- Email: [email protected]
Effect of surface treatments on the sorption of tritium on type-316 stainless steel
Journal of Nuclear Materials North-Holland, Amsterdam
EFFECT OF SURFACE STAINLESS STEEL T.
HIRABAYASHI,
Department Received
187
127 (1985) 187-192
TREATMENTS
M. SAEKI and
ON THE SORPTION
1984; accepted
ON TYPE-316
E. TACHIKAWA
of Chemrstty, Japan Atomrc Energy Research Institute, 15 March
OF TRITIUM
23 August
Tokar, Iharaki 319
- II. Japan
1984
The effect of surface treatments on the interaction between tritium and type-316 stainless steel was studied by a thermal desorption technique. The surface treatments caused changes in intensity for all of the four desorption peaks of tritium. The chemical passivation resulted in a decrease of the peak at 540 K (peak HTO) as well as the peak at 430 K (peak HT-I). The vacuum bake-out at 773 K led to a decrease of the peak at 750 K (peak HT-II) and an increase of the peak at 970 K (peak HT-III). The sputter-etching resulted in an increase of the peaks HTO and HT-I. These changes in the behavior of thermal desorption of tritium can primarily be attributed to the variation of the surface composition. It is suggested that the peaks HTO and HT-I are given by desorption of the tritium of which sorption is related to iron on the surface, while the peaks HT-II and HT-III are given by desorption of the tritium which is trapped on the sites of carbide and nickel, respectively, in the surface layer.
1. Introduction
sites
in the
peaks of
Austenitic the most
reactors
type-316 stainless steel is proposed as one promising structural materials in fusion
using
tritium
as
a fuel
[l].
It
has
treatment
in various
suitable
surface
against
corrosion.
surface treatments vacuum bake-out atmospheres
treatment
recently
[7-lo],
and
that
K
of atomic (peak
species,
which
and
at 970
HT-II)
gives K
the
(peak
The present research effects of various surface
was undertaken to clarify the treatments ~ chemical passiva-
tion,
and
vacuum
stainless
bake-out,
sputter
phenomenon
etching
of tritium
-
on the
on type-316
steel.
a
resistance
In the same way one may expect
form
sorption-desorption
such as chemi[4-61 and heat
can give a higher
750
HT-III).
become clear that the composition and the microstructure of the surface of stainless steel are markedly affected by the various cal passivation [2,3],
at
2. Experimental
that
the sorption phenomenon of tritium on stainless steel will also be dependent on the physicochemical surface
2.1. Materials
properties
A type-316 stainless steel plate (30 X 10 X 1.25 mm3) was used as a sample; its composition was 0.06% C,
modified
by the surface
treatments.
The surface conditioning of stainless steel is of great interest with respect to the lowering of tritium loss and
0.56%
contamination, and it is necessary to have a detailed knowledge concerning the effect of surface treatments on the sorption phenomenon of tritium. In previous studies [11,12], it has been reported that
16.60% Cr, 0.0009% B, 0.026% N and 2.24% MO with the balance of Fe. Diluted tritium gas (HT gas) with concentration of 64.38 TBq/mol was used as sorbing gas. The detailed procedure for the preparation of the
there were at least four distinct peaks desorption spectra of tritium desorbing
stainless steel sample scribed in the previous
in the thermal from stainless
steel exposed to gaseous tritium, and that the sorbed tritium could be classified into four groups as follows: (1) the tritium very weakly adsorbed on the topmost surface in the form of HT molecules or HT+ ions,
(North-Holland
0 Elsevier Physics
Publishing
Science
Publishers
Division)
Mn,
0.024%
P, 0.007%
and the HT paper [12].
gas
S, 13.10%
has
been
Ni,
de-
2.2. Surface treatments Prior to the sorption experiment, the sample was treated in the following three different manners. (a) Chemical passivation: The sample was chemically passivated in an aqueous solution containing 0.5%
which gives the peak at 430 K (peak HT-I); (2) the tritium sorbed on the surface and along a grain-boundary in the form of OT- ions, which gives the peak at 540 K (peak HTO); (3) the tritium sorbed on two different 0022-3115/85/$03.30
Si, 1.63%
K,Cr,O, B.V.
