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Oxidation performance of a Fe–13Cr alloy with additions of rare earth elements
Materials Science and Engineering A363 (2003) 15–19
Oxidation performance of a Fe–13Cr alloy with additions of rare earth elements A. Martinez-Villafañe a , J.G. Chacon-Nava a , C. Gaona-Tiburcio a , F. Almeraya-Calderon a , G. Dom´ınguez-Patiño b , J.G. Gonzalez-Rodr´ıguez b,∗ a
Centro de Investigación en Materiales Avanzados S.C. (CIMAV), Miguel de Cervantes No 120, Complejo Industrial Chihuahua, 31109, Chihuahua, Chih, Mexico b UAEM, Facultad de Ciencias Qu´ımicas e Ingenier´ıa, Av. Universidad s/n. Cuernavaca, Mor 62210, Mexico Received 28 May 2002; received in revised form 16 April 2003
Abstract The influence of rare earth elements (REE’s) i.e. Neodymium (Nd) and Praseodymium (Pr) on the oxidation behavior of a Fe–13Cr alloy has been studied, and its role on the oxidation rate and oxide morphology and formation is discussed. Specimens were isothermally oxidized in oxygen at 800 ◦ C for 24 h. It was found that a small addition (≤0.03 wt.%) of either Nd or Pr, reduced the oxidation rate of the Fe–13Cr base alloy. Moreover, the simultaneous addition of both elements to the alloy produced a dramatic reduction in the oxidation kinetics. Analysis by scanning electronic microscope (SEM) revealed that the morphology of oxides formed on Fe–13Cr specimens with and without REE’s specimens was very different. In fact, a fine-grained oxide morphology was observed for alloys with REE’s addition. For these alloys only, chromium enrichment at the metal/scale interface was observed. From transmission electronic microscope (TEM) analysis, it was found the following: at the early stages of oxide formation, after 0.25 h, Cr2 O3 , Fe3 O4 , ␣-Fe2 O3 and ␥-Fe2 O3 were formed; at 6 h, Cr2 O3 , FeCr2 O4 and ␣-Fe2 O3 were identified and, for exposure times greater than 6 h, Cr2 O3 , ␣-Fe2 O3 and a spinel which was presumably transformed into a solid solution (Fe2 O3 ·Cr2 O3 ) were found. © 2003 Elsevier B.V. All rights reserved. Keywords: High temperature oxidation; Iron–chromium alloys; Rare earth elements
1. Introduction The oxidation of metals or alloys at high temperatures can be treated as a special case of metallic corrosion, which consider the material destruction due to chemical causes, in which solid phases interact either with a liquid agent or a gaseous agent. The latter case involves reducing and oxidizing gases, steam and even free oxygen. The oxidation is determined by the internal mobility of the solid phases and mainly diffusion in solid state. It is well known that alloying with chromium improves the oxidation resistance of iron at high temperatures. The magnitude of this improvement depends primarily upon the chromium concentration. Above 570 ◦ C iron form three oxides: Fe1−x O, Fe3 O4 and Fe2 O3 . Of these oxides, Fe1−x O constitutes more than 90% of the oxide scale. Alloying with chromium suppresses the ∗ Corresponding author. Tel.: +52-777-39-7084; fax: +52-777-329-7084. E-mail address: [email protected] (J.G. Gonzalez-Rodr´ıguez).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00346-0
formation of wustite. Seybolt [1] showed that Cr2 O3 is the stable oxide when the chromium concentration at the alloy/oxide interface exceeds 13 wt.%. At lower chromium contents, the oxidation behavior of Fe–Cr alloys is very complex [2] i.e. scales consisting of Fe–Cr spinel of the type Fe(Fe2−x ,Crx )O4 and ␣-Fe2 O3 hematite. Wood [3] studied the oxidation of binary and ternary alloys at elevated temperatures, pointing out some factors that determine when an alloy has either a superficial oxide, internal oxidation, or both phenomena. He also indicated that many systems require time and a considerable thickness of the scale before the steady-state pattern could be fully reached. However, this behavior depends upon the initial exposure to the environment, and after loss of the scale (oxide spalling) induced isothermally or during thermal cycles. Further, he discussed the effect of third elements on scaling behavior. The benefits of rare earth elements (REE’s) addition are well-established [4–7]. It has been observed that small additions (0.01–0.5%) of elements such as yttrium, scandium
