- Email: [email protected]
High-temperature corrosion behavior of Kanthal alloy in molten silver under an oxidizing atmosphere
Corrosion Science xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
High-temperature corrosion behavior of Kanthal alloy in molten silver under an oxidizing atmosphere Gyu-Seok Lima, Suk-Cheol Kwona, Soo-Haeng Chob, Jong-Hyeon Leea,b,c,
⁎
a
Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Daejeon 34134, Republic of Korea c Graduate school of Energy Science and Technology, Chungnam National University, Daejeon 34134, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: High-temperature corrosion Anode current-collector Solid oxide membrane Kanthal alloy Liquid silver medium
The high-temperature corrosion behaviors of Kanthal alloy for an anode current-collector were investigated at 1150 °C in molten silver under an oxidizing atmosphere. The hot corrosion products of Kanthal alloy in the gas section were Al2O3 and Al2O3 and FeAl2O4 obtained after 1 d and 3, 5, and 7 d, respectively. Contrarily, only Al2O3 was identified in the liquid silver section. Isolated patches were observed beneath the outer oxide layer in the gas section, whereas there was no evidence of these in the liquid silver section, and a cohesive Ag-rich layer beneath the oxide layer was shown to retard the corrosion by acting as an effective barrier.
1. Introduction Solid oxide membranes (SOMs) have recently been applied to metal production using oxide reduction to achieve a low-cost development of primary products through simple pre-processing, low capital costs, small manufacturing plants, and low energy costs, without the expense required in collecting and treating the by-products. In particular, the SOM process is eco-friendly, which makes it even more attractive [1–7]. The SOM process uses a direct electrolytic reduction with a solid-oxide oxygen ion conducting membrane, for example, yttria-stabilized zirconia (YSZ) with high-temperature molten salts as a reaction medium owing to its high oxygen ion conductivity. In addition, as the SOMassisted electroreduction process, the anode reaction area can be separated from the molten salt using an SOM tube, allowing only O2− to pass through, which means that the SOM electrolysis process generally possesses a high reduction speed. Therefore, it has been used as an electrolyte, an oxygen sensor, and an anode for the production of highenergy content metals, alloys, and inter-metallics, such as magnesium, aluminum, chromium, tantalum, calcium, titanium, silicon, Ti-Fe alloys, and Ti-Si intermetallics, directly from their oxides and mixed oxides at low cost and in an environmentally friendly manner [8–22]. The SOM anode used in the direct electroreduction was composed of a solid-oxide oxygen-ion-conducting membrane tube filled with an electron conductive liquid silver medium for transforming oxygen ions generated from electrochemical reduction reactions into an oxygen gas (xO2− → x/2O2 + 2xe-), and an anode current-collector used to
⁎
conduct an electric current. The high-temperature liquid medium with dissolved oxygen can corrode the anode current-collector of the SOM tube system. A lengthy corrosion resistance is one of the important parameters determining the suitability of an anode current-collector application requiring continued exposure to an elevated-temperature liquid medium under a dissolved oxygen atmosphere, and encounters isothermal and electrochemical reactions at an elevated temperature in an SOM system. There have been several reports on hot-corrosion resistant anode current collectors in an SOM system, such as porous Ni-YSZ cermet [1], graphite with Fe-Csat melt [2], graphite with liquid Cu [6], Fe-Cr-Al/Mo with Sn-Csat melt [11–14], graphite with Cusat melt [15], LaMnO3/liquid silver [16], Pt with a platinized inner wall [17], and strontiumdoped lanthanum manganite (LSM) [18–20]. The high-temperature corrosion tests are therefore a mainstay of the material characterization and performance ranking for high-temperature materials. Hence, studies on the corrosion of materials used to handle a high-temperature liquid medium should be necessary. Among the current collector in an SOM system, because Fe-Cr alloy forms a conductive Cr-Al oxide protective layer in the presence of Al2O3 as an electrolyte, Fe-Cr-Al with liquid silver could be an ideal combination, although such use has yet to be reported [21]. In fact, there have been few reports on the isothermal high-temperature corrosion behaviors of Fe-Cr-Al-based alloys in a liquid silver medium, which is the subject of the present study. The oxygen ions transported from the cathode during the direct electroreduction reactions may create a
Corresponding author at: Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea. E-mail address: [email protected] (J.-H. Lee).