and 5% HN03
at 333 K for various
periods
188
T Hirahayashr et ul. / Sorption of rritium on type - 316 SS
(0.5-24 h), and then it was rinsed in distilled water and finally in acetone, followed by baking-out in vacuum for 2 h at 573 K. The thickness of oxide film formed on the surface by chemical passivation for 0.5 h can be estimated as several nm by assuming the same passivating rate as reported by Rhodin [13]. (b) Vacuum bake-out: The sample was baked out in a vacuum (ultimate vacuum < 10m3 Pa) for 2 h at a constant temperature in the region of 293-973 K. (c) Sputter etching: The surface of the sample was sputter-etched for 1 h with an Ar+ ion beam having a current of about 0.5 mA/cm2 under 2.4 kV. The etchdepth was estimated to be about 0.15 pm from the weight loss of the sample. The sample was further baked out in vacuum for 2 h at 573 K prior to the sorption experiment. 2.3. Sorption
and thermal
desorption
of tritium
The sorption of HT gas and its thermal desorption were carried out in a similar manner as previously reported [ll]. The stainless steel sample previously surface-treated was exposed to HT gas of 13.3 kPa (= 3 GBq) at 298 K for 7 days, and then it was heated in a He flow (30 ml/min) under a constant heating rate (5.00 K/min) in the temperature range of - 300-1173 K and the desorption rates of tritium in the forms of HT and HTO were measured as functions of the heating temperature to provide thermal desorption spectra of tritium.
4uu
WJ
800
1000
Temperature
IK)
Fig. 1. The thermal desorption spectra of tritium desorbing from the samples passivated for 0.5 h (A: HT, A: HTO) and 24 h (0: HT. n : HTO); not passivated(------: HT, -----: HTO).
3. Results The effect of chemical passivation on the sorption of tritium was first examined. Fig. 1 shows representative thermal desorption spectra of HT and HTO desorbing from the samples passivated and exposed to HT gas. The result without passivating, which was reported elsewhere [ll], is also included in fig. 1 for comparison. It is obvious that the prolonged passivation led to the decrease of the peaks HTO and HT-I, while the peak HT-III was scarecely affected by the passivation. This result shows a tendency for the chemical passivation of surface to lower the amount of sorbed tritium. Figs. 2 and 3 show the changes in the form of thermal desorption spectra of HT and HTO, respectively, caused by the variation in bake-out temperature; the spectra (b) in the figures are the results previously reported [ll]. As seen in fig. 2, the most intense peak in spectrum (a) is the peak HT-II, in spectrum (c) the peak
> , 400
,
1
600
,
,
800
Temperature F;lg. 2. The thermal desorption
,
I
I
1000 IK)
spectra of HT desorbing the samples baked out in vacuum at various temperatures: K (a), 573 K (b). 773 K (c) and 973 K (d).
from 293
T. Hirahayashi et al. / Sorption of tritium on type-316 SS
I
400
I
,
600
189
I
I
800
Temperature Temperature
(K 1
Fig. 3. The thermal desorption the samples as in fig. 1.
of HTO desorbing from
spectra
i0" -
io5
-
i06
z
io5
“E ,” g lo4
_s a,
.
L IO” lo”
.+:I’
s 2
TJ lo4 z 2 5
i0” IO2
lo’*
A I
I
300
500
1
I
700
Bakeout - temperature Fig. 4. The amount
1
900
(K)
of tritium released (a): HT+ HTO, (b): HTO and (c): HT from the samples baked out in vacuum at various temperatures; The sorbed tritium was released by thermal annealing (0, 0, a)) and by acid etching (A, A, A).
I
,
/
‘\.
-I
1000
(K 1
Fig. 5. The thermal desorption spectra of tritium desorbing from the sample sputter-etched for 1 h (A: HT. A: HTO); not sputter-etched (------: HT, -----: HTO).
HT-III becomes more intense than any other peaks, and in spectrum (d) the peak HT-I becomes the most intense one and the peak HT-II disappears completely. These results mean that a rise in the bake-out temperature increased the relative intensity of the peak HT-I but reduced that of the peak HT-II, whereas that of the peak HT-III had a maximum at a bake-out temperature of 773 K. The temperature of each peak-top varied only a little with bake-out temperature. As seen in fig. 3, the thermal desorption spectrum of HTO had a broader peak than that of HT, particularly in the case of the bake-out at 773 K. The amount of tritium released in the forms of HT or HTO was measured by means of either thermal desorption or chemical etching. The chemical etching was carried out with an acid solution in the same manner as previously reported [12]. The results obtained are summarized in fig. 4, where the amount of tritium released is also expressed as the number of hydrogen isotopes in the right ordinate. As shown in fig. 4(c), the amount of HT reaches a maximum at the bake-out temperature of 773 K. The effect of sputter etching with Ar+ ions was also examined and the spectra obtained are shown in fig. 5. The sputter etching of the surface resulted in a marked increase of the peaks HTO and HT-I and in a shift of the peak HT-III to the higher-temperature side accom-
T. Hirnhawshi et al. / Sorption of tritium on type _316SS 4.1. Efject of chemicui pussivution
400
600
800
Temperature Fig. 6. The thermal
desorption
1000 (K)
spectra
of tritium
desorbing
from the samples pretreated as follows: passivated for 15 h (a: HT. A: HTO); sputter-etched for 1 h after passivation m: HTO); not pretreated f------: HT, ------: HTO).