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lanthanum, cerium, or their oxides produced a remarkable improvement in oxidation resistance [8–11]. The effect of REE’s on the mechanical resistance of carbon steels has also been reported [12]. Chevalier et al. [13] and Bonnet et al. [14] studied the effect of Nd2 O3 coatings on the oxidation behavior of stainless steels. In other work, Fernandes and Ramanathan [15] reported the effect of Pr2 O3 coatings on the oxidation behavior of a Fe–20Cr alloy. Nevertheless, information on the effect of rare earth elements such as Nd and Pr in metallic form could not be found. Thus, the aim of the present work was to investigate the effect of small additions of Nd, Pr, and Nd + Pr, on the oxidation kinetics of a Fe–13Cr alloy.
2. Experimental procedure All the alloys, i.e. Fe–13Cr, Fe–13Cr–0.03Nd, Fe–13Cr– 0.03Pr and Fe–13Cr–0.01Nd–0.01Pr were prepared from high purity elements in an electrical arc furnace using a purified argon atmosphere. The alloys were cold rolled into a sheet, and rectangular strips of approximately 1.2 × 0.6 × 0.1 cm size were cut-off. They were cleaned and annealed at 1100 ◦ C for 3 h in an argon atmosphere. A hole of 0.158 cm in diameter was drilled at one end of each specimen. Afterwards, the surfaces were ground to 1200 grit paper, rinsed with distilled water and degreased with acetone. Each specimen was set on a platinum wire for weight-gain measurements in an electronic microbalance (sensitivity 10−6 g). Experiments were conducted in an atmosphere of dry oxygen at a partial pressure of 1 atm, a fixed temperature of 800 ◦ C (±2 ◦ C) and an exposure time of 24 h. The oxidized samples were analyzed in a scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) system in order to understand the oxidation phenomena in terms of scale morphology and oxidation products distribution. Further, the identification and structure of different oxides was made in a JEOL 200CX transmission electron microscope (TEM) with an accelerating voltage of 200 kV and a resolution of 2 Å, enough to identify the phases. Phases were identified either directly from oxide which came off the alloy or from crushed oxides.
3. Results The kinetic measurements indicated that, in general, the alloys presented a parabolic behavior, according to: (m/A) = Kp(t − to ), 2
Fig. 1. Kinetic data for the isothermal oxidation of Fe–13Cr and Fe–Cr–REE’s alloys versus time during oxidation at 800 ◦ C for 24 h.
that the Fe–13Cr base alloy has the greater oxidation kinetics, which decreases with the addition of REE’s in the following order: Fe–13Cr–0.03Nd, Fe–13Cr–0.03Pr and Fe–13Cr–0.01Nd–0.01Pr. Particularly for the latter alloy, a dramatic reduction in oxidation kinetics was noted. Rate constants values derived from Fig. 1, are given in Table 1. These values are in reasonable agreement from values reported in literature for Fe–Cr alloys with additions of yttrium and rare earth at 800 ◦ C [16,17]. kp1 is the parabolic rate constant at the beginning of the experiment, and kp2 is the value in a later stage, due to a change in the kinetics of the oxide formation. The results obtained by SEM on the oxidized specimens revealed different morphologies depending upon the alloy composition. For example, Fig. 2a shows a plain view of the oxide formed on the Fe–13Cr alloy. At this stage, the oxide is polycrystalline presenting a cone-shape morphology composed of ␣-Fe2 O3 , which became the major surface constituent in the latest period of oxidation. Fig. 2b and c shows the morphology observed for the Fe–13Cr alloy with addition of REE’s. In these cases, the cone-shape morphology was no longer observed, but a fine-grained oxide surface was present.
