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Selective oxidation of zirconium in Zr3Al
of the Less-Common
Journal
SELECTIVE M. PALJEVId Rudjer Science,
Metals,
(1985)
83
83
- 86
OXIDATION OF ZIRCONIUM IN Zr,AI and Z. BAN
BoSkovic’ Institute, P.O. Box University of Zagreb, Zagreb,
(Received
105
February
1016, Bijenicka C.54, Croatia (Yugoslavia)
YU-41000,
and
Faculty
of
6, 1984)
Summary The oxidation of ZrsAl in oxygen at temperatures ranging from 740 to 1050 K results in the formation of the following sequence of layers: cubic ZrO,, monoclinic ZrO*, Zr,Al and Zr,Al. During the oxidation, aluminium diffuses from the oxide layer into the Zr,Al bulk, thus forming a ZrzAl phase at some distance from the alloy-oxide interface. The excess zirconium is therefore selectively oxidized, yielding monoclinic and/or cubic ZrO, containing no alumina.
1. Introduction We have found in previous investigations [ 1, 21 that only monoclinic and/or cubic ZrOz were formed when ZrsAl was oxidized in oxygen at elevated temperatures. No A1203 was detected in the oxide layer. Since ZrsAl-based alloys are potential pressure tube materials for nuclear power reactors [3 - 151, a number of oxidation studies of these alloys have already been performed [ 1 - 61. Owing to the absence of A120s in the oxide scale, questions arise concerning the role and diffusion behaviour of aluminium during the oxidation process. The present work is an attempt to answer these questions.
2. Experimental
details
ZrsAl samples were prepared by arc melting appropriate amounts of iodide zirconium (zirconium refined by the van Arkel-de Boer hot-wire process) and aluminium (purity, 99.999%) in a water-cooled copper mould in a titanium-gettered argon atmosphere (pressure, 48 kPa). The samples were remelted three times to ensure homogeneity. The homogenization was performed in evacuated sealed quartz vials at 1193 K for about 40 days. X-ray and metallographic analysis confirmed that the samples consisted essentially of the Zr,Al phase. The samples were then cut into small flat 0022-5088/85/$3.30
@ Elsevier
Sequoia/Printed
in The Netherlands
84
specimens of dimensions 8 mm X 5 mm X 1 mm. After grinding, they were polished using a slurry of Cr203 and water with the addition of a few drops of 0.5% HF solution. The oxidation of Zr,Al in a stream of dry oxygen at atmospheric pressure was performed at temperatures in the range 740 - 1050 K. The oxygen absorption was followed using an automatic thermobalance. The surfaces of the oxidized specimens were examined using an X-ray diffraction technique. The surfaces were then ground and re-examined using X-ray diffraction analysis. This procedure was repeated several times. The changes in the zirconium, aluminium and oxygen concentrations were measured on cross sections of oxidized specimens by means of electron microanalysis.
3. Results and discussion Kinetic measurements have shown that the oxidation of Zr,Al obeys a parabolic rate law, which suggests that the rate-determining process is thermal diffusion [2]. Two values were determined for the activation energy: 114.0 kJ mol-’ for oxidation at temperatures lower than 800 K and 53.1 kJ mol-’ for oxidation at temperatures above 890 K. It was found that only cubic ZrOz was formed below 790 K. At temperatures above 800 K, monoclinic ZrOz was formed in addition to the cubic phase. The oxide layer is dark at lower temperatures, but at higher temperatures the colour changes to grey and finally becomes white. No Al,Os was detected in the oxide scale. In order to estimate the role and the behaviour of aluminium during oxidation, the concentrations of aluminium, zirconium and oxygen were measured for a series of samples by means of electron microanalysis. The results of these measurements are shown schematically in Fig. 1. The concentration of aluminium in the oxide layer is very low. It rapidly increases at the alloy-oxide interface and then remains almost
LO
60
‘O&
zro,
5
Zr
80
100
120
IpI
distance I
ZrsAl
cub.lmonocl.
