Simulating porous oxide films on zirconium alloys

ELSEVIER

Journal of Nuclear Materials 218 (1995) 324-334

Simulating porous oxide films on zirconium alloys B. Cox, Y.-M. Wong Centre for Nuclear Engineering, University of Toronto, Toronto, Ontario, Canada M5S 1A4

Received 8 June 1994; accepted 13 October 1994

Abstract This paper reports an attempt to simulate thick porous thermal oxide films on zirconium alloy specimens by successively applying layers of sol-gel zirconia to an initial thin oxide film. The intent was to aid in the interpretation of impedance spectra obtained from porous oxide films. To the extent that double-peaked Bode plots of phase angle, similar to those obtained from thermal oxides, were obtained in the impedance spectra the attempt was successful. However, the impedance spectra obtained on the simulated films were insensitive to the number of sol-gel layers. The film always appeared to be a double layer film, which was interpreted as the thermal oxide (produced during sintering the sol-gel oxide) under the sol-gel oxide. The impedance spectra were also insensitive to the nature of the porosity, which took the form of multiple high aspect ratio cracks in the simulated oxide films rather than the small pores observed in thermal oxide films. Accurate comparisons of oxide thicknesses by interferometry and impedance measurements with mercury contacts were rendered difficult by evidence for internal reflections (at the solgel/thermal oxide interface) in the former and incomplete contact problems in the latter.

1. Introduction The interpretation of the impedance spectra obtained from thick porous oxide films on zirconium alloys has presented a number of difficulties [1-3]. In particular, the translation of the complicated equivalent circuits, that can be developed to give good matches to the impedance spectra, into an understanding of the actual physical structure of the oxide film, still eludes a completely satisfactory solution. These difficulties arise from two primary factors. Firstly, although there is ample evidence of electrolyte absorption into porous oxides, in the presence of electrolytic double layers on the pore walls, it has not so far been possible to relate the characteristic differences in the departures of the impedance slopes and the phase angles (in a Bode plot) from - 1 . 0 and - 9 0 °, respectively, seen, for example, between oxides formed in water and in concentrated LiOH solution with a volume fraction of porosity in the different oxides. Secondly, when liquid (or evaporated) metal contacts are used, the results appear to be insensitive to the same porosity as that which affects measurements in an electrolyte. Thus, the con-

ductivity of the pore walls does not seem to contribute any characteristic features to the impedance spectra in the absence of an electrolyte filling them. In order to try to understand the physical implications of the impedance spectra, a series of porous oxide films was built up by successively depositing thin sol-gel zirconia films on zirconium alloy specimens, using the preparation method of Pascual et al. [4].

2. Experimental The specimens were paddle-shaped specimens, either of Zircaloy-2 sheet (batch Bh) or of Zr-2.5 wt% Nb (batch Aw) in the (ct+ I~) quenched condition (from ~ 880°C), the analyses for which have been given previously [5,6]. The initial surface preparation was by chemical polishing in mixed nitric/hydrofluoric acids (or nitric/hydrofluoric/sulphuric acids for Z r 2.5% Nb), and the specimens were coated either in the as-pickled condition, after a brief (24 h) thermal oxidation in 300°C air, or after forming a 100 V anodic oxide film on the pickled surface in a 1 molar sulphuric acid

