Temperature rise during anodic oxidation of zirconium

TEMPERATURE

RISE DURING ANODIC OXIDATION ZIRCONIUM

OF

J. S. L. LEACHand C. N. PANAGOPOULOS Department of Metallurgy and Materials Science, The University of Nottingham, Nottingham NG7 2RD, U.K. (Received 10 December 1984; in revisedform

7 May

1985)

temperature rise in the anodic oxide during its formation on zirconium has been measured using the anodizing specimen as a resistance thermometer. The temperature rise in the oxide formed on strip samples was reduced by electrolyte stirring. The temperature of an anodized wire was found to be less than that of the anodized strip under otherwise similar conditions. The temperature rise is shown to depend on the balance between the rate of heat generation in the oxide and the rate at which it is conducted to the surrounding electrolyte. This latter factor changes with geometry and with stirring of the solution. Abstract-The

INTRODUCTION The temperature rise during anodization of valve metals may influence several different phenomena occurring during the anodization process, including breakdown, luminescence and crystallization of the growing anodic oxide. The temperature of the oxide may also influence the growth kinetics. Vermilyea[ l] estimated the temperature rise during tantalum anodization from the relation between the rate of oxide growth and the applied electric field. Young[2] calculated the temperature rise during tantalum and niobium anodization from a thermal conduction model. Leach et al.[3] measured temperature rises of the order of 50°C for an aluminium wire during the formation of a barrier anodic oxide film using a sensitive electrical technique. Using a similar technique, Zahavi and Yahalom[rl] reported the temperature increase during tantalum anodization and found reasonable agreement between measured and calculated values which were the order of 70°C at SOrnAcm-* current density. Recently, Jouve and Leach[5] measured the temperature rise during titanium anodization in acid media, finding values up to 80°C. The temperature rise during zirconium anodization has not previously been reported. The purpose of the present investigation was to study the temperature of the anodic zirconium oxide during its formation under galvanostatic conditions.

EXPERIMENTAL The experimental technique described previously[3] was adopted in the present study. This technique uses a conventional anodizing circuit with a low thermal mass anode of a valve metal and a platinum foil cathode. The electrical resistance of the anode is measured using a Wheatstone bridge with an ac signal. The out of balance voltage of the bridge was detected and recorded, as a function of anodizing time, on a sensitive storage oscilloscope.

Before anodizing the zirconium samples, the Wheatstone bridge was balanced. Any imbalance caused by subsequent change in the resistance of the zirconium anode appeared as a voltage on the oscilloscope. The increase in the oscilloscope signal was calibrated either before or after the anodization by immersing the anode in a water bath of known temperature, relating the imbalance voltage of the bridge to the temperature of the metal. Assuming that during anodizing the temperature of the metal was equal to that of the oxide, the temperature of the oxide was deduced from the oscilloscope signal. The maximum error in measurement of the zirconium anode temperature was assessed as f 2°C. Two types of material were used in this investigation. Zirconium sheet of 99.8% purity with major impurities of iron and oxygen was used in the form of a strip 50 mm long, 0.125 mm thick and 1 mm wide. Wire of 99.9 % pure zirconium, 0.127 mm diameter and 100 mm long, was also used. Galvanostatic anodization was carried out in a 0.01 M NaJP04 solution, pH 11.8, at various applied current densities. In some tests with the strips, the solution was stirred, but stirring was not used in the tests with the thin wire specimens. The oxide growth kinetics during zirconium anodization in the phosphate solution were examined in additional experiments. Specimens of flag shape, 300 mm2 surface area, were produced from the same zirconium sheet from which the strip samples were made. These specimens were electropolished at 14 V in a mixture of 20 y0 vol. HClO, and 80 % vol. methanol cooled to -40°C. After electropolishing, the specimens were rinsed in methanol and dried. The impedances of the anodized samples were measured in an ammonium hydrogen tetraborate solution, pH 9, at f= 400 Hz and T = 17°C with a platinized platinum counter electrode. The thickness of those oxides, and the electric field for zirconium anodization, were calculated assuming the relative dielectric constant of the zirconium oxide = 21. Because of effects of hyd; ration on the capacitance of the oxide film measured at 400 Hz an additional correction corresponding to

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J. S. L. LEACH AND C. N.

