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Microstructural evaluation of oxide layers formed on Fe–22Cr–6Al metallic foam by pre-oxidization
Applied Surface Science 293 (2014) 255–258
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructural evaluation of oxide layers formed on Fe–22Cr–6Al metallic foam by pre-oxidization Jae Young Lee a , Hyung Giun Kim a , Mi Ri Choi a , Chang Woo Lee b , Man Ho Park b , Ki Hyun Kim c , Sung Hwan Lim a,∗ a
Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon 200-701, Republic of Korea Alantum, 5439-1 Sangdaewon 2, Seongnam 462-819, Republic of Korea c Production Technology Team, Samsung Display, Samsung st 181, Asan 336-741, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 20 December 2013 Accepted 24 December 2013 Available online 3 January 2014
a b s t r a c t Three types of oxide layers Al2 O3 , Al1.98 Cr0.02 O3 , and AlFeO3 phases on the surface of Fe–22Cr–6Al (FeCrAl) foam during pre-oxidation were characterized by means of transmission electron microscopy (TEM). The growth rate of each oxide layer was investigated using measurement of total weight gain. The Al2 O3 layer mainly affected the growth rate of the total oxide layer, and it hindered the diffusion of Fe and Cr atoms from the matrix to the oxide layers. © 2013 Elsevier B.V. All rights reserved.
Keywords: FeCrAl Foams Oxides Oxidation kinetics Transmission electron microscopy
1. Introduction Metallic foams are widely used in the energy and filtration industries. This is because their unique cellular structures provide surface area efficiency and because of the material properties (e.g., oxidation, thermal conductivity, and stiffness characteristics) of alloy such as FeCrAl, NiCrAl, and NiFeCrAl [1–6]. These metallic foams are generally exposed to extreme environments subjects to high temperature or other harsh conditions. For example, in automotive mufflers, they are used to reduce exhaust gases with catalysts coating the surface of the metallic foam. The ␥Al2 O3 is commonly used to coat Pt or Pd catalytic materials using wash-coating, sol–gel, or wetness impregnation methods [7–10]. However, since these coating layers would adhere only weakly to the smooth surface of pure metallic foam, it is first necessary to roughen the surface by pre-oxidization. Studies on oxidization phenomena on the surface have thus been conducted with diverse shapes of FeCrAl alloys by means of various analytical tools [11–16]. These oxide layers not only prevent internal oxidization but also improve adhesion properties [7]. In particular, for the alloy FeCrAl, diverse ␣-, ␥-, and -Al2 O3 phases were found, and
∗ Corresponding author. Tel.: +82 33 250 6267; fax: +82 33 250 6260. E-mail address: [email protected] (S.H. Lim). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.144
they also exhibited abnormal growth, morphologically and structurally [11]. Furthermore, the accumulation of Fe or Cr in oxide layers has been widely reported in the literature [13–16]. Most studies have been conducted with a long pre-oxidization step; nevertheless microstructural characterization at the beginning of the pre-oxidation step has not yet been studied in depth. Thus, we focused this study on the formation sequence and accurate phase characterization of the initial oxide layers. The oxide layers formed on a nano scale early in the process, and it was possible to characterize them by means of transmission electron microscopy (TEM). 2. Experimental procedure In the present work, Fe–22Cr–6Al (FeCrAl) foams with 450 m pores prepared using Alantum’s patented process [17,18] were pre-oxidized for 0, 1, 30, and 60 min at 1000 ◦ C in atmospheric conditions. The weight gain values of the pre-oxidized foam were measured by the difference of weight before and after preoxidization, since the specific surface area could not be accurately measured. Morphological surface changes such as abnormally grown oxide scales or defect formation after pre-oxidization were observed using scanning electron microscopy (SEM) with a SUPRA 55VP (Carl Zeiss, Germany). Cross-sectional analyses for accurate phase characterization and measurement of the growth rates of oxide layers were carried out utilizing a 200 kV field-emission
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J.Y. Lee et al. / Applied Surface Science 293 (2014) 255–258
Fig. 2. Sequential cross-sectional bright-field TEM images and elemental line analysis of the oxide layer on FeCrAl metallic foam surface after pre-oxidization at 1000 ◦ C for (a) 1, (b) 30, and (c) 60 min. Fig. 1. Sequential SEM images of the FeCrAl metallic foam surface after preoxidization at 1000 ◦ C for (a) 0, (b) 1, (c) 30, and (d) 60 min. SEM image of 450 m pores is inset.