0022-3115/95//$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00653-9

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995) 324-334 electrolyte (thickness ~ 250 nm). The possibility of differences in adhesion of the sol-gel film to these different surfaces was kept in mind, but no apparent differences were seen. The sol-gel coating was applied by dipping the specimen in a solution of zirconium (IV) propoxide in glacial acetic acid and 1-propanol; pulling the specimen up at a constant speed of 5 c m / m i n ; and drying after removal. The zirconia film was then partially sintered by heating at 300°C for ~ 6 h. To get complete crystallisation of these oxides, a temperature of 400-600°C has usually been employed [4]. When dipping the sample c,nly half of the ~ 2 × 2 cm area of the paddle was immersed. For each successive layer of zirconia applied the paddles were alternately dipped vertically or sideways, thus dividing the paddle area into four quadrants, one of which did not get coated with the sol-gel film until the last dip. This allowed for monitoring the growth of the thermal oxide on the substrate that resulted from the 300°C air treatment that the sol-gel film:; received. As a control a piece of platinum sheet was similarly dipped in the sol-gel solution and annealed at 300°C. After each of the deposition treatments the four quadrants of each specimen were examined in the optical microscope at up to 1500 × magnification. The interference colours of each quadrant were estimated by comparison with an anodic strip chart prepared in saturated ammonium borate solution [7]. The interference colours and the flaws that were visible in the films on the various quadrants were photographed in colour. The impedance spectra of all quadrants were measured with both mercury and 1.0 molar ammonium nitrate electrolyte contacts using small holes punched in an adhesive plastic film to localise the measurements. The areas of the surface exposed in these measurements were 7.4 mm 2 and this area was corrected for meniscus effects during the measurements with mercury. The electrolyte resistance measured on these spots was 25 q-: 2 1) during the various measurements, showing that the conditions for these small spot

Complete Surlece #S

~2

#5

#2

II #2 ffl

_

4 ~i

i~ ii

spec:rLmen Aw1068 Pickled

i~

s]pecJaten Bh1048 24 ht/300Oc a i r

sPecimelrt tW150 lOOv A n e d ~ ¢

Fig. 1. Orientation c,f successive sol-gel dipping events (#1, #2, #3, #4, #5) and identification of the four quadrants of each specimen on the front (numbered) face of each specimen (circled numbers 1-4).

325

measurements were reproducible. Interference fringes were then obtained from each quadrant using a Perkin-Elmer Model 30 U V / V I S . Spectrometer or, once the oxide became thick enough, with an Analect FTIR spectrometer. These techniques make measurements, respectively, on areas of ~ 1 cm and 0.1 mm diameter. The successive directions of dipping and the numbering of the quadrants are shown in Fig. 1, only at the last dip was the final quadrant of each specimen covered with a sol-gel film. After this, a further heat treatment of ~ 6 h at 300°C, without any further dipping, was given to each specimen. Finally, platinum spots (7.4 mm 2 area) were evaporated onto each quadrant in order to get the most accurate possible measure of the total oxide thickness from impedance measurements.

3. Results The sol-gel films applied to the platinum specimen were much more heavily cracked than those on the zirconium alloy specimens, and were very poorly adherent, flaking off extensively when the specimen was flexed. No useful thickness values for the sol-gel films alone could be obtained. Thus, the ambiguity about the thickness of the thermal oxide film under the sol-gel film on the zirconium alloy specimens remained, and interfered with the interpretation of the results. The visual appearance of the various quadrants of each specimen after each heat treatment are noted in Tables 1-3 together with estimated values of the total oxide thickness obtained from comparisons of the interference colours with an anodic calibration chart [7] (as long as clear interference colours were present), from interference fringes obtained using a U V / V I S spectrometer, or (when multiple layers and oxidations produced thicker oxides) from F F I R interferometry. It can be seen from the results that in many instances neither good interference colours nor interference fringes were obtained. The former tend to be faint pinks and greens that cannot easily be related to the order of interference while, for the latter, the local variations in thickness on a scale much smaller than the spot illuminated by the spectrometer wash out the interference fringes. This effect is much more evident with the U V / V I S spectrometer that irradiates an area close to 1 cm in diameter than for the FTIR where, by using the microscope attachment, a spot of ~ 40 i~m can be measured. One concern was that the oxide thicknesses measured by interferometry might only be part of the total thickness as a result of internal reflections either at the boundary between the sol-gel and the thermal oxide film, or between layers in the sol-gel film. Support for this view arose from the observation that in the menis-