1622

2: 80A[6, 71 was added to the thickness calculated from d = KSJ4nC

where d K S C

= oxide thickness = relative dielectric constant = oxide surface area = oxide capacitance.

RESULTS AND DISCUSSION Temperature rise

ofanodizingzirconium

The out of balance voltage of the Wheatstone bridge

was found to be proportional to the temperature change (Fig. 1). This is in accordance with the linear temperature coefficient of zirconium resistivity and the expected linear dependence of the out of balance voltage on the change in the value of one arm of the bridge. In the range 17-8o”C, the sensitivity was found to be 0.25 mV”C _ ’ for the conditions employed. Figure 2 shows a typical temperature profile as a function ofthe anodizing timeduring azirconium strip

-

20 .

15

;

10

t ,5 -

0

Fig. 1. Calibration of the imbalance voltage of the bridge as a function of temperature for a zirconium strip.

PANACOPOULOS

in an alkaline phosphate solution without stirring. The temperature rise during this anodization process closely follows the rise in the formation voltage which is almost, but not quite, linear with time. Generally, the resistive heat, Q,, generated by an anodization current is given by anodization

where V, = formation voltage I = applied current which is constant for galvanostatic conditions. Assuming a linear voltage rise, then v, = Et and we obtain IBC2

IV,t

tdt=2=2.

Taking as an example the temperature profile of Fig. 2 and using the following values for the experimental parameters V,=lOOV I = 0.081 A (anodized surface area S = 1.125 cm’) t=7s

then the heat generated is 28.35 J. The heat capacity, C,, of the strip is Y 0.011 J “C’. If all this heat generated in the oxide were kept in the strip, then the temperature would increase by about 25OO’C. However most of it is dissipated to the solution so the temperature of the strip and the oxide rises only 49°C. On switching off the current, the temperature of the strip falls to that of the solution in about l-2 s. The good thermal contact and low heat capacity of the metal ensure that the temperature of the strip and the oxide are the same to within a few tenths of a degree. Figure 3 shows the maximum temperature rise of anodic oxide formed to 100 V on thezirconium strip in an alkaline phosphate solution and the effect of stirring for a range of applied current densities. For a given formation voltage, the temperature of the oxide increases with increasing applied current density. This agrees with observations made previously[l-51 that

20

40

60

80

100

I,,

Fig. 2. Anodic ZrOz temperature as a function of time. Anodizing conditions: 0.01 M N+PO., solution without stirring. Zc,, = 72 mA cm-‘, electrolyte temperature x 17°C

and V, =

CtlOU

V.

(mA.rm-2)

Fig. 3. Anadic ZrOz temperature

as a function of applied current density for a zirconium strip anodized in 0.01 M Na,PO., solution, (a) without and (b) with stirring. V, x

100

V and electrolyte temperature o 17°C.

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Temperature rise during anodic oxidation of zirconium the temperature rise during anodization of tantalum, niobium, aluminium and titanium at 10 mA cm- * current density is only a few degrees. Comparing curves (a) and (b) shows that the temperature rise is much less if the solution is stirred, which increases the heat dissipation. Direct comparison between the results can be made by using the parameter AT where AT = temperature rise I,, = applied current density V, = formation voltage. For zirconium strip anodized in the phosphate solution this paramater has a value of 6.8”C A-’ cmm2 VW1for a static solution and 3°C A-’ cm-* V-’ when the solution is stirred (Fig. 3). The heat transfer coefficient may be easily calculated, as the heat absorbed by the metal and oxide is very small and may be neglected. Thus