Fig. 1. Concentration 918 K for 24 h.
profiles
across
the alloy
and &Al
scale
oxidized
in oxygen
at
85
constant. At some distance from the alloy-oxide interface it jumps to the level corresponding to the amount of aluminium in Zr,Al and then drops to the original value. The corresponding change in the zirconium concentration at the point where the aluminium concentration abruptly increases is gradual. The zirconium concentration in the oxide layer supplements the oxygen concentration. The oxygen concentration decreases through the oxide layer and continues to decline in the alloy. X-ray phase analysis provides more information about the crystallographic phases appearing at and below the surface of the oxidized specimen. It was found that the surface layer of the oxide scale of the ZraAl oxidized at temperatures below 790 K consisted of the cubic modification of ZrOz. After the surface layer had been removed by grinding, monoclinic ZrOz was detected followed by Zr,Al. The next layer consisted of Zr,Al. When ZrsAl was oxidized at temperatures above 800 K, the surface of the oxide scale consisted of both cubic and monoclinic ZrO*. The amount of monoclinic ZrOz increased in the subsurface region and appeared to be better crystallized than that at the surface. The amount of the cubic modification of ZrOz then began to decrease. As Zr,Al contains 25 at.% Al, aluminium oxide should be present in the oxidized specimen and thus should be detected by X-ray diffraction, as has been shown for fully oxidized material. A1203 is almost insoluble in ZrO, in the solid state [16 - 191, but a eutectic which contains 57.4 wt.% A1203 and 42.6 wt.% ZrOz and has a melting point of 1983 K is formed [19]. However, Cevales [19] has reported that ZrOz is slightly soluble in A1203 on the alumina-rich side with the formation of a new phase e-Al*Os containing about 99 wt.% Al*Os and 1 wt.% ZrO,. Since no Al,Os was detected in the oxide layer by X-ray diffraction examination and since electron microprobe analysis showed that the amount of aluminium in the oxide layer had undergone a significant decrease, the aluminium must have diffused to some other location because it could not have evaporated at the preparation temperature or have simply disappeared. Indeed, an increased aluminium concentration was found in the bulk ZraAl phase at some distance from the alloy-oxide interface and a ZrzAl phase was detected. Thus, when an oxide film is formed, both oxygen and aluminium diffuse through the film to the alloy. The remaining zirconium is selectively oxidized in this way to form ZrOz. Cowgill and Smeltzer [20] reported a similar oxidation product in the reaction of a Zr-2.7wt.%Nb alloy with oxygen at temperatures in the range 573 - 773 K. The major product of the reaction at all temperatures, irrespective of the alloy structure, was an oxide whose structure was identified as monoclinic ZrO*. Wood and Whittle [21, 221 found that Cr,Os in which small quantities of iron were dissolved was the only oxide formed in the oxidation of Fe-Cr alloys containing 22 - 68 wt.% Cr at 1073 - 1473 K. Crz03 was also formed, at least initially, on alloys containing 14 and 18 wt.% Cr. Iron only entered
86
