ORNL-4191 - the Molten Salt Energy Technologies Web Site

The third design, which is also a direct graphite-
to-Hastelloy N joint, is shown schematically in
Fig. 19.2. Once again we have the graphite in
compression; however, in this case allowance is
made for keeping the joint in compression even if
the graphite should shrink under irradiation.
Firozing Development
We are continuing work on the development of
alloys suitable for joining graphite to graphite and
to structural materials. We are currently looking at
several alloys based on the corrosion-resistant
Cu-Ni, Ni-Pd, Cu-Pd, and Ni-Nb binary systems.
Quaternary compositions were prepared containing
a carbide-forming element plus a meltingpoint de-
pressant. Preliminary discrimination between the
various alloys was obtained through wettability
tests on high-density graphite. Poor flowability
was obtained with the Ni-Nb and Cu-Pd alloys. In
the Pd-Ni system, the carbide-forming elements and
tnelting-point depressant were added to the 70-30,
60-40, and SO-SO binary alloys. In the Cu-Ni sys-
tem, the carbide-forming elements and melting-point
depressant were added to the 80-20 and 70-30 binary
alloys. Most of the alloys seemed to wet graphite
well at temperatures ranging from 2102 to 2192°F.
Concurrent with the brazing development work,
we are investigating the radiation stability of the
brazing alloys. We are currently irradiating four
batches of IJastelloy N Miller-Peaslee braze speci-
mens in the OKR. The specimens will receive a
dose (thermal) of approximately 2 r: 10
neutrons/cm2 at 1400OF.
19.2. COMPATISILITY OF GRAPHITE-
MOLYBDENUM BRAZED JOINTS WITH MOLTEN
FLUORIDE SALTS
W. H. Cook
The salt-corrosion studies of joints of grade CGB
graphite brazed to molybdenum with 60 Pd-35 Ni-5
Cr (wt %) have continued. The specimens are ex-
posed to static L,iF-BeF,-ZrF4-ThF4-UF4 (70-
23.6-5-1-0.4 mole %) at 1300'F in HasteIIoy N. We
237
have reported previously2P3 that there was no visible
attack on the braze after a 5000-hr exposure, but
there was a coating of palladium on the braze and
some Ct3C2 on the graphite. A 10,000-hr test has
now been concluded with similar results.
All salt-corrosion tests of this series were sealed
at room temperature with a pressure of approximately
4 x lowfi torr by TIG welding. A thermal control
for the 10,000-hr salt test was made in which the
test cotnponents and test history were the same except
that no salt was present. The results are
shown in the microstructures of the two joints in
Fig. 19.36 and c. The diffusion of the palladium
out of the brazing alloy to form a nearly pure
palladium coating on the surfaces of the braze occurred
both in the control (the one exposed to a
vacuum) and the one exposed to the salt. The
coating formed in the vacuum may be more uniform.
Formation of the coating in the vacuum eliminates
the salt as an agent in its formation. The more
probable explanation is that the palladium is
diffusing to the surface of the braze metal. 'The
thickness of the coating appeared to be a function
of time in the 5000-hr test, but this time dependence
does not seem to continue for as long as
10,000 hr.
There is some possibility that the palladium
coating may help prevent corrosion of the brazing
alloy by decreasing or preventing exposure of the
alloy to the salt.
The chemical analyses of the salts remained
essentially unchanged, as shown in Table 19.1.,
with the exception that the chromium content of the
salt in the 10,000-hr test rose sharply relative to
the others. This is higher than one would expect
with Hastelloy N in these types of tests.
This particular test series for this brazing alloy
will he terminated by a 20,000-hr test which is in
progress. Another corrosion test of this brazing
alloy in a similar joint configuration is being made
in the MSRE core surveillance assembly, where the
joint is being exposed to radiation and flowing fuel
salt.
Other potential brazing alloys will be subjected
to similar tests as they are developed. The most
promising alloys will be more rigorously tested in
dynamic salts and irradiation fields.
'W. H. Cook, MSR Program Semiann. Pro@. Rept.
Aug. 31, 1966, ORNL-4037, pp, 115-17.
3W. EI. Cook, MSR Program Serniann. Progr. Rept.
Feh. 28, 1967, ORNL-4419, pp. 111-15.