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995)324-334

326

Table 1 Visual appearance of each quadrant on specimen Aw 1068 (ct + 13 quenched Zr-2.5% Nb) after each dip Sequence

Heating conditions

Quadrant 1

2

3

4

0

Initial

Pickled, shiny

Dip # 1 (Q3,4)

+ 5.5 h, 300°C air

Metal grains clearly visible 1st order yellow-blue 25-75 nm U V / V I S thickness 30-95 nm

Grain structure, differently coloured 2nd order yellow-pink 130-170 nm U V / V I S thickness 150-200 nm

Dip # 2 (Q2/3)

Total 10.5 h, 300°C air + 0.25 h, 400°C air

Structure visible Grey oxide - little colour U V / V I S 0.4 ~m FTIR 0.83 ixm oxid. curve ~ 0.5 ~.m

3rd order green-pink No grain/grain diff. No U V / V I S fringes -

Grey blue/green No grain/grain diff. FTIR 0.9 p.m

U V / V I S 0.43 p.m

U V / V I S 0.38 p.m

Grey-blue No grain diff. FTIR 0.86 I~m

Dip # 3 / 4 (Q3,4)

Total 21 h, 300°C air + 0.25 h, 400°C air

Grey oxide ~ 1.5 ~m from oxid. curve

Dip #5 (Q1,4)

Total 26.5 h, 300°C air +0.25 h, 4000C air

Grey oxide > 2 ~m from oxid. curve

No U V / V I S fringes

No U V / V I S fringes

No U V / V I S fringes

No dip # 6

Total 31.75 h, 300°C air +0.25 h, 400°C air

Grey oxide FTIR 2.7 Ixm oxid. curve > 2 p.m

Grey oxide FTIR 3.0 p.m

U V / V I S 0.44 g m FTIR 3.0 g m

U V / V I S 0.68 wm FTIR no fringe

oxidation after a b o u t 6 h [6], so t h a t after t h e second " d i p a n d s i n t e r " cycle the t h e r m a l oxide is quite thick ( ~ 0.5 I~m) a n d grey. W i t h i n t h e s e cracks in the s o l - g e l

cus region, after e a c h dip a n d sinter, t h e oxides were heavily cracked (Fig. 2). Oxidation of (et + [3) q u e n c h e d Z r - 2 . 5 % N b in air at 300°C shows a severe b r e a k a w a y

Table 2 Visual appearance of each quadrant on specimen Bh 1048 (Zircaloy-2) after each dip Sequence

Heating conditions

Quadrant

0

Initial

24 hr 300"C air, 2nd order pink-blue, grain structure visible, ~ 200 nm

Dip #1 (Q3,4)

+5.5 h, 300°C air

More blue, less pink grains, colours in grains mottled by stresses U V / V I S thickness 0.27-0.29 ixm Colour chart 0.2 o.m

3rd order green-pink

Total 10.5 h, 300°C air + 0.25 h, 400°C air

Mainly 3rd pink Little yellow 0.30 ~m U V / V I S 0.39 0.m

Green-pink

Green-pink

Green-yellow-pink

U V / V I S 0.46 ~m

U V / V I S 0.59 ~m

U V / V I S 0.45 ~m

Dip # 3 / 4 (Q3,4)

Total 21 h, 3000C air + 0.25 h, 4000C air

Pink-green 0.35 ~m U V / V I S 0.42 o.m

Pink-green U V / V I S 0.54 I~m

Pink-green UV/VIS 0.81-0.89 I~m

Pink-green UV/VIS 0.75-0.79 ~m

Dip #5 (Q1,4)