J.Y. Lee et al. / Applied Surface Science 293 (2014) 255–258
TEM (JEOL-2100F, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector along with a scanning TEM. TEM specimens were prepared by a standard in-situ focused ion beam (FIB) TEM specimen preparation method with Pt or W coating to prevent oxide layer loss. Also, the Joint Committee on Powder Diffraction Standards (JCPDS) was referenced to identify the phases of the oxide layers. 3. Results and discussion After aging at 1000 ◦ C for 0, 1, 30, and 60 min, a FeCrAl metallic foam surface with 450 m pores was recorded in sequential SEM images (Fig. 1). During pre-oxidization, the porous shape of the FeCrAl metallic foam was not affected as shown in the insets of Fig. 1a–d. The surface of the FeCrAl foam before pre-oxidization was smooth, but a roughly formed and abnormally grown oxide layer with some defects, was observed after pre-oxidization. Fig. 1b shows cavities on the oxide layer; the formation of these cavities is attributed to rapid oxidation after 1 min. In some prominent regions, a few micro-cracks were also detected after 30 min, as shown in Fig. 1c. It was determined that these micro-cracks in the oxide layer occurred due to thermal shock caused by the difference in the thermal expansion coefficients of the matrix and the oxide layer. Moreover, it is suggested that the micro-cracks would hinder strong bonding between the FeCrAl foam surface and the catalytic materials, and could also serve as fracture starting points in the catalysis coated layer. Fig. 1d exhibits the rough surface of the oxide layer, which was observed in most areas of the FeCrAl foam after 60 min; although areas with abnormally grown oxide scales were also observed on the margins. The clearly observable grain boundaries on the oxide layer suggest that the oxide grains grew to a size greater than about 100 nm, and also indicate that the grain growth of the oxide layer started before 60 min. Fig. 2 presents sequential cross-sectional bright-field TEM images and an elemental line analysis of the oxide layers after preoxidization. After 1 min, as shown in Fig. 2a, the oxide layers were not dense and showed no defects such as pores, whereas the oxide
257
layers covered the whole surface of the FeCrAl foam. The elemental line analysis showed that the oxide layers formed separately in the order of Al–O, Al–Cr–O, and Al–Fe–O layers starting at the surface. The separate oxide layers were also maintained even after 60 min, as presented in Fig. 2b and c. In addition, with greater preoxidization time, the oxide layers grew and became denser. These oxide layers had distinctive chemical compositions, and could be considered Cr and Fe accumulated alumina layers according to previous studies [13–16]. However, the oxide layers exhibited unique phases, as characterized in Fig. 3 by a high-resolution TEM with digital diffractogram. Three types of oxide layers were characterized: AlFeO3 (JCPDS 84-2154, Orthorhombic, Pna21 ) with the zone axis ¯ direction, Al1.98 Cr0.02 O3 (JCPDS 88-0883, Rhombohein the [0 1 3] ¯ with the zone axis in the [0 1 0] direction and Al2 O3 (JCPDS dral, R3c) ¯ with the zone axis in the [4 4¯ 1] direc88-0826, Rhombohedral, R3c) tion. These phase characterization results of the oxide layers on the FeCrAl metallic foam after pre-oxidization are expected to be meaningful when choosing suitable coating materials for catalytic coating, because the bonding characteristics generally depend on the structural factors of each material. Meanwhile, the thickness of the AlFeO3 , Al1.98 Cr0.02 O3 , and Al2 O3 layers, which had been preserved from the Ga ion-damage during FIB sampling process by coating with Pt or W layers shown in Figs. 2 and 3, were investigated via TEM analyses as a function of the pre-oxidization time (min and s1/2 ). The results are plotted with the measured weight gain value in Fig. 4. Fig. 4a shows the weight gain value (%) and the thickness (nm) of the oxide layer of FeCrAl metallic foam as a function of the pre-oxidization time (min) with the standard B-spline curve. It is well known that weight gain generally corresponds with the quantity of oxide-gain after pre-oxidization, and that the value grows slowly after reaching a saturation point. In previous work, calculations of the oxidation rate from the weight gain in relation to unit surface area were not done for other FeCrAl alloy shapes such as bulk or sheet alloys; for which the surface area could be measured directly [12–14]. However, the oxidation rate was calculated from the growth rate of the oxide layers in the present study. The growth rate of oxide layers (or
Fig. 3. Bright-field and high-resolution TEM images of AlFeO3 , Al1.98 Cr0.02 O3 , and Al2 O3 oxide layers with digital diffractograms corresponding to the white square marked regions.