(0; HT,
panying a decrease in peak height. Fig. 6 shows the thermal desorption spectra which were obtained for two samples: one was chemically passivated for 15 h and the other was additionally sputter-etched for 1 h with Art ions prior to their exposure to HT gas. Although the chemical passivation caused a decrease in the peaks HTO and HT-I, the successive sputter etching caused a marked increase of the peaks. It is suggested that the action of the sputter etching counteracts that of the chemical passivation on the sorption of tritium.
4. Discussion In the present study it was observed that the various surface treatments markedly affect the sorption of tritium on stainless steel. As pointed out by many workers [2-10,13,14], the surface composition also varies with the surface treatments. Therefore, an attempt was made to explain the effects of surface treatments on the sorption of tritium on the basis of the relationship between the sorption phenomenon of tritium and the surface composition of stainless steel.
The surface composition of the stainless steel passivated in 30% HNO, at 333 K has been examined by X-ray photoelectron spectroscopy [Z] and Auger electron spectroscopy [3], and it has been found that the passivation caused the enrichment of chromium and the depletion of iron in the surface oxide film, whereas nickel was enriched a little in the film-substrate interface. If the surface composition of the passive film, which was formed in this work by immersion in the solution containing 0.5% K,Cr,O, and 5% HNO, at 333 K, is assumed to be similar to that described above, the change in the form of thermal desorption spectra caused by passivation may be related to the variation of the surface composition as follows. It is already known that the sorbed tritium forms predominantly OT- ions by combining with surface oxygen, which leads to the desorption of HTO on heating [Ill. Fig. 1 shows that the peak HTO is decreased by passivation. Since the passivation is accompanied by the depletion of iron on the surface, it appears that the iron plays an important role on the process of OT- ion formation through the combination of tritium with surface oxygen. In other words, the depletion of iron by passivation would result in a reduction of OT- ion formation and, consequently, in a decrease of the amount of HTO desorbed. This relation between the iron content and the amount of HTO desorbed is consistent not only with the proposal by Beavis [17] that the presence of iron oxide on the surface of stainless steel is associated with the generation of water, but also with the postulation by Staib et al. [IS] that the formation of is followed by that of aquo iron hydroxide, Fe(OH),, iron complex, from which water can be desorbed. The peak HT-I is decreased by passivation similarly to the peak HTO as shown in fig. 1, and the peak HT-I would be given by desorption of the HT molecule or molecular ion held at the adsorbate such as OT-- ion on iron. The peak HT-III is attributable to the tritium sorbed on the site of nickel as described below (section 3.2). Therefore it is reasonable that the height of the peak HT-III should be little changed before and after the passivation, because the nickel content is kept constant in the surface layer except for a slight enrichment in the oxide-film-substrate interface [3]. 3.2. Effect of vacuum bake-out Nonmetallic elements nitrogen have been found
such as oxygen, carbon, to be present in the surface
T. Hirabayashi et al. / Sorption 01 tritium on type- 316 SS
oxide layer which consists primarily of Cr,O, [12,14,15]. From the surface observation by soft X-ray appearance potential spectroscopy [5], it has been elucidated that Cr,O, was partially obscured by contaminant graphitelike carbon when baked out at 473 K, and that most of the carbon was eliminated by the bake-out at 773 K. In the present work, the peak HT-II, which was relatively high before the bake-out (spectrum (a) in fig. 2) disappeared completely after the bake-out at 973 K (spectrum (d) in fig. 2). These results suggest that the peak HT-II is related to some carbide. Furthermore, Otsubo et al. [16] has found that the “residual hydrogen in steel” related to Fe& was evolved at about 750 K which corresponds to the temperature of desorption for the peak HT-II. For the above reasons, it is presumed that the peak HT-II is given by desorption of the tritium trapped on the site of some carbide on the surface layer. Although the peak HT-III was given by desorption of the tritium dissociated into atomic species on the sorption process as