Table 1 Parabolic rate constant values, KP (g2 cm−4 s−1 ) for the oxidized alloys at 800 ◦ C Fe–13Cr
kp1 = 3.2 × 10−9 kp2 = 1.6 × 10−9
Fe–13Cr–0.03Pr
kp1 = 1.3 × 10−9 kp2 = 4.1 × 10−10
Fe–13Cr–0.03Nd
kp1 = 2.2 × 10−9 kp2 = 7.2 × 10−10
Fe–13Cr–0.01Pr–0.01Nd
kp1 = 3.7 × 10−12 kp2 = 8.5 × 10−12
(1)
where (m/A) is the weight-gain per unit area, t and to are the final and initial time respectively, and Kp is the parabolic rate constant. Fig. 1 shows the weight change per unit area against time recorded for the various alloys exposed in pure oxygen at 800 ◦ C for 24 h. From this figure, it can be seen
A. Martinez-Villafañe et al. / Materials Science and Engineering A363 (2003) 15–19
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Fig. 2. Typical morphology for: (a) Fe–13Cr; (b) Fe–13Cr–0.03Nd; (c) Fe–13Cr–0.01Nd–0.01Pr, oxidized at 800 ◦ C for 24 h.
Cross section of all specimens were further studied by SEM with electron probe microanalysis to determine the distribution of elements in the scales. Thus, Fig. 3a shows the scale formed on the Fe–13Cr–0.01Nd alloy, and their corresponding X-ray maps for Fe and Cr in Fig. 3b and c. On the other hand, Fig. 4a shows the scale formed on the Fe–13Cr–0.01Nd–0.01Pr, and their corresponding X-ray maps for Fe and Cr are shown in Fig. 4b and c. Furthermore, TEM analysis also revealed different oxide structures and compositions as a function of time. As an example, for the Fe–13Cr–0.01Nd–0.01Pr alloy, Fig. 5a shows an SEM micrograph of the oxide formed at 6 h of oxidation, whereas Fig. 5b shows its X-ray diffraction pattern. Here, Cr2 O3 , FeCr2 O4 and ␥-Fe2 O3 were identified. In similar way, after 0.25 h, Cr2 O3 , Fe3 O4 , ␣-Fe2 O3 and ␥-Fe2 O3 were identified and, for exposure times greater than 6 h, Cr2 O3 , ␣-Fe2 O3 and a spinel, probably transformed into a solid solution (Fe2 O3 ·Cr2 O3 ) were found.
4. Discussion The kinetic results are in agreement with those reported in the literature in the sense that the kinetic behavior is
parabolic [16,17], and only Nakamura [16] reported changes on the parabolic rate constant, as reported in the present work. SEM analysis in plain view for the oxidized surfaces showed interesting differences in morphology. For the base alloy, a cone-shape oxide morphology was observed, whereas, for the alloys with REE’s addition, a fined-grained oxide surface morphology was noted. These differences in morphology could be explained by selective oxidation of the protective oxide forming element, where chromium is oxidized during the initial exposure period. This reduces the amount of oxidation of the base metal prior to the establishment of the protective-oxide, and enables a continuous layer to develop at a lower chromium concentration in the alloy. The growth rate of Cr2 O3 scales at high temperatures is greatly reduced by the presence of rare earth elements. Stringer [18] suggested that the reactive elements of dispersed-oxide particles in the alloy surface act as nucleation sites for the first-formed oxides, thereby decreasing the inter-nuclei spacing. Since the distance between adjacent Cr2 O3 nuclei is decreased, the time required for such nuclei to grow laterally and link up to form a complete layer is reduced. The overall effect is enhanced selective oxidation of chromium and a finer grain size for the resulting Cr2 O3 layer.