the scale during the very early stages of oxidation, so that the weight percentage of iron in the oxide decreased with time. Such selective oxidation, in which the least noble constituent of the alloy is selectively or preferentially oxidized, is often observed, particularly when the oxides which should be formed are mutually insoluble and do not react with each other. The scale consists of only one oxide phase. However, the occurrence of selective oxidation depends not only on the type of alloying component but also on the concentration of the active alloying component, the temperature and the oxygen partial pressure. Selective oxidation will not take place under any conditions if the concentration of the “active” alloy component is less than a critical value. In view of the theory proposed by Wagner [23 - 251, who analysed the conditions required for selective oxidation in a binary alloy and derived a mathematical expression for the critical concentration, it would be interesting to examine the oxidation behaviour of other phases in the Zr-Al system.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
M. PaljeviE and Z. Ban, J. Nucl. Mater., 95 (1980) 253. M. PaljeviC1 and Z. Ban, J. Less-Common Met., 95 (1983) 105. E. M. Sehulson, J. Nuel. Mater., 50 (1974) 127. D. J. Cameron and A, E. Unger, Rep. A~CL-4662, 1974 (Atomic Energy of Canada Ltd.). D. J. Cameron and A. E. Unger, Rep. AECL-4665, 1974 (Atomic Energy of Canada Ltd.). E. V. Murphy,J. Nucl. Mater., 55 (1975) 117. L. M. Howe, M. Rainville and E. M. Schulson, J, Nucl. Muter., 50 (1974) 139. E. M. Schulson, J. Nucl. Mater., 56 (1975) 38. E. M. Schulson, J. Nuct. Mater., 57 (1975) 98. E. M. Schulson, J. Nucl. Mater., 57 (1975) 358. E. M. Schulson and J. A. Roy, J. Nucl. Mater., 60 (1976) 234. E, M. Schulson, J. Nucl. Mater., 66 (1977) 322. H. E. Rosinger, J. Nucl. Mater., 66 (1977) 193. D. J. Cameron, Rep. AECL-4669,1974 (Atomic Energy of Canada Ltd.). R. W. Cahn, Nature (London), 275 (1978) 176. H. v. Wartenberg, H. Linde and R. Jung, 2. Anorg. Allg. Chem., 176 (1928) 349. A. Dietzel and H. Tober, Ber. Dtsch. Keram. Ges., 30 (1953) 47. H. Suzuki, S. Kimura, H. Yamoda and T. Yamauchi, J. Ceram. Assoc. Jpn., 69 (1961) 52. G. Cevales, Ber. Dtsch. Keram. Ges., 45 (1968) 216. &I. G, Cowgill and W. W. Smeltzer, J. Electrochem. Sot., 114 (1967) 1089. G. C. Wood and D. P. Whittle, J. Electrochem. Sot., I15 (1968) 126. D. P. Whittle and G. C. Wood, J. Electrochem. Sot., 115 (1968) 133. C. Wagner, J. Electrochem. Sot,, 99 (1952) 369. C. Wagner, J. Electrochem. Sot., 103 (1956) 627. C. Wagner, 2. Eiektrochem., 63 (1959) 773.
Journal
SELECTIVE M. PALJEVId Rudjer Science,
Metals,
(1985)
83
83
- 86
OXIDATION OF ZIRCONIUM IN Zr,AI and Z. BAN
BoSkovic’ Institute, P.O. Box University of Zagreb, Zagreb,
(Received
105
February
1016, Bijenicka C.54, Croatia (Yugoslavia)
YU-41000,
and
Faculty
of
6, 1984)
Summary The oxidation of ZrsAl in oxygen at temperatures ranging from 740 to 1050 K results in the formation of the following sequence of layers: cubic ZrO,, monoclinic ZrO*, Zr,Al and Zr,Al. During the oxidation, aluminium diffuses from the oxide layer into the Zr,Al bulk, thus forming a ZrzAl phase at some distance from the alloy-oxide interface. The excess zirconium is therefore selectively oxidized, yielding monoclinic and/or cubic ZrO, containing no alumina.
1. Introduction We have found in previous investigations [ 1, 21 that only monoclinic and/or cubic ZrOz were formed when ZrsAl was oxidized in oxygen at elevated temperatures. No A1203 was detected in the oxide layer. Since ZrsAl-based alloys are potential pressure tube materials for nuclear power reactors [3 - 151, a number of oxidation studies of these alloys have already been performed [ 1 - 61. Owing to the absence of A120s in the oxide scale, questions arise concerning the role and diffusion behaviour of aluminium during the oxidation process. The present work is an attempt to answer these questions.
