NiO oxygen buffer

Journal

of Nuclear

North-Holland.

Matertals

135 (1985)

I1

11-17

Amsterdam

OXIDATION OF SUS-316 STAINLESS STEEL FOR FAST BREEDER REACTOR FUEL CLADDING UNDER OXYGEN PRESSURE CONTROLLED BY Ni/NiO OXYGEN BUFFER Minoru

SAITO,

Hirotaka

FURUYA

and

Masayasu

SUGISAKI

Received 5 January 1985: accepted 5 April 1985

Oxidation 843-1010

of SUS-316

stainless steel for a fast breeder reactor

K under the oxygen pressure of 10

‘7-1O-‘1

The formation of the duplex oxide layer, i.e. an outer

fuel cladding

was examined

Pa by use of an experimental

Fe,O,

constant

lO’/(

RT/J)].

k,( PC,,, T)

was

determlned

On the basis of the oxidation

outer oxide layer wx

as

follows:

k,( PC, ( T)jkg’.m-4.\

kinetics and the metallogaphic

assigned to he the rate-determining

1. Introduction Oxygen potentials at the fuel-cladding gap increase the increase of burn-up under operating conditions in fast breeder reactor (FBR) fuel pins. So, the oxidation behavior of the fuel cladding of SUS-316 stainless steel in a wide range of oxygen potentials is one of the important problems in~uencing fuel performance and safety of FBR fuel pins. With regard to oxidation behavior under oxygen potentials corresponding to relatively low burn-up, we have reported that the oxidation product is a mixture of Cr,O, and MnCr,O, and the oxidation rate is controlled by the outward diffusion of Cr in stainless steel with

11-21. On the other hand, with regard to oxidation behavior under oxygen potentials corresponding to high burn-up, investigations by Hales [3] and Smith [4] are available. They have studied the oxidation behavior of stainless steel from the aspect of corrosion of structural materials of high temperature advanced gas cooled reactors, and reported the formation of the duplex oxide layer, i.e. an outer Fe,O, and an inner (Fe, Cr)-spine1 oxide layers. According to Hales [3], in the case of a long oxidation time such as several 1000 h, the healing oxide layer is formed at the interface of the inner oxide and stainless steel matrix and the oxidation rate is controlled by the diffusion process of the oxidized element in the healing oxide layer. Smith [4] has examined the oxidation be0022-3115/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Pubiishing Division)

ranga of

oxygen buffer.

layer and an inner (Fe. Cr. Ni)-spine1 layer. has observed and the

oxidation kinetics was found to obey the parabolic rate law. The oxygen pressure and temperature rate

in the temperature

technique of a Ni/NiO

information.

’=

dependence of the parabolic

0.170( C;,

the outwar;

/Pa)“‘“’

rxp[ ~ 114x

diffuston of Fe m the

process.

havior in the case of relative short oxidation time of 45-1200 h and found that the oxidation rat< is controlled by the outward diffusion of Fe in the outer oxide layer in spite of the occurrence of the healing oxide layer. Thus, the oxidation behavior of stainless steel under relatively high oxygen potentials has not been clarified unequivocally. So, a more detailed investigation may be required, in particular at the early stage of oxidation where the healing oxide layer is not yet formed. Therefore, in the present study, the oxygen pressure and temperature dependence of the oxidation rate of SUS-316 stainless steel has been measured for a short oxidation time of 24 h under oxygen potential controlled by Ni/NiO oxygen buffer. On the basis of the obtained kinetic data and observation of the oxide layer by X-ray diffraction, SEM and EPMA, the oxidation behavior has been discussed in comparison with the data reported by Hales and Smith.

2. Experimental

The oxygen pressure at the fuel-cladding gap of FBR fuel pins is controlled by the buffer effect due to the non-stoichiometry (O/U + Pu) of the mixed oxide fuel. In the case of out-of-pile experiment, however, the usage of the mixed oxide fuel as an oxygen buffer may B.V.

12

M. Sartri “I ul, ,/ O\idarmn <# sG:T

be inappropriate. So, in the present experiment, the. equilibrium reaction between Ni and NiO was used to simulate the buffer effect of the fuel. The comparison of the oxygen potential of some kinds of metal oxides and that of the mixed oxide fuel is shown in fig. 1, as a function of temperature. This figure shows that the equilibrium oxygen pressure of Ni/NiO corresponds to that of the slightly hyper-stoichiometric mixed oxide fuel and that Fe can be easily oxidized under the oxygen pressure of Ni/NiO oxygen buffer.

