoroborate-type coolant salt system proposed for the
MSBR. The coolant salt proposed for an MSBR is the eutectic mixture of NaBF,-NaF (92-8 mole %), which
has a melting point (liquidus temperature) of 385 * 1°C (725 = 1.8°F). The Metals and Ceramics Division
had the responsibility for the analysis of the corrosion and mass transfer behavior of the system.

Design Approach

To simulate MSBR coolant circuit conditions, the salt was circulated in the loop piping at a velocity of
~10 fps at bulk fluid temperatures from 454 to 620°C. Corrosion test specimens were installed at three
locations in the loop circuit for exposure to the salt at three different temperatures (620, 537, and 454°C)
and at bulk flow velocities of 10 and 20 fps (4 gpm flow rate). The corrosion specimen installation and
removal system, shown in Fig. 1, is unique in that the corrosion specimens can be installed or removed
without draining the salt from the loop, although it is necessary to stop the pump and thaw the freeze
valve. The specimens were attached to the specimen rod by clips held in place by small wires. The clips and
wires could be attached or removed with a minimum of labor and without damaging the corrosion

specimens. Different cross sections of the specimen rod permitted the different salt velocities. Thus three

specimens were exposed to one velocity and three to another (10 and 20 fps). Access ports and salt
sampling equipment were provided to remove salt samples from the loop and sump tank for analysis and for
installation of cold-finger devices. An inert cover gas system (high-purity helium) was used to prevent salt
contamination by water and oxygen. -

An isometric schematic of MSR-FCL-2 is shown in Fig. 2, and an actual picture of the loop is shown in
Fig. 3. It consisted of approximately 80 ft of 0.50-in.-OD by 0.042-in.-wall Hastelloy N tubing through
which the salt was circulated by means of the salt pump, Fig. 4 (designated ALPHA). The new pump was
designed for variable salt flow rates up to 30 gpm, pressure heads to 300 ft, speeds to 6000 rpm, and

 
 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Fig. 1. Corrosion specimen installation and removal system for MSR-FCL-2.
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RESISTANCE HEATED SECTION NO.1

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DRAIN AND FILL LINE Y5-in.OD X 0.042~in, WALL

Q& BALL VALVE HASTELLOY N

AUXILIARY TANK N

 
     

HEATER LUGS (TYPICAL) RESISTANCE HEATED SECTION NO. 2

COOLER NO.1

    
  
 
    

   

FLOW RATE = ~4 gpm _
VELOCITY = ~10 fps IN %-in. TUBING
REYNOLDS NO, = ~37,000 TO 45,000

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* SPECIMENS \
COOLER NO.2

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g‘\ =~ Ya=in.OD X 0.035-in. WALL

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THERMOCOUPLE WELL

Fig. 2. Molten-salt forced-convection corrosion 1oop MSR-FCL-2.

 

 
 

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ALPHA pump and auxiliary tank of MSR-FCL-2.

 
 

 

PHOTO 78170

rin S

Fig. 5. Air-cooled finned radiators of MSR-FCL-2.

temperatures to 760°C.!~4 Thus the new pump allowed much flexibility in the operation of the loop.
Along with the pump was an auxiliary tank which could be used for sampling or additions. In addition to
the loop tubing, all parts of the loop systems which were contacted by salt were fabricated of Hastelloy N.
The salt was heated from 454 to 620°C in passing through two resistance-heated sections of the loop
tubing, and then was cooled to 454°C in passing through two air-cooled finned radiators (Fig. 5) before
returning to the pump suction.

Design Data

Some of the pertinent engineering design data for MSR-FCL-2 are given in Table 1. Heat transfer values
shown in Table 1 are based on the properties of NaBF,-NaF (92-8 mole %) and Hastelloy N (INOR-8) given
in Table 2. The calculated temperature profile around the loop circuit at design operating conditions is
shown in Fig. 6.

 

1. MSR Program Semiannu. Progr. Rep. Feb. 28, 1970, ORNL4548, p. 74.
2. MSR Program Semiannu. Progr. Rep. Aug. 31, 1970, ORNL-4622, p. 45.
3. MSR Program Semiannu. Progr. Rep. Feb. 28, 1971, ORNL-4676, p. 59.
4. MSR Program Semiannu. Progr. Rep. Aug. 31, 1971, ORNL4728, pp. 35-36.