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J.Y. Lee et al. / Applied Surface Science 293 (2014) 255–258
Table 1 Growth rate constants D1 and D2 for the oxide layers. Type of oxide layer AlFeO3 Al1.98 Cr0.02 O3 Al2 O3 Total oxide layer
D1 (cm2 /s) −14
3.04 × 10 3.75 × 10−14 8.97 × 10−14 1.58 × 10−13
D2 (cm2 /s) 1.90 × 10−15 3.22 × 10−15 2.27 × 10−14 2.78 × 10−14
0.84, 0.99, and 0.97, respectively. The growth rates of the AlFeO3 and Al1.98 Cr0.02 O3 layers decreased dramatically after 1 min, and that of the Al2 O3 layer also dropped slowly. The growth rate of the total oxide layer was considered to depend on the growth rate of the Al2 O3 layer. Consequently, it may be that the diffusion of Fe and Cr atoms from the matrix to the oxide layers; via the Al2 O3 layer, was hindered. However, demonstration of this possibility requires further in-depth study. 4. Conclusions In summary, using TEM, we characterized three types of oxide layers, formed in the order of Al2 O3 , Al1.98 Cr0.02 O3 , and AlFeO3 , from the surface of Fe–22Cr–6Al foam during pre-oxidation at 1000 ◦ C. The growth rate of each oxide layer was determined by measure of the total weight gain value. In particular, from the calculated growth rates, it appeared that the Al2 O3 layer mainly affected the growth of the total oxide layer and hindered diffusion of Fe and Cr atoms from the matrix to the oxide layers. Acknowledgments This study was supported by the BK21 Plus funded by the Ministry of Education, Korea (No.31Z20130012978). The use of the electron microscopes at the National NanoFab Center and the Central Laboratory, Kangwon National University, is also greatly appreciated. References
Fig. 4. Weight gain value (%) and thickness (nm) of total oxide layer of FeCrAl metallic foam corresponding to the pre-oxidization time in minutes (a) and (b). Thickness of each layer and total oxide layers versus the pre-oxidization time (s1/2 ).
oxidation rate) can generally be expressed by the following parabolic equation [19]: x = (Dt)1/2
(1)
where x represents the thickness of the oxide layer (nm), D is the growth rate of the oxide layers (cm2 /s), and t is the pre-oxidization time (sec). Fig. 4b indicates that the growth rates for each oxide layer and the total oxide layers were separated into two sections owing to the initial reaction being faster than the following reaction. The growth rate constants D1 and D2 for the oxide layers were calculated as noted in Table 1. Moreover, the R-squared values (R2 ), which indicate the soundness of fit of the leaner trend line at D2 for AlFeO3 , Al1.98 Cr0.02 O3 , Al2 O3 , and the total oxide layer, were 0.44,
