Kinetics of formation of solid iodides on zirconium

Journal of the Less-Common

241

Metals, 77 (1981) 241 - 250

KINETICS OF FORMATION

OF SOLID IODIDES ON ZIRCONIUM

DANIEL CUBICCIOTTI and A. C. SCOTT SRI International,

Menlo Park, CA 94025

(U.S.A.)

(Received June 5,198O)

Summary

The reaction of zirconium with gaseous Zr14 was studied in the temperature range 300 - 500 “C. Specimens of zirconium were heated in sealed glass tubes with solid ZrI, which was kept at a lower temperature to maintain a fixed ZrI, pressure. The solid zirconium iodides that formed on the zirconium surface were examined by scanning electron microscopy. Initially, isolated clusters of ZrI crystals formed which subsequently coalesced into a layer of ZrI. A layer of diiodide formed on top of the layer of monoiodide in later stages. Preformed oxide layers less than 0.2 pm thick on the zirconium surface had no effect on the rate of the reaction with gaseous Zr14 but an oxide layer about 4 E.tmthick slowed the reaction markedly. The amount of iodide on the surface was determined by chemical analysis. In one series of timed experiments the amount of reaction was found to obey the cubic law for metal-gas reactions. The kinetics of the reaction resemble those of the Zr-0, reaction.

1. Introduction

The kinetics of formation of solid iodides on zirconium metal are involved in the mechanism of iodide attack on the metal. These processes are probably involved in the iodide-induced stress corrosion cracking of zirconium alloys as well as in the iodide refining of the metal. Several studies of the reaction of the metal with iodine vapor have been made in relation to the iodide-refining process [ 1 - 31. Bus01 [4] has studied the rate of reaction of gaseous ZrI, with zirconium metal in the form of filings. He has measured the rate of disappearance of gaseous ZrI, by a manometric method and has found that it is parabolic at temperatures from 306 to 500 “C. We report in this work a study of the formation of the iodide products from the reaction with gaseous ZrI1. Our earlier investigations of the thermodynamics of the Zr-I, system [ 5,6] have shown that, of the gaseous species, ZrI* is dominant in the range of conditions pertinent to the present work. We have also obtained [ 7 - lo] thermodynamic information about the equilibrium solids which show phases 0022-5088/81/0000-0000/$02.50

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242

approximating the mono-, di-, tri- and tetraiodide. We have also reported [ 111 information about the chemisorption of iodine on zirconium.

2. Expe~en~ Pieces of zirconium foil were exposed to Zr14 vapor in evacuated sealed Pyrex tubes maintained in a two-temperature furnace, with the foil in the hotter zone and solid ZrI, in the cooler zone. After a fixed time at the high temperature a tube would be brought to room temperature in such a manner as to ensure that the end containing ZrI* was always the cooler, in order to avoid conden~tio~ of ZrId on the z~conium specimen. The bulb was then opened in an argon-purged dry box, and a comer of the foil was cut and mounted on a holder which was transferred to a greased-joint weighing bottle. The closed weighing bottle was taken to the scanning electron microscope, where it was opened in a rapid stream of dry nitrogen issuing from the stage opening of the microscope. The sample was quickly transferred and the scanning electron microscope system was evacuated. The transfer was thus accomplished with a rn~~~ exposure of the sample to atmospheric moisture. The surfaces of specimens were examined by scanning electron microscopy (SEM) for topographical features and spot analyses of elemental composition were made by energy-dispersive analysis of the X-ray fluorescence excited by the 30 kV electron beam using a Kevex Ge-Li detector system. At that electron energy we estimated that the X-ray fluorescence originated from a layer of material of the order of 1 I.rmthick, so that analyses of features of that size or smaller included some of the substrate. The elemental compositions determined in this way are dependent to some degree on the geometry of the surface in relation to the detector. We found generally good correspondence between the compositions of individual crystals, larger than a few microns, observed in this work and the compositions of solid solution phases found in our vapor pressure studies [ 8,9] . Thus we feel the ratios of I/Zr reported in the present work are accurate to +O.l. The main portions of the reacted foils were also removed from the dry box in wei~~g bottles to analyze for iodide. They were then quickly immersed in acetic acid solution and the solution was titrated with standard silver nitrate, using an iodide specific electrode to determine the end point. The procedure had a sensitivity of about 0.005 (mg I) (cm2 Zr)-‘. The zirconium was foil 0.25 mm thick of 99.9% purity obtained from the Alfa-Ventron Corporation (Danvers, Massachusetts). For most experiments the foil was cleaned in “bright-dip” solution (45% HNOs, 10% HF, 45% of 3% H,Oz) at room temperature for a few seconds. The process caused the formation of a passive film which was a thin oxide containing a few per cent fluoride 1121. For some experiments the foil was oxidized in air under various conditions of temperature and time to form thicker oxide

243

films. The thicknesses of these preformed oxides were estimated from the information given by Kofstad [ 131 for the cubic oxidation of zirconium. ZrI, was made by the reaction of iodine with excess zirconium powder in Pyrex tubes at elevated temperature. Sublimation of Zr14 from the excess zirconium in the reaction tubes produced lumps of pure ZrI, at the cool end. The purity of the material made by this procedure has been reported earlier 171.