 
 

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Table 1. Engineering design data for MSR-FCL-2

 

. Materials, temperatures, velocities

Tubing and corrosion specimens, standard Hastelloy N
Nominal tubing size, %-in. OD by 0.042-in. wall
Approximate tubing length, 90 ft

Bulk fluid temperatures, 454 to 620°C (850 to 1150°F)
Bulk fluid AT, 166°C (300°F)

Flow rate, 4 gpm

Fluid velocity, 10 fps (to 20 fps past corrosion specimens)
System AP at 4 gpm, 82 psi

. System volumes

Piping ( /,-m OD by 0.042-in. wall, 147 in.3 (2400 cm3)

Pump (at 1%-in. salt depth), 30 in.? (492 cm?)

Auxiliary tank (at 4-in. salt depth), 80 in.3 (1311 cm?)

Drain lines (¥} in. by 0.035-in. wall), 10 in.3 (164 cm?)

Total salt volume in loop, 267 in.3 (4380 cm3)

Capacity of fill and drain tank (5-in. sched 40 pipe by 22 in. long), 440 in.3 (7200 cm?)

. Heat transfer (heaters and coolers)

1. Coolers
Material, %-in.-OD by 0.042-in.-wall Hastelloy N tubing with 1, ¢-in. -tthk nickel fins
Number of cooler sections, 2
Finned length of cooler No. 1, 18.8 ft
Finned length of cooler No. 2, 17.9 ft
Coolant air flow, 2000 cfm per cooler
Cooling capacity No. 1 cooler (1000 2-in.-OD by y s-in.-thick fins), 101 kW (345,000 Btu/hr)
Cooling capacity No. 2 cooler (645 1 /g'll'l 0D by /15-m -thick fins), 43 kW (146,000 Btu/hr)
Total heat removal (both coolers), 144 kW (491,000 Btu/hr)

Heat flux (based on tube ID)

Cooler 1, 168,000 Btu hr ™! ft =2

Cooler 2, 71,000 Btu hr~! ft~2

Inside wall temperature at outlet, cooler No. 2, 425°C (798°F)

2. Heaters .
Material, %-in.-OD by 0.042-in.-wall Hastelloy N _
Number of heated sections, 2
Length of each heater, 12 ft
Heat input, each heater, 62.5 kW (213,000 Btu/hr)
Total, 125 kW (426,000 Btu/hr)
Inside wall temperature at outlet heater No. 2, 680°C (1256°F) .
Qutside wall temperature at outlet heater No. 2 (maximum pipe wall temperature) 692 C (1278°F)
Heat flux, 162,000 Btu hr~! ft~2 .
Salt Reynolds numbers in loop piping at 4 gpm, 31,000 to 51,000

 

 
 

Table 2. Properties of NaBF4-NaF (92-8 mole %) and Hastelloy N kr
(tisin °F; Tisin °R)

 

A. Properties of NaBF4-NaF (92-8 mole %)
Density (Ib/ft3), 142.6-0.025¢
Heat capacity, Cp, (BtuIb™! CF)™'], 0.36
Thermal conductivity, k¥ {Btu hr™? ft™! (°F)71], 0.23
Viscosity (Ib ft~1 hr™1), 0.2121 exp (4032/7)
Melting point (°F), 725

B. Properties of Hastelloy N \ o
Thermal conductivity, k {Btu hr™! ft™! CF)™1], 6.057 + 2453 X 1073£+1.931 x 1076 2
(at 850°F = 9.54) (at 1150°F = 11.43)
Electrical resistivity (u2-in.), 47.5 at 75°F, 49.6 at 1300°F
Mean coefficient of thermal expansion {107¢ in. in.”! CF)™], 7.8 (70-1200°F)
Chemical composition (%)
; Chromium, 6.00-8.00
i Molybdenum, 15.00—18.00
Iron, 5.00, max
Silicon, 1.00, max
Manganese, (.80, max
Carbon, 0.04-0.08 : ~
Nickel, balance - :

 

ORNL-DWG 70-5628

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

' | | ! I
PUMP I.HEATER-1| IHEATER-Zl |  COOLER-1 I | COOLER-2 | RETURN
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| | ] | | | | PUMP
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LENGTH (ft)

_Fig. 6. Temperature piofile of MSR-FCL-2 st design operating conditions.
 

