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Observations concerning the transitional oxidation behaviour of titanium when exposed to carbon dioxide at elevated temperatures
Journal oJ’ the ~e.~s-Com~n Metais, 37 (1974) 299-302 (:a Elsevier Sequoia %A.. Lausanne - Printed in The Netherlands
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Observations concerning the transitional oxidation behaviour of titanium when exposed to carbon dioxide at elevated temperatures I. A. MENZIES ~epartmento~ Muter~a~sTech~~olffg~,~oughboroughUfzi~ers~t~o~ Technolog~i,~~l~ghbor~ugh{Ct. ~rita~iz~ K. N. STRAFFORD Deparfment ef Materials Science, Newcastle upon Tyne Poiytechnic, Newcastle rcpn Tjlne j&t. &it&n/ (Received February 4. 1974)
Although the oxidation behaviour of titanium in oxygen and air has been extensively studied’, the behaviour of the metal in carbon dioxide has received less attention. In two recent papers 2*3 Menzies and Strafford have discussed in detail the oxidation characteristics of titanium in this gas at temperatures between 675 and lOWC, ascertained by an integrated experimental approach involving kinetic, metallographic, microhardness and electron-probe microanalyser studies. At a reaction tem~rature of lOOO”C,paralinear kinetics were observed; that is, the kinetic behaviour was characterised by an initial period during which a parabolic weight gain/time relationship was obeyed which was followed by a second period when linear kinetics prevailed. In some cases the change from parabolic to linear kinetics was abrupt, whereas with other samples a smooth transition in behaviour was observed. Extensive oxygen dissolution in the metal substrate was evident from microhardness measurements, the oxygen gradients being observed to extend more deeply into the metal with increasing time of exposure: there was also a steep rise in the microhardness values near to the metal/oxide interface. Interestingly, however, after a certain critical time these microhardness contours merged, indicating oxygen saturation, and, significantly, this saturation was observed to occur after a time interval at which the kinetics changed from parabolic to linear. These observations in many ways parallel those reported during studies of the oxidation of titanium in oxygen and air at a similar temperature, and a feature of common interest and importance in these two environments is the transition in overall kinetic behaviour4-7. The oxidation behaviour of titanium in oxygen is known to be strongly influenced by the large amount of oxygen which the metal may dissolve (up to 14.5 wt.%). In particular it has been suggested by many authors4.5*7-10 that the transition in kinetic behaviour is associated with saturation of the metal in the vicinity of the metal/scale interface, and the subsequent break up of the brittle substrate under the influence of growth stresses, brought about by metal/oxide, stress/strain interactions: the fragmented metal is then rapidly converted to a characteristic, porous, layered oxide giving rise to the linear (possibly “breakaway”) kinetics.
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This interpretation also broadly appears to be operative during the oxidation of the metal in carbon dioxide233 but it is the object of the present paper to highlight a number of unusual metallographic features observed in oxidised substrates/scales which, it is believed, are of particular significance to the mechanism of substrate degeneration and the onset of linear kinetics. Full experimental details have been given elsewhere29 3,1‘3 ’ 2’. Briefly, small rectangular coupons (1 x 0.5 x 0.1 cm) of a commercial grade titanium sheet were exposed to pure, dry carbon dioxide at 1000°C. In Fig. 1. are shown microstructural features which were characteristic of samples after oxidation for periods within which linear kinetics were observed, i.e., X4$ h. In this photomicrograph several elongated areas (“fingers”) are evident within the a (oxygen stabilised) region of the titanium substrate, and, in addition, corresponding (partial) “ghost” mirror images are also present in the adjacent oxide areas, within the striated oxide scale.
Fig. 1. Photomicrograph of section of titanium coupon after exposure to carbon dioxide for 8 h at lOOO”C, showing outer duplex oxide scale, cc-titanium (oxygen enriched) case, areas of internal oxidation (“fingers”), and inner transformed /? core (etched).
Examination of these “lingers” at high magnification (Fig. 2.) suggested that they were regions of high porosity where, apparently, some kind of localised internal oxidation had taken place. It was interesting to note the strong demarcation line between these porous areas and the neighbouring c( grains in the etched structure, indicating a strong electrochemical difference between these adjacent areas. In many cases the axes (or centre lines) of the porous areas were coincident with the grain
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oxidised area
Fig. 2. Microhardness region.