previously reported [ll], this type of sorbed tritium did not form HTO but HT on the desorption process in contrast to the tritium sorbed on the site of iron (section 3.1). This suggests that the peak HT-III is given by desorption of the tritium sorbed on the site containing no available oxygen to form OTions. Park et al. [5] has observed the surface of stainless steel baked out at various temperatures and reported that, while the chromium content on the surface changed little in the range of bake-out temperatures of 573-1273 K, the bake-out at 773 K gave a maximum in the nickel content accompanying a balanced minimum in the iron content. As can be seen from the spectrum (c) in fig. 2, the bake-out at 773 K caused a marked increase of the peak HT-III. On the basis of the phenomenon that the intensity of the peak HT-III reached a maximum at the maximal nickel content, it can be presumed that the peak HT-III is given by desorption of the tritium sorbed on the site of nickel in the surface layer. This is also supported by the fact that the amount of tritium released in a form of HT has a tendency to reach a maximum at the bake-out temperature of 773 K as shown in fig. 4(c). On the other hand, the minimizing of the iron content at this bake-out temperature should reduce the amount of HTO formed by the same mechanism as described in section 3.1., and this tendency for the reduction of HTO at the bake-out temperature of 773 K is seen in fig. 4(b). 3.3. Effect of sputter etching
It is generally accepted that sputter etching generates a rough surface of composition close to that of the bulk due to removal of the surface layer. Hence the sputter-
191
etched surface of stainless steel may apparently contain more iron than the unetched surface, which has been confirmed by Tanabe et al. [7]. Such a roughened and iron-enriched surface may make it possible to sorb a larger amount of tritium, and the peaks HTO and HT-I increased as seen in fig. 5. It is not clear why the peak HT-III was shifted to the higher-temperature side by the sputter etching, and a detailed study is required to explain this. The effects of sputter etching and chemical passivation on the sorption of tritium are obviously opposite to each other. The result in fig. 6 indicates that the loss and contamination of tritium will be depressed by the passive film on the surface, whereas sputter etching leads to the sorption of a larger amount of tritium. The results obtained in the present work suggest that the surface conditioning should be practically important for the stainless steel to be utilized on the handling of tritium especially in a high specific activity, and that first walls made of stainless steel in a fusion reactor will sorb a much larger quantity of tritium after the discharge of plasma than before.
4. Conclusions Chemical passivation resulted in a decrease of intensity for the peak HTO(540 K) as well as the peak HT-I(430 K). Vacuum bake-out at 773 K led to a decrease of the peak HT-11(750 K) and an increase of the peak HT-III(970 K). Sputter etching resulted in a marked increase of the peaks HTO and HT-I. It is suggested that the effect of surface treatment on the sorption of tritium is closely related to the variation of the surface composition of stainless steel.
References
111K. Shiraishi, J. At. Energy Sot. Japan 25 (1983) 617. [in Japanese] Corros. Sci. 19 (1979) 1007. PI K. Asami and K. Hashimoto, (31 M. Seo and N. Sato, Trans. Japan Inst. Met. 21 (1980) 805. [41 G. Hultquist and C. Leygraf, Mater. Sci. Eng. 42 (1980) 199. [51 R.L. Park, J.E. Houston and D.G. Schreiner, J. Vat. Sci. Technol. 9 (1972) 1023. and Shimodaira, Corros. Sci. 18 161 K. Asami, K. Hashimoto (1978) 125. [71 T. Tanabe and S. Imoto, J. Nucl. Mater. 80 (1979) 361. PI T. Tanabe and S. Imoto, Trans. Japan Inst. Met. 20 (1979) 507.
192 [9] G. Hulquist
T. Hirabu~as~~et al. / Sorption of tntrunz on type _216 SS
and C. Leygraf, J. Vat. Sci. Technol. 17 (1980) 85. IlO] G. Betz, G.K. Wehner, L. Toth and A. Joshi. J. Appl. Phys. 45 (1974) 5312. [II] T. Hirabayashi, M. Saeki and E. Tachikawa, J. Nucl. Mater. 126 (1984) 38. [12] T. Hirabayashi and M. Saeki, J. Nucl. Mater 120 (1984) 309.
[13] [14] [15] [16]
T.N. Rhodin, Corros. 12 (1956) 123. R.O. Adams, J. Vat. Sci. Technol. A, 1 (1983) 12. R. Schubert, J. Vat. Sci. Technol. 12 (1975) 505. T. Otsubo, S. Goto and H. Sato, Proc. 2nd JIM int. Symp. on Hydrogen in Metals, Trans. JIM Suppl. 21 (1980) 241. 1171 L.C. Beavis, J. Vat. Sci. Technol. 10 (1973) 386. (181 Ph. Staib, H.F. Dylla and SM. Rossnagel, J. Vat. Sci. Technol. 17 (1980) 291.