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OPERATION
Preliminary Operation

Salt and gas purification. The salt charge for MSR-FCL-2 was processed to improve its purity prior to
transfer into the loop. The first routine processing was done after 25 kg of the salt components were mixed
and sealed in a vessel. The mixed powders were heated to 149°C and evacuated with a mechanical vacuum
pump for 45 hr to remove moisture. The salt was then heated to' 500°C, and 16.7 kg was transferred to a

- small filling pot. The salt in the filling pot was heated under static vacuum to check for further impurity

outgassing. The pressure rose from —28.7 in. Hg at 149°C to —26.4 in. Hg at 470°C, which indicated little
outgassing above the expected BF; vapox pressure of the salt. An equilibrium mixture of He-BF3; was
bubbled through the salt at 480°C at about 100 cc/min for 60 hr, and the effluent gas was passed through a
cold trap at 0°C. This test was run to check for impurity collections such as were noted during operation of
a liquid level bubbler in the PKP loop.5 The inlet gas mixture was prepared for the experiment by mixing
dried helium (<1 ppm H;O) with BF; in the proper ratio to balance the vapor pressure of the hot salt.

Impurities were found in the gas mixture after it had bubbled through the salt. A gas rotameter
downstream of the cold trap initially became fouled with a clear liquid. A white film also deposited on the
walls of the glass cold trap. A total of 0.15 cc of brown liquid was collected in the trap after 60 hr, and the
materials collected appeared similar to those noted by.Smith5 during operation of the PKP loop. Analysis
was inconclusive as it only disclosed major quantities of Na, B, and F, with ppm values for many other
elements. These instances of impurity collection show that presently available sodium fluoroborate systems
can be expected to give problems with acid collection and fouling when devices such as liquid level bubblers
are used. From the small amount of material collected, we concluded that our method was not practical for
removing large amounts of impurities. However, we feel that unprovements could be made on the basic
process which could provide more efficient removal.

Heat transfer measurements. During hot shakedown operations we measured the heat transfer
performance of the coolant salt. Heat transfer data were obtained at Reynolds moduli from 4200 to
48,000, salt velocities from 1.25 to 10.6 fps, heat fluxes from 43,000 to 157,000 Btu hr™! ft72, film
coefficients from 230 to 2100 Btu hr™! ft™ (°F) ™!, and at salt temperatures from 454 to 616°C. The test
section was '4-in.-OD tubing with a measured ID of 0.416 in. Resistance heating was supplied by a three-lug
system with voltage applied to the center lug and the two exterior lugs at. ground potential. Therefore, there
was a section at the center lug of the heated length which had an interrupted heat flux. The actively heated
length was 11.5 ft, which gives an L/D ratio of 331. Guard heaters were located on the heater lugs and
along the resistance-heated tube to reduce heat losses during the heat transfer tests.

The temperature of the bulk fluid was measured by three thermocouples located in wells at the inlet
and exit to the test section. Wall temperatures along the heated section were measured by thermocouples

. that were wrapped at 180° intervals circumferentially around the tubing and clamped against the wall at

about 1-ft intervals. The above-mentioned thermocouples were all sheathed, insulated junction, 0.040-in.-
OD Chromel-Alumel and were precalibrated. Four bare wire thermocouples (0.010-in.-OD Chromel-Alumel)
were also placed on the heated wall and read out on a potentiometer for comparison with the sheathed
thermocouples recorded by the DEXTIR data logger The readlngs of the two types of thermocouples were
in good agreement. : : \

The heat transfer data obtained with sodmm fluoroborate in MSR-FCL-2 were put in dimensionless
form and are shown in Fig. 7. The data are in good agreement with the empirical correlation of Sieder and |

 

5. A.N. Smith et al., MSR Program Semiannu. Progr. Rep. Feb. 28, 1969, ORNL-4369, pp. 102-6.

 
 

10

ORNL-DWG 74-9642

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Ny, = 0027/v°°/v’3( ")

HEATED SECTION £/0 RATIO = 334

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0.14
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103 2 5 10* 2 5 10°
| REYNOLDS NUMBER

Fig. 7. Heat transfer characteristics of NaBF4-NaF (92-8 mole %) flowing in 0.416-in.-ID tube

Tate,5 which is shown by the solid line. The plotted data points were obtained with more precise
thermocouple techniques than those used in earlier work by Huntley;? therefore the data of Fig. 7 are
considered the best available evidence that sodium fluoroborate does indeed perform as a typical heat
transfer fluid.