https://doi.org/10.1016/j.corsci.2019.108247 Received 5 March 2019; Received in revised form 24 September 2019; Accepted 25 September 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Gyu-Seok Lim, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108247
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
examined visually. The specimens were then ultrasonically cleaned in deionized water, and before being subjected to the characterization experiments the corroded specimens were cut into two sections (one for the liquid silver region, while the other for the gas region at the nearest distance from the interface) using a diamond cutter and ultrasonically cleaned in deionized water and dried. The objective of this study was to identify the corrosion products of the Kanthal alloy regions exposed to liquid silver and oxygen gas. Therefore, the interface between the gas and liquid silver phases was carefully separated in order to avoid crosstalk during the analysis. Some specimens were prepared for metallographic examination using cold mounting, grinding, and polishing. The microstructure, morphology, chemical composition, and elemental distribution of the external scale and subscale of the corroded layers were examined using a field emission scanning electron microscope (FE-SEM, JEOL, JSM-7000 F) equipped with an energy dispersive X-ray spectroscopy (EDS) system. X-ray diffraction (XRD, Rigaku International Corporation, D/MAX-2200 Ultima/PC) was applied to analyze the structural phase evolution of the tested specimen surface and layers.
Table 1 Chemical compositions of the tested alloys (wt.%). Alloy
Fe
Cr
Al
Mn
Si
C
Ti
Mg
Kanthal A-1
Bal.
22.53
5.80
0.40
0.70
0.08
–
–
chemically aggressive environment, which can be corrosive for ordinary structural materials [23,24]. Therefore, there is a need for a high-temperature corrosion behavior of Fe-Cr-Al alloy in a high-temperature liquid medium under a dissolved oxygen atmosphere. In this study, the high-temperature corrosion behavior of Kanthal alloy, which is a typical commercially available Fe-Cr-Al alloy, was investigated at 1150 °C in a liquid silver medium under an oxidizing atmosphere. The high-temperature corrosion phenomena, surface and cross-sectional morphologies, and characteristics and elemental distribution of the corroded layers were discussed. 2. Experimental This study was conducted using a Kanthal rod, the chemical compositions of which are listed in Table 1. Specimens were prepared using dimensions of 20 mm (D) × 30 (L) mm. Before the high-temperature corrosion tests, the specimens were ground using SiC paper (#2000), polished with diamond paste, and cleaned using deionized water and acetone. The experimental apparatus is shown in Fig. 1 (the inset shows the layout of the specimen in liquid silver). One portion of the anode current-collector, which constituted the SOM system, is immersed in liquid silver, while the other is exposed to oxygen gas. Hence, the corrosion test apparatus was devised considering the actual device configuration, as shown in Fig. 1. The portion exposed to gas is denoted as the gas section, while that immersed in liquid silver is denoted as the liquid silver section. As-purchased silver powders and a thermocouple shielded with an alumina tube were introduced into a high-density MgO crucible and then heated at 300 °C for 3 h under an argon atmosphere to remove any possible moisture. After reaching the desired temperature (1150 °C), the specimens with an alumina tube were immersed in a liquid silver medium, and oxygen (4 ml/min) was supplied through the alumina tube to simulate the actual SOM process. The isothermal hightemperature corrosion exposures were 1, 3, 5, and 7 d at 1150 °C in an argon atmosphere. Following the isothermal high-temperature corrosion tests, the specimens were withdrawn from the liquid silver medium and kept under an argon atmosphere, and the furnace was cooled to room temperature. After the reactor was opened, the specimens were removed and
3. Results and discussion 3.1. High-temperature corrosion phenomena Fig. 2 shows the morphologies of the cross-sectional corrosion layers of the Kanthal alloy specimens exposed to 1150 °C in molten silver under an oxidizing atmosphere for 1 (2a), 3 (2b), 5 (2c), and 7 d (2d) in the gas section and for 1 (2e), 3 (2f), 5 (2 g), and 7 d (2 h) in the liquid silver section. As shown in Fig. 2, the high-temperature corrosion phenomenon of Kanthal alloy after the isothermal corrosion tests at 1150 °C in liquid silver and gas atmosphere at each exposure time varied significantly. In the case of the gas section, a dark area developed, with some isolated patches, consisting of an Al-rich phase as shown in Fig. 2a–d. With continued exposure, they changed into an oxide phase, namely Al2O3 as shown in Fig. 2d. The composition of the dark area is presented in Section 3.3. The dark area beneath the surface of the specimens in the gas section is expected to effect the internal corrosion due to the depletion of the alloying elements. This may be due to the inward diffusion of oxygen through the cracks by the depletion of major alloying elements, such as Cr and Fe, as well as the different thermal-expansion characteristics of the different components of the localized attack area. In addition, the internal attack depth was 9.6, 15.1, 16.8, and 22.4 μm, after 1, 3, 5 and 7 d, respectively, that could be characterized by rapid kinetics followed by a stage of parabolic-rate
Fig. 1. Schematic diagram of the apparatus used for the corrosion tests (inset shows the layout of the specimen in liquid silver). 2
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 2. Cross-sectional micrographs of Kanthal alloy corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a, e) 1, (b, f) 3, (c, g) 5, and (d, h) 7 d ((a–d) in the gas section and (e–h) in the liquid silver section).
