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The two other experiments, one with Ilastelloy N
and the other with iron and molybdenum, were not
monitored for vapor pressure. After desorbing the
gaseous impurii ies from the sample charge (metal
plus salt), the vessels weie brought to the tempera-
tures shown in Table 13.1 and were kept there for
the indicated time. After completing these two
experiments the samples were examined; very little
or no visible attack was apparent on the metal
specimens. Also, there were no visible color
changes in the salt mixtures. However, the weight
losses in the Iiastelloy and in the iron specimens
(see Table 13.1) suggest the need for more thorough
investigation of the corrosion read ions of chromium
and iron with fluoroborate salt melts.
Apparent Mass Transfer cif Nickel
Afier completing vapor pressure measurements
on each fluoroborate mixture, the nickel vessels
are always cut open for exarninatiori oE the con-
tents. In almost every case, the top surface of the
solidified salt contains a small amount of black
material. In all cases the inner metal surfaces iue
shiny. The top portions of two salt cakes (one
97.5-2.5 mole %, the other 65-35 mole % NaBF4-
Nap) when dissolved in water yielded silvery
residues, These residues were ferromagnetic, and,
by x-ray diffraction, were identified as nickel rneial.
The reason that nickel metal particles appeared
on the melt is not readily evident. It is unlikely
that metal particles were present in the vessel
prior to loading with salt, nor is it likely that the
stock salts contained nickel metal. The luster of
the walls jn contact with salt suggests that nickel
inay have been mass transferred. Further investiga-
tion should provide additional information on this
unexpected deposition of nickel metal on the salt.
13.4 REACTION OF BF, WITH CHROMIUM
METAL AT 650°C
J. Is. Shaffer I-I. F. Mclhffie
Current platis to replace the secondary coolant
of the MSRE with a fluoroborate mixture have
prompted studies of the compatibility of these
materials with structural metals of the reactor
system. Since lluoroborates of interest to this
program exert measurable vapor pressure of BF,
at operating temperatures, covering atmospheres
163
containing equivalent concentrations of BF’,3 must
be maintained in the free volume of the pump bowl
of dynamic systems or used for gas sparge operations.
This experimental program will examine
the reactions of BF3 with various structural metals
and alloys that might be applicable to the program.
Preliminary results obtained by contacting BF3
with chromium metal at 650°C are presented here.
The experimental method utilizes a 30-in. length
of 2-in. IPS nickel pipe, mounted horizontally in a
341. tube furnace, as the reaction chamber. The
reacting gases are admitted through a penetration
in the end plate that is welded tu one end of the
nickel pipe. A sheathed thermocouple also penetrated
the end plate and extended into the central
region of the heat zone. The other end of the teaction
chamber, which extends sotile 10 in. out of the
tube furnace, is closed by Teflon in a threaded pipe
cap. The gas manifold system provides for the
introduction of helium, BF or mixtures of these
?’
gases at known flow rates into the reaction chamber.
The system is sealed from the atmosphere by
bubbling the gas effluent through a fluorocarbon oil.
Concentrations of HF, in helium can be determined
continuously from the recorded signal of a calibrated
thermal conductivity cell. Metal samples or specimens
are carried in nickel boats inserted through
the threaded access port.
The reaction of BF, with chromium metal was
followed by periodic determination of weight gain
of about 10.88 g of prepared chromium flakes
during a 60hr reaction period ai 650°C. The
chromium metal was prepared by electrolytic deposition
on a copper sheet which was subsequently dissolved
by an acid leach. The thiti film of chromium
metal which remained was broken into small flakes
for this investigation. Reaction periods commenced
by introducing WF3 at 1 to 2 cc/min after heating
the sample to 650°C in flowing helium, The sample
was also cooled to room temperature under flowing
helium to permit inspection and weight gain determinations.
As shown by Fig. 13.8, the weight gain of the
chromium sample showed a lineiir dependence on
the square root of reaction time. The overall reaction
period showed that the chromium sample increased
its weight by about 4%; there was no
significant difference in the weight or appearance
of the nickel boat during thi:; experiment. Examination
by x-ray diffraction techniques showed that the
chromium sample contained substantial qunntities
of Cr203; minor fractions of the mixed fluoride,