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A. Martinez-Villafañe et al. / Materials Science and Engineering A363 (2003) 15–19
Fig. 3. Secondary electron image of the cross section of the: (a) Fe–13Cr–0.01Nd alloy oxidized at 800 ◦ C for 24 h; (b) X-ray map for Fe; (c) X-ray map for Cr.
Fig. 4. Secondary electron image of the cross section of the: (a) Fe–13Cr–0.01Nd–0.01Pr alloy oxidized at 800 ◦ C for 24 h; (b) X-ray map for Fe; (c) X-ray map for Cr.
A. Martinez-Villafañe et al. / Materials Science and Engineering A363 (2003) 15–19
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Fig. 5. (a) Dark field technique of an SEM micrograph of the oxide formed at 6 h of oxidation; (b) X-ray diffraction pattern taken from the oxide scale formed on the alloy Fe–13Cr–0.01Nd–0.01Pr oxidized at 800 ◦ C for 6 h.
Cross section analysis by SEM/EDX on the scale formed on all alloys suggested the formation of iron and chromium rich oxides. The oxide scale on the Fe–13Cr–0.03Nd alloy was somewhat thicker than that observed on the Fe–13Cr–0.01Nd–0.01Pr alloy, confirming the oxidation kinetics results. For the latter alloy, oxidized chromium was observed near the metal surface as internal oxides. However, in both alloys, chromium enrichment at the scale/metal interface was clear, (see Figs. 3 and 4). Indeed, this was not the case for the Fe–13Cr base alloy. These observations are in agreement with the results obtained by TEM. For the base alloy, analysis of the scale showed the presence of Cr2 O3 , Fe3 O4 and ␣-Fe2 O3. For the REE’s containing alloys, it is thought that the better oxidation performance was afforded by the presence of Cr2 O3 , a solid solution (Fe2 O3 ·Cr2 O3 ), FeCr2 O4 and Fe2 O3 . It is worth noticing that the amount of chromium at the metal/scale interface decreases rapidly towards the gas/scale interface. Overall, the rare earth effect can be observed on the REE’s containing alloys by considering the formation of a rich chromium layer at the metal/scale interface. This is agreement with the work of Nakamura [16] and Felten [19] whose findings showed that the rare earth acts as nucleation sites for the formation of a protective Cr2 O3 scale.
5. Conclusions Based on the previous results, the following conclusions can be drawn: The addition of 0.03% wt of either Nd or Pr reduced the oxidation rate of the Fe–13Cr base alloy. This beneficial
effect became more evident with the addition of 0.01% wt Nd or 0.01 Pr. The morphology of the oxides formed on specimens with and without REE’s was varied significantly. In the first case, a fine-grained oxide morphology was observed, whereas, for the latter, a coned-shaped surface oxide was noted. Chromium enrichment at the metal/scale interface was observed only for the alloys with REE’s addition.