Pump seal leak. A gas leak occurred at an O-nng seal within the ALPHA pump on July 7, 197 1, as the
test was being brought to design operating conditions. At that time the salt had been in the loop for 510 hr
and had been circulated by> the pump for 56 hr. The salt inventory was sampled to check for impurity
additions resulting from the gas leak, and none were found. The system was then drained to repair the
pump. The gas leak resulted from BF; attack of a Buna-N O-ring in the mechanical seal cartridge. The
manufacturer of the mechanical seal had indicated during early engineering discussions that Viton (a Du
Pont fluoroelastomer which is more resistant to BF; than Buna-N) would be used in this applicaiion, but
this was changed during actual assembly of the seals. The pump was reassembled with new seal cartridges
containing Viton O-rings.

Corrosion. Corrosion test specimens were in the loop while the heat transfer measurements were made.
These specimens were examined after total salt exposure of 510 hr and exposure to circulating salt with the
'ALPHA pump for 56 of those hours. During the heat transfer measurements the pump operated at speeds
between 1400 and 5400 rpm, and the specimens were exposed to temperatures between 440 and 620°C.

~ The weight changes of the corrosion specimens were higher than expected (maximum weight loss of 1.5
mg/cm?) and showed a definite velocity effect. The specimens exhibited expected temperature-gradient
mass transfer behavior of weight losses in the high-temperature region, very little change in the
medium-temperature region, and weight gains in the cold region. -

 

6. E. N. Sieder and G. E. Tate, “Heat Transfer and Pressure Drop of Liquids in Tubes » Ind. Eng. Chem. 28(12),
142935 (1936). ,
7. W.R. Huntley, MSR Program Semmnnu Progr. Rep. Feb. 28, 1969, ORNL-4369 p. 254.
 

11

Analyses of salt samples indicated that chromium in the salt increased from 64 to 82 ppm, and the iron
changed from 359 to 347 ppm. The indicated oxide content of salt samples varied from 500 to 800 ppm,
and the H* content ranged from 27 to 31 ppm. Mass transfer was not considered excessive, and the decision
was made to go to design conditions.

Design Operation

Corrosion loop MSR-FCL-2 began routine operation on September 1, 1971, and operated for 5283 hr
with very little difficulty. Salt was not drained from the loop during this time period. On April 28, 1972, an
oil leak occurred at a soft-soldered seal plug in the upper end of the pump shaft. This leak necessitated
draining the salt into the dump tank.

After repairs to the pump shaft, the loop was refilled on May 25, 1972. After operating only a few
hours, an alarm was noted. A low level of salt was found in the auxiliary tank, and the loop was dumped.
We found that salt was above the freeze valve on all three metallurgical specimen stations and had traveled
200 in. into the gas lines above station 2. The salt was forced into the gas lines because some of the salt
dump lines were plugged and the pressure could not equalize as the loop was being filled. The ball sections
of all the ball valves above the metallurgical specimens were removed and replaced with flat copper washers,
heaters were installed near plugged lines, and the salt around the specimens was melted so the specimens
could be removed, examined, and weighed. All gas lines containing salt were replaced. The loop had
operated only 186 hr since April 20, 1972. After all ;epairs and replacement of the specimens, the loop was
again refilled June 29, 1972.

Operational problems with the ALPHA pump resulted in the final shutdown of the corrosion test
facility on October 23, 1972. The pump had operated successfully without maintenance for more than
6800 hr prior to this incident. Disassembly disclosed that salt deposits were found above the normal sait
operating level in the pump bowl. Salt had entered and frozen in the shaft annulus and helium purge inlet
line which resulted in abnormal operation. The exact time of the salt level excursion into these regions is
not known. However, level surging and gas trapping did occur in earlier operation when plugged filling lines
between the dump tank and system piping resulted in improper salt filling. This appears to be the most
likely time for the salt level excursion to have occurred within the pump.