T,, = initial oGde temperature, ie when the anodization was stopped I”i,= temperature in the bulk of the electrolyte, ie 17°C. Selecting a similar initial temperature T,, = 45°C for both conditions of zirconium anodization, ie with and without stirred electrolyte, and introducing values for the various parameters, we find dT=28”Cexp-1% for the case of static electrolyte and 6T = 28°C exp - 33% for the case of stirred electrolyte. For various values of cooling time, the calculated solutions of the above two equations are given in Fig. 4. Figure 5 shows an oscilloscope trace obtained during a zirconium strip anodization. The transients

V,l a HSAT VI orH=i=_ SAT

Vf I,, AT

where H = transfer coefficient and the other symbols have been defined previously. The heat transfer coefficient is seen to be the reciprocal of the experimentally obtained parameter AT I,,’ The heat transfer coefficient has the following values: H Nst= 0.15 J cme2 s-’ "C-l for a static solution and 2 s- 10 C- 1

H,=0.33Jcm-

for a stirred solution. The important role played by stirring the electrolyte can also be seen when we calculate the oxide temperature during the cooling process, ie when the zirconium anodization was stopped. Assuming that the following equation describes the cooling process -HSAT=C

Fig. 4. Calculated temperature difference between oxide and electrolyte us cooling time after the end of zirconium strip anodization with (--) and without (-) stirring in 0.01 M Na3P0, solution. The temperature difference observed experimentally c .)from the zirconium anodization of Fig. 5.

da’ ’ dt

_*T-C,E -HS

dt

-l:dt=gIsg T-T, -_t =C,lnHS T,,-T, T=T,+(T,,-T,)expC or

6T = (Ti,-T,)exp--

- HSt P

- HSt

CP where 6T = temperature difference between oxide and electrolyte during the cooling process

0

2

4

6

8

10

Tlme(ser)

Fig. 5. Oscilloscope trace of imbalance voltage for a zirconium strip anodization in the alkaline phosphate solution without stirring, I, = 58 mA cm-‘, I’, = 040 and electrolyte temperature 2 17°C.

J. S. L.LEACHAND C.N. PANAGOFQULOS

1624

observed at the start and the end of the process are due to the switching of the constant current source. During cooling time, the out of balance signal decreases approximately exponentially with time to a constant residual signal. This residual signal is larger than that before anodization, due to the increased resistance of the specimen since the cross-section of metallic anode was decreased as a result of the anodization process. From the calibration (Fig. 1) we obtain the various oxide temperatures during this cooling time which are given in Fig. 4. The experimental cooling of the oxide is seen to be much slower than that predicted. This difference may be attributed to the value used for the heat capacity. In our previous calculations, we adopted the value of the heat capacity corresponding to the zirconium anode which is six to seven times lower than that of the surrounding liquid medium. The liquid layer around the anodized specimen plays an important role for the cooling of the oxide as this layer stores up considerable heat which is slow to dissipate. The temperature reached by a zirconium wire anodized in the alkaline phosphate solution, without stirring at different applied current densities, is shown in Fig. 6. In this case, the value of the parameter is AT x 29°C A-’ cm-l Vf I,, and for the heat transfer coefficient H;$,x

ization in an alkaline phosphate solution at a constant applied current density. The value of current density, 7 mA cm-‘, is chosen so that the temperature rise due to the anodization may be ignored. As expected from the equation of high field ionic conduction

r,

W- qoE

Temperature rise and oxide growth Figure 7 shows the measured electric field as a function of bath temperature during zirconium anod-

100

80

120

I,, Cm.4 cm.'1

Fig. 6. Anodic 550, temperature as a function of applied current density for a thin zirconium wire anodized in 0.01 M NQPO, solution without stirring. V, = 180 V and eleer 17°C.