Total 26.5 h, 300°C air + 0.25 h, 400°C air

Faded pink 0.44 ~ m U V / V I S 0.48-0.51 ~m

Faded pink UV/VIS 0.86-0.93 Ixm

Grey U V / V I S 1.0-1.1 ~.m

Faded pink UV/VIS 0.74-0.82 )~m

No dip #6

Total 31.75 h, 3000C air + 0.25 h, 400°C air

Faded pink U V / V I S 0.52-0.54 o.m FTIR 0.49 I~m

UV/VIS 0.55-0.70 p.m FFIR 0.53 ~.m

Grey U V / V I S 1.02 ~ m FTIR 1.02 I~m

1

Dip # 2 (Q2,3)

2

3

4

U V / V I S 0.38-0.43 p.m Colour chart 0.33-0.38 i~m

UV/VIS 0.79-0.83 p.m FTIR 0.74 I~m

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995) 324-334

327

Table 3 Visual appearance of each quadrant on specimen #150 (Zircaloy-2) after each dip Sequence

Heating conditions

Quadrant 1

2

3

4

Initial

100 V anodic oxide in 1 M H2SO4, 2nd order green, ~ 240 nm

Dip #1 (Q3,4)

+ 5.5 h, 2~00°Cair

Shades of green-yellow Grain structure visible Colour chart 0.25 ixm

Dip #2 (Q2,3)

Total 10.5 h, 300°C aii 0.25 h, 400°C air

Yellow > pink 0.28 ~m UV/VIS 0.31 0.m

Pink > yellow 0.30 p~m UV/VIS 0.35 ~m

Green-pink UV/VIS int. refl.

Green-yellow-pink UV/VIS 0.36 p.m

Dip # 3 / 4 (Q3,4)

Total 21 h, 300°C air + 0.25 h, 400°C air

Green-pink UV/VIS poor fringes

Green-pink UV/VIS poor fringes

Green-pink U V / V I S 0.45 0.m

Green-pink UV/VIS 0.46 I~m

Dip #5 (all Q)

Total 26.75 h, 300°C air + 0.25 h, 400°C air

Green-pink U V / V I S 0.34 p~m

UV/VIS 0.34 ~m

UV/VIS 0.45 ~m

U V / V I S 0.44 txm

No Dip #6

Total 31.75 h, 300°C air + 0.25 h, 400°C air

Faded pink UV/VIS 0.37 ~m FFIR 0.35 o.m

UV/VIS 0.46 ~m FTIR 0.38 ~m

UV/VIS 0.42 I~m FFIR 0.84 ixm

UV/VIS poor fringes FTIR 0.70 p.m

film a grey oxide, apparently identical with that on the uncoated quadrant, could be seen while the coated region adjacent to such cracks still showed an interference colour film much thinner than the thermal oxide.

Pink-green colour Differently coloured grains UV/VIS 0.37 ~m; colour chart 0.35 ~m

One interpretation of this film inhibited the growth other explanation would colours came only from the

could be that the sol-gel of the thermal oxide, the be that the interference s o l - g e l oxide because of an

Fig. 2. Heavy cracking in the meniscus area of a sol-gel zirconia film after sintering for 5-6 h at 300°C in air, a + 13-quenched Zr-2.5 wt% Nb specimen Aw1068.

328

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995) 324-334 5.0.

0.5

•

r.

~

3.0.

0.0

1.0

I 200

I

I

I

I 500

~

'

'

'

'~ ta ~a

-0.5 500

800

WAVm..]ViGTH (~)

800

[~]

Fig. 3. Refractive index and phase shift on reflection measured for combined sol-gel and thermal oxide film on specimen Bh1048 (quadrant 4). Sol-gel oxide >__0.3 ixm, thermal oxide < 0.5 Ixm thick.