References [1] A.J. Seybold, J. Eletrochem. Soc. 107 (1960) 147. [2] P.A. Labun, J. Covington, K. Kuroda, G. Welsh, T.E. Mitchel, Met. Trans. A13 (1982) 2103. [3] G.C. Wood, Oxid. Met. 2 (1970) 11. [4] H. Hindam, D.P. Whittle, Oxid. Met. 18 (1982) 245. [5] D.P. Whittle, J. Stringer, Phil. Trans. Roy. Soc. London A295 (1980) 309. [6] S.K. Mitra, S.K. Roy, S.K. Bose, Oxid. Met. 34 (1990) 101. [7] M.J. Bennett, D.P. Moon, Mat. Sci. Forum 51 (1999) 135. [8] D.T. Hoelzer, B.A. Pint, I.G. Wright, J. Nucl. Mat. 283 (2000) 1306. [9] N. Hiramatsu, F.H. Stott, Oxid. Met. 51 (1999) 479. [10] H.J. Grabke, Intergranular and Interphase Mat. Sci. Forum 294 (1999) 135. [11] J. Deakin, V. Prunier, G.C. Wood, F.H. Stott, Mat. Sci. Forum 251 (1997) 41. [12] B.T. Kilbourn, J. Metals, 22–27, May, 1988. [13] S. Chevalier, P.G. Bonnet, K. Przybylski, J.C. Colson, J.P. Larpin, Oxid. Met. 54 (2000) 527. [14] G. Bonnet, J.P. Larpin, J.C. Colson, Solid State Ionics 51 (1992) 11. [15] S.M.C. Fernandes, L.V. Ramanathan, Surf. Eng. 16 (2000) 327. [16] Y. Nakamura, Metallurgical Trans. 5 (1974) 909. [17] F.A. Golightly, G.C. Wood, F.H. Stott, Oxidation Metals 14 (3) (1980) 217. [18] J. Stringer, B.A. Wilcox, R.I. Jaffe, Oxidation Metals 5 (1972) 11. [19] E.J. Felten, J. Electrochem. Soc. 108 (6) (1961) 490.
Oxidation performance of a Fe–13Cr alloy with additions of rare earth elements A. Martinez-Villafañe a , J.G. Chacon-Nava a , C. Gaona-Tiburcio a , F. Almeraya-Calderon a , G. Dom´ınguez-Patiño b , J.G. Gonzalez-Rodr´ıguez b,∗ a
Centro de Investigación en Materiales Avanzados S.C. (CIMAV), Miguel de Cervantes No 120, Complejo Industrial Chihuahua, 31109, Chihuahua, Chih, Mexico b UAEM, Facultad de Ciencias Qu´ımicas e Ingenier´ıa, Av. Universidad s/n. Cuernavaca, Mor 62210, Mexico Received 28 May 2002; received in revised form 16 April 2003
Abstract The influence of rare earth elements (REE’s) i.e. Neodymium (Nd) and Praseodymium (Pr) on the oxidation behavior of a Fe–13Cr alloy has been studied, and its role on the oxidation rate and oxide morphology and formation is discussed. Specimens were isothermally oxidized in oxygen at 800 ◦ C for 24 h. It was found that a small addition (≤0.03 wt.%) of either Nd or Pr, reduced the oxidation rate of the Fe–13Cr base alloy. Moreover, the simultaneous addition of both elements to the alloy produced a dramatic reduction in the oxidation kinetics. Analysis by scanning electronic microscope (SEM) revealed that the morphology of oxides formed on Fe–13Cr specimens with and without REE’s specimens was very different. In fact, a fine-grained oxide morphology was observed for alloys with REE’s addition. For these alloys only, chromium enrichment at the metal/scale interface was observed. From transmission electronic microscope (TEM) analysis, it was found the following: at the early stages of oxide formation, after 0.25 h, Cr2 O3 , Fe3 O4 , ␣-Fe2 O3 and ␥-Fe2 O3 were formed; at 6 h, Cr2 O3 , FeCr2 O4 and ␣-Fe2 O3 were identified and, for exposure times greater than 6 h, Cr2 O3 , ␣-Fe2 O3 and a spinel which was presumably transformed into a solid solution (Fe2 O3 ·Cr2 O3 ) were found. © 2003 Elsevier B.V. All rights reserved. Keywords: High temperature oxidation; Iron–chromium alloys; Rare earth elements
1. Introduction The oxidation of metals or alloys at high temperatures can be treated as a special case of metallic corrosion, which consider the material destruction due to chemical causes, in which solid phases interact either with a liquid agent or a gaseous agent. The latter case involves reducing and oxidizing gases, steam and even free oxygen. The oxidation is determined by the internal mobility of the solid phases and mainly diffusion in solid state. It is well known that alloying with chromium improves the oxidation resistance of iron at high temperatures. The magnitude of this improvement depends primarily upon the chromium concentration. Above 570 ◦ C iron form three oxides: Fe1−x O, Fe3 O4 and Fe2 O3 . Of these oxides, Fe1−x O constitutes more than 90% of the oxide scale. Alloying with chromium suppresses the ∗ Corresponding author. Tel.: +52-777-39-7084; fax: +52-777-329-7084. E-mail address: [email protected] (J.G. Gonzalez-Rodr´ıguez).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00346-0