impressions
in areas
of oxygen-enriched
a-titanium
near
to internally
oxidised
boundaries of the cc-titanium, suggesting that the porous areas had developed and grown preferentially along grain boundaries. Some further interesting information regarding the nature of these internally oxidised areas was also obtained using electron-probe microanalysis and microhardness techniques. On scanning the oxidised areas for titanium a concentration of about 79 wt.% Ti was detected, corresponding to an oxygen concentration (by difference) of about 21%. It is, of course, possible that such titanium concentrations might well be slightly low on account of the porosity of such regions, but the values were reproducible from area to area. Such concentrations correspond to material of a composition Ti00,79. In spite of the approximate nature of these measurements, it was nevertheless clear that the “lingers” represented areas where the oxygen concentration was well above that of the oxygen solubility limit in atitanium (Ti0,,,)13. It is pertinent to recall that the oxide, TiO, has been reportedi to exist between the composition limits TiO,.,, to Ti0,,33. Thus, such areas represented regions which were in the early stages of being converted to rutile and seem to provide evidence for the nucleation and growth of “TiO”. Curiously, although the presence of “TiO” would be expected on thermodynamic grounds during the oxidation of titanium, in practice it has rarely been observed”. This is perhaps partly because (at least at high temperatures) the rutile TiO, scale is known to be very porous so that “short circuit” diffusion of (molecular) oxygen can take place, thus raising the oxygen activity in the vicinity of the metal/scale interface. The location of “TiO” within the metal substrate in the present work is, as expected, in a region of high titanium and low oxygen activities. The microhardness measurements confirmed that the metal immediately adjacent to the “fingers” was enriched in oxygen in relation to more distant areas. A variation in microhardness indentation size was apparent on making measurements approximately parallel to the metal/oxide interface at a depth of about 140 pm from the interface. The variation in indentation size is apparent from Fig. 2(a), where indentations near to the internally oxidised area are evidently smaller than those at distances away from this area. In particular (Fig. 2(b)),
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microhardnesses varying from about 850 to 1370 kg/mm2 were observed in the indicated areas. There was thus an oxygen gradient almost parallel to the metal/ oxide interface close to these oxidised areas, i.e., at right angles to those gradients already observed and discussed in earlier papers. This indicates that oxygen has diffused inwards from areas of the localised attack. These observations provide an interesting additional facet of information concerning the mechanism of oxidation of titanium in carbon dioxide. It was pointed out earlier that the significance of oxygen saturation of the metal at the metal/oxide interface in relation to the kinetic parabolic-+linear transition can be taken as established2q3. However, the present observations suggest, in particular, that the mechanical degradation of the oxygen-saturated substrate may occur via nucleation of “TiO” as an intermediate step, to be followed by the conversion of this “TiO” to Ti02. Since the conversion of titanium to oxide in this way involves a large increase in volume, it can be postulated that such regions would lead to the build up of localised stresses in a region already embrittled by virtue of its high oxygen content. Such areas would effectively act as “wedges” in the grain-boundary regions, leading to localised break up of the metal and allowing production of porous, layered Ti02 oxide, possibly in a manner similar to the model suggested by Stringer a number of years ago7 when discussing the oxidation behaviour of titanium in oxygen. A somewhat analogous behaviour has also been reported in studies concerning the oxidation of zirconium in oxygen, where “lingers” of oxide were observed locally to penetrate into the metal, leading to substrate degradation15. Here, however, ZrO, is formed directly, no stable monoxide being known in the Zr/O system.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
P. Kofstad, The High Temperature Oxidation of Metals, Chapter 6, Wiley, New York, 1966. I. A. Menzies and K. N. Strafford, Corrosion Sci., in press, April, 1974. I. A. Menzies and K. N. Strafford, Corrosion Sci., in press, April, 1974. P. Kofstad, K. Hauffe and H. Kjollesdal, Acta Chem. Stand., 12 (1958) 239. P. Kofstad, P. B. Anderson and 0. J. Krudtaa, J. Less-Common Met&, 3 (1961) 89. A. E. Jenkins, J. Inst. Metals, 84 (1955) 1. J. Stringer, Acta Met., 8 (1960) 758. A. E. Jenkins, J. Inst. Metals, 82 (1953) 213. T. Hurlen, J. Inst. Metals, 89 (1960) 128. G. R. Wallwork and A. E. Jenkins, J. Electrochem. SOL, 106 (1959) 10. I. A. Menzies and K. N. Strafford, J. Less-Common Metals, 12 (1967) 85. I. A. Menzies and K. N. Strafford, Corrosion Sci., 7 (1967) 23. E. S. Bumps, H. D. Kessler and M. Hansen, Trans. Amer. Sot. Metals, 45 (1953) 1008. P. Erlich, Z. Electrochem., 45 (1939) 363; Z. Anorg. Allg. Chem., 247 (1941) 53. G. R. Wallwork, C. J. Rosa and W. W. Smeltzer, Corrosion Sci., 5 (1965) 113.
