Effect of oxygen in sodium on vanadium and vanadium alloys

Journal of the Less-Common Metals Elsevier Sequoia S.X., Lausanne ~ Printed in The Netherlands

EFFECT AND

OF OXYGEN

VANADIUM

R. L. KLUEH Metals

ON VANADIUM

ALLOYS*

AND J. H. DE\‘AS

and Ceramics

(Rcccivcd

IN SODIUM

389

Division,

Oak Ridge Satioual

Laboratovy,

Oak Ridge,

Term.

37830 (U.S..4.)

June rgth, 1970)

SUMMARY

Vanadium-base

alloys are being considered

as potential

fuel cladding materials

for sodium-cooled fast-breeder reactors. The mechanical and corrosion properties of vanadium alloys in sodium are strongly affected by oxygen impurities in the sodium. Vanadium

and vanadium

alloys readily absorb oxygen from liquid sodium at 500’ to

7oo’C when the sodium contains 5 to IO ppm of oxygen, try of the alloy. Alloys containing simultaneous

formation

titanium

of a subsurface

depending

and aluminium

upon the chemis-

absorb oxygen with the

hardened zone. The depth of this zone depends

on time, temperature, oxygen content of the sodium and alloy chemistry. In the case of pure vanadium or vanadium containing chromium or niobium, oxygen absorption occurs somewhat

faster and without the formation

of a pronounced

hardened

zone.

We have examined the published corrosion results for vanadium-titanium and vanadium-aluminum in terms of internal oxidation. Internal oxidation occurs when oxygen dissolves in the alloy and reacts with the dissolved titanium

or aluminum

to form TiOz or AlgOa, which precipitates. This precipitate gives rise to the hardened zone. We have shown that the kinetics of hardened zone formation obey the internal oxidation external

model proposed by WAGKER,

when no vanadium

oxide scale forms on the

surface, and obey the model proposed by RAPP AKD COLSON

when an external

scale forms.

INTRODUCTION

Vanadium alloys possess several advantages as fuel cladding materials for sodium-cooled fast-breeder reactors. Compared with currently used stainless steels, vanadium affords a lower neutron absorption cross section and better high-temperature mechanical properties. For this reason, installations throughout the world have been developing and testing vanadium alloys for cladding applications. A major part of this testing program has concerned the compatibility of vanadium alloys with sodium in the temperature range 5oo”-8oo’C. In general, the results from the various corrosion studies on vanadium alloys are in qualitative agreement and show that compatibility is strongly dependent on * Research sponsored Carbide Corporation.

by the U.S. Atomic

Energy Commission

under contract

J. Less-Common Metals,

with the Union

22 (1970) 389-398

R. L. KLUEH, J. H. DEVAN

390

Fig. I. Vanadium-so% titanium exposed for 600 h at 600°C to static sodium containing Note the different polishing characteristics of the band adjacent to the surface.

oxygen.

the oxygen concentration in the sodium. When exposed to sodium that has been continuously hot-trapped*, vanadium and its alloys show very small weight variations at temperatures up to 650°C; small weight gains are usually observed. In cold-trapped sodium, however, vanadium and several of the alloys usually have exhibited heavy weight losses as a result of the formation and spallation of a surface scale. In both types of systems, V-zo%Ti** develops a band with different polishing and etching characteristics (Fig. I) immediately below the surface when the specimen has no surface scale, and below the scale when one is present. The depth of this band increases with time of exposure to sodium, and has been labeled by most investigator+* as the “hardened zone” or “hardened layer” because of its extreme hardness relative to the unaffected interior (Fig. 2). Some investigators have assumed that the hardened zone in V-zo%Ti is due to the solution of oxygen in the alloyl-4. Based on this assumption, diffusion coefficients for oxygen have been calculated from solutions of FICK’S second law as applied to the rate of advance of the hardened layer 193.The calculated values are always much less than those measured in unalloyed vanadium, and the difference is attributed to * Oxygen is removed from sodium by hot trapping or cold trapping. In hot trapping, the sodium is passed over an active metal (e.g., zirconium chips at 600°C) that getters the oxygen from the sodium. Sodium is cold trapped by passing it through a low-temperature (I 1o’~175’C) zone where sodium oxide precipitates (the oxygen concentration of sodium decreases with decreasing temperature). The oxygen concentration of the system is therefore maintained in equilibrium with sodium oxide at the cold-trap temperature. * * Unless otherwise noted, alloy compositions are in percent by weight. J. Less-Common Metals,