Fig. 3. XRD patterns of the gas sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
behavior as presented by the internal attack depth with exposure time. For a liquid silver section, an Ag-rich layer retards the progress of the corrosion reactions owing to the formation of a cohesive Ag-rich layer beneath an oxide layer acting as an effective barrier toward the inward diffusion of oxygen and outward diffusion of alloying elements in the Kanthal alloy. This phenomenon was reported in Fe-20Cr-4Al alloyed with noble metals (Pd, Pt, and Au) and yttrium, which contributed through the adherence of the Al2O3 passivation layer [25]. Hence, silver effectively protects the specimens from continuous oxidation in a corrosion environment by enhancing the bonding strength of the Al2O3
corrosion layer. In general, under an oxidative environment with an elevated temperature, high-temperature corrosion phenomena can be significantly affected by the adhesive strength between the corrosion layer and base metal, considering the formation, maintenance, dissolution, and spallation of the corrosion layer, as well as the stability of the growth of the corrosion layer. 3.2. High-temperature corrosion products Fig. 3 shows the XRD patterns of the high-temperature corrosion 3
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 4. XRD patterns of the liquid silver sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Fig. 5. Surface SEM images and EDS point analysis results for Kanthal alloy corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a, c) 1 and (b, d) 7 d ((a, b) in the gas section and (c, d) in liquid silver section).
peaks of Ag, Al2O3, and FeAl2O4 for 3, 5, and 7 d. The surface SEM images and EDS analysis results of the Kanthal alloys corroded at 1150 °C in molten silver under an oxidizing atmosphere for 1 and 7 d in the gas section are shown in Fig. 5a and b, respectively. As shown in Fig. 5a and b, the localized EDS results indicate that the alloys contained 91.71–99.22 wt.% Ag from S1 and S2; and 47.21–50.06 wt.% Al and 42.31–49.00 wt.% O from S3 to S6 for 1 d; and 97.16 wt.% Ag from S6; 38.24–43.85 wt.% Al and 47.40–50.38 wt.% O from S1, S3, and S5;
products on the top surface of the specimen after the high-temperature corrosion tests at 1150 °C in molten silver under an oxidizing atmosphere for 1, 3, 5, and 7 d in the gas section. Fig. 4 shows the XRD patterns of the high-temperature corrosion products on the bottom surface of the specimen after the high-temperature corrosion tests at 1150 °C in molten silver under an oxidizing atmosphere for 1, 3, 5, and 7 d in the liquid silver section. In the case of a gas atmosphere, as shown in Fig. 3, the XRD patterns showed peaks of Ag and Al2O3 for 1 d, and 4
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Chemistry software version 7.11 (Outotec). The initial element composition is 66.44 mol% Fe, 22.43 mol% Cr, and 11.13 mol% Al according to the Kanthal alloy composition. The concentration of metallic Al rapidly decreases with an increase in oxygen, which causes a rapid increase in Al2O3. From a thermodynamic perspective, Al2O3 is an extremely stable oxide in Fe-Cr-Al based alloys at above 1000 °C. Hence, for application in an oxygen-rich environment at higher temperatures, namely, up to 1200 or 1300 °C, alloys and alloy coatings are often designed to develop the surface layers of Al2O3 that do not form higher volatile oxides [26]. Hence, the corrosion product of the specimens immersed in liquid silver with a limited oxygen concentration is only Al2O3 without a spinel formation, as confirmed in Fig. 4. With an increase in oxygen concentration, metallic Al and Cr are completely converted into FeAl2O4 and FeCr2O4, respectively, as shown in Fig. 6. In order to form FeCr2O4 phase, it is necessary to continuously supply oxygen, but due to the formation of the dense FeAl2O4 phase, sufficient oxygen is not supplied and then only FeAl2O4 phase was observed in the corrosion products, as shown in Fig. 3b–d. Consequently, in the tested alloys exposed to a gas section, Al2O3 was initially formed under a hot corrosion environment followed by the formation of FeAl2O4 spinel through a solid-phase reaction between FeO and Al2O3 [27].
