oxydation zircaloy 4

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Journal of Nuclear Materials 202 (1993) 239-244

North-Holland

Improvement in oxidation resistance of Zircaloy-4

by surface alloying with a thin layer of palladium

G.A. Eloff a, C.J. Greyling a and P.E.Viljoen b

aAt omic Energy Corporation of South Afr ica Lt d., P.O. Box 582, Pretori a 0001, South Afri ca

b Uni versit y of t he Or ange Free State, P.O. Box 339, Bloemfont ein 9300, South Af ri ca

Received 13 October 1992; accepted 18 December 1992

Economic considerations and environmental pressures have forced reactor operators world-wide to consider higher

burnup, to the extent that standard Zircaloy-4 fuel cladding is now approaching its design limitations. One of the main

factors limiting the safe in-reactor lifetime of PWR fuel is the waterside corrosion of the fuel cladding. In this investigation

the oxidation resistance of Zircaloy-4, surface alloyed with a thin layer of palladium by annealing in vacuum at 95O”C,was

studied. Short term oxidation in air shows a retardation of the transition to linear oxidation kinetics. It is argued that this

retardation is caused by the incorporation of intermetallic particles into the oxide layer which increases the ductility of the

oxide, thereby retarding the onset of cracking from the outside surface of the oxide layer.

1 Introduction

The performance of Zircaloy4 fuel cladding tubes

in pressurized water reactors (PWRs) all over the world

has been extremely satisfactory over the past two

decades. In the past decade, however, economic con-

siderations have been forcing reactor operators to con-

sider higher bumup levels of their fuel in order to

contain reload cost and to minimize reactor downtime.

An added attraction of higher burnup is that the

amount of spent fuel is accordingly reduced. The latest

PWRs are designed to operate at higher coolant tem-

peratures than the previous generations of PWRs with

longer fuel cycles than before, which means that the

fuel is now required to withstand a much more hostile

environment for considerably longer periods [l].

It is generally accepted that waterside corrosion

(oxidation and hydriding) of fuel cladding represents

the single mosti important limitation on the safe in-re-

actor lifetime of fuel [2-41. The developments referred

to in the previous paragraph have spurred various

programs to develop alternative zirconium base alloys

capable of meeting the demands of reactor operators

through the next decade. Considerable improvements

in corrosion resistance have already been achieved

with new alloys such as ZirloTM (Zr-l%Nb-l%Sn-

O.l%Fe) [5,6]. The search for better materials, mean-

while, continues unabated as economics and environ-

mental concerns about nuclear safety keep the pres-

sure up. This investigation was carried out with the

purpose of determining whether the oxidation resis-

tance of Zircaloy-4 can be improved by surface alloying

with palladium.

2. Background

The effect of a small addition of one of the PGMs

to some alloys can be quite dramatic, as evidenced by

the significant improvement in corrosion resistance ob-

tained by alloying pure titanium with a small amount of

palladium [7]. This effect, generally referred to as

cathodic modification or cathodic alloying, is normally

brought about by either adding a small amount of the

PGM to the alloy, or by ion implantation of the desired

PGM into the surface where the cathodic protection is

required [8,9].

The comparable chemical and metallurgical be-

haviour of titanium and zirconium suggests that addi-

tion of palladium to Zircaloy-4 may have a similarly

beneficial effect. Very little reference to any such work

can, however, be found in the literature. This is proba-

0022-3115/93/ 06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved


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240

G.A. Eloff et al. / improv ement in oxidat ion resistance of Zircaloy- 4

bly due to the results published by Dixon [lo] who

found no improvement in the corrosion resistance of

Zircaloy-2 enriched with small amounts of palladium.

Alloying pure zirconium with varying small amounts of

palladium, according to Schleicher [ll], brought about

an improvement in corrosion resistance through anodic

protection by galvanic corrosion of Zr,Pd intermetallic

precipitates.

3. Experimental procedure

3 I Sample preparation

A number of 10 mm pieces were cut from a stan-

dard Zircaloy-4 fuel cladding tube in the cold-worked,

stress-relieved condition. A 2-p,rn thick palladium layer

was electroplated onto the surfaces of these samples.

They were divided into five groups of four each, and

these groups, designated Al-A4 through El-E4, were

then separately vacuum-annealed at 950°C for 30 min,

1, 2, 4 and 8 h, respectively, inside a quartz tube which

was evacuated to a vacuum of better than 10m4 Pa.

After annealing the samples were quenched, still in

vacuum, to room temperature by withdrawing the evac-

uated tube from the furnace. The average cooling rate

through the (a + p&phase was approximately 18 K/s.

It was essential to perform the annealing in vacua to

contain the high affinity of zirconium for oxygen at

elevated temperatures.

Fig. 1. Cross section of a Zircaloy-4 tube, surface alloyed with

a 2 km thick palladium layer, after vacuum annealing at

950°C for an hour (sample B2) (200

x 1.