22

(1970)

389-398

EFFECT OF OXYGEN IN SODIUM ON v AND v ALLOYS

Fig. 2. VanadiumPzoOh titanium exposed for 40 h to sodium at 650’ C containing 6 ppm 0. xote the Iinoop hardness indentations and the extreme hardness of the band adjacent to the surface. (b.rom ref. 2, reproduced by permission of Intern. .\tomic Energy .L\gcincy,Vienna.)

the presence of titanium. sibility

Only recently

that the hardened

have investigators

zone is caused by internal

begun to examine

the pos-

oxidations.

This paper will review some of the previous work on the oxidation of vanadium and vanadium alloys in sodium and show how an internal oxidation model can be used to explain the results on certain tion is due to internal

oxidation

vanadium

alloys, that is, hardened

and the increased hardness

or non-internal oxidizing vanadium precipitation hardening.

zone forma-

(over unalloyed

vanadium

alloys similarly exposed to sodium) resulting

from

INTERNAL OXIDATION

Internal oxidation is encountered when alloys contain components that are less noble than the base metal-A., the free energy of formation of the alloy component oxide is more negative

than the free energy of formation

of the base metal oxide.

Under certain oxidizing conditions, the more active alloying elements selectively react with oxygen and precipitate as oxide particles within the metal matrix. This internal oxidation can proceed with or without the formation of an external oxide of the base metal, depending on whether the oxygen potential of the oxidizing atmosphere is greater or less than the oxygen potential of the lowest base metal oxide. Furthermore, oxygen must be soluble and diffuse readily in the base metal, and the oxygen flux into the alloy must be greater than the alloying element flux in the opposite direction. specimen

If the latter is not true, an oxide layer of the alloying element forms on the surface. Indeed, when the solute exceeds some critical concentration (de].

Less-COWWLON

~&dS,

22

(1970)

389-398

R. L. KLUEH, J, H. DEVAN

392

pending upon the alloy system, oxygen pressure, etc.), solute oxidation proceeds on the external surface and may become a barrier to internal oxide formation, The simplest case is one where no external base metal oxide forms. This case is also informative for examining the oxidation effects to be expected during sodium exposure. WAGNER~ and others’-9 have treated this case analytically, and the WAGNER model has been reviewed recently by RAPP~O. Basically, the model assumes that oxygen diffuses into the metal and reacts with the dissolved solute to form the insoluble oxide, which precipitates. Precipitation occurs at an advancing reaction front which, assuming there are no short-circuiting paths in the alloy, is parallel to the external surface. WAGNER” examined the case where growth of the internal oxidation zone is controlled by the diffusion of oxygen through the zone as opposed to the oxide precipitation rate. Assuming that an insoluble oxide precipitate forms--&., no alloying component remains in solution in the presence of excess oxygen - and that the precipitate does not affect the diffusion of oxygen in the base metal, WAGNER showed that the distance from the external surface to the reaction front (t) can be assumed to be a parabolic function of time t, [=zy(Dot)

1;

(1)

y is a dimensionless constant which depends on the oxygen diffusion coefficient in the unalloyed base metal (Do), the effusion coefficient of the alloy component, the alloying element concentration, and the oxygen concentration at the specimen surface. Within the diffusion zone, the concentration profile for oxygen, No, is determined by solving the one-dimensional diffusion equation, GNo

8No dt=DoF

(2)

using eqn. (I) and the boundary t >o

No=N&

for x=0,

No=o

forx>%,t>o,

where x is the distance oxygen at the external initially at concentration XVB -=

DB

at

conditions (3)

from the external surface and NO(B) is the atom fraction of surface. Similarly, if the alloying element being oxidized is NBO its concentration profile follows by solving

@Ns -

ax2

for the boundary

(4)

conditions

Nn-.NgO

for x >o, t==o

Ne=o

forxcE,

(5)

t>o.

