Sorption of tritium on surface-modified type-316 stainless steel

Fusion Engineering and Design 10 (1989) 287-291 North-Holland, Amsterdam

SORPTION

OF TRITIUM

Takakuni HIRABAYASHI,

287

ON SURFACE-MODIFIED

TYPE-316 STAINLESS

STEEL

M a s a k a t s u S A E K I , T e i k i c h i A. S A S A K I a n d K _ i - W o u n g S U N G

Department of Chemistry, Japan Atomic Energy Research Institute, Tokai, lbaraki 319-11, Japan

An attempt has been made to depress the sorption of tritium on stainless steel. After the surface of the stainless steel was modified either by heating in air or by chemical passivation, the chemical properties of the modified surfaces were examined and the sorption phenomenon of tritium on the surface was studied. The thermal oxidation in air at 423-773 K caused the formation of a surface oxide layer consisting of about 90% of Fe(lll) with a slight amount of Cr(IV) in an Fe203/CrO2-1ike state. The amount of tritium sorbed was found to be smaller on the above surface than on that oxidized at lower or higher temperature. The formation of a surface layer by chemical passivation led to an appreciable decrease in the amount of tritium sorbed. The depression of tritium sorption was attributed to the formation of a surface oxide layer constituted of an outer thin layer of Cr(lll)/Fe(ll) in the Cr2Oa/FeO-like state and an inner layer of CrO2-1ike Cr(IV) with metallic Fe(0) and Ni(0).

I. Introduction The sorption of tritium on material surfaces will d o m i n a t e the tritium c o n t a m i n a t i o n of the inner walls of a p p a r a t u s and measuring instruments for h a n d l i n g of a large quantity of gaseous tritium of high specific activity. The tritium sorbed on a material surface is gradually, but continuously, released and often causes serious problems in the course of the experiments. This induced the authors to attempt to suppress the sorption of tritium by modification of the surface of austenitic type-316 stainless steel, which is one of the most imp o r t a n t structural materials. In the present work, the surface of type-316 stainless steel was modified either by heating in air or by chemical passivation a n d the chemical surface properties were examined by an X-ray photoelectron spectroscopy, and the sorption of tritium on the well-characterized surface was studied to clarify the relationship between the sorption p h e n o m e n o n of tritium and the surface condition of the stainless steel.

2. Experimental 2.1. Materials and preparation of specimens The specimen used in this experiment was cut from a 1.25 m m thick sheet of type-316 stainless steel, a n d its chemical composition by weight was 0.06% C, 0.56% Si, 0920-3796/89/$03.50

1.63% Mn, 0.024% P, 0.007% S, 13.10% Ni, 16.60% Cr, 0.0009% B, 0.026% N a n d 2.24% M o with the balance of Fe. After mechanical grinding to a b o u t 1.0 m m thick and polishing with a suspension of a l u m i n a (0.05 t t m in grain diameter), the surface of the specimen was modified by the two different oxidizing procedures: (a) thermal oxidation in air (about 50% relative h u m i d i t y at room temperature) for 2 h at a c o n s t a n t t e m p e r a t u r e in the region of 293-1073 K, a n d (b) chemical passivation for various times in an aqueous solution c o n t a i n i n g 0.5% K2Cr207 a n d 5% H N O 3 at 333K. Tritium, purchased from New England Nuclear, was diluted with purified p r o t i u m to a c o n c e n t r a t i o n of 51.7 T B q / m o l (T : H = 1 : 40.5), which was used as a sorbing gas (HT gas) for the sorption experiments.

2.2. X-ray photoelectron spectroscopic analysis X-ray photoelectron spectroscopic(XPS) analysis of the specimen surface was carried out with an ESCALAB-5 spectrometer (Vacuum G e n e r a t i o n Scientific Co. Ltd.) employing M g K,, X-rays (1253.6 eV); all b i n d i n g energies of core-line electrons were calibrated with the binding energy of Au 4f7/2 electrons of gold, 84.0 eV. Chemical shifts in binding energy were evaluated on the basis of the metallic state in the substrate of the stainless steel. D e p t h profiles of the c o m p o s i t i o n a n d chemical state of the surface layer were also e x a m i n e d by the c o m b i n e d technique of XPS analysis a n d sputter-etching with Ar ÷ ions.

© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.

288

T. Hirabayashi et a L / Sorption of tritium

2.3. Sorption and thermal desorption of tritium

The sorption of H T gas on the specimen was carried out in a glass vessel that was connected to a high-vacuum apparatus; the specimen baked out for 2 h at 573 K in vacuum was exposed to H T gas of 13.3 kPa for 7 days at 298 K and then evacuated for 20 min at room temperature. The tritium still remaining on the specimen after the above procedures is d e f i n e d as sorbed tritium in this paper. The a m o u n t of sorbed tritium was determined by the thermal desorption method, since almost all the sorbed tritium could be released from the specimen by heating up to 1273 K in an helium flow; the a m o u n t obtained was expressed in the n u m b e r of atoms evaluated for the carrier-free tritium gas because of the dependence of the a m o u n t on the specific activity of the sorbing gas.

3. Results and discussion

5 u. 4 "1_J

< 5 t.)

. i°'~

-r 2 tD

1 0

/

/Z

/ '

300

i

~

I

,

t

,

t

500 700 900 1100 OXIDATIONTEMPERATURE(K)

Fig. 2. Chemical shift in binding energy of core-line electrons of the surface atoms of type-316 stainless steel oxidized in air for 2 h at various temperatures; e: Fe, ©:Cr, zx: Ni, and ( ): approximate value.

3.1. Sorption of tritium on thermally-oxidized surface

A n XPS examination was made for the surface of stainless steel thermally oxidized in air for 2 h at various temperatures. Figure 1 shows the atomic ratios of the metallic elements of the surface as a function of the oxidation temperature; the atomic ratios can be compared with those of the bulk (Fe: 0.666, Cr: 0.179,

1.0

o 0.8 ¢

~

~

C--0.6 n," U

o.4 F

300

500 700 900 1100 OXIDATIONTEMPERATURE(K)

Fig. 1. Surface composition of type-316 stainless steel oxidized in air for 2 h at various temperatures; e: Fe, o: Cr, zx: Ni, x : Mn, ,t: Mo.

Ni: 0.125, Mn: 0.017 and Mo: 0.013). Figure 2 shows the chemical shifts of the binding energies of the coreline electrons of the main surface atoms, namely Fe 2p3/2, Cr 2p3/2 and Ni 2P3/2. In the case of a surface oxidized at 293 K, as can be seen from fig. 2, there is no chemical shift in binding energy for the surface-constituent atoms, except for 2.9 eV of Cr 2p3/2 which was attributable to Cr(lII) [1-3]; the surface of the stainless steel proved to consist of roughly 30% of Cr203-1ike Cr(III) and 50% of metallic Fe(0) and 20% of metallic Ni(0). W h e n the stainless steel was modified by thermal oxidation in air at a temperature of 573-773 K, the surface iron was markedly enriched and the other surface c o m p o n e n t s such as c h r o m i u m and nickel were depleted. The chemical shift of the binding energy was 3.9-4.5 eV for Fe 2P3/2 and a b o u t 1.6 eV for Cr 2p3/2, which were attributable to Fe(III) [4-5] a n d Cr(IV) [2,6-7], respectively. From these results, the modified surface proved to consist of about 90% of Fe(III) with a slight a m o u n t of Cr(IV) in an Fe203/CrO2-1ike state. The thickness of the surface oxide layer was estimated to be a b o u t 0.2 /.tin from the c o m p o s i t i o n - d e p t h profile reported elsewhere [8]. Raising the oxidation temperature to 1073 K led to a depletion of iron with an e n r i c h m e n t of c h r o m i u m as well as manganese. Figure 3 shows the a m o u n t s of sorbed tritium on the surface as a function of the oxidation temperature of the stainless steel. As can be clearly seen from the figure, the oxidation at 423-773 K led to a marked

T. Hirabayashi et al.

/

Sorption of tritium

289

1.0

C E (t)

1,5

0.8 o

0

2~ ~ 10 i.--

0.6

rl,-

n,pi 1El u..I ¢'n

o 0.4

0 o3

w

u_ 5

0.2

0 z 0

k. . . . . . . . . . . .