Fig. 4. "Ghost cracks" in the sol-gel layer after a second coating was applied over the first (cracked) layer in the meniscus region of Zircaloy-2 specimen Bh1048. internal reflection at the sol-gel/thermal oxide interface. The latter conclusion appeared to be borne out by the large discrepancies between oxide thicknesses measured by U V / V I S and FTIR interferometry when both were possible on the same area (Table 1). The origins of the various interference colours and fringes was less easily resolved for the Zircaloy-2 specimen with the initial thermal oxide present, since this oxide remained in the interference colour region from thermal oxidation alone, as Zircaloy-2 is not susceptible to the rapid breakaway oxidation shown by the Zr-2.5% Nb alloy specimens in 300°C air [6]. For the Zircaloy-2 specimen the agreement between U V / V I S and FTIR measurements was very good (Table 2). There was little evidence of any internal reflections and all optical methods of assessing oxide thickness gave reasonable agreement. The Zirealoy-2 specimen that started with the 100 V anodic oxide gave mixed results (Table 3). On some quadrants the agreement between U V / V I S and FTIR interferometry was good,

whereas on others it was poor. That these differences might be the result of internal reflections resulting from poorer adhesion between some of the oxide lay-

Fig. 5. "Newton's rings" (top left corner) resulting from imperfect adhesion between oxide layers on Zircaloy-2 specimen #150.

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995)324-334

329

Fig. 6. Arrays of small cracks in the centre of sol-gel coated quadrant on specimen Bh1048 after application of one coating.

8

9o

I

6 ¸

45 4 ¸ N

~1048 q~lad.

3 I

4.

821 . . . . . . . .

i

~

#150 ~.umd. 3

i

,5 - -

i

,/,. J

.

~

-45

'

'

~o

4

.45

4

a

i

'

~,'

~

'

,~

~ ' 2 z,ocj Z~'R,B00"~qeX (Hz)

, 4

•

, 5

Fig. 7. Impedance spectra of the same quadrants after successive sol-gel coatings (using mercury contacts). Numbers refer to sequence in Tables 1-3.

330

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995) 324-334 90

45

-

--0

21

. . . . . . .

--....~-,2

,

~1048

~d.3

2 I

45

o].........

90

~r~

~150

~2.4,6

Quad. 3

6 45 4¸

2

2

3

4~5

1 log ~u~qcY

'

'

'

,i.

'

60

(Hz)

Fig. 8. Impedance spectra of the same quadrants as those in Fig. 7 after saturating with ammonium nitrate electrolyte. Numbers refer to sequence in Tables 1-3.

ers on some specimens was supported by the observation that specimens Awl068 and #150 showed the highest incidence of small areas of "Newton's rings", whereas no "Newton's rings" were seen on specimen Bh1048. Specimen Bh1048 was also the only specimen that gave a consistent set of well behaved fringes at all angles of incidence in the U V / V I S spectrometer from 45-85 °, and thus allowed the refractive index to be measured (Fig. 3). The value of 2.0 obtained for the refractive index, even though only ~ 0.3 I~m out of a total of ~ 0.9 Ixm of oxide was the sol-gel film, suggests that sol-gel films have refractive indices identical with those of thermal oxide films (i.e., ~ 2.0 + 0.05), even when sintered at a low temperature (300°C) where crystallisation of the oxide will be limited. Further support for the argument that better adhesion was achieved between the sol-gel film and the initial thermal oxide on specimen Bh1048, than with either of the other two initial oxides, may be indicated by the prevalence of "ghost cracks" in this specimen. The prevalence of cracks in the menisci of the sol-gel layers has already been noted. When a second layer is put on top of the first layer, these cracks become

covered by another sol-gel layer. The cracks in the initial sol-gel layer are usually still visible through the second sol-gel layer as differences in local interference colour, along with the second array of cracks produced in the second layer. These cracks in the first layer were prominent sites for "Newton's rings" on specimen AWl068. In specimen Bh1048, however, not only were no "Newton's rings" seen, but cracks in the first layer apparently filled with well bonded oxide during the application of the second sol-gel layer, so that they were only visible as "ghosts" (black lines indicating their edges, but with no disturbance of the interference colour across the crack). Unfortunately, these "ghost cracks" are very difficult to reveal in black and white (Fig. 4) although they are very evident in colour. "Newton's rings" are equally difficult to reproduce, but an attempt is made in Fig. 5. Within the central region of each quadrant, away from the menisci, there were few cracks in the sol-gel coatings and individual cracks were small. An example of such cracks is shown in Fig. 6. In general, the frequency of such cracks appeared to increase as successive sol-gel layers were deposited on the same area.