[1] M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams, A Design Guide, Butterworth-Heinemann, Boston, 2000. [2] J. Banhart, Prog. Mater. Sci. 46 (2001) 559–632. [3] E.J. Cookson, D.E. Floyd, A.J. Shih, Int. J. Mech. Sci. 48 (2006) 1314–1322. [4] H. Yu, H. Chen, M. Pan, Y. Tang, K. Zeng, F. Peng, H. Wang, Appl. Catal. A: Gen. 327 (2007) 106–113. [5] H. Choe, D.C. Dunand, Acta Mater. 52 (2004) 1283–1295. [6] G. Walther, B. Klöden, B. Kieback, R. Poss, Y. Bienvenu, J.D. Bartout, World powder metallurgy congress & exhibition, in: Proceedings of the PM2010 Powder Metallurgy World Congress, Florence, 2010, p. 109-116. [7] D.H. Kim, B.Y. Yu, P.R. Cha, W.Y. Yoon, J.Y. Byun, S.H. Kim, Surf. Coat. Tech. 209 (2012) 169–176. [8] J.E. Samad, J.A. Nychka, N.V. Semagina, Chem. Eng. J. 168 (2011) 470–476. [9] G. Walther, B. Klöden, T. Büttner, T. Weissgärbe, B. Kieback, A. Böhm, D. Naumann, S. Saberi, L. Timberg, Adv. Eng. Mater. 10 (2008) 803–811. [10] J. Jia, J. Zhou, J. Zhang, Z. Yuan, S. Wang, Appl. Surf. Sci. 253 (2007) 9099–9104. [11] H.E. Kadiri, R. Molins, Y. Bienvenu, M.F. Horstemeyer, Oxid. Met. 64 (2005) 63–97. [12] C.S. Chang, B. Jha, Corrosion 98, San Diego, 1998, 540 pp. [13] H. Josefsson, F. Liu, J.E. Svensson, M. Halvarsson, L.G. Johansson, Mater. Corros. 56 (2005) 801–805. [14] C. Badini, F. Laurella, Surf. Coat. Technol. 135 (2001) 291–298. [15] J. Engkvist, U. Bexell, M. Grehk, M. Olsson, Mater. Corros. 60 (2009) 876–881. [16] J. Engkvist, U. Bexell, T.M. Grehk, M. Olsson, Appl. Surf. Sci. 231-232 (2004) 850–853. [17] D. Naumann, G., Walther, A. Böhm, WO Patent 2005/037467 A3 (2005). [18] D. Naumann, T. Weissgärber, H.D. Böhm, B. Engelmann, G. Walther, A. Böhm, EP Patent 1 620 370 B1 (2004). [19] A.T. Fromhold, J. Chem. Phys. 41 (1964) 509–514.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructural evaluation of oxide layers formed on Fe–22Cr–6Al metallic foam by pre-oxidization Jae Young Lee a , Hyung Giun Kim a , Mi Ri Choi a , Chang Woo Lee b , Man Ho Park b , Ki Hyun Kim c , Sung Hwan Lim a,∗ a
Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon 200-701, Republic of Korea Alantum, 5439-1 Sangdaewon 2, Seongnam 462-819, Republic of Korea c Production Technology Team, Samsung Display, Samsung st 181, Asan 336-741, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 20 December 2013 Accepted 24 December 2013 Available online 3 January 2014
a b s t r a c t Three types of oxide layers Al2 O3 , Al1.98 Cr0.02 O3 , and AlFeO3 phases on the surface of Fe–22Cr–6Al (FeCrAl) foam during pre-oxidation were characterized by means of transmission electron microscopy (TEM). The growth rate of each oxide layer was investigated using measurement of total weight gain. The Al2 O3 layer mainly affected the growth rate of the total oxide layer, and it hindered the diffusion of Fe and Cr atoms from the matrix to the oxide layers. © 2013 Elsevier B.V. All rights reserved.