2. Experimental
details
ZrsAl samples were prepared by arc melting appropriate amounts of iodide zirconium (zirconium refined by the van Arkel-de Boer hot-wire process) and aluminium (purity, 99.999%) in a water-cooled copper mould in a titanium-gettered argon atmosphere (pressure, 48 kPa). The samples were remelted three times to ensure homogeneity. The homogenization was performed in evacuated sealed quartz vials at 1193 K for about 40 days. X-ray and metallographic analysis confirmed that the samples consisted essentially of the Zr,Al phase. The samples were then cut into small flat 0022-5088/85/$3.30
@ Elsevier
Sequoia/Printed
in The Netherlands
84
specimens of dimensions 8 mm X 5 mm X 1 mm. After grinding, they were polished using a slurry of Cr203 and water with the addition of a few drops of 0.5% HF solution. The oxidation of Zr,Al in a stream of dry oxygen at atmospheric pressure was performed at temperatures in the range 740 - 1050 K. The oxygen absorption was followed using an automatic thermobalance. The surfaces of the oxidized specimens were examined using an X-ray diffraction technique. The surfaces were then ground and re-examined using X-ray diffraction analysis. This procedure was repeated several times. The changes in the zirconium, aluminium and oxygen concentrations were measured on cross sections of oxidized specimens by means of electron microanalysis.
3. Results and discussion Kinetic measurements have shown that the oxidation of Zr,Al obeys a parabolic rate law, which suggests that the rate-determining process is thermal diffusion [2]. Two values were determined for the activation energy: 114.0 kJ mol-’ for oxidation at temperatures lower than 800 K and 53.1 kJ mol-’ for oxidation at temperatures above 890 K. It was found that only cubic ZrOz was formed below 790 K. At temperatures above 800 K, monoclinic ZrOz was formed in addition to the cubic phase. The oxide layer is dark at lower temperatures, but at higher temperatures the colour changes to grey and finally becomes white. No Al,Os was detected in the oxide scale. In order to estimate the role and the behaviour of aluminium during oxidation, the concentrations of aluminium, zirconium and oxygen were measured for a series of samples by means of electron microanalysis. The results of these measurements are shown schematically in Fig. 1. The concentration of aluminium in the oxide layer is very low. It rapidly increases at the alloy-oxide interface and then remains almost
LO
60
‘O&
zro,
5
Zr
80
100
120
IpI
distance I
ZrsAl
cub.lmonocl.
Fig. 1. Concentration 918 K for 24 h.
profiles
across
the alloy
and &Al
scale
oxidized
in oxygen
at
85
constant. At some distance from the alloy-oxide interface it jumps to the level corresponding to the amount of aluminium in Zr,Al and then drops to the original value. The corresponding change in the zirconium concentration at the point where the aluminium concentration abruptly increases is gradual. The zirconium concentration in the oxide layer supplements the oxygen concentration. The oxygen concentration decreases through the oxide layer and continues to decline in the alloy. X-ray phase analysis provides more information about the crystallographic phases appearing at and below the surface of the oxidized specimen. It was found that the surface layer of the oxide scale of the ZraAl oxidized at temperatures below 790 K consisted of the cubic modification of ZrOz. After the surface layer had been removed by grinding, monoclinic ZrOz was detected followed by Zr,Al. The next layer consisted of Zr,Al. When ZrsAl was oxidized at temperatures above 800 K, the surface of the oxide scale consisted of both cubic and monoclinic ZrO*. The amount of monoclinic ZrOz increased in the subsurface region and appeared to be better crystallized than that at the surface. The amount of the cubic modification of ZrOz then began to decrease. As Zr,Al contains 25 at.% Al, aluminium oxide should be present in the oxidized specimen and thus should be detected by X-ray diffraction, as has been shown for fully oxidized material. A1203 is almost insoluble in ZrO, in the solid state [16 - 191, but a eutectic which contains 57.4 wt.