331

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995) 324-334

In order to investigate the degree of interconnectedness between these cracks, measurements of the impedance spectra of each quadrant of each specimen were made after each sol-gel coating using both mercury and aqueous ammonium nitrate contacts [8]. In addition, the impedance as a function of time was followed at 103 Hz while the ammonium nitrate soaked into the oxide layers. Results for the impedance spectra of the same area after successive coatings are shown in Fig. 7 for the mercury contacts, and Fig. 8 for the ammonium nitrate saturated oxides. Fig. 9 shows the change in impedance with time of soaking for quadrants having the least (quadrant 1) and the most (quadrant 3) sol-gel layers on them.

Zr-2.5% Nb in air at 30&C. The estimated thicknesses, based on a dielectric constant e = 22, are listed in Table 4, and plotted on an oxidation curve measured for similarly heat-treated specimens (Fig. 10). The trends are generally the same for the other two specimens, but are smaller and less regular as a result of the slower change in thermal oxide thickness with time, and the larger variation in thickness from spot to spot (at least for Bh1048). The impedance spectra measured with mercury were generally well behaved with high phase-angles, and log I Z I versus log f slopes near - 1. Only for a few spots on the initial surfaces was shortcircuiting observed. The final thickness measurements made with platinum contacts (Table 4) were only about 60% of those obtained with mercury. This seems to be an effect of surface roughness on the effectiveness of the contacts made by the mercury [7], and earlier mercury measurements in this series should probably be divided by the ratio of the final mercury and platinum results for each quadrant. The impedance spectra of the oxides saturated with ammonium nitrate (Fig. 8) developed double peaks in the phase angle plots indicative of double layer films

4. Discussion

The impedance increased steadily a n d / o r hearings at in agreement with

thicknesses measured with mercury with successive sol-gel coatings 300°C for specimen Awl068. This is the rapid oxidation of 13-quenched

40-

-4Q

4

,2 ~5

Bh1048 ~tad. 1

Bh1048 -'

20

15

Quad. 3

i

m

,° ~

MIO ~.

~6

2

~ i

|

i

i

e i

o I

i

o ,

,

'

#150 ~d.1

[~o

'

#150

Quad.

3

4

0

18 XMII~ION T ~

18 (mlne.)

Fig. 9. Change in impedance with time of immersion in ammonium nitrate for quadrants with minimum (1) and maximum (4) number of sol-gel coatings.

332

B. Cox, Y.-M. Wong/Journal of Nuclear Materials 218 (1995)324-334

for specimens Aw1068 and Bh1048, but not for #150. Since visually there were many cracks in the sol-gel films on this specimen and it showed the most evidence for poor bonding as indicated by the frequency of "Newton's rings" this major difference in the impedance spectra cannot be explained on the basis of the cracking in the sol-gel layers. The oxide thicknesses calculated from the impedance spectra in the aqueous electrolyte appear to be more typical of barrier layer thicknesses in the underlying thermal oxide, and may be insensitive to the sol-gel layers. In response to a suggestion that peaks corresponding to the sol-gel layers might be evident at higher frequencies some repeat measurements in electrolyte were made at frequencies up to 107 Hz. No evidence for an additional high frequency peak was found over this range, and only a small inductive loop resulting from the inductance of the leads was seen. Thus, it may not be possible to identify the impedance of the sol-gel layers from measurements in electrolytes, and only when using metallic contacts are they measurable.