Keywords: FeCrAl Foams Oxides Oxidation kinetics Transmission electron microscopy
1. Introduction Metallic foams are widely used in the energy and filtration industries. This is because their unique cellular structures provide surface area efficiency and because of the material properties (e.g., oxidation, thermal conductivity, and stiffness characteristics) of alloy such as FeCrAl, NiCrAl, and NiFeCrAl [1–6]. These metallic foams are generally exposed to extreme environments subjects to high temperature or other harsh conditions. For example, in automotive mufflers, they are used to reduce exhaust gases with catalysts coating the surface of the metallic foam. The ␥Al2 O3 is commonly used to coat Pt or Pd catalytic materials using wash-coating, sol–gel, or wetness impregnation methods [7–10]. However, since these coating layers would adhere only weakly to the smooth surface of pure metallic foam, it is first necessary to roughen the surface by pre-oxidization. Studies on oxidization phenomena on the surface have thus been conducted with diverse shapes of FeCrAl alloys by means of various analytical tools [11–16]. These oxide layers not only prevent internal oxidization but also improve adhesion properties [7]. In particular, for the alloy FeCrAl, diverse ␣-, ␥-, and -Al2 O3 phases were found, and
∗ Corresponding author. Tel.: +82 33 250 6267; fax: +82 33 250 6260. E-mail address: [email protected] (S.H. Lim). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.144
they also exhibited abnormal growth, morphologically and structurally [11]. Furthermore, the accumulation of Fe or Cr in oxide layers has been widely reported in the literature [13–16]. Most studies have been conducted with a long pre-oxidization step; nevertheless microstructural characterization at the beginning of the pre-oxidation step has not yet been studied in depth. Thus, we focused this study on the formation sequence and accurate phase characterization of the initial oxide layers. The oxide layers formed on a nano scale early in the process, and it was possible to characterize them by means of transmission electron microscopy (TEM). 2. Experimental procedure In the present work, Fe–22Cr–6Al (FeCrAl) foams with 450 m pores prepared using Alantum’s patented process [17,18] were pre-oxidized for 0, 1, 30, and 60 min at 1000 ◦ C in atmospheric conditions. The weight gain values of the pre-oxidized foam were measured by the difference of weight before and after preoxidization, since the specific surface area could not be accurately measured. Morphological surface changes such as abnormally grown oxide scales or defect formation after pre-oxidization were observed using scanning electron microscopy (SEM) with a SUPRA 55VP (Carl Zeiss, Germany). Cross-sectional analyses for accurate phase characterization and measurement of the growth rates of oxide layers were carried out utilizing a 200 kV field-emission
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J.Y. Lee et al. / Applied Surface Science 293 (2014) 255–258
Fig. 2. Sequential cross-sectional bright-field TEM images and elemental line analysis of the oxide layer on FeCrAl metallic foam surface after pre-oxidization at 1000 ◦ C for (a) 1, (b) 30, and (c) 60 min. Fig. 1. Sequential SEM images of the FeCrAl metallic foam surface after preoxidization at 1000 ◦ C for (a) 0, (b) 1, (c) 30, and (d) 60 min. SEM image of 450 m pores is inset.
J.Y. Lee et al. / Applied Surface Science 293 (2014) 255–258
TEM (JEOL-2100F, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector along with a scanning TEM. TEM specimens were prepared by a standard in-situ focused ion beam (FIB) TEM specimen preparation method with Pt or W coating to prevent oxide layer loss. Also, the Joint Committee on Powder Diffraction Standards (JCPDS) was referenced to identify the phases of the oxide layers. 3. Results and discussion After aging at 1000 ◦ C for 0, 1, 30, and 60 min, a FeCrAl metallic foam surface with 450 m pores was recorded in sequential SEM images (Fig. 1). During pre-oxidization, the porous shape of the FeCrAl metallic foam was not affected as shown in the insets of Fig. 1a–d. The surface of the FeCrAl foam before pre-oxidization was smooth, but a roughly formed and abnormally grown oxide layer with some defects, was observed after pre-oxidization. Fig. 1b shows cavities on the oxide layer; the formation of these cavities is attributed to rapid oxidation after 1 min. In some prominent regions, a few micro-cracks were also detected after 30 min, as shown in Fig. 1c. It was determined that these micro-cracks in the oxide layer occurred due to thermal shock caused by the difference in the thermal expansion coefficients of the matrix and the oxide layer. Moreover, it is suggested that the micro-cracks would hinder strong bonding between the FeCrAl foam surface and the catalytic materials, and could also serve as fracture starting points in the catalysis coated layer. Fig. 1d exhibits the rough surface of the oxide layer, which was observed in most areas of the FeCrAl foam after 60 min; although areas with abnormally grown oxide scales were also observed on the margins. The clearly observable grain boundaries on the oxide layer suggest that the oxide grains grew to a size greater than about 100 nm, and also indicate that the grain growth of the oxide layer started before 60 min. Fig. 2 presents sequential cross-sectional bright-field TEM images and an elemental line analysis of the oxide layers after preoxidization. After 1 min, as shown in Fig. 2a, the oxide layers were not dense and showed no defects such as pores, whereas the oxide