3. Results and discussion The initial experiments in this study were made with iodine and zirconium. The reaction products are shown in Fig. 1. The presence of more than two phases with different compositions in close contact implied that the system was far from equilibrium. Subsequent experiments were made with Zr14 instead of iodine, and with the ZrIl maintained at lower temperatures than the zirconium. Under these conditions the effective iodine partial pressure was much smaller and more uniform layers of reaction products were formed. In Table 1 we present a summary of the experimental conditions and the results of the two-temperature capsule runs performed. The first few runs were made to determine the conditions for obtaining measurable surface layers. Under mild conditions (i.e. low zirconium temperature and small ZrI, pressure) clusters of acicular ZrI crystals grew from isolated spots; the surface between the clusters was covered with a thin film that contained iodide. Analyses of these films indicated I/Zr ratios of the order of 0.1 or smaller

Fig. 1. Reactionproductsformed underseverereactionconditions.Zirconiumexposed to iodine for 17 h at 400 “C. X-ray fluorescence analysis showed that the acicular crystals (arrow 1) were ZrI4 and that the thicker crystals (arrow 2) were ZrIla9. (Width of photograph, 0.2 mm.)

500 500

500

300

300

B B

B

Blk Ox

A

B

A

B Blk Ox Blk Ox

Zr Zirc

Zr

Zr

Zr

Zr

Zr

Zr Zr Zr

11 12

13

14

15

16

17

18 19 20

500 500 500

500

300

300 300 300 400 400 500

B B B B B B

Zr Zr Zirc Zr Zire Zirc

2 5 6 7 8 10

Metal temperature (“C)

Initial surface conditionb

Metal samplea

Run no.

400 400 400

400

300

300

300

~250

300 = 250

300 200 200 400 406 300

(“C)

Zrl* temperatureC

Experimental conditions and results of capsule tests

TABLE 1

ZrI2.8

zrf2.8

zr12.8

Zrf2.8

ZrI*

ZrI4

24 24 24

24

24

24

24

17

ZrI,

ZrI4

3 I7

17 40 40 65 65 3

Time (h)

ZrI2 ZrI2

Zr14 ZrI4 Zr12

zr12.8

Zrl2.8

ZrI*

Highest iodided

2.0 co.01 <0.0x

2.1

0.18

0.23

co.01

0.14

0.67 0.12

ND ND ND ND ND 0.6

Layer of ZrI1.a over a layer of ZrI1.r See Fig. 2 Similar to run 5 See Fig. 3 Similar to run 7 Clusters of separated crystals; composition of tops w ZrIg,a Poorly defined structure Separated clusters of crystals; like Fig. 2 but each clump was more dense, = ZrI The number of clumps per unit area was variable over the surface and completely covered the surface in some areas Similar to run 19 hut fewer iodide crystals Isolated clusters of crystals of ZrI similar to Fig. 2 Similar to run 15, but crystals not well defined Layers of crystals of ZrIg.2 on top and ZrI beneath; like Fig. 3 Similar to run 17 See Fig. 4 Similar to run 19

SEM observations Amount of surface io+dinee tmgcm )

L

Zr

Zr Zr Zr Zr Zr Zr Zr

22

23 24 25 26 27 28 29

500

500 357 357 357 357 357 357

B B B B B B B

500

Gld Ox

Blu Ox ZrI2.8

400 400 258 258 258 258 258 258

ZrkL8

400

24

24 0.25 0.5 1.2 2.3 3.5

24

0.31

2.5
2.5 2.8 i Similar to Fig. 3 This series of experiments shows the evolution with time of the iodide layers; see Fig. 5 i The surface of the foil from run 24 looked like Fig. 5(a) but with fewer I crystals

All three specimens had an outer layer of crystals of ZrI.27 + 0.2, an inner layer ! of ZrI1.o t:0.2 and a thin poorly defined layer of about Mr.8 between

aZr, zirconium sheet; Zirc, pieces cut from commercial Zircaloy-2 tubing. bA, as received; B, bright dipped; Blk Ox, black oxide, specimen preoxidized in air at 500 “C for 120 h, oxide thickness estimated from ref. 13 to be 4 pm; Blu Ox, blue oxide, specimen preoxidized in air at 300 “C for 24 h, oxide thickness estimated from ref. 13 to be 0.2 /.&n; Gld OX, gold oxide, specimen preoxidized in air at 300 “C!for 3 h, oxide thickness estimated from ref. 13 to be 0.1 p. ?ressures of gaseous Zr14 calculated from the data of ref. 7 were 3 Pa for 200 C, 95 Pa for 258 C, 800 Pa for 300 “C and 4 x lo* Pa for 400 “C. dThe highest iodide is the equilibrium composition of iodide expected for the given sample temperature and ZrI4 pressure based on information from refs. 8 and 9. eND, not determined.