2..?. Specimen prepurution

Table I

TI 600

K 1200

1000

800

I

I

1

\talniesa
C‘hemical compoaltion of SUS-316

___.-.-Element --..-

c SI

Compositwn _______~_~... 0.92 I.74

P

o.tis?

s

0.002

NI

13.x3

(‘I

16.55

Mo

2.56

N

O.OO?f

Fe

O.OO?I hal.

_--

and NiO (99.9%); the mixture was dried remove the small quantity of moisture. 2.3. Appuratus and experimental

insulator II

-- Specimen .--quartz

--

(sus-316)

capsule

c-heat

insulator

quartz

tube

+--furnace

1

oxygen

buffer

heat

reserve:

heat

insulator

r

1000

800 T I “C

Fig. 1, Comparison

of oxygen potential of some kinds of metal

oxides and that of mixed (U. Pu)Oz

,

fuel.

to

The arrangeInent of the heating caps&e and furnace is shown in fig. 2. The stainless steel specimen is placed at the top of a quartz tube and the oxygen buffer is set at the bottom, which is filled with pure Ar gas of 380 Torr. The apparatus is designed so as to control separately the temperatures of the specimen and the oxygen

--heat

600

in vacua

procedarre,~

+ --turnace

400

.__

0.053

IMn

R

The specimen of SUS-316 stainless steel with the chemical composition of table 1 (Sumitomo Steel Co. Ltd.) was prepared in the cylindrical form (9 mm in diameter and 1 mm in thickness). A solution treatment was carried out at 1300 K for 30 min. The specimen surface was polished by abrasive papers and then finished by alumina buffing. The oxygen buffer of Ni/NiO was prepared by mixing eq~librium moles of the powders of Ni (99.99%)

.?I!? srutnle.~.~.irel?1

Fig. 2. Schematic diagram of heating capsule and furnace.

buffer. The variation of the oxygen pressure can be carried out by adjusting the temperature of the oxygen buffer. In this study. three kinds of measurements were carried out: (1) Time dependence of oxidation: The oxygen uptake by the specimen was measured as a function of time at two temperatures of 873 and 937 K under an oxygen pressure of 1.05 X lo-l4 Pa. (2) Oxygen pressure dependence of the oxidation rate: The oxygen uptake by the specimen for 24 h was measured at X73 K by varying the oxygen pressure in the range of 10~“7-10~‘3 Pa. (3) Temperature dependence of oxidation rate: The oxygen uptake by the specimen for 24 h was measured under a fixed oxygen pressure of 1.OS X 10’ I4 Pa. by varying the temperature of the specimen in the range of

843.-1010 K. The oxygen uptake by specimen was determined bymeans of the carrier gas extraction method using a tflermal-conductivity cell. the details of which are described in our previous paper ]I]. In addition to the measurement of the oxygen uptake, the microstructure and composition changes of a vertical cross section of the oxidized specimen were examined by X-ray diffraction, SEM and EPMA.

Fig. 3. Time dependence of oxygen uptake under an oxygen pwhure of 1.05 x 10 ” Pa at temperatures of X73 and 937 K.

3, Results

pressed

Since the oxidation behavior of metals has been successfully explained by the parabolic rate law in many previous studies [S]. the present data were also analyzed on the basis of the parabolic rate law. The experimental results of oxygen uptake at temperatures of 873 and 937 K are shown in fig. 3. The square of the oxygen uptake is plotted as a function of time in order to test the validity of the parabolic rate law. The oxygen uptake is expressed in terms of the mass gain per unit area of the specimen surface. As is seen in this figure, experimental results are successfully summarized by the equation

where E is the activation energy of oxidation (J/mol). P,, the oxygen pressure (Pa), T the temperature (K), R the gas constant (J/K I mol), and k. and )I constants. The values of E and ~2 can be determined from the experimental data of oxygen pressure and temperature dependence of k,.