257
layers covered the whole surface of the FeCrAl foam. The elemental line analysis showed that the oxide layers formed separately in the order of Al–O, Al–Cr–O, and Al–Fe–O layers starting at the surface. The separate oxide layers were also maintained even after 60 min, as presented in Fig. 2b and c. In addition, with greater preoxidization time, the oxide layers grew and became denser. These oxide layers had distinctive chemical compositions, and could be considered Cr and Fe accumulated alumina layers according to previous studies [13–16]. However, the oxide layers exhibited unique phases, as characterized in Fig. 3 by a high-resolution TEM with digital diffractogram. Three types of oxide layers were characterized: AlFeO3 (JCPDS 84-2154, Orthorhombic, Pna21 ) with the zone axis ¯ direction, Al1.98 Cr0.02 O3 (JCPDS 88-0883, Rhombohein the [0 1 3] ¯ with the zone axis in the [0 1 0] direction and Al2 O3 (JCPDS dral, R3c) ¯ with the zone axis in the [4 4¯ 1] direc88-0826, Rhombohedral, R3c) tion. These phase characterization results of the oxide layers on the FeCrAl metallic foam after pre-oxidization are expected to be meaningful when choosing suitable coating materials for catalytic coating, because the bonding characteristics generally depend on the structural factors of each material. Meanwhile, the thickness of the AlFeO3 , Al1.98 Cr0.02 O3 , and Al2 O3 layers, which had been preserved from the Ga ion-damage during FIB sampling process by coating with Pt or W layers shown in Figs. 2 and 3, were investigated via TEM analyses as a function of the pre-oxidization time (min and s1/2 ). The results are plotted with the measured weight gain value in Fig. 4. Fig. 4a shows the weight gain value (%) and the thickness (nm) of the oxide layer of FeCrAl metallic foam as a function of the pre-oxidization time (min) with the standard B-spline curve. It is well known that weight gain generally corresponds with the quantity of oxide-gain after pre-oxidization, and that the value grows slowly after reaching a saturation point. In previous work, calculations of the oxidation rate from the weight gain in relation to unit surface area were not done for other FeCrAl alloy shapes such as bulk or sheet alloys; for which the surface area could be measured directly [12–14]. However, the oxidation rate was calculated from the growth rate of the oxide layers in the present study. The growth rate of oxide layers (or
Fig. 3. Bright-field and high-resolution TEM images of AlFeO3 , Al1.98 Cr0.02 O3 , and Al2 O3 oxide layers with digital diffractograms corresponding to the white square marked regions.
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Table 1 Growth rate constants D1 and D2 for the oxide layers. Type of oxide layer AlFeO3 Al1.98 Cr0.02 O3 Al2 O3 Total oxide layer
D1 (cm2 /s) −14
3.04 × 10 3.75 × 10−14 8.97 × 10−14 1.58 × 10−13
D2 (cm2 /s) 1.90 × 10−15 3.22 × 10−15 2.27 × 10−14 2.78 × 10−14
0.84, 0.99, and 0.97, respectively. The growth rates of the AlFeO3 and Al1.98 Cr0.02 O3 layers decreased dramatically after 1 min, and that of the Al2 O3 layer also dropped slowly. The growth rate of the total oxide layer was considered to depend on the growth rate of the Al2 O3 layer. Consequently, it may be that the diffusion of Fe and Cr atoms from the matrix to the oxide layers; via the Al2 O3 layer, was hindered. However, demonstration of this possibility requires further in-depth study. 4. Conclusions In summary, using TEM, we characterized three types of oxide layers, formed in the order of Al2 O3 , Al1.98 Cr0.02 O3 , and AlFeO3 , from the surface of Fe–22Cr–6Al foam during pre-oxidation at 1000 ◦ C. The growth rate of each oxide layer was determined by measure of the total weight gain value. In particular, from the calculated growth rates, it appeared that the Al2 O3 layer mainly affected the growth of the total oxide layer and hindered diffusion of Fe and Cr atoms from the matrix to the oxide layers. Acknowledgments This study was supported by the BK21 Plus funded by the Ministry of Education, Korea (No.31Z20130012978). The use of the electron microscopes at the National NanoFab Center and the Central Laboratory, Kangwon National University, is also greatly appreciated. References
Fig. 4. Weight gain value (%) and thickness (nm) of total oxide layer of FeCrAl metallic foam corresponding to the pre-oxidization time in minutes (a) and (b). Thickness of each layer and total oxide layers versus the pre-oxidization time (s1/2 ).
