Permeation experiments after nitriding the inlet surface of a hollow cylindrical membrane were conducted. Permeation data on two additional specimens given pre-oxidation or pre-nitriding treatments at both inlet and outlet surfaces are presented in terms of Arrhenius plots. Additionally, an analysis of geometry dependence on permeation rate was made for several specimens including the ones mentioned above. For simplicity in this report, the term as-polished is used to refer to a specimen which is either as-polished or is as-polished, pre-oxidized and annealed

Johnson, D. L.

Shah, K. K.

English

NASA Technical Reports Server

ANNUAL PROGRESS REPORT
NASA-Ames Grant NGR 28-004-025
"Hydrogen Transport in Alpha Titanium"
Donald L. Johnson and Kirit K. Shah
August 25, 1974
.ef .-
Signed _riaV_3 2hvi ,*i,'c--
Donald L. Johnson, P/-:ncipal Investigator
Associate Professor of Mechanical Engineering
Metallurgy Program
Department of Mechanical Engineering
University of Nebraska
Lincoln, Nebraska 68508
(NASA-CR-139632) HYDROGEN TRANSPORT IN N74-3198 4
ALPHA TITANIUM Annual Progress Report
(Nebraska Univ.) 20 p HC $4.00 CSCL 11F
G3/17 47093
https://ntrs.nasa.gov/search.jsp?R=19740023871 2020-03-23T04:06:29+00:00Z
Introduction
A semi-annual progress report was submitted in January outlining the
results of permeation experiments after nitriding the inlet surface of a
hollow cylindrical membrane. In this report, permeation data on two addi-
tional specimens given pre-oxidation or pre-nitriding treatments at both inlet
and outlet surfaces are presented in terms of Arrhenius plots. Additionally,
an analysis of geometry dependence on permeation rate was made for several
specimens including the ones mentioned above. For simplicity in this report,
the term as-polished is used to refer to a specimen which is either as-polished
or is as-polished, pre-oxidized and annealed.
Experimental
Geometry Dependence
Permeation can be defined by
J = 2wtDC2 [tn(b/a)] -1  (1)
for the case where diffusion through the metal is slow and surface reactions
-1
are relatively fast (ref. 1). A plot of J versus [tn(b/a)] -1 is linear in such
a case. However, if the surface reactions are slow and diffusion through the
metal is relatively fast, the relationship is non-linear. In the latter case,
it is also expected that J would not necessarily be constant for a constant
value of the b/a ratio. For example, if the surface at a were more important
than b in controlling the rate, the permeation rate would be more sensitive to
change in a with b constant than it would be to change in b with a constant.
To observe how equation (1) would apply to the data, hydrogen flow rate
was plotted in terms of [Zn(b/a)]-l in Figure 1. The profile is non-linear,
2thus confirming that surface reactions are important during the transport
process in a-titanium. As expected for the case where surface reactions are
important, values of J appear to increasingly vary as [n(b/a)]-l increases
at a particular b/a ratio. For example, at [En(b/a)]l 1 = 7 the spread in
J x 108 (mol s-1) is about 13. At [in(b/a)]-1 = 9.0 the spread in J x 108
is about 31. Because of the limited number of specimens studied, it is impos-
sible to determine which surface predominates in controlling the hydrogen flow
rate. A systematic study of geometry dependence is planned using about 20
additional specimens.
Geometry dependence may also be characterized in a different way using
equation (2). Setting a constant in equation (1) and putting the resulting
equation in linear form:
/J =n(b) _n(a)2TrDC2  2nZDC2
= Kn(b) - K' (2)
If permeation is diffusion controlled, equation (2) is linear just as it
would be in equation (1). However, if diffusion is not controlling, a plot
of l/J versus tn(b) is non-linear. At this point, equation (2) has no advantage
over equation (1) in the analysis unless a similar plot is made between J and
1/a where b is constant according to equation (3):
1/J = Kkn(l/a) + K" (3)
If the plot of equation (3) is less linear than equation (2), then the surface
at a is more important than the surface at b in controlling the rate and the
location of the important surface reaction is established. Due to a lack of
data on specimens of constant b/a ratios for various b and a values, an inter-
pretation based on equations (2) and (3) cannot be made at this time.
Another way that geometry dependence can be characterized is from Fick's
second law solution for hydrogen transport through hollow cylindrical membranes.
3Lag-time expressions have been developed for (1) diffusion controlled trans-
port, (2) phase boundary reaction control at the inlet surface, and (3) phase
boundary reaction control at the outlet surface (ref. 2). These expressions
yield lag-time equations of the general form:
1/tL = =D (4)