The parameter y is determined by the conditions of equivalent oxygen and I3 that reach E react to form oxide or

fluxes-namely,

where Y is the ratio of oxygen to B atoms in the BOY oxide. In theory, therefore, when the system parameters are known, J. Less-~o~~~~

Metals,

a2

(1970)

389-398

all

it is possible to

EFFECT OF OXYGENIN SODIUMON Jr ANDIT ALLOYS

393

calculate the concentration profiles for oxygen and the alloying element B and then to calculate y and 6. When comparisons are made for conditions of relatively high temperature and moderate stability of the internal oxide, the theoretical expressions agree with the experimentally observed kinetic+?

N8

-----_^_ ho.

N0

or

No

Ehk Alloy

or

N00. f xFig. 3. Concentrationprofiles for the exclusive internal oxidationof alloys. (From ref. 10, corlrtesp by Corrosion.) A special case, and one that is of interest for the vanadium alloys of this discussion, is the situation shown in Fig. 3 where $<

No(“) -j$Q+r.

In this case,

and it can be shown that10 (7) and

Physically, this case corresponds to one where the B is oxidized in place and only oxygen diffuses. RAPPERdiscussed internal oxide microstructures, and of importance here are the following: (I) for a given oxidizing potential, the precipitate size increases as one proceeds from the specimen surface to the interior of the cross section; (2) the more negative the free energy of formation of the oxide, the smaller the precipitate particles; (3) the precipitate particle size decreases with decreasing temperature and increasing No@). J. Less-Common Metals,

22

(1970)

389-398

R. L. KLUEH, J. H. DEVAN

394

Internal oxidation may also occur with the simultaneous growth of an external oxide scale. The kinetics for this case have been derived under two conditions. In the first, the scale is assumed to thicken parabolica~y~l and in the second, linearlylz. The internal oxide microstructures are similar with or without an external scale. VANADIUM

AND VANADIUM

ALLOYS IN SODIUM

A quantitative comparison of the results for vanadium alloys in cold-trapped sodium is hampered by the inability to correlate analytical techniques used by different investigators for measuring oxygen in sodium. The general findings, however, have been in qualitative agreement. RUTHER~~ reports that no external vanadium oxide forms at temperatures of 600” to 700°C if the oxygen concentration of the sodium is less than I ppm. This is in agreement with calculations where the free energies of formation of the lowest v~a~urn oxide are compared with the free energies of formation calculated for various concentrations of oxygen in sodium. RUTKER~~ has also stated that, when a carbon source is present in the sodium circuit, the first scale that forms is a vanadium oxycarbide; this compound also appears to form at about I ppm 0 in sodium. Since diffusion of oxygen in vanadium is much faster than carbon, this should not affect the subsequent formation of an internal oxide. TABLE

I

ALLOYSTESTED

BY CHAMPEIX

Alloying

Percent

element

Weight

Atomic

Al Si Ti

IO

IQ 0.91

Cr

Zr Nb MO

0.5 20 6

et n1.4~*

21

I.5 5

5.9 0.84 2.7

5

2.7

* Reproduced by permission of Intern. Atomic

Energy Agency, Vienna.