Fig. 6. Calculated equilibrium composition of the reaction system based on the amount of oxygen at 1150 °C (initial composition of 66.44 mol% Fe, 22.43 mol % Cr, and 11.13 mol% Al).
and 34.99–36.51 wt.% Fe, 24.79–33.67 wt.% Al, and 24.95–25.46 wt.% O from S2 and S4 for 7 d, respectively. By combining the XRD (Fig. 3a and d) and EDS analysis results, these were confirmed as Ag and Al2O3 for 1 d, and Ag, Al2O3, and FeAl2O4 for 7 d. Fig. 5c and d show the surface SEM images and EDS analysis results, respectively, of the liquid silver section of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for 1 and 7 d. As shown in Fig. 5c and d, the localized EDS results indicate that they contained 100 wt.% Ag from S1 and S4; 45.36–49.12 wt.% Al; and 43.53–45.45 wt.% O from S2, S3, S5, and S6 for 1 d, and 93.54–100 wt. % Ag from S2 to S4, and 46.37–49.82 wt.% Al and 43.98–45.26 wt.% O from S1, S5, and S6 for 7 d, respectively. By combining the XRD (Fig. 4a and d) and EDS analysis results, these were confirmed as Ag and Al2O3 for 1 and 7 d, respectively. Fig. 6 shows the equilibrium composition of the reaction system along with the amount of oxygen at 1150 °C as calculated using HSC
3.3. High-temperature corrosion behavior Figs. 7 and 8 show the cross-sectional SEM images and elemental mapping as well as EDS point analysis results for the gas section of the Kanthal alloy specimens exposed to 1150 °C in molten silver under an oxidizing atmosphere for 1 (Figs. 7a and 8 a), 3 (Figs. 7b and 8 b), 5 (Figs. 7c and 8 c), and 7 d (Figs. 7d and 8 d). As shown in Figs. 7 and 8, the dark area occurred beneath the surface of the specimens, which caused an effect of the internal corrosion based on the different thermal-expansion characteristics between the dark area and the base metal. For example, the localized EDS results obtained from S6 (Fig. 8a) indicate that it contained 14.59 wt.% Al, 19.16 wt.% Cr, and 66.25 wt. % Fe, the depletions of which would increase the chemical potential, and thus lead to a higher corrosion rate [28]. Figs. 7b and 8 b show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the gas section of
Fig. 7. Cross-sectional SEM images and elemental mapping for the gas sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d. 5
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 8. Cross-sectional SEM images and EDS point analysis results for the gas sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Fig. 9. Cross-sectional SEM images and elemental mapping for the liquid silver sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Figs. 9a and 10 a show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the liquid silver section of the Kanthal alloy specimen exposed to 1150 °C in molten silver under an oxidizing atmosphere for 1 d. As can be observed from Fig. 9a, there was no evidence of dark area beneath the outer oxide layer and/or silver layer (Fig. 7a). As mentioned above, a cohesive Agrich layer beneath the oxide layer retards the corrosion progress by acting as an effective barrier. Figs. 9b–d and 10 b–d show the cross-sectional SEM images and elemental mapping as well as the EDS point analysis results for the liquid silver sections of the Kanthal alloy specimens exposed to 1150 °C in molten silver under an oxidizing atmosphere for 3 (Figs. 9b and 10 b), 5 Figs. 9c and 10 c), and 7 d (Figs. 9d and 10 d). As shown in Figs. 9 and 10, there was no evidence of dark area beneath the outer oxide layer and/or silver layer. The SOM anode simulation experiments showed that the Kanthal
the Kanthal alloy exposed to 1150 °C in molten silver under an oxidizing atmosphere for 3 d. As shown in Figs. 7b and 8b, much dark area occurred than those for 1 d (Figs. 7a and 8 a), and the amount of porous corrosion composed of Al and O was greater. Figs. 7c and 8 c show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the gas section of the Kanthal alloy specimen exposed to 1150 °C for 5 d. This specimen showed more dark area than that exposed for 3 d (Fig. 7b). Figs. 7d and 8 d show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the gas section of the Kanthal alloy specimen exposed to 1150 °C in molten silver under an oxidizing atmosphere for 7 d. As can be observed from Fig. 7d, this specimen showed more dark area than that exposed for 5 d (Fig. 7c). As shown in Fig. 7a–c, the dark area comprised an Al-rich phase; however, it subsequently developed into an oxide phase, Al2O3, with an increase in the exposure time as shown in Fig. 7d. 6
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 10. Cross-sectional SEM images and EDS point analysis results for the liquid silver sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Acknowledgments
alloy specimens exhibited completely different corrosion behaviors in the gas and liquid silver sections at 1150 °C in molten silver under an oxidizing atmosphere. Based on the fact that a well-defined protective corrosion layer is formed on the Kanthal alloy immersed in molten silver, a greater corrosion resistance is expected than that achievable with other current collector systems. This simple and durable SOM anode system will enhance the performance of the metal production process in a benign environment.