where (a) a is defined as aD (functional radii dependence for diffusion
control applies).
(b) a is defined as ea (functional radii dependence for inlet
surface control applies).
(c) a is defined as ab (functional radii dependence for outlet
surface control applies).
If equation (4) is valid for the properly selected value of =, a log-log plot
of 1/tL versus I/= yields a straight line of slope n = -1. The correct
functional value of = then identifies the predominant controlling mechanism
and its location. Again, an attempt was made to apply equation (4) but the
results were inconclusive because of the limited lag-time data available for
varying geometries.
Oxide Films
A modified form of Richard's equation is given by
J = KpH2 (0 < n 1) (5)
Taking the logarithm of both sides, a plot of log J versus log pH2 yields a
straight line of slope n where n = 1/2 for diffusion control and n = 1 for
surface reaction control. The effect of inlet surface pre-oxidation and pre-
nitriding on pressure dependence is shown in Figure 2. Comparison with the
as-polished condition shows both the decrease in permeation rate and the typical
4first power pressure dependence normally observed before pre-oxidation. Per-
meation rate is markedly influenced by the location of the oxide film. As
can be seen in Figure 3, oxide film at the outlet surface results in a permeation
rate increase, whereas at the inlet surface, pre-oxidation results in a permea-
tion rate decrease. In the latter case, activation energy increases relative
to the as-polished condition as illustrated on the Arrhenius plot in Figure 4.
Such an observation was observed on a different pre-oxidized specimen and was
reported on earlier (ref. 3). In Figure 4, a very thin oxide film (' 50A) on
the inlet surface is effective in reducing the rate whereas about the same
film on the outlet surface increases the rate. Above 500 - 6000C the permea-
tion rate decreases to the value it would have if the specimen had not been
pre-oxidized at the outlet surface. After annealing at about 7000C, the process
was repeated and the data reproduced.
As observed in both Figures 3 and 4, an increase in permeation rate was
observed after pre-oxidation at the outlet surface. The same observation is
noted in Figure 5, except that the permeation rate is slightly less for a
thicker oxide film at the outlet surface. Figures 5 and 6 show that first
power pressure dependency is maintained regardless of the pre-treatment.
Nitride Films
The permeation rate is much more sensitive to nitride film at the inlet
surface than it is at the outlet surface. Figure 7, presented earlier in the
semi-annual report, is an Arrhenius plot of the permeation rate through
progressively thicker nitride films on the inlet surface. As the film thick-
ness increases, the permeation rate decreases. Permeation is first power
dependent upon pressure as shown in both Figures 7 and 8. Very thin nitride
films (. 1-2A) at the inlet surface are effective in reducing the rate as
shown in Figure 8. Figures 9 and 10 represent two different sets of data
5that compare the effect of pre-nitriding at inlet and outlet surfaces. Pre-
nitriding at the outlet surface promotes only slight change in the rate.
Discussion
The decrease in permeation rate in the presence of oxide at the inlet
surface is consistent with an interpretation put forth that steady state is
established between gas-phase molecular hydrogen, a discontinuous layer of
TiH 2 and atomic hydrogen in the titanium lattice (ref. 4.) It is suggested
that this steady state condition is modified by oxygen in such a way that
partial filling of available chemisorbed sites by oxygen reduces the number
of sites available to hydride. Therefore, the actual concentration of hydrogen
in a-phase titanium is lower than it was for the case where the surface was
not pre-oxidized. While the process is surface controlled, this decrease in
concentration is equivalent to a reduced concentration gradient across the
membrane and a reduced hydrogen flow rate. Other information which adds
credibility to this explanation is noted in the literature. Caskey interprets
lack of hydride formation on titanium as due to the formation of a stable oxide
film (ref. 5). Reichardt comments on the inhibiting effect of contaminants on
reaction between hydrogen and titanium (ref. 6) and Schoenfelder and Swisher
account for their results in terms of the retarding effect of oxide films on
the rate of hydriding (ref. 7). Wasilewski and Kehl recognized the effect of
oxide on reaction rates in titanium in 1953 (ref. 8).
This interpretation of surface controlled permeation of hydrogen through
a-titanium is consistent with experimental observations assuming hydride forms
as an intermediate product at the inlet surface. This interpretation can be