CHAMPEIX et cd.4 compared the behavior of vanadium and vanadium binaries of Ti, Al, Cr, Nb, MO, Si, and Zr in cold-trapped sodium at 600°C under forced-flow conditions; Table I shows the compositions of the alloys studied. The alloys divided into two groups, based on hardness changes occurring at the center of x-mm test specimens, After IWO h, the center microhardness increased si~ificantly in unalloyed vanadium and all alloys except V-1oo?A1 and V-zo%Ti. The authors attributed this hardness change to an increase in dissolved oxygen. The aluminum and titanium alloys, however, displayed the “hardened zone” previously described, withno change in center hardness. If the standard free energies of formation of the oxides of the alloying elements tested by CHAMPEIXare compared14 with the free energy of formation of the vanadium oxides, it is found that Al, Ti, Si, and Zr form oxides more stable than the lowest vanadium oxide. These are then the alloys which would be expected to oxidize internally. V-zo%Ti and V-IO%A~ give every indication in the above study of having

EFFECT OF OXYGEN IN SODIUM ON V AND V ALLOYS formed an internal fraction

oxide. Furthermore,

3%

if we compare

the alloys tested on an atom

basis, it is obvious why the center hardness of the zirconium and silicon alloys

might have increased even though the alloys oxidized internally. Whereas aluminum and titanium were at the 20 at.% level, silicon and zirconium alloys were present to less than I at.%. A rough calculation using eqn. (8) shows that, at the latter concentration

(< I at.%),

the reaction

zone would proceed throughout

the cross section in

the test times employed. For the silicon and zirconium alloys, therefore, hardness increases are due to oxygen solution and internal oxidation. It should be noted that metallographic

observations

as reported

the center

by CHAMPEIX

et al.4 (and the other work discussed

phase within the hardened diffusion of titanium the precipitates

below) gave no evidence of a distributed second zone. This is not too surprising, however, because at boo”C

is extremely

sluggish, and the titanium

formed would have been extremely

ning of the small oxide particles

that form.

RUTHER and co-workers1,2,13 at the Argonne National the effect of both hot- and cold-trapped V-2o0/bTi exposed

to hot-trapped

gained weight and initially profiles

taken

through

is oxidized in place and

small. There is little or no coarseLaboratory

sodium on vanadium

have studied

alloys in some detail.

sodium developed no surface oxides. All specimens

the weight gains followed a parabolic

the hardened

zone showed a dramatic

law. Microhardness hardness

decrease

in

going from the hardened zone, which was easily discernible metallographically (Fig. 2)) to the unaffected zone where the hardness is the same as material given similar vacuum heat treatment--i.e.,

not exposed to sodium. This behavior

contrasted

with unalloyed

vanadium, similarly exposed to sodium, where the microhardness profiles were smooth curves into the interior of the specimensi, and the vanadium showed uniform polishing and etching characteristics

across the specimen cross section.

At a given temperature,

the higher the oxygen content of the sodium, the higher the parabolic rate constant and the thicker the hardened zone. For a given temperature and oxygen concentration in the sodium, the thickness of the hardened zone increased with time. i\ll of these observations agree with what is expected for internal oxidation. For this system,

as in many internal oxidation

known and the possibility

exists

that

studieslo,

no is a function

determine if the system is consistent with the theoretical product ‘\To(s)Do which from eqn. (8) becomes

The ratio of this product for different should be unity. Argonne investigators1 dened layer for V-Io%Ti Since the only difference the ratio of the product,

examined

concentrations

the difference

Hence,

model, we can examine

(constant

=

to the

temperature)

between the depth of the har-

and V--2o%Ti in hot-gettered sodium after nine days. between the two specimens is the titanium concentration,

t102NoTi-10

~ro(s)~ollo

NO(S

titanium

No(s) is not accurately

of concentration.

(10)

520"N"Ti-20 ’

should be unity. We determined

[ from the photomicrographs

(penetrations

of 2g and

,T. Less-Common Metals, 2.~ (1970) 389-398

K. L. KLUEH, J. H. DEVAN

396

18 pm were measured) and hardness profiles given by LEVIN ANDGREENBERG~, and calculated a value of the ratio of about 3. In similar tests by BORGSTEDT AND FREEST in Germany, on vanadium alloys containing 5, IO, and ZOO/~ Ti exposed to hot-trapped sodium for 500 h at Goo”C, the hardened-zone penetrations of 200, IOO, and 70 pm, respectively, were reported. If these penetrations are compared using eqn. (II), it is found that

and No’Wol~

N*w+

r 2R

in good agreement with theory.