= exp W-

is much lower and the value of the heat transfer coefficient is correspondingly larger than those found for the zirconium strip anodization. The efficiency of radial heat conduction from the wire is greater than that for planar conduction away from a strip.

trolyte temperature

I'CI

Fig. 7. Electric field (E) US temperature during zirconium anodization in 0.01 M Na,PO, solution, pH = 11.8 at I,, = 7mAcum2 and V, = 5OV.

Ww

Icr, Vf

60

60 Temp.

0.34 Jcn~‘-~ s-’ “C-‘.

40

40

V-l

During the zirconium wire anodization, the value of the parameter AT

20

20

0

E=

&?& (

43 > q*

assuming constant values for the various parameters (Cr: a, q, I,, v, q) then the electric field decreases by about 3 % for each 10°C increase in temperature in the region of 300 K. A similar decrease (2-4%) has also been observed during zirconium anodization in acid[S] and alkaline sulphate solutions[9] and citrate and KOH solutions[lO]. Consequently, the increase of temperature of a growing oxide due to the Joule heat effect would be expected to cause a decrease in the electric field of a zirconium anodization process as the oxide thickens. Figure 8 shows the electric field dependence on applied current density during zirconium anodization in the alkaline phosphate solution. For the current densities used in these anodizations, the oxide temperature rise was found from Fig. 3. Then correcting for temperature rise new values of electric field were calculated relating electric field and applied current density as if the oxide temperature were constant. For these thin foil specimens the effects of the Joule heating on the electric field are small below 5 mA cm- 2 but at 5OmA cm-’ the electric field is reduced by about 4% even for small film thicknesses. For massive metal samples, the temperature rise will be lower than for foil samples and so measurements may not be comparable. Also calculations, involving the electric field, which assume that the temperature of the oxide films is constant during anodization and independent of current density, will be in error to an extent depending on the current density. Finally, the crystallization of anodic oxides has also been attributed to the temperature rise during anodization. In the experiments performed in this study,

Temperature rise during anodic oxidation of zirconium

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density and was higher in an unstirred than in a stirred solution. The transfer coefficient of heat generated in the oxide was also calculated and found to be lower with a stirred solution. For a thin zirconium wire, the temperature rise was found to be. lower than that for a zirconium strip, other conditions being equivalent. The observed temperature rise is significant, especially at high growth rates and at large oxide thicknesses. This may limit the validity of deductions made assuming that the oxide temperature is always the same as the solution temperature.

I 1

5

10

50

REFERENCES

I,, ImA.cm-'I

Fig. 8. Electric field (E) us log I, relationship during zirconium anodization in 0.01 M NalPOl solution. oH = 11.8. V, p. 50 V andelectrolyte temper&re~z 17°C(i&n circles): The same relationship recalculated to eliminate the influence of Joule heat e&ct on the oxide growth kinetics (filledcircles). temperatures sufficient to cause the transformation of amorphous to crystalline oxide were never obtained even for very thick oxide films and high current densities.

1. D. A. Vermilyea, Actn Metal 1, 282 (1953). 2. L. Young, Trans. Faraday Sot. 53, 229 (1957). 3. F. R. Applewhite, I. S. L. Leach and P. Neufeld. Corros. Sci. 9, 305 (3969). 4. J. Zahavi and J. Yahalom, Electrochim.Acta 16,89 (1971). 5. G. Jouve and J. S. L. Leach, Thin Solid Films 110, 263 (1983). 6. J. Richardson, Private communication, The University of

Nottineham

119831

A. Bov.&, J. ?. Hubbard and I. S. L. Leach, Proc. 167th Meeting of the Electrochemical

Society, Toronto (1985).

8. G. C. Willis, G. B. Adams and P. Van Rysselberge, CONCLUSIONS khe temperature rise observed during zirconium anodization increased with increasing applied current

Elecrrochim. Acra 9, 79 (1964). Ph.D. thesis, The University of Nottingham (1983). 10. C. Ortega and J. Siejka. J. electrochem. Sot. 129, 1895

9. C. N. Panagopoulos, (1982).