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Observations concerning the transitional oxidation behaviour of titanium when exposed to carbon dioxide at elevated temperatures I. A. MENZIES ~epartmento~ Muter~a~sTech~~olffg~,~oughboroughUfzi~ers~t~o~ Technolog~i,~~l~ghbor~ugh{Ct. ~rita~iz~ K. N. STRAFFORD Deparfment ef Materials Science, Newcastle upon Tyne Poiytechnic, Newcastle rcpn Tjlne j&t. &it&n/ (Received February 4. 1974)
Although the oxidation behaviour of titanium in oxygen and air has been extensively studied’, the behaviour of the metal in carbon dioxide has received less attention. In two recent papers 2*3 Menzies and Strafford have discussed in detail the oxidation characteristics of titanium in this gas at temperatures between 675 and lOWC, ascertained by an integrated experimental approach involving kinetic, metallographic, microhardness and electron-probe microanalyser studies. At a reaction tem~rature of lOOO”C,paralinear kinetics were observed; that is, the kinetic behaviour was characterised by an initial period during which a parabolic weight gain/time relationship was obeyed which was followed by a second period when linear kinetics prevailed. In some cases the change from parabolic to linear kinetics was abrupt, whereas with other samples a smooth transition in behaviour was observed. Extensive oxygen dissolution in the metal substrate was evident from microhardness measurements, the oxygen gradients being observed to extend more deeply into the metal with increasing time of exposure: there was also a steep rise in the microhardness values near to the metal/oxide interface. Interestingly, however, after a certain critical time these microhardness contours merged, indicating oxygen saturation, and, significantly, this saturation was observed to occur after a time interval at which the kinetics changed from parabolic to linear. These observations in many ways parallel those reported during studies of the oxidation of titanium in oxygen and air at a similar temperature, and a feature of common interest and importance in these two environments is the transition in overall kinetic behaviour4-7. The oxidation behaviour of titanium in oxygen is known to be strongly influenced by the large amount of oxygen which the metal may dissolve (up to 14.5 wt.%). In particular it has been suggested by many authors4.5*7-10 that the transition in kinetic behaviour is associated with saturation of the metal in the vicinity of the metal/scale interface, and the subsequent break up of the brittle substrate under the influence of growth stresses, brought about by metal/oxide, stress/strain interactions: the fragmented metal is then rapidly converted to a characteristic, porous, layered oxide giving rise to the linear (possibly “breakaway”) kinetics.
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This interpretation also broadly appears to be operative during the oxidation of the metal in carbon dioxide233 but it is the object of the present paper to highlight a number of unusual metallographic features observed in oxidised substrates/scales which, it is believed, are of particular significance to the mechanism of substrate degeneration and the onset of linear kinetics. Full experimental details have been given elsewhere29 3,1‘3 ’ 2’. Briefly, small rectangular coupons (1 x 0.5 x 0.1 cm) of a commercial grade titanium sheet were exposed to pure, dry carbon dioxide at 1000°C. In Fig. 1. are shown microstructural features which were characteristic of samples after oxidation for periods within which linear kinetics were observed, i.e., X4$ h. In this photomicrograph several elongated areas (“fingers”) are evident within the a (oxygen stabilised) region of the titanium substrate, and, in addition, corresponding (partial) “ghost” mirror images are also present in the adjacent oxide areas, within the striated oxide scale.
Fig. 1. Photomicrograph of section of titanium coupon after exposure to carbon dioxide for 8 h at lOOO”C, showing outer duplex oxide scale, cc-titanium (oxygen enriched) case, areas of internal oxidation (“fingers”), and inner transformed /? core (etched).
Examination of these “lingers” at high magnification (Fig. 2.) suggested that they were regions of high porosity where, apparently, some kind of localised internal oxidation had taken place. It was interesting to note the strong demarcation line between these porous areas and the neighbouring c( grains in the etched structure, indicating a strong electrochemical difference between these adjacent areas. In many cases the axes (or centre lines) of the porous areas were coincident with the grain
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oxidised area
Fig. 2. Microhardness region.