oxidation rate) can generally be expressed by the following parabolic equation [19]: x = (Dt)1/2
(1)
where x represents the thickness of the oxide layer (nm), D is the growth rate of the oxide layers (cm2 /s), and t is the pre-oxidization time (sec). Fig. 4b indicates that the growth rates for each oxide layer and the total oxide layers were separated into two sections owing to the initial reaction being faster than the following reaction. The growth rate constants D1 and D2 for the oxide layers were calculated as noted in Table 1. Moreover, the R-squared values (R2 ), which indicate the soundness of fit of the leaner trend line at D2 for AlFeO3 , Al1.98 Cr0.02 O3 , Al2 O3 , and the total oxide layer, were 0.44,
[1] M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams, A Design Guide, Butterworth-Heinemann, Boston, 2000. [2] J. Banhart, Prog. Mater. Sci. 46 (2001) 559–632. [3] E.J. Cookson, D.E. Floyd, A.J. Shih, Int. J. Mech. Sci. 48 (2006) 1314–1322. [4] H. Yu, H. Chen, M. Pan, Y. Tang, K. Zeng, F. Peng, H. Wang, Appl. Catal. A: Gen. 327 (2007) 106–113. [5] H. Choe, D.C. Dunand, Acta Mater. 52 (2004) 1283–1295. [6] G. Walther, B. Klöden, B. Kieback, R. Poss, Y. Bienvenu, J.D. Bartout, World powder metallurgy congress & exhibition, in: Proceedings of the PM2010 Powder Metallurgy World Congress, Florence, 2010, p. 109-116. [7] D.H. Kim, B.Y. Yu, P.R. Cha, W.Y. Yoon, J.Y. Byun, S.H. Kim, Surf. Coat. Tech. 209 (2012) 169–176. [8] J.E. Samad, J.A. Nychka, N.V. Semagina, Chem. Eng. J. 168 (2011) 470–476. [9] G. Walther, B. Klöden, T. Büttner, T. Weissgärbe, B. Kieback, A. Böhm, D. Naumann, S. Saberi, L. Timberg, Adv. Eng. Mater. 10 (2008) 803–811. [10] J. Jia, J. Zhou, J. Zhang, Z. Yuan, S. Wang, Appl. Surf. Sci. 253 (2007) 9099–9104. [11] H.E. Kadiri, R. Molins, Y. Bienvenu, M.F. Horstemeyer, Oxid. Met. 64 (2005) 63–97. [12] C.S. Chang, B. Jha, Corrosion 98, San Diego, 1998, 540 pp. [13] H. Josefsson, F. Liu, J.E. Svensson, M. Halvarsson, L.G. Johansson, Mater. Corros. 56 (2005) 801–805. [14] C. Badini, F. Laurella, Surf. Coat. Technol. 135 (2001) 291–298. [15] J. Engkvist, U. Bexell, M. Grehk, M. Olsson, Mater. Corros. 60 (2009) 876–881. [16] J. Engkvist, U. Bexell, T.M. Grehk, M. Olsson, Appl. Surf. Sci. 231-232 (2004) 850–853. [17] D. Naumann, G., Walther, A. Böhm, WO Patent 2005/037467 A3 (2005). [18] D. Naumann, T. Weissgärber, H.D. Böhm, B. Engelmann, G. Walther, A. Böhm, EP Patent 1 620 370 B1 (2004). [19] A.T. Fromhold, J. Chem. Phys. 41 (1964) 509–514.