The double peaked impedance spectra only appear clearly after about three sol-gel layers have been applied, and remain double peaked when further sol-gel layers are applied. Thus, since such oxides continue to behave as double-layer oxides, irrespective of the number of sol-gel layers, it is tempting to interpret these results as representing the impedance of the underlying thermal oxides and not the sol-gel films. A two layer thermal oxide film would be a plausible result for the Zr-2.5% Nb specimen, where a breakaway oxidation process occurs in moist air at 300°C. However, the double peaked Bode plots are even more prominent for the Zircaloy-2 specimen where such an effect is not expected. For specimen #150 there was only a significant decline in the phase angles at the highest frequencies employed (Fig. 8). However, the apparent barrier layer thickness, measured with the aqueous electrolyte, is in this instance much thinner than the apparent initial thickness of the anodic oxide film. Perhaps the thermal treatments have severely degraded this initial oxide.

Table 4 Oxide thicknesses estimated from impedance measurements (e = 22) at 2 × 105 Hz with either mercury or ammonium nitrate contacts Sequence Aw1068 Bh1048 #150

Q1

Q2

Q3

Q4

Q1

Q2

Q3

04

Q1

Q2

Q3

Q4

0.35

0.35

0.26

0.26

0.57

0.57

0.57

0.57

0.11

0.23

0.39 [ 0.54

0.54

0.48

0.81

0.74

Apparent oxide thickness in mercury (Ixm)

0

-

Short•0.11

1b

1

Short

2

1.12 I - 0.92 -

0.80.32

0.47

0.28

~

3/4

1.53 ]

1.88

0.83

0.52

~

0.44 0.37 Short

0.65 0.45 0.51

0.12

5 6 Pt

a

1.98 4.13 2.44

1.40

2.17 3.50 2.0

[ 2.30 I 1.38

1.84 4.52 2.47

4

I

6.40

~ 0.43

~

0.40

--

_

0.65 0.36 0.90

0.53 0.26 0.89

1.18 0.67 0.15

1.49 1.25 0.27

0.20 0.97 0.32

0.16 0.41 0.39

0.22

0.22

0.02

0.02

I 0.21

0.22

0.11

0.11

Apparent oxide thickness in NH4NO 3 (v,m)

0 1

0.01

0.04 I

2

0.39

0.36

3/4

0.41

5 6

0.21 0.30

0.04 ~

0.30 I 0 ~ . 2 5 0.18 0.20 l[ 0.17

0.04 0.36 0.30 0.18 0.20

a Final measurement with evaporated Pt contact. b Number of sol-gel coats applied.

0.10

0.10

~

0.13

0.13

~

0.18 0.11

0.15 0.15

0.11 0.10

0.14

-

0.14 0.10

0.06 0.04

~0.11 7-~

0.01 0.02

0.01 0.003

0.01 0.002 0.008

B. Cox, Y.-M. Wong /Journal of Nuclear Materials 218 (1995)324-334

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7.5

Fig. 10. Comparison of data obtained in this work with previous data. (a) Results obtained for Zr-2.5% Nb 13-q, batch Aw (Awl068, quadrant 1) by impedance and interferometry compared with earlier data [6] for weight gains and impedance after immersion (103 Hz). (b) Results obtained for Zircaloy-2 (Bh1048, quadrant 1) by impedance and interferometry compared with previous weight gain curles and interference colour thicknesses.

The thicknesses of the barrier oxide layers indicated by the electrolyte soaking measurements increase steadily for all specimens to begin with. The barrier thickness then begins to decrease for the Zr-2.5% Nb alloy specimen in line with the breakaway in the oxidation kinetics in air (Fig. 10). For the Zircaloy-2 specimens, the barrier thickness increases up to an apparent plateau for the thermal oxide films (Bh1048), but only rises initially and then falls to a lower plateau for the initially anodised surface (#150). These trends follow the same pattern observed from the impedance spectra (above) and again suggest that the initial anodic oxide may be degrading duiing the subsequent heating periods in 300°C air. These changes in apparent barrier oxide thickness as a function of heating time are shown for the quadrants receiving the least (#1) and the most (#3) sol-gel coatings in Fig. 9. The initial oxide thicknesses indicated by these measurements are typical of those expected for th,~ initial condition of the surface [6,8] for the anodic oxide film. The thicknesses measured are somewhat less than expected for the thermal film on Zircaloy-2 (Bh1048) when measured in ammonium nitrate, but are close to or a bit higher than the

