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0.250in. It
4 0.250in. I-
ORNL-DWG 67-14844
Fig. 18.17. Geometry of Diffusion Specimen.
(at 1110"F, AFo for TiF, is -90 kcal per gramatom
of F and AFOfor CrF, is -77 kcal per gramatom
of F). Since protective films do not seem
to form in fluoride systems, we would assume that
both the chromium and the titanium will be removed
as rapidly as these elements can diffuse.
Our previous studies indicate that the corrosion
rate of the standard alloy is acceptable, and the
question of paramount importance is whether the
addition of titanium will accelerate the corrosion
rate.
To answer this question we are measuring the
rate of titanium diffusion in modified Hastelloy N.
The following techniqueg is being used. Diffusion
samples, 0.625-in.-diam cylinders (Fig.
18.17), were machined from small commercial heats
of modified Hastelloy N (heat 66-548). They were
heat treated 1 hr at 2400°F to establish a stable
grain size and impurity distribution. The radioactive
44Ti isotope was deposited on the polished
face of the sample using a micropipet. The isotope
was supplied in an HF-HC1 acid solution;
therefore, additions of NH40H were made after
depositing the isotope to neutralize the solution.
The sample was then heated in vacuum for 0.5
hr at 932°F to decompose the mixture to leave
a thin layer of 44Ti.
Each sample was given a diffusion anneal for
appropriate times in flowing argon at precisely
controlled temperatures. Sections were then
taken on a lathe at 0,001-in. increments, and the
activity of the turnings was measured using a
single-channel gamma spectrometer with an NaI(T1)
scintillation crystal detector. At lower diffusion
anneal temperatures, a hand-grinding technique
'A. Glassner, The Thermodynamic Properties of the
Oxides, Fluorides, and Chlorides to 250@K, ANL-5750.
'J. F. Murdock, Diffusion of Titanium-44 and Vana-
dium-48 in Titanium, ORNL-3616 (June 1964).
232
10.'
5
TEMPERATURE
1250 1200 1150 It00 1000
Fig. 18.18.
Hastelloy N.
ORNL-DWG 67.11845
Diffusivity of Titanium in Modified
was used to obtain smaller increments since the
penetration distances were less. From a plot of
the specific activity of each section against the
square of the distance from the original interface,
a value of titanium diffusivity was obtained for
the diffusion anneal temperature of that specimen.
To date we have determined the diffusivity of
titanium at five temperatures from 2282 to 1922°F.
The results of these measurements are shown in
Fig. 18.18. Although the temperature range over
which we have data is considerably higher than
the proposed reactor operating temperature, we
can compare the rates of titanium diffusion with
that of chromium to get some ideas of the relative
mobilities of the two constituents. At
2012°F the diffusion rate of chromium in an Ni-20%
Cr alloy is reported to be approximately 8 x 10- l1
cm2/sec. lo From our data at 2012°F the diffusivity
of titanium in modified Hastelloy N is 3.9 x
lo-' cm '/set, which is a factor of 2 lower than
for chromium at this temperature.
Thus on a very rough basis there does not appear
to be too much difference in the rate of diffusivity
of these constituents at these higher temperatures.
However, we cannot conclude at this
lop. L. Gruzin and G. B. Federov, Dokl. Akad. Nauk
SSSR 105, 264-67 (1955).