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Cr
Metal
Hastelloy N
Fe
Mo
~
162
Table 13.1. Summary of Initial Corrosion Experiments
All experiments were carried out in nickel vessels wit? 92-8 mole Yo NaHF,-NaF
Form of Specimen
-
Large irregdar-
shaped chips
Cylindrical rods
\virea
strips a
the same experiment.
~
Total Hours at Maximum
500°C or Above Temperature (OC)
-
a chromium concentration approximately twice as
large as for the sample as a whole. Boron, a dif-
ficult element to determine by microprobe analysis,
was not detected in this latter scan.
Thus, this first experiment indicates rather
substantial chemical reaction of chromium with
molten NaBF,-NaF. Two certain reaction products
are Na3CrF, and BF,. The chemical nature of the
black inclusions is not readily apparent. The
presence of boron and the high concentration of
chromium, not as metal, suggest that the black
inclusions (in the green solid) may be a boride of
chromium whose x-ray diffraction pattern is as yet
NaBF,
Na3CrF6
BF 3
CrxB
26
59
28
28
710
Results
Increase of vapor pressure with
time. Formation of gieen crystals,
largely Na CrF,, but containing
3
black inclusions whose Cr content
is higher than in pure Na3CrF3;
green crystals also contain boron
(valence unknown). Relatively
extensive corrosion - loss of
0.56 g of Cr out of 20 g.
600 Small weight loss (0.017 gout
of 37.8 g of metal). No visible
color in salt.
6 SO Loss of 20% by weight of wire .-
no change in salt color.
650 No weight loss or discernible
attack.
unknown. A chemical reaction that describes the
above observations may be written as follows:
3NaBF4(1) - (1 + x)Cr(s) ~ Na,CrF,(s)
i 2BF,(g) * CrxB(s) ,
where x > 2. For this reaction, AG~,,,, the
standard free energy at 1000°K, is estimated to he
-14 5 20 kcal. The following table gives the free
energies of formation:
--AC,~,,~~ (Formation) Reference or Method of Estimation
368 * 4 From RF dissociation equilibria
581 f 15
256 + 1
25 * 10
By analogy with formation data of Na3AIF,
JANAF Thermochemical Tables
Based on estimate of CrZD by K. E. Spear,
Metals and Ceramics Division