Zr

2x

246

which probably resulted from a layer of ZrI that was much less than 1 pm thick, Figures 2 and 5(a) illustrate the structure of the surface after mild attack. {Runs 5,6,15,16,24 and 25 showed this kind of surface.) As the conditions became more severe the density of each cluster increased and the number of clusters per unit area also increased until the surface became completely covered by a layer of ZrI. (Runs 1213 and 27 showed this stage.) Additions attack led to the formation of a layer of diiodide (about ZrIa) on top of the ZrI (runs 17 and 18). Further attack produced material of approximate composition of the triiodide (about ZrIz7) on top (runs 7,8, 21, 22 and 23) as shown in Fig. 3. The highest iodide that could form was a function of the pressure of the ZrId gas and the temperature of the sample; it corresponded to the equilibrium composition for the solid under those conditions. The sixth column of Table 1 indicates the stoichiometry of the anticipated equilibrium solid derived from results given in refs. 8 and 9. The composition of the outpost Iayer attained the behest value in some of the 24 h tests. In some of the tubes a specimen of Zircaloy-2 alloy, cut from samples of commercial nuclear reactor cladding, were included with the zirconium specimens. No significant differences in behavior of the two metals were detected (see runs 5 and 6,7 and 8,lO and 11, and 12 and 13). The presence of a preformed oxide film on the surface might be expected to act as a barrier to the reaction between the metal and the gaseous ZrI*. Accordingly, several experiments were made with preformed oxide films. In runs 14,15 and 16 the metal specimens were placed side by side in

Fig. 2. Reaction products formed under very mild conditions (run 5). The needles were ZrIl.0 and the surface between the needles had detectable amounts of iodide. (Width of photograph, 0.02 mm,) Fig. 3. A layer of products that had curled up from the metal substrate after run 7: bottom layer {arrow l), = ZrIxo; middle of upper layer (arrow Z), -Z&.Q; tops of domes, = ZrI2.7. (Width of photograph, 0.02 mm.)

247

a tube and were tested under the conditions indicated in Table 1 whilst runs 17,18 and 19 were made in another tube. The results of these experiments, both SEM and analysis for iodide, show that there was little difference in attack on the as-received surface and on the surface after bright dip. The results also show that the preformed oxide layer 4 pm thick markedly reduced the amount of reaction with ZrI, gas; however, the reaction was not completely stopped as evidenced by the presence of some ZrI crystals on the surface (Fig. 4). Additional experiments were made to investigate the influence of thin oxide films. In runs 20,21,22 and 23 specimens with different oxide thicknesses were exposed in the same glass tube. The results indicate that thin oxide films do not affect the rate of the reaction. The amounts of iodide on the surfaces, determined by titration and SEM, were the same for both the bright-dipped surface and the surfaces with preformed oxide layers about 0.1 and 0.2 I.trnthick. A series of experiments was performed to investigate the reaction as a function of time. The results are given in runs 24 - 29. The SEM observations are illustrated in Fig. 5. They indicate that the formation of the surface iodide layers involves several steps. (1) A thin layer of iodide is formed and clusters of acicular ZrI crystals grow from that surface (Fig. 5(a)). (2) The clusters of crystals increase in density until the surface becomes covered with a layer of ZrI (Fig. 5(b)). (3) Further reaction leads to the formation of a layer of approximately diiodide on top of the monoiodide. The diiodide layer has a columnar struc-

Fig. 4. The surface of preoxidized zirconium exposed to ZrI4 gas (run 19). Analysis showed that the crystals were ZrI 0.9. The cracks in the underlying oxide layer were presumably formed when the specimen was cut after the reaction. (Width of photograph, 0.02 mm.)