- CORROSION RESULTS

Cumulative weight changes subsequent to the beginning of design operation are shown in Fig. 8.
(Changes during 500 hr of startup testing ranged from +0.9 to —-1.5 mg/cm? and are not included.) Table 3
gives the incremental and total weight changes for the specimens. The total average weight loss at 20.8 fps
and 620°C was 16.6 mg/cm? and at 10.9 fps and 620°C was 13.1 mg/cm?. The total average weight gairi at
20.8 fps and 454°C was 5.7 mg/cm? and at 10.9 fps and 454°C was 1.58 mg/cm?. Each point is the average
for more than one specimen exposed to a particular combination of temperature and velocity. These results
ind_icéte temperature-gradient mass transfer at a fsteadilyl decreasing rate, with a sighificant veldcity effect.
Speciméns in the hottest position remained fairly bi‘ighi, the specimens in the middle temperature position
were darker, and those in the coldest position were the darkest (Fig. 9). Differences in appearance such as
these have been previously observed.8 |

 

8; 1. W. Koger, MSk Program Semiannu. Progr. Rep. Aug. 31, 1970, 0RNL-4622, pp. 175-76.

 
 

12

ORNL~DWG 72— 5246R

 

20
VELOCITY
- o 20.8 fps
€ ® 10.9 fps
&
} 10
£
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0 1000 2000 3000 4000 5000 €000 7000 8000
' TIME {hr)

Fig. 8. Weight changes of Hastelloy N specimens exposed to NaBF4—NaF (92-8 mole %) in MSR-FCL-2 as a function of
tnne, temperature, and velocity.

Table 3. Weight changes of specimens exposed to NaBF-NaF (92-8 mole %) in MSR-FCL-2

 

 

 

 

 

L | Average weight change (mg/cm?)
Operating time () 620°C 537°C. 454°C
Period , ——
end Interval at 10.9 at 20.8 at 10.9 at 20.8 at10.9 at20.8
fps fps fps fps fps fps

450 450 -3.13 —4.40 0 0 +0.76 +2.9
914 464 -1.40 -1.76 0 -0.10 +0.10 +0.6
1780 866 -1.11 -1.53 0 0 +0.06 +0.2
2774 994 -0.28 -0.58 0 0 0 -0.1
3846 1072 -0.29 -0.38 0 0. 0 -0.2
5102 1256 -0.16 -0.16 0 0 -0.03 0
5283¢ 181 —-0.26 02 - 0 0 -0.10 -0.2
5792 509 -5.70 —6.76 +0.46 +0.61 +0.90 +2.7
6272 480 —-0.55 -057 . +0.04 —0.06 -0.03 -0.1
6806 534 -0.20 -0.20 -0.05 —0.07 —-0.08 —0.1
Totals 6806 -13.1 -16.6 - +0.37 +0.38 +1.58 +5.7

 

%Three-month shutdown after 5283 hr;'loop drained.

Corrosion-product concentrations in the salt showed no anomalous behavior, either during the startup
or in subsequent operation. As shown in Fig. 10 the chromium concentrations increased by about 60 ppm
during the startup period, rose another 70 ppm or so during the first few hundred hours of routine
operation, and thereafter changed very little. The iron concentration decreased, and molybdenum and
nickel remained low.

In sets of specimens exposed at the same temperature to the same superficial salt velocity, the upstream
specimen generally showed the greatest weight change, the second specimen in line showed less, and the
" downstream specimen showed the least. The differences (on the order of 10% between the first and last
specimen) were clear early in ‘the operation when weight changes were relatively large, but became
practically indistinguishable later when all weight changes approached the limits of precision of
measurement. Changes of this nature are quite common in liquid metals systems (*“downstream” effect) and
are most likely due to a concentration gradient dnvmg-force effect between the wall and the fluid, although
turbulence may also be a contributing factor.
 

-~

 

 

13

PHOTO Y-107703

 

Fig. 9.- Removable corrosion specimens from MSR-FCL-2.

The dependence' of the rates of weight change on time and velocity that is evident in Fig. 9 is revealed
more sharply in Table 4, which shows weight losses of the high-temperature specimens from one
measurement to the next. For the sake of comparison with other corrosion data, the weight losses are also
expressed as an equivalent corrosion rate during the interval. The overall average corrosion rate of the
hottest specimen (6_20°C) was 0.94 mil/year at 20.8 fps and 0.74 mil/year at 10.9 fps.