4. Results

4.1. M icrostructur e

3.2. Sample characteri zati on

One sample of each group (Al-El) was subjected

to Auger line scan analysis over a cross section of the

tube, to determine the depth distribution of palladium

in the samples. Another sample of each group (A2-E2)

was mounted end-on in resin, abraded down to 4000

grit finish, and given a 15-s etch using a solution of 5%

HF/45% HN0,/50% H,O. The microstructures of

these samples were then studied under a metallograph-

ical microscope.

The microstructures of samples B2, D2 and E2 are

shown in figs. 1 to 3, respectively. In fig. 1, the zirco-

nium/palladium interface is difficult to distinguish.

The outer bands of the zirconium matrix consists of a

fine a-Widmannstltten structure with a high concen-

tration of intermetallic precipitates on the sub-grain

boundaries of the WidmannstHtten structure, while the

inner band consists mostly of equiaxed a-zirconium

grains. There is a clear distinction between the two

types of microstructure. As may be seen in fig. 2, there

is no more evidence of a separate surface layer after 4

h of annealing, and the band of the Widmannstgtten

structure is considerably wider than is the case of fig. 1.

Some grains in the inner band, closer to the boundary

between the two types of microstructure, have also

transformed to the WidmannstPtten structure. In fig. 3

it is shown that the bands of the WidmannstHtten

structure are even wider and that several of the

equiaxed grains throughout the inner band have also

transformed.

3.3. Oxidat ion

4.2. Auger el ectron spectro scopy l i ne prof i l es

The other two samples of each group (A3,A4-

E3,E4) were oxidized at 450°C in a quartz tube furnace

open to the atmosphere. During oxidation the samples

were periodically removed from the furnace and

weighed to determine the oxide weight gain as a func-

tion of time.

Two AES

line profiles, obtained on samples Al and

Dl, are shown in figs. 4 and 5, respectively. In both

cases, both the zirconium and the palladium profiles

are shown for scans from outside of the outer surfaces

of the samples, up to several microns into the bulk of

the sample. From fig. 4 it appears that there is still a


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242 G.A. Eloff et al. / Improv ement in oxidat ion resistance of Zircaloy- 4

50

100

150 200 250 300 350

POSITION pm)

Fig. 5. Auger line scan of a cross section of sample Dl,

showing complete dissolving of the initial palladium surface

layer into the zirconium matrix after 4 h of annealing at 950°C

in vacuum.

h of oxidation at a weight gain of approximately 40

mg/dm*. This is still considerably better than the

performance of the as-received samples, which started

1

I

I DI

OXIDATION TlME (h)

Fig. 6. Average weight gain curves as indicated for groups A

to E respectively. Curve F represents the average weight gain

of two “as-received” samples. Error bars have not been in-

cluded as they are smaller than the symbols in the graph.

I I I I I I I I I

0 10 20 30 40 50 60 70 60 90 100

OXIDATION TIME (h)

Fig. 7. Average weight gain curves showing the initial oxida-

tion kinetics for two sets of palladium-treated samples (curves

A and B) and the as-received samples (curve F).

breaking away at the expected weight gain of 30

mg/dm’ which was reached after about 80 h.

5 Discussion

The solid solubility of palladium in a-zirconium is

virtually zero at room temperature and is still lower

than 0.2 at% at 800°C. In the P-phase, however, the

solid solubility of palladium is almost two orders of

magnitude higher and reaches a maximum of 11.5 at%

at 1030°C [12]. The (a + P)/p-phase boundary temper-

ature is lowered with increasing palladium content to a

minimum of 755°C at 7 at% palladium. It is therefore

expected that the palladium which had gone into solu-

tion in p-zirconium during annealing, would virtually

completely precipitate in the form of one or more of

the known Zr-Pd intermetallic compounds. This is

confirmed by the dense agglomeration of intermetallic

precipitates observed on the sub-grain boundaries of

the a-Widmannstltten structure.

The distinct boundary between the two observed

types of microstructure observed in figs. 1, 2 and 3 is a


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G.A. Eloff et al. / Improv ement in oxidati on resistance of Zi rcaloy-4 243

manifestation of the concentration gradient created by

the inward diffusion of palladium from the sample

surfaces. This boundary represents the distance from

the surface at which the palladium concentration falls

below the value needed to cause complete transforma-

tion to the P-phase.

From fig. 5 one may assume a significant diffusion

distance of approximately 200 pm after 4 h at 950°C.

Using the well-known solution for Fick’s second law

x=J2Dt,

(I)

the diffusion coefficient of palladium in B-zirconium

was calculated to be 1.4 x 10m8 cm’/s, which is an

order of magnitude higher than the self-diffusion coef-

ficient of 95Zr in pure p-zirconium at this temperature

[12], and which is comparable to the self-diffusion

coefficients of many metals close to their melting points.