The determined parabolic rate constants are summarized in table 2, and plotted as a function of the oxygen pressure in fig. 4. The oxygen pressure depend-

W’ = Rp( PO:. T) f.

ence of k, was determined as k,tP,,?)a: P&t4’, which

(1)

I

2

3

fimel?05s

by the equation

by the least-squares fitting is in good agreement with

that obtained by Smith [4]. where W is the oxygen uptake by the specimen (kg/m’), t the time (s) and k,(Po?. T) the parabolic rate constant (kg’jm’ f s). Usually. the oxygen pressure and temperature dependence of the parabolic rate constant can be ex-

The results are summarized in table 3. The Atrhenius plot of &,( ?‘) is shown in fig. 5, in which the solid line

x

uptake

Oxygen

r (kg’/m”a)

cku’m2) 2.1xX 10~

I7

2.71x10

1.43x10

lh

3.00 x 10

2.05 x 10

I5

3.50x

10

1.05X

‘4

4.X7X

10

10

3.16xlo-‘~ 0.48X

4.62 X 10

1 1 ’ ’ ’

7.47 x 10

“’

’

I.Y?X

Ii)

“’

1.45 i( IO

‘/I

10

“l

4.07 x IO

I.91 >: 10 _-

Ii

5.36 i( IO z .-_

was determined

by the least squares

The

microstructure

specimens

oxidized

1.05 X 10

l4

oxide

layer.

he seen that

0.85x 10 1”

of for

Pa is shown

i.e. an outer

in both

the outer

3

2.74 x 10

j0

z,

crc\s

section K

in fig. 6, in which

layer

It

follows:

h at 973

und an inner

photographs.

oxide

120

I0 “I

fitting

vertical

24 and

1.04 X IO 1.42X 10

the duplex

oxide

can also

CA

under

layers.

can

he observed

has a comparatively

uniform

T I ‘C 700

650

8

at 873K

kpz215

kpoi

550

/

Poz=l.05x101‘Pa

-9 ; v* ‘E tv W Y

600

x10-'expt-II‘, X 10"RT)

0.1&l

PO,

a Y $

-10

1

I

I

I

I

I

-13

-15

-17

iog(Po2/Pa) Fig. 4. Oxygen at lo-

pressure

a temperature ”

lo-

‘? Pa.

of

dependence 873

K

in

12

11 T-II lo- ‘K-I

of parabolic oxygen

1

IO

rate L‘onst;ml

pressure

range

01

Fig. under

i.

Temperature an oxygen

range of X43-1010

dependence of parabolic

pressure K.

of

1.05’~ IO -”

rate

comtant

Pa 111 temprraturc

Fig. 6. Micrograph

of vertical cross section of oxidized

specimen

surface. is rich in Cr. Ni and 0 and depleted in Fe. The interface of outer and inner oxide layers is located at the position of the original specimen surface. The healing oxide layer, which was reported by Hales and Smith. was not observed.

(a) at 973 K for 24 h: (b) at 973 K for 120 h.

Ry an X-ray diffraction analysis. the outer oxide layer was identified as Fe,O,. and the inner oxide layer as the spine1 oxide consisting of Fe. Cr. Ni and Mn.

Fig. 7. X-ray distribution images of vertical cross section of oxidized specimen 1.05x IO-“’ Pa for 24 h.

at a temperature

of 973 K undrr

an oxygen pressure

of

4. Discussion The key information obtained in the present experiment to clarify the oxidation mechanism may be summarized as follows: (1) Formation of the duplex oxide layer. i.e. the outer Fe,O, layer and the inner (Fe. Cr. Ni)-spine1 oxide layer, is observed. (2) Validity of the parabolic rate law: this suggests that the oxidation rate is closely related to the diffusion of certain constituents in the oxide layers. (3) The outer Fe,O, layer is formed outside the original specimen surface: this suggests that Fe has to migrate outwards out of the original specimen surface. (4) The inner oxide layer is formed in the interior side of the original specimen surface. (5) The shape of the distribution map of oxygen in the inner oxide layer just corresponds to that of the Fe-depleted zone. Judging from above key information, the formation of the outer oxide layer ma); be due to the outward diffusion of Fe. and this may result in the formation of the inner oxide layer as follows: When the outward diffusion of Fe occurs. the activity of Fe in alloy matrix decreases and subsequently increasing Cr activity makes it possible to permit the penetration of oxygen by the dissociation of an outer Fe,O, layer. Microfissure in the outer oxide layer is considered to give also the additive penetration path of gaseous oxygen. Thus. the inner oxide layer grows in the Fe-depleted zone and as a result the shape of the inner oxide layer is identical with that of the Fe-depleted zone. Therefore. it may be straightforwardly supposed that the growth rate of the outer and inner oxide layer is controlled by the outward diffusion of Fe through the outer or the inner oxide layer. So, the next problem is to determine which diffusion stage of the outer and inner layers governs the oxidation rate. For the solution of this problem, the data of oxygen pressure dependence of the rate constant k, may be useful. As was summarized in the previous section, the oxygen pressure dependence of k, can be expressed by ” 14’ So. the rate-determining stage the equation k, a PO2 of the diffusion of Fe by vacancy mechanism has to satisfy this relation. The thermodynamical consideration shows that the dissociation pressure at the interface of the outer and inner oxide layers is maintained constant even when the ambient oxygen pressure is varied. So. the oxidation rate of the inner oxide layer is not influenced by the ambient oxygen pressure: This fact may exclude the possibility that the diffusion of Fe in the inner oxide layer governs the oxidation rate. Conse-