This work was supported by the Technology Innovation Program (10063427, Development of Eco-Friendly Smelting Technology for the Production of Rare Metal Production for Lowering Manufacturing Costs Using Solid Oxide Membrane) funded by the Ministry of Trade, Industry, and Energy (MOTIE, Korea). References
4. Conclusions
[1] U.B. Pal, D.E. Woolley, G.B. Kenney, Emerging SOM technology for the green synthesis of metals from oxides, JOM 53 (2001) 32–35. [2] Y.M. Gao, B. Wang, S.B. Wang, S. Peng, Study on electrolytic reduction with controlled oxygen flow for iron from molten oxide slag containing FeO, J. Min, Metall. Sect. B-Metall. 49 (1) (2013) 49–55 B. [3] A. Aytimur, I. Uslu, S. Kocyigit, F. Ozcan, Magnesia stabilized zirconia doped with boron, ceria and gadolinia, Ceram. Int. 38 (2012) 3851–3856. [4] S. Das, Primary magnesium production costs for automotive applications, JOM 60 (2008) 63–69. [5] D.J.L. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Intermediate temperature solid oxide fuel cells, Chem. Soc. Rev. 37 (2008) 1568–1578. [6] A. Krishnan, X.G. Lu, U.B. Pal, Solid oxide membrane process for magnesium production directly from magnesium oxide, Metall. Mater. Trans. B 36 (2005) 463–473. [7] S. Selvasekarapandian, M.S. Bhuvaneswari, M. Vijayakumar, C.S. Ramya, P.C. Angelo, A comparative study on ionic conductivity of Sr and Mg stabilized zirconia by impedance spectroscopy, J. Eur. Ceram. Soc. 25 (12) (2005) 2573–2575. [8] U.B. Pal, A.C. Powell IV, The use of solid-oxide-membrane technology for electrometallurgy, JOM 59 (2007) 44–49. [9] S. Li, X. Zou, K. Zheng, X. Lu, Q. Xu, C. Chen, Z. Zhou, Direct production of TiAl3 from Ti/Al-containing oxides precursors by solid oxide membrane (SOM) process, J. Alloys Compd. 727 (2017) 1243–1252. [10] A. Krishnan, X.G. Lu, U.B. Pal, Solid oxide membrane process for magnesium production directly from magnesium oxide, Metall. Mater. Trans. B 36 (2005) 463–473. [11] X. Lu, X. Zou, C. Li, Q. Zhong, W. Ding, Z. Zhou, Green electrochemical process solid-oxide oxygen-ion-conducting membrane (SOM): direct extraction of Ti-Fe alloys from natural ilmenite, Metall. Mater. Trans. B 43 (2012) 503–512. [12] X. Zou, X. Lu, Z. Zhou, C. Li, W. Ding, Direct selective extraction of titanium silicide Ti5Si3 from multi-component Ti-bearing compounds in molten salt by an electrochemical process, Electrochim. Acta 56 (2011) 8430–8437. [13] B. Zhao, X. Lu, Q. Zhong, C. Li, S. Chen, Direct electrochemical preparation of CeNi5 and LaxCe1-xNi5 alloys from mixed oxides by SOM process, Electrochim. Acta 55 (2010) 2996–3001. [14] X. Zou, X. Lu, C. Li, Z. Zhou, A direct electrochemical route from oxides to Ti-Si intermetallics, Electrochim. Acta 55 (2010) 5173–5179. [15] X. Su, X.G. Lu, C.H. Li, W.Z. Ding, X.L. Zou, Y.H. Gao, Q.D. Zhong, Preparation of TiFe based hydrogen storage alloy by SOM method, Int. J. Hydrogen Energy 36 (2011) 4573–4579. [16] U.B. Pal, A lower carbon footprint process for production of metals from their oxide sources, JOM 60 (2008) 43–47. [17] A. Martin, D. Lambertin, J.-C. poignet, M. Allibert, G. Bourges, L. Pescayre, J. Fouletier, The electrochemical deoxidation of metal oxides by calcium using a solid oxide membrane, JOM 55 (2003) 52–54.
