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ANP PROJECT PROGRESS REPORT
lnconel contains 15 wt ?6 G or a chromium mole
fraction of about 0.16. The assumption that the
activity of chromium in lnconel is roughly equal to
its mole fraction appears to be rather good. The
compounds in the melts, which were treated with
pure chromium at 600 and 80O0C, are shown in
Table 2.2.5, along with corresponding values cal-
culated for equilibration with lnconel. Experimental
values obtained from corrosion experiments with
lnconel and these melts are usually slightly lower
than these calculated values, presumably because
in the experiments equilibrium is dttained with an
lnconel surface layer significantly depleted in
chromium. When lnconel powder is used in experi-
mental equilibration apparatus, side reactions that
involve oxides of the metals complicate the situa-
tion; equilibrium concentrations of chromium cow
pounds higher than those calculated are often
observed. In general, it appears that the calcu-
lations are good only for the idealized case de-
scribed.
From the data in Table 2.2.5 it is obvious that
lnconel exposed to the NaF-KF-LiF-UF4 melt will
support a higher concentration of CrF,-GF, in
equilibrium at 800OC than pure Go is able to
support at 600°C. Accadingly, chromium is re-
moved from lnconel in the hot zone of a loop and
deposited as essentially pure chromium in the cold
zone. Since no diffusion process is necessary in
the cold zone (the chromium can deposit at the sur-
face of the Go crystals), the rate of attack is con-
trolled simply by the rate of diffusion of chromium
to the metal-salt interface in the hot zone.
98
The data in Table 2.2.5 also reveal, however, bB
thut lnconel exposed to NaF-ZrF4-UF4 mixtures is
in equilibrium with much lower GF concentrations
than pure CrO is in equilibrium wit when exposed
to the fluoride mixture under the same conditions.
Accordingly, it is not possible for chromium to
dissolve from 800OC lnconel and to deposit at
600OC as Go when NaF-ZrF UF, mixtures of
4:
this general composition are circulated. Loops
of pure Go would, of course, mass-transfer in
this medium; moreover, mass transfer can occur
if a sufficiently dilute alloy of chromium can be
formed in the cold zone. This suggests that the
mass-transfer process takes place in the following
general way.
The molten salt in the hot zone reaches equi-
librium with the 8oo°C lncanel and passes with
the dissolved GF, to the cold zone. In the cold
zone, equilibrium is established by deposition of
a small amount of the chromium (by reversal of the
reaction) in the surface layer of the metal to form
an alloy containing slightly more chromium than is
normally present in Inconel. If no diffusion of
chromium were possible, a true equilibrium would
be reached shortly, with the hot surfaces slightly
depleted and the colder surfaces slightly en-
riched in chromium. The process would then
stop (except for the exchange process which has
no net effect), with negligible corrosion of the
metal. Since diffusion does take place, however,
the process continues. In the hot zme the con-
centration gradient causes a flow of chromium to
TABLE 2.2.5. EQUILIBRIUM CONCENTRATIONS OF CHROMIUM FLUORIDES WITH
ALKALI FLUORIDE AND ZrF4-BEARING FUEL MIXTURES
Experimental results for melt treated with
pure chromtum, (G) = 1.0*
Chromium Concentration (ppm)
In Na F-K F-LI F-U F4 In Na F -t F4-UF4
At 6OO0C 1100 2400
At 8WoC 260 2550
Results calculated for equilibration of melt
with Inconel, (G) 0.16*
At 6OO0C
At 8OO0C
*Concentration of chromium in mole fraction.
7 10
1660
1320
1400
.
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