EFFECT OF SURFACE STAINLESS STEEL T.
HIRABAYASHI,
Department Received
187
127 (1985) 187-192
TREATMENTS
M. SAEKI and
ON THE SORPTION
1984; accepted
ON TYPE-316
E. TACHIKAWA
of Chemrstty, Japan Atomrc Energy Research Institute, 15 March
OF TRITIUM
23 August
Tokar, Iharaki 319
- II. Japan
1984
The effect of surface treatments on the interaction between tritium and type-316 stainless steel was studied by a thermal desorption technique. The surface treatments caused changes in intensity for all of the four desorption peaks of tritium. The chemical passivation resulted in a decrease of the peak at 540 K (peak HTO) as well as the peak at 430 K (peak HT-I). The vacuum bake-out at 773 K led to a decrease of the peak at 750 K (peak HT-II) and an increase of the peak at 970 K (peak HT-III). The sputter-etching resulted in an increase of the peaks HTO and HT-I. These changes in the behavior of thermal desorption of tritium can primarily be attributed to the variation of the surface composition. It is suggested that the peaks HTO and HT-I are given by desorption of the tritium of which sorption is related to iron on the surface, while the peaks HT-II and HT-III are given by desorption of the tritium which is trapped on the sites of carbide and nickel, respectively, in the surface layer.
1. Introduction
sites
in the
peaks of
Austenitic the most
reactors
type-316 stainless steel is proposed as one promising structural materials in fusion
using
tritium
as
a fuel
[l].
It
has
treatment
in various
suitable
surface
against
corrosion.
surface treatments vacuum bake-out atmospheres
treatment
recently
[7-lo],
and
that
K
of atomic (peak
species,
which
and
at 970
HT-II)
gives K
the
(peak
The present research effects of various surface
was undertaken to clarify the treatments ~ chemical passiva-
tion,
and
vacuum
stainless
bake-out,
sputter
phenomenon
etching
of tritium
-
on the
on type-316
steel.
a
resistance
In the same way one may expect
form
sorption-desorption
such as chemi[4-61 and heat
can give a higher
750
HT-III).
become clear that the composition and the microstructure of the surface of stainless steel are markedly affected by the various cal passivation [2,3],
at
2. Experimental
that
the sorption phenomenon of tritium on stainless steel will also be dependent on the physicochemical surface
2.1. Materials
properties
A type-316 stainless steel plate (30 X 10 X 1.25 mm3) was used as a sample; its composition was 0.06% C,
modified
by the surface
treatments.
The surface conditioning of stainless steel is of great interest with respect to the lowering of tritium loss and
0.56%
contamination, and it is necessary to have a detailed knowledge concerning the effect of surface treatments on the sorption phenomenon of tritium. In previous studies [11,12], it has been reported that
16.60% Cr, 0.0009% B, 0.026% N and 2.24% MO with the balance of Fe. Diluted tritium gas (HT gas) with concentration of 64.38 TBq/mol was used as sorbing gas. The detailed procedure for the preparation of the
there were at least four distinct peaks desorption spectra of tritium desorbing
stainless steel sample scribed in the previous
in the thermal from stainless
steel exposed to gaseous tritium, and that the sorbed tritium could be classified into four groups as follows: (1) the tritium very weakly adsorbed on the topmost surface in the form of HT molecules or HT+ ions,
(North-Holland
0 Elsevier Physics
Publishing
Science
Publishers
Division)
Mn,
0.024%
P, 0.007%
and the HT paper [12].
gas
S, 13.10%
has
been
Ni,
de-
2.2. Surface treatments Prior to the sorption experiment, the sample was treated in the following three different manners. (a) Chemical passivation: The sample was chemically passivated in an aqueous solution containing 0.5%
which gives the peak at 430 K (peak HT-I); (2) the tritium sorbed on the surface and along a grain-boundary in the form of OT- ions, which gives the peak at 540 K (peak HTO); (3) the tritium sorbed on two different 0022-3115/85/$03.30
Si, 1.63%
K,Cr,O, B.V.