The high-temperature corrosion behaviors of Kanthal alloy were investigated under an oxidizing atmosphere. The high-temperature corrosion behavior of the specimens showed that the gas section of Kanthal alloy undergoes much corrosion than the liquid silver section owing to the localized corrosion in the former. A localized corrosion region is thought to effect the internal corrosion, depending on the dark area formed on the outermost layer of the substrate, and is related to the region and content of the major element depletion. The isothermal corrosion test revealed that the hot corrosion of Kanthal alloy in the gas section resulted in the formation of Al2O3 (1 d) and Al2O3 and FeAl2O4 (3, 5, and 7 d). On the other hand, only Al2O3 was identified as the hot corrosion product in the liquid silver section owing to its limited oxygen solubility. There was dark area beneath the outer oxide layer in the gas section, which cause to effect the internal corrosion by the different thermal-expansion characteristics between the dark area and base metal, whereas there was no evidence of dark area beneath the outer oxide layer in the liquid silver section, and the cohesive Ag-rich layer acting as an effective barrier beneath the oxide layer retards the corrosion progress.
Declaration of Competing Interest ✓ All authors have participated in analysis and interpretation of the data; drafting the article or revising it critically for important intellectual content; and approval of the final version. ✓ This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. ✓ The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript. 7
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
[24] J.E. Indacochea, J.L. Smith, K.R. Litko, E.J. Karell, Corrosion performance of ferrous and refractory metals in molten salts under reducing conditions, J. Mater. Res. 14 (1999) 1990–1995. [25] T. Amano, High-temperature oxidation resistance of Al2O3-forming heat-resisting alloys with noble metal and rare earth additions, Mater. Corros. 62 (2011) 659–667. [26] F.H. Stott, G.C. Wood, J. Stringer, The influence of alloying elements on the development and maintenance of protective scales, Oxid. Met. 44 (1995) 113–145. [27] Y. Harada, High temperature oxidation and high temperature corrosion of metallic materials: I high temperature oxidation, Jpn. Therm. Spraying Soc. 33 (1996) 128–134. [28] D.A. Jones, Principles and Prevention of Corrosion, Macmillan Publishing Company, New York, 1992.
[18] X. Guan, U.B. Pal, Design of optimum solid oxide membrane electrolysis cells for metals production, Prog. Nat. Sci. 25 (2015) 591–594. [19] S. Su, U.B. Pal, X. Guan, Solid oxide membrane electrolysis process for aluminum production: experiment and modeling, J. Electrochem. Soc. 164 (2017) F248–F255. [20] X. Guan, U.B. Pal, Y. Jiang, S. Su, Clean metals production by solid oxide membrane electrolysis process, J. Sustain. Metall. 2 (2016) 152–166. [21] A. Allanore, L. Yin, D.R. Sadoway, A new anode material for oxygen evolution in molten oxide electrolysis, Nature 497 (2013) 353–356. [22] G.Z. Chen, D.J. Fray, T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride, Nature 407 (2000) 361–364. [23] J.E. Indacochea, J.L. Smith, K.R. Litko, E.J. Karell, A.G. Raraz, High-temperature oxidation and corrosion of structural materials in molten chlorides, Oxid. Met. 55 (2001) 1–16.