This is not surprising when considering that there is a

eutectic point at 24.5 at% palladium on the

zirconium-palladium phase diagram, and that the

melting point of zirconium decreases almost linearly

from 1852°C for pure zirconium to 1030°C at the

eutectic point.

The oxidation kinetics of zirconium-based alloys are

usually described by a rate law equation of the general

form

(AW)” = kt,

2)

where n is the exponent which is characteristic of the

rate law being followed. If n = 2 then eq. (1) is the

well-known parabolic rate law of Tammann, Pilling and

Bedworth [13] which indicates that oxide growth takes

place by a simple diffusion mechanism. More often

than not, however, published results fit a cubic (n = 3)

rate law which, though many possible explanations

have been offered, still remains without a solid theoret-

ical base [14]. The experimental data represented in

fig. 6 was fitted to eq. (2) by linearizing the model and

applying a “least squares” fitting technique. For the

purpose of comparison, only the data points up to

weight gains of 30 mg/dm* were used. The results

obtained in this way correspond more closely to a

fourth power rate law (n = 4) as can be seen in table 1.

The improvement in the oxidation resistance

brought about by surface alloying Zircaloy4 with palla-

dium is quite substantial, and can be associated with an

extension of the protective nature of the oxide layer to

a substantially thicker oxide. The results suggest that

the most favourable palladium surface concentration

was obtained after 2 h of annealing, as further anneal-

ing resulted in an earlier transition to linear oxidation

kinetics.

Table 1

Kinetic parameters n, k obtained by fitting data to eq. (1)

Sample Annealing

n

group

time (h)

A 0.5 3.92 10.2

B

1

3.94 9.6

C

2

3.86 9.8

D

4 3.70

9.6

E

8 3.51 9.2

As received

_

2.85 6.8

A possible explanation for the delay of the transi-

tion to linear oxidation may be found by considering

the role of intermetallic precipitates in preventing

cracks in the oxide layer. The ratio between the molar

volumes of ZrO, and o-zirconium (the so-called

Pilling-Bedworth ratio) is 1.56, which means that the

growing oxide is under a compressive stress at the

metal-oxide interface. The formation of new oxide at

this interface creates a radially outward pressure on

the oxide layer, which eventually causes a tensile stress

on the oxide-atmosphere interface. Although the tran-

sition to linear oxide kinetics is still not fully under-

stood, it is generally accepted that this is caused, at

least partially, by cracks or pores in the oxide which

develop as a result of the tensile stress in the outer

part of the oxide layer [15].

Douglass [16] has shown that the presence of impu-

rity atoms increases oxide plasticity, presumably by

enhancing dislocation mobility in the oxide matrix.

This enhancement in dislocation mobility could be due

to a lowering of the Peierls-Nabarro stress, which

depends on the atomic structure and the nature of the

atomic bonding forces. An increase in oxide plasticity

or ductility implies that the oxide should accommodate

higher stress conditions by plastic deformation before

cracking commences.

Bangaru et al. [17] suggested that a homogenous

distribution of fine precipitates, such as is obtained by

P-quenching, gives rise to the formation and mainte-

nance of a fine-grained structure for the oxide layer,

which results in a tougher oxide with improved crack

resistance under tensile stress conditions; a view shared

by Glazkov et al. [18]. In addition, a fine oxide grain

structure implies a high diffusion rate for oxygen ions

through the oxide layer, as oxygen transport occurs

mainly through grain boundary diffusion. According to

Nowok [19], this would benefit the formation of new

oxide at the metal-oxide interface at the expense of

internal oxidation of intermetallic precipitates within


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G.A. Eloff et al. / Improvement in oxidation resistance of Zircaloy 4

the oxide layer, thereby leading to a reduction in the

internal stresses in the oxide layer.

All the above-mentioned arguments support the

idea that the improvement in oxidation resistance

achieved by surface alloying with a thin layer of palla-

dium is brought about by postponing the transition to

linear oxidation kinetics through the formation, during

B-quenching, of a high concentration of intermetallic

particles near the surface, which act to prevent crack-

ing of the oxide layer by increasing oxide plasticity and

reducing stresses in the oxide layer.

6. Conclusion

This investigation has demonstrated that the oxida-

tion rate of Zircaloy-4 in air can be significantly re-

duced by surface alloying with a thin layer of palla-

dium. The microstructural effect of this treatment is

the formation of a region of a-Widmannstatten struc-

ture near the surface with an abundance of intermetal-

lit precipitates on the subgrain boundaries which, when

incorporated in the oxide layer during oxidation, act to

retard the transition to linear oxidation kinetics. This

retardation is ascribed to increased ductility of the

oxide which allows a higher deformation tolerance

before cracking.

Acknowledgements

The authors are indebted to Dr. W.J. de Wet, Dr.

E.T. van der Kouwe, Mr. J.G.M. Bresser and Miss M.

van Reenen for their contributions to this study.

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