6used to explain the permeation mechanism at the outlet surface if it is'assumed
that hydride forms at this location also. That this is a logical assumption
can be explained in terms of Barrer's interpretation of surface dependent
permeation (ref. 9). Since the actual concentration just inside the inlet
surface is below equilibrium, much of the hydrogen is removed by diffusion to
the outlet surface. At this surface, the slow rate of transfer across the
interface but relatively high diffusion rate through the solid leads to an
accumulation of hydrogen just inside the outlet surface. The actual concen-
tration is therefore greater than the equilibrium (zero) concentration ex-
pected at the outlet side in contact with vacuum. Thus, at the outlet surface,
it is logical to expect hydride formation and a permeation mechanism identical
to that described for the inlet surface. When the outlet surface is pre-
oxidized, the available sites for hydride decrease, and the concentration
just inside the outlet surface decreases just as it did at the inlet surface.
However, contrary to the decrease in the concentration gradient and equivalent
decrease in hydrogen flow rate after oxidation at the inlet surface, the
concentration gradient now increases and the equivalent result is an increase
in the hydrogen flow rate after oxidation at the outlet surface. The gradi-
ents (greatly magnified) for the three cases (1) Inlet oxidized (2) Outlet
oxidized and (3) As-polished are depicted in the following sketch:
7;nle sur-ace cS/e7 soace
I
First power pressure dependence has been consistently observed regard-
less of pre-treatment, temperature or pressure. This suggests that the boundary
reactions at each surface are identical to what they are in the as-polished
condition and the only variable is the degree of surface coverage by hydride.
First power pressure dependence results, not because the process is surface
dependent, but because molecular hydrogen dissolves in the TiH 2 on a 1:1 basis.
This is equivalent to solution as molecular hydrogen for the purpose of this
discussion. The process is first power pressure dependent regardless of the
location of the surface treatment since in the above interpretation TiH 2
exists at both surfaces. It is quite possible that the controlling surface
switches from one surface to another depending upon the contaminant on the
surface and its relative thickness. As mentioned earlier, for example, oxide
8at the outlet is more effective than nitride in varying the rate whereas the
opposite is true at the inlet surface.
It is interesting to note that Smithells and Ransley observed differences
in the hydrogen permeation rate through aluminum depending upon which surface
was abraded (ref. 9). Abrasion of the outlet surface reduced the permeability
whereas abrasion of the inlet surface increased permeability.
,Conclusions
1. Pre-oxidation, pre-nitriding, and film thickness have no effect on the
usual dependence observed between permeation rate and first power of
pressure.
2. Irrespective of location, thicker oxide and nitride films reduce the
permeation rate.
3. Hydrogen flow rate is more sensitive to nitride film at the inlet surface
than the outlet surface whereas the opposite is true for oxide film.
4. Analysis of geometry dependence and effects of oxide-nitride films pro-
vides further evidence that permeation of hydrogen through a-titanium is
surface reaction controlled.
5. Nitrogen is more effective as a hydrogen "barrier" than oxygen under the
same conditions of pressure and temperature.
Nomenclature
J - Hydrogen flow rate or permeation rate
i - Reduced gage section length
C2 - Hydrogen concentration just inside inlet surface
b - Outlet radius
a - Inlet radius
9References
1. D. L. Johnson and H. G. Nelson: Met. Trans., Vol. 4, Feb. 1973, pp. 569-573.
2. K. K. Shah, H. G. Nelson, D. L. Johnson and F. M. Hamaker: "A Generalized
Expression for Lag-Time in the Gas Phase Permeation of Hollow Tubes,"
submitted for publication.
3. D. L. Johnson and K. K. Shah: Proceedings of the Conference, Hydrogen in
Metals, American Society for Metals, Mar. 1974.
4. Private communidation, H. G. Nelson: April 5, 1971.
5. G. R. Caskey, Jr.: Proceedings of the Conference, Hydrogen in Metals,
American Society for Metals, Mar. 1974.
6. J. W. Reichardt: J. Vac. Sci. and Tech., 1972, Vol. 9, p. 548.
7. C. W. Schoenfelder and J. H. Swisher, Sandia Laboratories, SCL-DC-720100,
Jan. 1973.
8. R. J. Wasilewski and G. L. Kehl: Diffusion of Hydrogen, Nitrogen and Oxygen
in Titanium, WAL 401/149-11A, Watertown Arsenal Laboratory, July 1953.
9. R. M. Barrer: Diffusion in and through Solids, Cambridge University Press,
London, 1941, p. 175, p. 195.
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Hydrogen transport in alpha titanium

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