< 0 0

e

:.'.'.'300

I

500

J

I

700

I

I

I

900

I

1100

OXIDATION TEMPERATURE (K)

Fig. 3. Tritium sorption on the surface of type-316 stainless steel before (o) and after (©) thermal oxidation in air at various temperatures.

reduction in the a m o u n t of sorbed tritium. This signifies that the surface oxidation at this temperature was effective irr~depressing tritium sorption. W h e n the oxidation temperature was raised to 1073 K, the surface of the specimen was observed to become dark brown a n d the a m o u n t of sorbed tritium was markedly increased.

3.2. Sorption of tritium on chemically-passioated surface The change in chemical structure of the stainless steel surface by chemical passivation was examined a n d the results obtained were shown as a function of passivation time in figs. 4 and 5. It can be seen from fig. 4 that the surface atomic ratio of c h r o m i u m / i r o n / n i c k e l reached roughly 0 . 6 / 0 . 3 / 0 . 1 in the first stage of passivation and this ratio stayed a c o n s t a n t even after passivation for 24 h. As shown in fig. 5, the chemical shift of Cr 2p3/2 of the surface was 2.8-3.4 eV which is attributable to Cr(llI) [1-3], while that of Fe 2p3/2 was increased by passivation from 0 to a b o u t 2.5 eV which signifies the formation of Fe(II) [9-10]. It may be said from these results that a b o u t 60% of the surface was Cr(III) in the Cr203-1ike state and the passivation led to the gradual oxidation of the surface iron from metallic Fe(0) to FeO-like Fe(II). Figure 6 shows the a m o u n t of sorbed tritium as a function of passivation time. By comparison of fig. 6

"M'-'t . . . . . . . . . . . . . . . .

6

I. . . . . . . . . . . . . . .

12

I. . . . . . . . . . . . . . .

18

i

24

PASSIVATION TIME (h)

Fig. 4. Surface composition of type-316 stainless steel passivated for various times; e: Fe, o: Cr, zx: Ni, A: Mo.

with figs. 4 a n d 5, it can be p r e s u m e d that the depression of tritium sorption was caused by the increase of C r 2 0 : l i k e Cr(III), which c o m p e n s a t e d for the decrease of iron, a n d p r o b a b l y also by the f o r m a t i o n of FeO-like Fe(II) on the surface. A small a m o u n t of metallic Ni(0) appears to have n o m a r k e d influence on the tritium sorption. The surface layer which was effective in depressing tritium sorption was further examined b y the c o m b i n e d technique of XPS analysis a n d A r ÷ ion etching. T h e sputtering rate by A r + ion etching was estimated to be roughly 2 × 10-15 n m cm 2 i o n - l , assuming the sputtering yield was unity. Figures 7 a n d 8, respectively, show

4

i- 3 LL -'r (./3 /

2

.=,1 "1" (J

I

i

I

-~x

6

12

18

24

PASSIVATION TIME (h)

Fig. 5. Chemical shift in binding energy of core-line electrons of the surface atoms of type-316 stainless steel passivated for various times; e: Fe, o: Cr, A: Ni.

290

T. Hirabayashi et aL / Sorption of tritium

~3

u E

oE

10

LL "l-

m 2

.t-o

.J

O

~8

I IJJ "1-

o ~ . ~ _ . _ _ i 0 1 2 FLUENCE (10~ Ar" ions.cm-')

F-_ ~6 I Iii 113 tr"

Fig. 8. Chemical shifts of binding energies of Fe 2p3/2 (o), Cr 2p3/2 (o) and Ni 2p3/2 (zx) during ion beam profiling with Ar + ions; type-316 stainless steel after passivation for 24 h.