For the first 5100 hr (before the extended shutdown) we saw continuously decreasing corrosion rates
(Table 5). Over the last 1250 hr of the 5100-hr run, the 'co'f_rosion_ rate of the hottest specimens (620°C)
was 0.05 mil]year (assuming uniform dissolution). No velocity effect was seen for the last 2330 hr of the
5100-hr run. The average corrosion rate at 620°C for the first 5100 hr was 0.48 mil/year for specimens
exposed to salt at 10.8 fps and 0.67 mil/year for specimens exposed to salt at 20.9 fps. Half the entire
weight loss occurred during the first 450 hr. Almost no weight changes were measured for the specimens
 

METAL {ppm) OXYGEN {ppm)

HYDROGEN (ppm)

14

ORNL-DWG 72~7740

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1200
800 2 . . . o-
. . e
* e .l e '
400 OXYGEN
p
0
°r I I
400 Z;/?/%V —— CORROSION PRODUCTS
.// /4 ' ¢ Cr
* F
300 4////2% .
zw A < :
o o o ° o
00 | %7 2 A,//' . 7 4
099 7 % / % 7 //
0 Po . % /
40 T I
.o'.c . HYDROGEN AS BFSOH"
20 P© * - -
. * .
0
0 800 1600 2400 3200
STARTI UP OPERATING TIME {(hr)
Fig. 10. Results of MSR-FCL-2 salt analyses.
Table 4. Weight losses and equivalent corrosion rates of hottest (620°C) '
specimens exposed to NaBF4-NaF (92-8 mole %) in MSR-FCL-2
Operating time Average weight los.s Equivalent corrosion
(hr) (mg/cm?) rate? (mils/year)
Period at 10.9 at 20.8 at 10.9 at 20.8
Interval
end fps fps fps fps
450 450 3.13 4.40 2.7 3.8
914 464 140 1.76 1.17 1.46
1780 866 1.11 1.53 0.49 0.68
2774 994 0.28 0.58 0.11 0.23
3846 1072 0.29 - 038 0.10 0.14
5102 1256 0.16 - 0.16 0.05 0.05 .
5283 181 0.26 - 0.26 0.55 0.55
5792 509 5.70 6.76 4.32 5.12
6272 480 0.55 0.57 0.44 0.46
6806 534 0.20 0.20 Q_E_ 0.14
) Totals 6806 13.1 16.6 0.74 0.94

 

4 Assuming uniform material removal of all constituents.
BThree-month shutdown after 5283 hr;loop drairied.

4000
 

15

Table 5. Corrosion rate of Hastelloy N specimens
exposed to NaBF4-NaF (92-8 mole %) at 620°C
as a function of time and velocity

 

Corrosion rate? (mils/year)

 

 

Time interval
at 10.9 fps at 20.8 fps

Entire 5100 hr 048 0.67
Last 4650 hr 0.27 0.36
Last 4200 hr 0.17 0.25
Last 3320 hr 0.09 ’ 0.13
Last 2330 hr . 0.08 _ 0.08
Last 1250 hr 0.06 0.06

 

" @Assuming uniform material removal.

exposed at 454 and 537°C for the last 1250 hr. These data indicate that the controlling kinetic corrosion
mechanism over the 2330 hr mentioned above (no velocity effect) was probably solid-state diffusion of
constituent elements of the alloy to the surface.? The weight changes for the S09 hr after the extended
shutdown were quite large, and there was a velocity effect, which was indicative of general solution-rate
control of corrosio'n' (caused by impurities), as opposed to selective removal of the least-noble alloy
constituent. The velocity effect was seen early in the experiment and essentially disappeared as the rate of
mass transfer decreased. o ' '

Our overall corrosion rate at the highest velocity before shutdown was less than 0.7 mil/year for 5100
hr. The corrosion rate for this 509-hr time period was 5.2 mils/year, the largest measured in Loop
MSR-FCL-2. The corrosion rate for the first 450 hr of operation was 3.8 mils/year. It is obvious that
impurities, which probably entered the system during the four months that the loop was down, grossly
affected the corrosion. Within 1000 hr after the second startup, the corrosion rate was quite low, and no
velocity effect was evident. Almost 4000 hr was required after the initial startup to approach this low
corrosion-rate condition. We saw no evidence of increased corrosion rate due to our pump difficulties.