formation of wustite. Seybolt [1] showed that Cr2 O3 is the stable oxide when the chromium concentration at the alloy/oxide interface exceeds 13 wt.%. At lower chromium contents, the oxidation behavior of Fe–Cr alloys is very complex [2] i.e. scales consisting of Fe–Cr spinel of the type Fe(Fe2−x ,Crx )O4 and ␣-Fe2 O3 hematite. Wood [3] studied the oxidation of binary and ternary alloys at elevated temperatures, pointing out some factors that determine when an alloy has either a superficial oxide, internal oxidation, or both phenomena. He also indicated that many systems require time and a considerable thickness of the scale before the steady-state pattern could be fully reached. However, this behavior depends upon the initial exposure to the environment, and after loss of the scale (oxide spalling) induced isothermally or during thermal cycles. Further, he discussed the effect of third elements on scaling behavior. The benefits of rare earth elements (REE’s) addition are well-established [4–7]. It has been observed that small additions (0.01–0.5%) of elements such as yttrium, scandium
16
A. Martinez-Villafañe et al. / Materials Science and Engineering A363 (2003) 15–19
lanthanum, cerium, or their oxides produced a remarkable improvement in oxidation resistance [8–11]. The effect of REE’s on the mechanical resistance of carbon steels has also been reported [12]. Chevalier et al. [13] and Bonnet et al. [14] studied the effect of Nd2 O3 coatings on the oxidation behavior of stainless steels. In other work, Fernandes and Ramanathan [15] reported the effect of Pr2 O3 coatings on the oxidation behavior of a Fe–20Cr alloy. Nevertheless, information on the effect of rare earth elements such as Nd and Pr in metallic form could not be found. Thus, the aim of the present work was to investigate the effect of small additions of Nd, Pr, and Nd + Pr, on the oxidation kinetics of a Fe–13Cr alloy.
2. Experimental procedure All the alloys, i.e. Fe–13Cr, Fe–13Cr–0.03Nd, Fe–13Cr– 0.03Pr and Fe–13Cr–0.01Nd–0.01Pr were prepared from high purity elements in an electrical arc furnace using a purified argon atmosphere. The alloys were cold rolled into a sheet, and rectangular strips of approximately 1.2 × 0.6 × 0.1 cm size were cut-off. They were cleaned and annealed at 1100 ◦ C for 3 h in an argon atmosphere. A hole of 0.158 cm in diameter was drilled at one end of each specimen. Afterwards, the surfaces were ground to 1200 grit paper, rinsed with distilled water and degreased with acetone. Each specimen was set on a platinum wire for weight-gain measurements in an electronic microbalance (sensitivity 10−6 g). Experiments were conducted in an atmosphere of dry oxygen at a partial pressure of 1 atm, a fixed temperature of 800 ◦ C (±2 ◦ C) and an exposure time of 24 h. The oxidized samples were analyzed in a scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) system in order to understand the oxidation phenomena in terms of scale morphology and oxidation products distribution. Further, the identification and structure of different oxides was made in a JEOL 200CX transmission electron microscope (TEM) with an accelerating voltage of 200 kV and a resolution of 2 Å, enough to identify the phases. Phases were identified either directly from oxide which came off the alloy or from crushed oxides.
