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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