Chromium is the least-noble constituent of Hastelloy N; thus it is selectively oxidized from the alloy.
This selective oxidation can occur through reduction of a metel fluoride impurity in the salt; that is,

FeF, + Cr==CrF,; + Fe . ' . o o ‘ (1)

(The FeF, and CrF, are probably actually NaF-FeF,; and NaF-CrF, type compounds.) Our experience
indicates that water is responsible for most corrosion in fluoroborate systems. Reactions that are believed
to be involved include the following: :

'

H,0 + NaBF, = NaBF;0H + HF, | S . 2
NaBF,OH<NaBF,0+HF, = . o 3)
6HF + 6NaF + 2Cr = 2Na; CrF¢ + 3H, . (4)

i~

 

9. J. W. Koger, MSR Program Semiannu. Progr. Rep. Aug. 31, 1972, ORNL4832, p. 124.

 
 

16

Under certain conditions, HF will attack all the alloy constituents; so instead of Cr, we can write Fe, Ni, or
Mo. Equations (2), (3), and (4) may be combined to give .

6NaBF OH + 6NaF + 2Cr = = 6NaBF, 0 + 2Na, CrF +3H, , | (5)

3H,0 + 3NaBF, + 6NaF + 2Cr = 3NaBF, 0 + 2Na, CiF, + 3H, . )
Thus, both the NaBF3OH initially in the salt and any H, O that may be admitted later result in corrosion
and the production of hydrogen, which can diffuse through the hot metal walls and be lost from the
system. Removal of hydrogen from the system by diffusion through the metal would drive reactions (5)
and (6) to the right. The results of the tritium addition to a thermal convection loop supported the
contention that little if any hydrogen-containing impurities existed in the molten fluoroborate.!? Because
of the connection with corrosion (and, as discussed below, with tritium transport), samples of salt from
MSR-FCL-2 were routinely analyzed for oxygen and for BF;OH ™, The oxygen results shown in Fig 10 do
not reveal any trend with time. The measured concentrations of hydrogen as BF; OH", on the other hand
clearly indicate a downward trend for about 2800 hr, then little change over the next 1000 hr.
The hydrogen behavior in the fluoroborate salt in Loop MSR-FCL-2 is of great interest because of its
implications for tritium transport in an MSBR. The concentrations shown in Fig. 10 indicate a much less

rapid loss of hydrogen than would be expected on the basis of the interrelations of BF3 OH concentration,

hydrogen pressure, and diffusion through Hastelloy N observed in other experiments.

CONCLUSIONS

1. The pump loop system used in this experiment is ideally suited for quite sophlstxcated corrosion
studies and can be adapted to any flowing liquid.

2. Measurable temperature gradient mass transfer did occur in this experiment. Our measurements
disclosed weight losses for the hot specimens, weight gains for the cold specimens, with a balance point (no
gain or loss) at a temperature halfway between the maximum and minimum. :

3. A velocity effect existed under certain conditions and resulted in higher weight losses and gains for
the specimens exposed to the higher velocity salt. In time the velocity effect disappeared and mass transfer
rates were quite low.

4. Specimen examination early in the life of the loop disclosed a definite downstream effect. The first
specimen of three exposed to the salt (same temperature, same velocity for each specimen) showed the
largest -gain or loss, the next specimen showed the next largest change, and the last specimen showed the
smallest change of the three. o

5. Specimens in the hottest position rémained fairly bright, with the specimens in the middle
temperature position darker, and the specimens in the coldest position the darkest Differences in
appearance such as these have been previously observed. ' : '

6. Overall average corrosion rate of the hottest specimen (620°C) was 0.94 mfl/year at 20.8 fps and
0.74 mil/year at 10.9 fps. An average corrosion rate of 0.05 mil/year for the hottest specimen was found in
one 1256-hr increment.

 

10. J. W. Koger, MSR Program Semiannu. Progr. Rep. Feb. 28, 1971, ORNL-4676, pp. 210-11.

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