243

(4

lb)

Fig. 5. The evolution of iodide surface layers on zirconium at 351 “c exposed to 95 Pa of ZrI4 gas: (a) after 0.5 h, separated crystals of ZrI 0.9 (run 25); (b) after 2.3 h, crystals of ZrIo.9 almost completely cover the surface (run 27); (c) after 24 h, crystals completely cover the surface. Analysis of the tops of crystals in the area marked 1 showed ZrI2.3, the remainder showed ZrI,_a. Presumably there was an underlying layer of ZrI (run 29). (Width of photographs, 0.02 mm.)

ture that appears to be porous (Fig. 5(c)). With additional reaction there is evidence for the formation of triiodide (Fig. 3). The first thin layer of iodide to cover the surface had too little iodide to be determined by our analysis; it was less than 0.01 (mg I) cmw2 (run 24). Peehs et al. [ 141 have shown that iodide coverages of the order of 3 X lop3 mg cmp2 can induce stress corrosion cracking of Zircaloy whereas smaller coverages do not. A surface covered with that amount of iodide would be similar to that shown in Fig. 2 or Fig. 5(a). We suspect that the minimum

249

iodide required to induce the stress corrosion cracking as observed by Peehs et d. corresponds to a surface in which clusters of ZrI crystals begin to form. That is, with smaller amounts of surface iodide there is probably only a very thin uniform layer of iodide (i.e. the surface between needles in Fig. 2); with larger amounts clusters of crystals begin to form. We speculate that the needles of ZrI serve as a source of iodide that can react with fresh metal surface made available through the straining of the metal and that they can continue to supply iodide as a crack propagates in the metal. The rate of the reaction of gaseous Zr14 with zirconium, based on the measured amounts of surface iodide, decreased with time, typical of a gasmetal reaction in which a protective layer is formed. The amount of reaction as a function of time is shown both in a parabolic plot and in a cubic plot in Fig. 6. The results are in better accord with the cubic plot, with a small induction period. An induction period is understandable in view of the change in surface morphology during the early stages from scattered clusters of ZrI to a continuous layer. Bus01 [4] found that
Fig. 6. Amount of reaction as a function of time in a parabolic plot (upper curve) and in a cubic plot (lower curve). The zirconium was kept at 357 “c and the ZrI4 was kept at 258 “C (runs 24 - 29).

250

in this treatment but has only a minor effect on the constants.) For 500 “C, 357 “C and 300 “C the constants are 1.6 X 10P1’ kg me2 s-l, 3.4 X lo-l3 kg me2 s-l and 9.3 X lo-l4 kg mP2 s-l respectively. These values are consistent with an activation energy of about 140 f 5 kJ mol-‘. This value is comparable with the activation energy for the cubic reaction of zirconium with oxygen in this temperature range, namely 155 kJ mol-’ [ 131. Thus the kinetics of reaction for zirconium with gaseous Zr14 are very similar to those for the Zr-O2 reaction both in the cubic form of the reaction and in the activation energy.

Acknowledgments The authors are grateful to Jan Terry for the SEM studies. This research was funded by the Department of Energy, Office of Basic Energy Studies, under Contract EY-76-S-03-1339.

References 1 Z. M. Shapiro, Iodide decomposition process for production of zirconium. In B. Lustman and F. Kerze (eds.), Metallurgy of Zirconium, McGraw-Hill, New York, 1955, Chap. 5. 2 R. M. Horton and R. L. Kinney, Kinetics of formation of Zr14 from Zr and 12. In Z. A. Foroulis and W. W. Smeltzer (eds.),Metal Slag Gas Reactions and Processes, The Electrochemical Society, Princeton, New Jersey, 1975, p. 317. 3 L. N. Shelest, E. K. Safronovand A. S. Mikhailova, Russ. J. Znorg. Chem., 18 (1973) 9. 4 F. I. Busol, Russ. J. Phys. Chem., 33 (1959) 799. 5 D. Cubicciotti, D. L. Hildenbrand, K. H. Lau and P. D. Kieinschmidt, Thermodynamics of zirconium iodides at elevated temperatures. In D. L. Hildenbrand and D. Cubicciotti (eds.), High Temperature Metal Halide Chemistry, The Electrochemical Society, Princeton, New Jersey, 1978, p. 217. 6 P. D. Kleinschmidt, D. Cubicciotti and D. L. Hildenbrand, J. Electrochem. SOC., 125 (1978) 1543. 7 D. Cubicciotti, K. H. Lau and M. J. Ferrante, J. Electrochem. Sot., 125 (1978) 975. 8 D. Cubicciotti and K. H. Lau, J. Electrochem. Sot., 126 (1979) 771. 9 D. Cubicciotti and K. H. Lau, J. Electrochem. Sot., 128 (1981) 196. 10 R. H. Lamoreaux and D. Cubicciotti, Thermodynamic properties of the solid zirconium iodides, submitted to J. Electrochem. SOC. 11 G. N.Krishnan,B. J. Wood and D. Cubicciotti, J. Electrochem. Sot., 127 (1980) 2738. 12 D. R. Knittel and D. Cubicciotti, J. Electrochem. SOC., 127 (1978) 75. 13 P. Kofstad, High Temperature Oxidation of Metals, Wiley, New York, 196$, P. 179. 14 M. Peehs, F. Garzaroiii, R. Hahn and E. Steinberg, J. Nucl. Mater., 87 (1979) 274.