impressions
in areas
of oxygen-enriched
a-titanium
near
to internally
oxidised
boundaries of the cc-titanium, suggesting that the porous areas had developed and grown preferentially along grain boundaries. Some further interesting information regarding the nature of these internally oxidised areas was also obtained using electron-probe microanalysis and microhardness techniques. On scanning the oxidised areas for titanium a concentration of about 79 wt.% Ti was detected, corresponding to an oxygen concentration (by difference) of about 21%. It is, of course, possible that such titanium concentrations might well be slightly low on account of the porosity of such regions, but the values were reproducible from area to area. Such concentrations correspond to material of a composition Ti00,79. In spite of the approximate nature of these measurements, it was nevertheless clear that the “lingers” represented areas where the oxygen concentration was well above that of the oxygen solubility limit in atitanium (Ti0,,,)13. It is pertinent to recall that the oxide, TiO, has been reportedi to exist between the composition limits TiO,.,, to Ti0,,33. Thus, such areas represented regions which were in the early stages of being converted to rutile and seem to provide evidence for the nucleation and growth of “TiO”. Curiously, although the presence of “TiO” would be expected on thermodynamic grounds during the oxidation of titanium, in practice it has rarely been observed”. This is perhaps partly because (at least at high temperatures) the rutile TiO, scale is known to be very porous so that “short circuit” diffusion of (molecular) oxygen can take place, thus raising the oxygen activity in the vicinity of the metal/scale interface. The location of “TiO” within the metal substrate in the present work is, as expected, in a region of high titanium and low oxygen activities. The microhardness measurements confirmed that the metal immediately adjacent to the “fingers” was enriched in oxygen in relation to more distant areas. A variation in microhardness indentation size was apparent on making measurements approximately parallel to the metal/oxide interface at a depth of about 140 pm from the interface. The variation in indentation size is apparent from Fig. 2(a), where indentations near to the internally oxidised area are evidently smaller than those at distances away from this area. In particular (Fig. 2(b)),
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microhardnesses varying from about 850 to 1370 kg/mm2 were observed in the indicated areas. There was thus an oxygen gradient almost parallel to the metal/ oxide interface close to these oxidised areas, i.e., at right angles to those gradients already observed and discussed in earlier papers. This indicates that oxygen has diffused inwards from areas of the localised attack. These observations provide an interesting additional facet of information concerning the mechanism of oxidation of titanium in carbon dioxide. It was pointed out earlier that the significance of oxygen saturation of the metal at the metal/oxide interface in relation to the kinetic parabolic-+linear transition can be taken as established2q3. However, the present observations suggest, in particular, that the mechanical degradation of the oxygen-saturated substrate may occur via nucleation of “TiO” as an intermediate step, to be followed by the conversion of this “TiO” to Ti02. Since the conversion of titanium to oxide in this way involves a large increase in volume, it can be postulated that such regions would lead to the build up of localised stresses in a region already embrittled by virtue of its high oxygen content. Such areas would effectively act as “wedges” in the grain-boundary regions, leading to localised break up of the metal and allowing production of porous, layered Ti02 oxide, possibly in a manner similar to the model suggested by Stringer a number of years ago7 when discussing the oxidation behaviour of titanium in oxygen. A somewhat analogous behaviour has also been reported in studies concerning the oxidation of zirconium in oxygen, where “lingers” of oxide were observed locally to penetrate into the metal, leading to substrate degradation15. Here, however, ZrO, is formed directly, no stable monoxide being known in the Zr/O system.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
P. Kofstad, The High Temperature Oxidation of Metals, Chapter 6, Wiley, New York, 1966. I. A. Menzies and K. N. Strafford, Corrosion Sci., in press, April, 1974. I. A. Menzies and K. N. Strafford, Corrosion Sci., in press, April, 1974. P. Kofstad, K. Hauffe and H. Kjollesdal, Acta Chem. Stand., 12 (1958) 239. P. Kofstad, P. B. Anderson and 0. J. Krudtaa, J. Less-Common Met&, 3 (1961) 89. A. E. Jenkins, J. Inst. Metals, 84 (1955) 1. J. Stringer, Acta Met., 8 (1960) 758. A. E. Jenkins, J. Inst. Metals, 82 (1953) 213. T. Hurlen, J. Inst. Metals, 89 (1960) 128. G. R. Wallwork and A. E. Jenkins, J. Electrochem. SOL, 106 (1959) 10. I. A. Menzies and K. N. Strafford, J. Less-Common Metals, 12 (1967) 85. I. A. Menzies and K. N. Strafford, Corrosion Sci., 7 (1967) 23. E. S. Bumps, H. D. Kessler and M. Hansen, Trans. Amer. Sot. Metals, 45 (1953) 1008. P. Erlich, Z. Electrochem., 45 (1939) 363; Z. Anorg. Allg. Chem., 247 (1941) 53. G. R. Wallwork, C. J. Rosa and W. W. Smeltzer, Corrosion Sci., 5 (1965) 113.