8
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
High-temperature corrosion behavior of Kanthal alloy in molten silver under an oxidizing atmosphere Gyu-Seok Lima, Suk-Cheol Kwona, Soo-Haeng Chob, Jong-Hyeon Leea,b,c,
⁎
a
Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Daejeon 34134, Republic of Korea c Graduate school of Energy Science and Technology, Chungnam National University, Daejeon 34134, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: High-temperature corrosion Anode current-collector Solid oxide membrane Kanthal alloy Liquid silver medium
The high-temperature corrosion behaviors of Kanthal alloy for an anode current-collector were investigated at 1150 °C in molten silver under an oxidizing atmosphere. The hot corrosion products of Kanthal alloy in the gas section were Al2O3 and Al2O3 and FeAl2O4 obtained after 1 d and 3, 5, and 7 d, respectively. Contrarily, only Al2O3 was identified in the liquid silver section. Isolated patches were observed beneath the outer oxide layer in the gas section, whereas there was no evidence of these in the liquid silver section, and a cohesive Ag-rich layer beneath the oxide layer was shown to retard the corrosion by acting as an effective barrier.
1. Introduction Solid oxide membranes (SOMs) have recently been applied to metal production using oxide reduction to achieve a low-cost development of primary products through simple pre-processing, low capital costs, small manufacturing plants, and low energy costs, without the expense required in collecting and treating the by-products. In particular, the SOM process is eco-friendly, which makes it even more attractive [1–7]. The SOM process uses a direct electrolytic reduction with a solid-oxide oxygen ion conducting membrane, for example, yttria-stabilized zirconia (YSZ) with high-temperature molten salts as a reaction medium owing to its high oxygen ion conductivity. In addition, as the SOMassisted electroreduction process, the anode reaction area can be separated from the molten salt using an SOM tube, allowing only O2− to pass through, which means that the SOM electrolysis process generally possesses a high reduction speed. Therefore, it has been used as an electrolyte, an oxygen sensor, and an anode for the production of highenergy content metals, alloys, and inter-metallics, such as magnesium, aluminum, chromium, tantalum, calcium, titanium, silicon, Ti-Fe alloys, and Ti-Si intermetallics, directly from their oxides and mixed oxides at low cost and in an environmentally friendly manner [8–22]. The SOM anode used in the direct electroreduction was composed of a solid-oxide oxygen-ion-conducting membrane tube filled with an electron conductive liquid silver medium for transforming oxygen ions generated from electrochemical reduction reactions into an oxygen gas (xO2− → x/2O2 + 2xe-), and an anode current-collector used to
⁎
conduct an electric current. The high-temperature liquid medium with dissolved oxygen can corrode the anode current-collector of the SOM tube system. A lengthy corrosion resistance is one of the important parameters determining the suitability of an anode current-collector application requiring continued exposure to an elevated-temperature liquid medium under a dissolved oxygen atmosphere, and encounters isothermal and electrochemical reactions at an elevated temperature in an SOM system. There have been several reports on hot-corrosion resistant anode current collectors in an SOM system, such as porous Ni-YSZ cermet [1], graphite with Fe-Csat melt [2], graphite with liquid Cu [6], Fe-Cr-Al/Mo with Sn-Csat melt [11–14], graphite with Cusat melt [15], LaMnO3/liquid silver [16], Pt with a platinized inner wall [17], and strontiumdoped lanthanum manganite (LSM) [18–20]. The high-temperature corrosion tests are therefore a mainstay of the material characterization and performance ranking for high-temperature materials. Hence, studies on the corrosion of materials used to handle a high-temperature liquid medium should be necessary. Among the current collector in an SOM system, because Fe-Cr alloy forms a conductive Cr-Al oxide protective layer in the presence of Al2O3 as an electrolyte, Fe-Cr-Al with liquid silver could be an ideal combination, although such use has yet to be reported [21]. In fact, there have been few reports on the isothermal high-temperature corrosion behaviors of Fe-Cr-Al-based alloys in a liquid silver medium, which is the subject of the present study. The oxygen ions transported from the cathode during the direct electroreduction reactions may create a
Corresponding author at: Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea. E-mail address: [email protected] (J.-H. Lee).