3. Results The kinetic measurements indicated that, in general, the alloys presented a parabolic behavior, according to: (m/A) = Kp(t − to ), 2
Fig. 1. Kinetic data for the isothermal oxidation of Fe–13Cr and Fe–Cr–REE’s alloys versus time during oxidation at 800 ◦ C for 24 h.
that the Fe–13Cr base alloy has the greater oxidation kinetics, which decreases with the addition of REE’s in the following order: Fe–13Cr–0.03Nd, Fe–13Cr–0.03Pr and Fe–13Cr–0.01Nd–0.01Pr. Particularly for the latter alloy, a dramatic reduction in oxidation kinetics was noted. Rate constants values derived from Fig. 1, are given in Table 1. These values are in reasonable agreement from values reported in literature for Fe–Cr alloys with additions of yttrium and rare earth at 800 ◦ C [16,17]. kp1 is the parabolic rate constant at the beginning of the experiment, and kp2 is the value in a later stage, due to a change in the kinetics of the oxide formation. The results obtained by SEM on the oxidized specimens revealed different morphologies depending upon the alloy composition. For example, Fig. 2a shows a plain view of the oxide formed on the Fe–13Cr alloy. At this stage, the oxide is polycrystalline presenting a cone-shape morphology composed of ␣-Fe2 O3 , which became the major surface constituent in the latest period of oxidation. Fig. 2b and c shows the morphology observed for the Fe–13Cr alloy with addition of REE’s. In these cases, the cone-shape morphology was no longer observed, but a fine-grained oxide surface was present.
Table 1 Parabolic rate constant values, KP (g2 cm−4 s−1 ) for the oxidized alloys at 800 ◦ C Fe–13Cr
kp1 = 3.2 × 10−9 kp2 = 1.6 × 10−9
Fe–13Cr–0.03Pr
kp1 = 1.3 × 10−9 kp2 = 4.1 × 10−10
Fe–13Cr–0.03Nd
kp1 = 2.2 × 10−9 kp2 = 7.2 × 10−10
Fe–13Cr–0.01Pr–0.01Nd
kp1 = 3.7 × 10−12 kp2 = 8.5 × 10−12
(1)
where (m/A) is the weight-gain per unit area, t and to are the final and initial time respectively, and Kp is the parabolic rate constant. Fig. 1 shows the weight change per unit area against time recorded for the various alloys exposed in pure oxygen at 800 ◦ C for 24 h. From this figure, it can be seen
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17
Fig. 2. Typical morphology for: (a) Fe–13Cr; (b) Fe–13Cr–0.03Nd; (c) Fe–13Cr–0.01Nd–0.01Pr, oxidized at 800 ◦ C for 24 h.
Cross section of all specimens were further studied by SEM with electron probe microanalysis to determine the distribution of elements in the scales. Thus, Fig. 3a shows the scale formed on the Fe–13Cr–0.01Nd alloy, and their corresponding X-ray maps for Fe and Cr in Fig. 3b and c. On the other hand, Fig. 4a shows the scale formed on the Fe–13Cr–0.01Nd–0.01Pr, and their corresponding X-ray maps for Fe and Cr are shown in Fig. 4b and c. Furthermore, TEM analysis also revealed different oxide structures and compositions as a function of time. As an example, for the Fe–13Cr–0.01Nd–0.01Pr alloy, Fig. 5a shows an SEM micrograph of the oxide formed at 6 h of oxidation, whereas Fig. 5b shows its X-ray diffraction pattern. Here, Cr2 O3 , FeCr2 O4 and ␥-Fe2 O3 were identified. In similar way, after 0.25 h, Cr2 O3 , Fe3 O4 , ␣-Fe2 O3 and ␥-Fe2 O3 were identified and, for exposure times greater than 6 h, Cr2 O3 , ␣-Fe2 O3 and a spinel, probably transformed into a solid solution (Fe2 O3 ·Cr2 O3 ) were found.