expected value when measured in mercury. They become progressively less than expected and show more indications of porosity with increasing numbers of thermal treatments. For the Zr-2.5% Nb alloy specimen, the initial changes in barrier layer impedance suggest that the thermal oxide film started forming very similarly to those observed before [6]. The impedance measurements in mercury at high frequencies may show differences from the expected thicknesses because of imperfect contact with the whole spot area. The erratic nature of these differences, sometimes compounded by the apparently partial short-circuiting of the oxides on Zircaloy-2[7], makes it difficult to correlate the impedance measurements with Hg or Pt and the interference results. The large differences from the high frequency data in ammonium nitrate strongly suggest the presence of porosity [7]. In general, however, the high frequency impedance data in mercury gave good correlations with the F T I R data, while the barrier layer thicknesses obtained from the ammonium nitrate results are lower than these numbers by about the oxide thickness indicated by the U V / V I S interferometry. This strongly supports the

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view that internal reflections at the sol-gel/thermal oxide interface are the cause of the discrepancies between the U V / V I S and FTIR interferometry results. It is not unreasonable to expect that the long wavelength infra-red would be less likely to suffer total internal reflection at this interface than would the shorter wavelength U V / V I S light.

5. Conclusions (i)

(ii)

(iii)

(iv) (v)

The simulation of thick porous oxide films by coating thermal or anodic oxides with successive sol-gel zirconia layers has shown that the impedance spectra observed for thick porous thermal oxide films indicating a two-layer structure can be simulated in this way. The impedance spectra tend to show no more than two layers, indicating that the sol-gel films are porous and that the results in aqueous electrolyte were insensitive to the presence of the sol,gel layers, even when measured at higher frequencies. The nature of the characteristic phase angle peaks and the apparent oxide thicknesses calculated from them suggests that they represent the characteristics of the thermal barrier films under the sol-gel layers. The oxides behaved as two layer thermal oxide films covered with a uniformly porous sol-gel film. Cracks that are visible in the sol-gel layers are large compared with those that are observed in thermal oxides (in fact recent studies of thick

thermal oxides [3] have shown little or no evidence for cracks, only small roughly cylindrical pores). (vi) It does not appear that the impedance spectra are very sensitive to either the size, shape or numbers of the cracks or pores present, but only to the depth that these penetrate into the oxide. Thus, we will have to rely on other techniques, such as transmission electron microscopy to advance our knowledge of the precise morphology of these features.

Acknowledgements The authors are indebted to the CANDU Owner's Group and the Natural Sciences and Engineering Research Council of Canada for the funding that has permitted the conduction of this research.

References [1] [2] [3] [4] [5] [6] [7] [8]

O. Gebhardt, Electrochim. Acta 38 (1993) 633. O. Gebhardt, J. Nucl. Mater. 203 (1993) 17. B. Cox and Y. Yamaguchi, J. Nucl. Mater. 210 (1994) 303. R. Pascual, M. Sayer, G. Yi and C. Baker, J. Can. Ceram. Soc. 60 (1991) 43. B. Cox, J. Electrochem. Soc. 117 (1970) 654. B. Cox, Long-Term Oxidation of Zr-2.5 wt% Nb Alloy, Canadian Report, AECL-5610 (1976). B. Cox, F. Gascoin and Y.-M. Wong, J. Nucl. Mater., to be published. B. Cox, The Use of Electrical Methods for Studying the Growth and Breakdown of Oxide Films on Zirconium Alloys, Canadian Report, AECL-2668 (1967).