Id. O I-Z

1.0

layer was higher but the iron content was lower than that in the bulk. As can be seen from fig. 8, the chemical shift of Cr 2p3/2 is 3.0 eV at the topmost surface and a b o u t 2 eV in the inner surface; the former is attributable to Cr(IIl) [1-3] and the latter to Cr(IV) [2,6-7]. There is no chemical shift of Fe 2p3/2, except for 2.7 eV at the topmost surface which may be attrib u t e d to Fe(II) [9,10]. As already pointed out by Seo a n d Sato [11], a slight e n r i c h m e n t of nickel was shown at the surface l a y e r - s u b s t r a t e interface, while no chemical shift of Ni 2p3/2 was observed. It may be concluded that the depression of the tritium sorption on stainless steel was attained by the formation of a surface layer constituted of Cr203-1ike Cr(III) with a little a m o u n t of FeO-like Fe(II) at the topmost surface and of CrO2-1ike Cr(IV) with metallic Fe(0) and Ni(0) in the inner layer.

0.8

References

I

I

i

I

6

12

18

24

PASSIVATIONTIME (h) Fig. 6. Tritium sorption on the surface of type-316 stainless steel before (e) and after (o) passivation.

the c o m p o s i t i o n - d e p t h and the chemical s h i f t - d e p t h profiles for iron, c h r o m i u m and nickel in the oxide layer formed by passivation for 24 h. The thickness of the surface layer was estimated to be a b o u t 0.02 ~ m, which was about one tenth of that of the thermally oxidized surface layer [8]. The c h r o m i u m content in the surface

•

0

~< 0.6 11: (..)

O~0.4

0.Z _.~

0

t-o-" ~ " ~-'-~I-~,--

1 2 6 FLUENCE (10" Ar" ions-cm"2)

Fig. 7. Composition-depth profile of type-316 stainless steel passivated for 24 h; e: Fe, O: Cr, zx: Ni, x : Mn, A: Mo.

[1] G.C. Allen and P.M. Tucker, Multiplet splitting of X-ray photoelectron lines of chromium complexes. The effect of covalency on the 2p core level spin-orbital separation, Inorg. Claim. Acta 16 (1976) 41-45. [2] 1. lkemoto, K. Ishii, S. Kinoshita, H. Kuroda, M.A.A. Franco and J.M.Thomas, X-ray photoelectron spectroscopic studies of CrO 2 and same related chromium compounds, J Solid State Chem. 17 (1976) 425-430. [3] G.C. Allen, M.T. Curtis, A.J. Hooper and P.M. Tucker, X-ray photoelectron spectroscopy of chromium-oxygen systems, J. Chem. Soc. Dalton (1975) 1675-1683. [4] B.S. Covino, Jr. M. Rosen, T.J. Driscoll, T.C. Murphy and C.R. Molock, The effect of oxygen on the open-circuit passivity of Fe-18Cr, Corrosion Sci. 26 (1986) 95-107. [5] K. Asami, K. Hashimoto and S. Shimodaira, An ESCA study of the Fe2+/Fe 3÷ ratio in passive films on iron-chromium alloys, Corrosion Sci. 16 (1976) 387-391.

7". Hirabayashi et al. // Sorption of tritium [6] N.S. Mclntyre, D.G. Zetaruk and D. Owen, XPS study of the initial growth of oxide films on inconel 600 alloy, Appl. Surf. Sci. 2 (1978) 55-73. [7] F. Garbassi, E. Mello Ceresa, G. Basile and G.C. Boero, The mechanism of the surface stabilization of CrO 2 magnetic'powders, Appl. Surf. Sci. 14 (1982-83) 330-350. [8] T. Hirabayashi, K-W Sung and M. Saeki, Change of tritium sorbability of Type 316 stainless steel by thermal oxidation (1989), to appear.

291

[9] G.C. Allen, M.T. Curtis, A.J. Hooper and P.M. Tucker, X-ray photoelectron spectroscopy of iron-oxygen systems, J. Chem. Soc. Dalton (1974) 1525-1530. [10] N.S. Mclntyre and D.G. Zetaruk, X-ray photoelectron spectroscopic studies of iron oxides, Anal. Chem. 49 (1977) 1521-1529. [11] M. Seo and N. Sato, Surface characterization of stainless steels prepared with various surface treatments, Trans. Jpn. Inst. Metals 21 (1980) 805-810.