4. Discussion The kinetic results are in agreement with those reported in the literature in the sense that the kinetic behavior is
parabolic [16,17], and only Nakamura [16] reported changes on the parabolic rate constant, as reported in the present work. SEM analysis in plain view for the oxidized surfaces showed interesting differences in morphology. For the base alloy, a cone-shape oxide morphology was observed, whereas, for the alloys with REE’s addition, a fined-grained oxide surface morphology was noted. These differences in morphology could be explained by selective oxidation of the protective oxide forming element, where chromium is oxidized during the initial exposure period. This reduces the amount of oxidation of the base metal prior to the establishment of the protective-oxide, and enables a continuous layer to develop at a lower chromium concentration in the alloy. The growth rate of Cr2 O3 scales at high temperatures is greatly reduced by the presence of rare earth elements. Stringer [18] suggested that the reactive elements of dispersed-oxide particles in the alloy surface act as nucleation sites for the first-formed oxides, thereby decreasing the inter-nuclei spacing. Since the distance between adjacent Cr2 O3 nuclei is decreased, the time required for such nuclei to grow laterally and link up to form a complete layer is reduced. The overall effect is enhanced selective oxidation of chromium and a finer grain size for the resulting Cr2 O3 layer.
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Fig. 3. Secondary electron image of the cross section of the: (a) Fe–13Cr–0.01Nd alloy oxidized at 800 ◦ C for 24 h; (b) X-ray map for Fe; (c) X-ray map for Cr.
Fig. 4. Secondary electron image of the cross section of the: (a) Fe–13Cr–0.01Nd–0.01Pr alloy oxidized at 800 ◦ C for 24 h; (b) X-ray map for Fe; (c) X-ray map for Cr.
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Fig. 5. (a) Dark field technique of an SEM micrograph of the oxide formed at 6 h of oxidation; (b) X-ray diffraction pattern taken from the oxide scale formed on the alloy Fe–13Cr–0.01Nd–0.01Pr oxidized at 800 ◦ C for 6 h.
Cross section analysis by SEM/EDX on the scale formed on all alloys suggested the formation of iron and chromium rich oxides. The oxide scale on the Fe–13Cr–0.03Nd alloy was somewhat thicker than that observed on the Fe–13Cr–0.01Nd–0.01Pr alloy, confirming the oxidation kinetics results. For the latter alloy, oxidized chromium was observed near the metal surface as internal oxides. However, in both alloys, chromium enrichment at the scale/metal interface was clear, (see Figs. 3 and 4). Indeed, this was not the case for the Fe–13Cr base alloy. These observations are in agreement with the results obtained by TEM. For the base alloy, analysis of the scale showed the presence of Cr2 O3 , Fe3 O4 and ␣-Fe2 O3. For the REE’s containing alloys, it is thought that the better oxidation performance was afforded by the presence of Cr2 O3 , a solid solution (Fe2 O3 ·Cr2 O3 ), FeCr2 O4 and Fe2 O3 . It is worth noticing that the amount of chromium at the metal/scale interface decreases rapidly towards the gas/scale interface. Overall, the rare earth effect can be observed on the REE’s containing alloys by considering the formation of a rich chromium layer at the metal/scale interface. This is agreement with the work of Nakamura [16] and Felten [19] whose findings showed that the rare earth acts as nucleation sites for the formation of a protective Cr2 O3 scale.
5. Conclusions Based on the previous results, the following conclusions can be drawn: The addition of 0.03% wt of either Nd or Pr reduced the oxidation rate of the Fe–13Cr base alloy. This beneficial
effect became more evident with the addition of 0.01% wt Nd or 0.01 Pr. The morphology of the oxides formed on specimens with and without REE’s was varied significantly. In the first case, a fine-grained oxide morphology was observed, whereas, for the latter, a coned-shaped surface oxide was noted. Chromium enrichment at the metal/scale interface was observed only for the alloys with REE’s addition.
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