TIME, days Fig. 4. Corrosion of V-zo%Ti in 650°C sodium (cold-trapped approximately 175°C). The ordinate scaIes are equivalent, based on a meta! density of 5.7. (From ref. 13. reproduced by permission of Argonne National Laboratory.)

The Argonne investigators 2313found that the vanadium and vanadium alloys exposed to coId-trapped sodium developed an external scale, and that the corrosion kinetics for V-2o”hTi passed from parabolic weight gains to Iinear weight losses. Figure 4 shows measured penetrations, external losses, and weight losses as a function of time 13.The total penetration is the sum of the metal removed with the external scale-which was loosely adhering and brushed off before taking measurements -and the internal hardened zone. RUTHER~~states that the resultant slopes for the weight loss and penetration curves are equivalent because thickness of the internally hardened zone was relatively constant. This is shown in Table II where the thickness of the hardened zone is given as a function of time. A constant thickness of about 50 pm was approached in about 14 days. The internal oxidation model that assumes the presence of a linearly thickening 12. This treatment predicts that the thickoxide film was treated by RAPP ANDCC&SON ness of the internal oxidation zone (E-X) will approach a constant value; X is the J. Less-Cordon

~e~~~s, 22 (1970)

389-398

EFFECT

OF OXYGEN

TABLE

II

397

IN SODIUM ON v ASD v ALLOYS

DEPTH OF HARDENED LAYER ON V-zo%Ti

CORRODED

6j0°C

IN SODIUM

.4T

Agency,

Vienna.

(ref. z)*

Time (davs)

'7.0 14.0

38-4’ 49-55 45-58 44-57 45-60 5-65

‘5 ‘5 ‘4 16 16

21.1

28.1

35.’ 4’. I

12

* Reproduced

by permission

thickness of the external zone is given by12

of Intern.

Atomic

Energy

oxide. The limiting thickness

([ -X)

* of the internal oxidation

where b=dt,

dX

(13)

(i.e., the rate of thickening

of the external

scale).

The data in Fig. 4 can be used to compare the observed and calculated limiting thicknesses (t--X)*. The value of b is just the slope of the curves of Fig. 4; b gz.9 x 10-9

cm/set. If we then assume D0zr.8

x

cm-2/set

10-9

(ref. IS),

and No(s)

v=2,

~0.005 (solubility of oxygen in alpha vanadium at 650°C (ref. 16), eqn. (12) gives a thickness for the internal oxidation zone of 77 pm, in good agreement with the 50 pm observed (Table II). TAHLE

III

THICKNESS TRAPPED

OF

HARDENED

SODIUM

AT

SURFACE

600°C

ZONE

Thxkness

f h)

V-IO wt.o/, Al

v-20

50

30

2jO

El

1000

80

* Reproduced

ON

v-10

l;/o:\l and

V-20y/,Ti

ALLOYS

IN

COLD-

of hardened zone (pm)

