ORNL-4191 - the Molten Salt Energy Technologies Web Site

1000
- E
. E
0“
500
200
200
SO
10
5
2
TEMPERATURE (“C)
GRNl -D’W 67-11829
I
OR 09 40 ti 12 13
‘QOO/T (Ok)
Fig. 13.7. Equilibrium Quotlsnt As a Function of
Temperature for the Reaction NcrBF,if) - NaF(1) +
BF@.
Values of Q vs temperature are plotted in Fig.
P
13.7; as might be expected, log Q is linear with
P
1/ 7‘( i‘ K).
From the BF pressure data, thermodynamic
activities of NaRF, and NaF can be evaluated.
This evaluation will be carried forth in the very
near future.
16 1
Stanley Cantor
This investigation seeks 1.0 determine the corrosion
reactions of Wastelloy N and its constituents
with fluoroborate salt melts, Thus far, three
initial experimen4.s have been carried out; the
conditions and results are summarized in Table
1.3.1. The nickel containment vessels and the
procedure for desorbing gases other than BY3 used
in these experiments were the saiiie as those used
in the vapor pressure measurements (see Sect.
13.2). The salt mixture was composed of 92-8 mole
% NaBF4-NaF; this mixture, which is the only
know11 eutectic in the NaBF,-NaF system, is under
consideration as a substitute coolant for the MSRE.
In the experiineist containing chromium metal
chips (see Table 13.1), the vapor pressures were
measured at several temperatures between 500 and
7QOOC. After the sample had been af. 500°C or
above for 26 hr, EF, vapor pressures were twice
those from the melt alone; that is, the reaction
between chromium metal and salt hiad yielded
BF, gas.
In the postexperimental examination of the vessel’s
contents, the chromium metal chips appeared
blackened and corroded and weighed less than
prior to the experiment. ‘The solidified salt mixture
contained a green solid. Both the chromium metal
chips and the green solid were separated from
adhering fluorobotate by dissolving; the watersoluble
Eluoroborate. The green solid, when
examined microscopically, showed relatively large
(as much as 50% in volume) black inclusioris. From
an x-ray powder pattern the green solid was identified
as predominantly crystalline Na,CrF,. Two
moderately intense lines in the pattern could not
be identified; these, however, cannot be attributed
to chromium metal or lo any boride of chromium for
which an x-ray diffraction pattern is available.
Chemical analysis of the green solid showed
27.2% chromium and 0.64% boron. In stoichiometric
Na,3CrF,, the chromium content is 22.1%. Presumably,
a11 the boron and the chromium in excess of
22.1% can be assigned to thc: black inclusions.
An electron microprobe examination of the green
solid, scanning 360 I,’, confirmed the presence
of H, Cr, Na, and F. A limited area scan directed
at :i black surface (of I !L’) on one crystal yielded