and 5% HN03
at 333 K for various
periods
188
T Hirahayashr et ul. / Sorption of rritium on type - 316 SS
(0.5-24 h), and then it was rinsed in distilled water and finally in acetone, followed by baking-out in vacuum for 2 h at 573 K. The thickness of oxide film formed on the surface by chemical passivation for 0.5 h can be estimated as several nm by assuming the same passivating rate as reported by Rhodin [13]. (b) Vacuum bake-out: The sample was baked out in a vacuum (ultimate vacuum < 10m3 Pa) for 2 h at a constant temperature in the region of 293-973 K. (c) Sputter etching: The surface of the sample was sputter-etched for 1 h with an Ar+ ion beam having a current of about 0.5 mA/cm2 under 2.4 kV. The etchdepth was estimated to be about 0.15 pm from the weight loss of the sample. The sample was further baked out in vacuum for 2 h at 573 K prior to the sorption experiment. 2.3. Sorption
and thermal
desorption
of tritium
The sorption of HT gas and its thermal desorption were carried out in a similar manner as previously reported [ll]. The stainless steel sample previously surface-treated was exposed to HT gas of 13.3 kPa (= 3 GBq) at 298 K for 7 days, and then it was heated in a He flow (30 ml/min) under a constant heating rate (5.00 K/min) in the temperature range of - 300-1173 K and the desorption rates of tritium in the forms of HT and HTO were measured as functions of the heating temperature to provide thermal desorption spectra of tritium.
4uu
WJ
800
1000
Temperature
IK)
Fig. 1. The thermal desorption spectra of tritium desorbing from the samples passivated for 0.5 h (A: HT, A: HTO) and 24 h (0: HT. n : HTO); not passivated(------: HT, -----: HTO).
3. Results The effect of chemical passivation on the sorption of tritium was first examined. Fig. 1 shows representative thermal desorption spectra of HT and HTO desorbing from the samples passivated and exposed to HT gas. The result without passivating, which was reported elsewhere [ll], is also included in fig. 1 for comparison. It is obvious that the prolonged passivation led to the decrease of the peaks HTO and HT-I, while the peak HT-III was scarecely affected by the passivation. This result shows a tendency for the chemical passivation of surface to lower the amount of sorbed tritium. Figs. 2 and 3 show the changes in the form of thermal desorption spectra of HT and HTO, respectively, caused by the variation in bake-out temperature; the spectra (b) in the figures are the results previously reported [ll]. As seen in fig. 2, the most intense peak in spectrum (a) is the peak HT-II, in spectrum (c) the peak
> , 400
,
1
600
,
,
800
Temperature F;lg. 2. The thermal desorption
,
I
I
1000 IK)
spectra of HT desorbing the samples baked out in vacuum at various temperatures: K (a), 573 K (b). 773 K (c) and 973 K (d).
from 293
T. Hirahayashi et al. / Sorption of tritium on type-316 SS
I
400
I
,
600
189
I
I
800
Temperature Temperature
(K 1
Fig. 3. The thermal desorption the samples as in fig. 1.
of HTO desorbing from
spectra
i0" -
io5
-
i06
z
io5
“E ,” g lo4
_s a,
.
L IO” lo”
.+:I’
s 2
TJ lo4 z 2 5
i0” IO2
lo’*
A I
I
300
500
1
I
700
Bakeout - temperature Fig. 4. The amount
1
900
(K)
of tritium released (a): HT+ HTO, (b): HTO and (c): HT from the samples baked out in vacuum at various temperatures; The sorbed tritium was released by thermal annealing (0, 0, a)) and by acid etching (A, A, A).
I
,
/
‘\.
-I
1000
(K 1
Fig. 5. The thermal desorption spectra of tritium desorbing from the sample sputter-etched for 1 h (A: HT. A: HTO); not sputter-etched (------: HT, -----: HTO).
HT-III becomes more intense than any other peaks, and in spectrum (d) the peak HT-I becomes the most intense one and the peak HT-II disappears completely. These results mean that a rise in the bake-out temperature increased the relative intensity of the peak HT-I but reduced that of the peak HT-II, whereas that of the peak HT-III had a maximum at a bake-out temperature of 773 K. The temperature of each peak-top varied only a little with bake-out temperature. As seen in fig. 3, the thermal desorption spectrum of HTO had a broader peak than that of HT, particularly in the case of the bake-out at 773 K. The amount of tritium released in the forms of HT or HTO was measured by means of either thermal desorption or chemical etching. The chemical etching was carried out with an acid solution in the same manner as previously reported [12]. The results obtained are summarized in fig. 4, where the amount of tritium released is also expressed as the number of hydrogen isotopes in the right ordinate. As shown in fig. 4(c), the amount of HT reaches a maximum at the bake-out temperature of 773 K. The effect of sputter etching with Ar+ ions was also examined and the spectra obtained are shown in fig. 5. The sputter etching of the surface resulted in a marked increase of the peaks HTO and HT-I and in a shift of the peak HT-III to the higher-temperature side accom-
T. Hirnhawshi et al. / Sorption of tritium on type _316SS 4.1. Efject of chemicui pussivution
400
600
800
Temperature Fig. 6. The thermal
desorption
1000 (K)
spectra
of tritium
desorbing
from the samples pretreated as follows: passivated for 15 h (a: HT. A: HTO); sputter-etched for 1 h after passivation m: HTO); not pretreated f------: HT, ------: HTO).