% A1203 and 42.6 wt.% ZrOz and has a melting point of 1983 K is formed [19]. However, Cevales [19] has reported that ZrOz is slightly soluble in A1203 on the alumina-rich side with the formation of a new phase e-Al*Os containing about 99 wt.% Al*Os and 1 wt.% ZrO,. Since no Al,Os was detected in the oxide layer by X-ray diffraction examination and since electron microprobe analysis showed that the amount of aluminium in the oxide layer had undergone a significant decrease, the aluminium must have diffused to some other location because it could not have evaporated at the preparation temperature or have simply disappeared. Indeed, an increased aluminium concentration was found in the bulk ZraAl phase at some distance from the alloy-oxide interface and a ZrzAl phase was detected. Thus, when an oxide film is formed, both oxygen and aluminium diffuse through the film to the alloy. The remaining zirconium is selectively oxidized in this way to form ZrOz. Cowgill and Smeltzer [20] reported a similar oxidation product in the reaction of a Zr-2.7wt.%Nb alloy with oxygen at temperatures in the range 573 - 773 K. The major product of the reaction at all temperatures, irrespective of the alloy structure, was an oxide whose structure was identified as monoclinic ZrO*. Wood and Whittle [21, 221 found that Cr,Os in which small quantities of iron were dissolved was the only oxide formed in the oxidation of Fe-Cr alloys containing 22 - 68 wt.% Cr at 1073 - 1473 K. Crz03 was also formed, at least initially, on alloys containing 14 and 18 wt.% Cr. Iron only entered
86
the scale during the very early stages of oxidation, so that the weight percentage of iron in the oxide decreased with time. Such selective oxidation, in which the least noble constituent of the alloy is selectively or preferentially oxidized, is often observed, particularly when the oxides which should be formed are mutually insoluble and do not react with each other. The scale consists of only one oxide phase. However, the occurrence of selective oxidation depends not only on the type of alloying component but also on the concentration of the active alloying component, the temperature and the oxygen partial pressure. Selective oxidation will not take place under any conditions if the concentration of the “active” alloy component is less than a critical value. In view of the theory proposed by Wagner [23 - 251, who analysed the conditions required for selective oxidation in a binary alloy and derived a mathematical expression for the critical concentration, it would be interesting to examine the oxidation behaviour of other phases in the Zr-Al system.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
M. PaljeviE and Z. Ban, J. Nucl. Mater., 95 (1980) 253. M. PaljeviC1 and Z. Ban, J. Less-Common Met., 95 (1983) 105. E. M. Sehulson, J. Nuel. Mater., 50 (1974) 127. D. J. Cameron and A, E. Unger, Rep. A~CL-4662, 1974 (Atomic Energy of Canada Ltd.). D. J. Cameron and A. E. Unger, Rep. AECL-4665, 1974 (Atomic Energy of Canada Ltd.). E. V. Murphy,J. Nucl. Mater., 55 (1975) 117. L. M. Howe, M. Rainville and E. M. Schulson, J, Nucl. Muter., 50 (1974) 139. E. M. Schulson, J. Nucl. Mater., 56 (1975) 38. E. M. Schulson, J. Nuct. Mater., 57 (1975) 98. E. M. Schulson, J. Nucl. Mater., 57 (1975) 358. E. M. Schulson and J. A. Roy, J. Nucl. Mater., 60 (1976) 234. E, M. Schulson, J. Nucl. Mater., 66 (1977) 322. H. E. Rosinger, J. Nucl. Mater., 66 (1977) 193. D. J. Cameron, Rep. AECL-4669,1974 (Atomic Energy of Canada Ltd.). R. W. Cahn, Nature (London), 275 (1978) 176. H. v. Wartenberg, H. Linde and R. Jung, 2. Anorg. Allg. Chem., 176 (1928) 349. A. Dietzel and H. Tober, Ber. Dtsch. Keram. Ges., 30 (1953) 47. H. Suzuki, S. Kimura, H. Yamoda and T. Yamauchi, J. Ceram. Assoc. Jpn., 69 (1961) 52. G. Cevales, Ber. Dtsch. Keram. Ges., 45 (1968) 216. &I. G, Cowgill and W. W. Smeltzer, J. Electrochem. Sot., 114 (1967) 1089. G. C. Wood and D. P. Whittle, J. Electrochem. Sot., I15 (1968) 126. D. P. Whittle and G. C. Wood, J. Electrochem. Sot., 115 (1968) 133. C. Wagner, J. Electrochem. Sot,, 99 (1952) 369. C. Wagner, J. Electrochem. Sot., 103 (1956) 627. C. Wagner, 2. Eiektrochem., 63 (1959) 773.