Time

5oo 750

FORMED

(ref. 4)*

______ by permission

70

wt.o/,O Ti

~~~

of Intern.

~_ htomic

Energy

Agency,

Vienna

Table III shows data taken from CHAMPEIX Et al.4 on the thickness of the hardened zone in V-1o%A1 and V-ao’$/,Ti tested at 600°C in cold-trapped sodium. External scales were formed and we can see that the thickness of the hardened zone of theV-~o~/~Alalsoapproachesaconstant. Since there are no data for intermediate times the results for V-zo%Ti are inconclusive. Ternary

systems

of the V-Ti-X

type that

have been tested2,3

.J. Less-Common

M&ls,

showed

“no

.zz (1970) 389-398

398

R. L. KLUEH, J. H. DEVAN

order of magnitude improvement” over V-zo%Ti. In general, the hardened zone was not much changed (for equivalent titanium concentrations) by these additions. For example, BORGSTEDT AND FREEST compared V-5%Ti-ro%Nb and V-Io%TiIo%Nb and found penetrations of approximately 170 and go ,um after 500 h at 600°C compared with 200 and IOO pm for V-s%Ti and V-ro”/OTi. The Argonne authors2 found some improvement in that certain ternary additions reduced the external scale loss. In this respect, they concluded that chromium was more effective than molybdenum, tantalum, and niobium. This conclusion, however, was made by comparing V-zo%Ti-5% Cr with V-zo%Ti to which only 2% MO, Ta, or Nb had been added. On an atom fraction basis, there is about five times as much chromium present as molybdenum and niobium and about ten times as much as chromium as tantalum. Even on that basis, however, the alloy with 2% MO apparently offered oxidation resistance comparable to that provided by 5% Cr. Indeed, this was borne out by the limited data of BORGSTEDT AND FREEST.Their gravimetric comparison of V-ro%Tiz%Cr and V-Io%Tij”hMo (somewhat closer to equal atom fractions of molybdenum and chromium) showed a slight advantage for molybdenum. SUMMARY AND CONCLUSIONS

It was proposed that the hardened zone observed (below the specimen surface or the external scale, depending on the oxygen concentration of the sodium) when V-Ti and V-Al alloys are exposed to sodium containing oxygen is the result of internal oxidation. The conditions for internal oxidation were reviewed, and kinetic data on the oxidation of V-Ti alloys was compared with the internal oxidation models proposed for cases where no external scale forms and where a scale forms with linear kinetics. In both cases, there was semiquantitative agreement between theory and experiment. REFERENCES I H. A. LEVIN AND S. GREENBERG, An exploratory study of the behavior of niobium- and vanadium-base alloys in oxygen-contaminated sodium, ANL Rep. 6982, January, 1968. 2 S. GREENBERG, W. E. RUTHER AND H. A. LEVIN, in Alkali Metal Coolants, (Proc. Symposium, Vienna, 28 Noel.-2 Dec. 1966), Intern. Atomic Energy Agency, Vienna, 1967, p. 63. 3 H. U. BORCSTEDT AND G. FREES, Corrosion, 2q (1968) 209. 4 L. CHAMPEIX, R. DARRAS AND 1. SANNIER.~~ Alkali Metal Coolants (Proc. Symposium, Vienna, 28 Nov.-z Dec. 1966), Intern. “Atomic Energy Agency, Vienna, 1967, p. 45. . 5 H. U. BORGSTEUT AND G. FREES, Werkstoffe Korrosion, IO (1968) 862. 6 C. WAGNER, Z. Elektrochem., 63 (1959) 77;: 7 F. N. RHINES, W. -4. JOHNSON AND W. A. ANDERSON, Trans. AIME, 147 (1942) 205. 8 L. S. DARKEN, Trans. AIME, 150 (1942) 157. g J. L. MEIJERING .\ND M. J, DRUYVESTEYN, Phili@ Res. Refit., 2 (1947) 81 and 260. IO R. A. RAPP, Corrosion, 21 (1965) 382. II F. MAAK, Z. Metallk., 52 (1961) 545. 12 R. A. RAPP AND H. D. COLSON, Trans. AIME, 236 (1966) 1616. 13 W. E. RUTHER, The corrosion of vanadium and vanadium-base alloys in sodium, in Proc. Intern. Conf. on Sodium Technology and Large Fast Reactor Design, (ANL-7520, Part I).

14 .4. GLASSNER, The thermochemical properties of the oxides, fluorides, and chlorides to 2500”K, ANL Rept. 5750, 1959. 15 R. W. POWERS AND M. V. DOYLE, J. A#. Phys., 30 (1959) 514. 16 J. STRINGER, J, Less-Common Metals, 8 (1965) 1. J. Less-Common Metals, 22 (1970) $39-398