(0; HT,
panying a decrease in peak height. Fig. 6 shows the thermal desorption spectra which were obtained for two samples: one was chemically passivated for 15 h and the other was additionally sputter-etched for 1 h with Art ions prior to their exposure to HT gas. Although the chemical passivation caused a decrease in the peaks HTO and HT-I, the successive sputter etching caused a marked increase of the peaks. It is suggested that the action of the sputter etching counteracts that of the chemical passivation on the sorption of tritium.
4. Discussion In the present study it was observed that the various surface treatments markedly affect the sorption of tritium on stainless steel. As pointed out by many workers [2-10,13,14], the surface composition also varies with the surface treatments. Therefore, an attempt was made to explain the effects of surface treatments on the sorption of tritium on the basis of the relationship between the sorption phenomenon of tritium and the surface composition of stainless steel.
The surface composition of the stainless steel passivated in 30% HNO, at 333 K has been examined by X-ray photoelectron spectroscopy [Z] and Auger electron spectroscopy [3], and it has been found that the passivation caused the enrichment of chromium and the depletion of iron in the surface oxide film, whereas nickel was enriched a little in the film-substrate interface. If the surface composition of the passive film, which was formed in this work by immersion in the solution containing 0.5% K,Cr,O, and 5% HNO, at 333 K, is assumed to be similar to that described above, the change in the form of thermal desorption spectra caused by passivation may be related to the variation of the surface composition as follows. It is already known that the sorbed tritium forms predominantly OT- ions by combining with surface oxygen, which leads to the desorption of HTO on heating [Ill. Fig. 1 shows that the peak HTO is decreased by passivation. Since the passivation is accompanied by the depletion of iron on the surface, it appears that the iron plays an important role on the process of OT- ion formation through the combination of tritium with surface oxygen. In other words, the depletion of iron by passivation would result in a reduction of OT- ion formation and, consequently, in a decrease of the amount of HTO desorbed. This relation between the iron content and the amount of HTO desorbed is consistent not only with the proposal by Beavis [17] that the presence of iron oxide on the surface of stainless steel is associated with the generation of water, but also with the postulation by Staib et al. [IS] that the formation of is followed by that of aquo iron hydroxide, Fe(OH),, iron complex, from which water can be desorbed. The peak HT-I is decreased by passivation similarly to the peak HTO as shown in fig. 1, and the peak HT-I would be given by desorption of the HT molecule or molecular ion held at the adsorbate such as OT-- ion on iron. The peak HT-III is attributable to the tritium sorbed on the site of nickel as described below (section 3.2). Therefore it is reasonable that the height of the peak HT-III should be little changed before and after the passivation, because the nickel content is kept constant in the surface layer except for a slight enrichment in the oxide-film-substrate interface [3]. 3.2. Effect of vacuum bake-out Nonmetallic elements nitrogen have been found
such as oxygen, carbon, to be present in the surface
T. Hirabayashi et al. / Sorption 01 tritium on type- 316 SS
oxide layer which consists primarily of Cr,O, [12,14,15]. From the surface observation by soft X-ray appearance potential spectroscopy [5], it has been elucidated that Cr,O, was partially obscured by contaminant graphitelike carbon when baked out at 473 K, and that most of the carbon was eliminated by the bake-out at 773 K. In the present work, the peak HT-II, which was relatively high before the bake-out (spectrum (a) in fig. 2) disappeared completely after the bake-out at 973 K (spectrum (d) in fig. 2). These results suggest that the peak HT-II is related to some carbide. Furthermore, Otsubo et al. [16] has found that the “residual hydrogen in steel” related to Fe& was evolved at about 750 K which corresponds to the temperature of desorption for the peak HT-II. For the above reasons, it is presumed that the peak HT-II is given by desorption of the tritium trapped on the site of some carbide on the surface layer. Although the peak HT-III was given by desorption of the tritium dissociated into atomic species on the sorption process as