quently. the diffusion process of i-‘c m the outer i~yer can be considered to be the rate-determining process of the oxidation. So, the diffusion coefficient of Fe in the outer Fe,O, layer must be dependent upon Pi:,‘“‘. It should be noticed here that this oxygen pres.sure drpendence is not in agreement with the relation II,, a I:(:_: reported by Schmalzried et al. [6 X]. This disagreement may be due to the difference in the grain size of Fe,<), phases: The grain size of the Fe,04 layer formed in the present study is about 5 pm and that of the Fe,<), crystal used by Schmalzried [6] wax ahnut 1OLlOO (em. ‘Then. the grain-boundary diffusion of Fe must contrrbute appreciably to the outward diffusion in the Fe,O, la>er of the present study. This appreciable contribution of the grain boundary diffusion must reduce the actl\.;ltion energy of the outward diffuslorr of Fe apparenti? ob~rved in the presenl stud\. In fact. the actl\atlolf cncrgh c>f I14 kJ/mol obtained in the present esperimcnt i> amaller than that ol’ 134 -1 37 kJ/mol in :I xinglc Fe,O, cr\\tal [7 91. Unfortunatelv. ;Ib there ;Irc no reliable data of the acti\cltion energy of Fe 111;I p~>l\ cr.b\talllnc Fc,O,. the influence of grain-houndarirs on the activalion energy cannot bc :lccuratel! predlctccl ;\I ,~n> rate, the difference het\reen [hc’,c activation cncrgis*L suggest\ that the grain-boundary diffusion contribute\ to the outward diffusion of Fe in the outer Fc,O, layer C’onaeyuently. it can be safely concluded th:rt Ihc out&ard diffusion of Fe in the outer oxide layer contrl)lb the oxidation rate and that this diffusion proL’e,%. includes the appreciable conrrlbutlon of the grain-. boundary diffusion.

5. Conclusions (1) Oxidation of SUS-316 stainless steel for ;I fast breeder reactor fuel cladding was examined in the oxygen pressure range of 10 “mmIO ” Pa and in the temperature range of 843-1010 K. (2) The formation of the duplex oxide layer. IX. 3111 outer Fe,O, and an inner (Fe. Cr. Ni)-spine]. ~13s observed. (3) The oxidation kinetics was found to obey the parabolic rate law. (4) The oxygen pressure and temperature dependence of the parabolic rate constant kr( P,,,. 7‘) W:I\ determined as follows:

(5) The rate-determining process was concluded be the outward diffusion of Fe in the outer Fe,O,.

to

M. Sorto et 01. / Omlution

oJ SUS-3/n

.sturnless steel

Acknowledgements

References

The authors would like to thank Prof. Y. Kawai for use of oxygen analyzer and Prof. E. Izawa for use of EPMA. They are also grateful to Mr. K. Koga’s assistance in a part of the experiment. This work was supported by a Grant-in-Aid Scientific Research from the Ministry of Education, Science and Culture (No. 58780209).

[II M. Salto. Y. Ishikawa, K. Ikeda and H. Furuya. VI 131 [41 151 161 [71 PI 191

17

J. Nucl. Sci. Technol. 21 (1984) 356. M. Saito. H. Furuya. K. Koga and M. Suglsaki. J. Nucl. Sci. Technol. 22 (1985) 153. R. Hales. Werkstoffe Korros. 29 (197X) 393. A.F. Smith, Corros. Sci. 22 (1982) 857. P. Kofstad. High-Temperature Oxidation of Metals (Wiley. New York. 1966). H. Schmalzried. Z. Phys. Chem. NF31 (1962) 184. R. Dieckmann and H. Schmalzried, Ber. Bunsenges. Phya. Chem. 81 (1977) 344. R. Dieckmann and H. Schmalzried. Ber. Bunsenges. Phys. Chem. 81 (1977) 414. A. Nakamura. S. Yamauchi, K. Fueki and T. Mukaibo. J. Phys. Chem. Solids 39 (1978) 1203.