previously reported [ll], this type of sorbed tritium did not form HTO but HT on the desorption process in contrast to the tritium sorbed on the site of iron (section 3.1). This suggests that the peak HT-III is given by desorption of the tritium sorbed on the site containing no available oxygen to form OTions. Park et al. [5] has observed the surface of stainless steel baked out at various temperatures and reported that, while the chromium content on the surface changed little in the range of bake-out temperatures of 573-1273 K, the bake-out at 773 K gave a maximum in the nickel content accompanying a balanced minimum in the iron content. As can be seen from the spectrum (c) in fig. 2, the bake-out at 773 K caused a marked increase of the peak HT-III. On the basis of the phenomenon that the intensity of the peak HT-III reached a maximum at the maximal nickel content, it can be presumed that the peak HT-III is given by desorption of the tritium sorbed on the site of nickel in the surface layer. This is also supported by the fact that the amount of tritium released in a form of HT has a tendency to reach a maximum at the bake-out temperature of 773 K as shown in fig. 4(c). On the other hand, the minimizing of the iron content at this bake-out temperature should reduce the amount of HTO formed by the same mechanism as described in section 3.1., and this tendency for the reduction of HTO at the bake-out temperature of 773 K is seen in fig. 4(b). 3.3. Effect of sputter etching
It is generally accepted that sputter etching generates a rough surface of composition close to that of the bulk due to removal of the surface layer. Hence the sputter-
191
etched surface of stainless steel may apparently contain more iron than the unetched surface, which has been confirmed by Tanabe et al. [7]. Such a roughened and iron-enriched surface may make it possible to sorb a larger amount of tritium, and the peaks HTO and HT-I increased as seen in fig. 5. It is not clear why the peak HT-III was shifted to the higher-temperature side by the sputter etching, and a detailed study is required to explain this. The effects of sputter etching and chemical passivation on the sorption of tritium are obviously opposite to each other. The result in fig. 6 indicates that the loss and contamination of tritium will be depressed by the passive film on the surface, whereas sputter etching leads to the sorption of a larger amount of tritium. The results obtained in the present work suggest that the surface conditioning should be practically important for the stainless steel to be utilized on the handling of tritium especially in a high specific activity, and that first walls made of stainless steel in a fusion reactor will sorb a much larger quantity of tritium after the discharge of plasma than before.
4. Conclusions Chemical passivation resulted in a decrease of intensity for the peak HTO(540 K) as well as the peak HT-I(430 K). Vacuum bake-out at 773 K led to a decrease of the peak HT-11(750 K) and an increase of the peak HT-III(970 K). Sputter etching resulted in a marked increase of the peaks HTO and HT-I. It is suggested that the effect of surface treatment on the sorption of tritium is closely related to the variation of the surface composition of stainless steel.
References
111K. Shiraishi, J. At. Energy Sot. Japan 25 (1983) 617. [in Japanese] Corros. Sci. 19 (1979) 1007. PI K. Asami and K. Hashimoto, (31 M. Seo and N. Sato, Trans. Japan Inst. Met. 21 (1980) 805. [41 G. Hultquist and C. Leygraf, Mater. Sci. Eng. 42 (1980) 199. [51 R.L. Park, J.E. Houston and D.G. Schreiner, J. Vat. Sci. Technol. 9 (1972) 1023. and Shimodaira, Corros. Sci. 18 161 K. Asami, K. Hashimoto (1978) 125. [71 T. Tanabe and S. Imoto, J. Nucl. Mater. 80 (1979) 361. PI T. Tanabe and S. Imoto, Trans. Japan Inst. Met. 20 (1979) 507.
192 [9] G. Hulquist
T. Hirabu~as~~et al. / Sorption of tntrunz on type _216 SS
and C. Leygraf, J. Vat. Sci. Technol. 17 (1980) 85. IlO] G. Betz, G.K. Wehner, L. Toth and A. Joshi. J. Appl. Phys. 45 (1974) 5312. [II] T. Hirabayashi, M. Saeki and E. Tachikawa, J. Nucl. Mater. 126 (1984) 38. [12] T. Hirabayashi and M. Saeki, J. Nucl. Mater 120 (1984) 309.
[13] [14] [15] [16]
T.N. Rhodin, Corros. 12 (1956) 123. R.O. Adams, J. Vat. Sci. Technol. A, 1 (1983) 12. R. Schubert, J. Vat. Sci. Technol. 12 (1975) 505. T. Otsubo, S. Goto and H. Sato, Proc. 2nd JIM int. Symp. on Hydrogen in Metals, Trans. JIM Suppl. 21 (1980) 241. 1171 L.C. Beavis, J. Vat. Sci. Technol. 10 (1973) 386. (181 Ph. Staib, H.F. Dylla and SM. Rossnagel, J. Vat. Sci. Technol. 17 (1980) 291.