https://doi.org/10.1016/j.corsci.2019.108247 Received 5 March 2019; Received in revised form 24 September 2019; Accepted 25 September 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Gyu-Seok Lim, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108247
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
examined visually. The specimens were then ultrasonically cleaned in deionized water, and before being subjected to the characterization experiments the corroded specimens were cut into two sections (one for the liquid silver region, while the other for the gas region at the nearest distance from the interface) using a diamond cutter and ultrasonically cleaned in deionized water and dried. The objective of this study was to identify the corrosion products of the Kanthal alloy regions exposed to liquid silver and oxygen gas. Therefore, the interface between the gas and liquid silver phases was carefully separated in order to avoid crosstalk during the analysis. Some specimens were prepared for metallographic examination using cold mounting, grinding, and polishing. The microstructure, morphology, chemical composition, and elemental distribution of the external scale and subscale of the corroded layers were examined using a field emission scanning electron microscope (FE-SEM, JEOL, JSM-7000 F) equipped with an energy dispersive X-ray spectroscopy (EDS) system. X-ray diffraction (XRD, Rigaku International Corporation, D/MAX-2200 Ultima/PC) was applied to analyze the structural phase evolution of the tested specimen surface and layers.
Table 1 Chemical compositions of the tested alloys (wt.%). Alloy
Fe
Cr
Al
Mn
Si
C
Ti
Mg
Kanthal A-1
Bal.
22.53
5.80
0.40
0.70
0.08
–
–
chemically aggressive environment, which can be corrosive for ordinary structural materials [23,24]. Therefore, there is a need for a high-temperature corrosion behavior of Fe-Cr-Al alloy in a high-temperature liquid medium under a dissolved oxygen atmosphere. In this study, the high-temperature corrosion behavior of Kanthal alloy, which is a typical commercially available Fe-Cr-Al alloy, was investigated at 1150 °C in a liquid silver medium under an oxidizing atmosphere. The high-temperature corrosion phenomena, surface and cross-sectional morphologies, and characteristics and elemental distribution of the corroded layers were discussed. 2. Experimental This study was conducted using a Kanthal rod, the chemical compositions of which are listed in Table 1. Specimens were prepared using dimensions of 20 mm (D) × 30 (L) mm. Before the high-temperature corrosion tests, the specimens were ground using SiC paper (#2000), polished with diamond paste, and cleaned using deionized water and acetone. The experimental apparatus is shown in Fig. 1 (the inset shows the layout of the specimen in liquid silver). One portion of the anode current-collector, which constituted the SOM system, is immersed in liquid silver, while the other is exposed to oxygen gas. Hence, the corrosion test apparatus was devised considering the actual device configuration, as shown in Fig. 1. The portion exposed to gas is denoted as the gas section, while that immersed in liquid silver is denoted as the liquid silver section. As-purchased silver powders and a thermocouple shielded with an alumina tube were introduced into a high-density MgO crucible and then heated at 300 °C for 3 h under an argon atmosphere to remove any possible moisture. After reaching the desired temperature (1150 °C), the specimens with an alumina tube were immersed in a liquid silver medium, and oxygen (4 ml/min) was supplied through the alumina tube to simulate the actual SOM process. The isothermal hightemperature corrosion exposures were 1, 3, 5, and 7 d at 1150 °C in an argon atmosphere. Following the isothermal high-temperature corrosion tests, the specimens were withdrawn from the liquid silver medium and kept under an argon atmosphere, and the furnace was cooled to room temperature. After the reactor was opened, the specimens were removed and
3. Results and discussion 3.1. High-temperature corrosion phenomena Fig. 2 shows the morphologies of the cross-sectional corrosion layers of the Kanthal alloy specimens exposed to 1150 °C in molten silver under an oxidizing atmosphere for 1 (2a), 3 (2b), 5 (2c), and 7 d (2d) in the gas section and for 1 (2e), 3 (2f), 5 (2 g), and 7 d (2 h) in the liquid silver section. As shown in Fig. 2, the high-temperature corrosion phenomenon of Kanthal alloy after the isothermal corrosion tests at 1150 °C in liquid silver and gas atmosphere at each exposure time varied significantly. In the case of the gas section, a dark area developed, with some isolated patches, consisting of an Al-rich phase as shown in Fig. 2a–d. With continued exposure, they changed into an oxide phase, namely Al2O3 as shown in Fig. 2d. The composition of the dark area is presented in Section 3.3. The dark area beneath the surface of the specimens in the gas section is expected to effect the internal corrosion due to the depletion of the alloying elements. This may be due to the inward diffusion of oxygen through the cracks by the depletion of major alloying elements, such as Cr and Fe, as well as the different thermal-expansion characteristics of the different components of the localized attack area. In addition, the internal attack depth was 9.6, 15.1, 16.8, and 22.4 μm, after 1, 3, 5 and 7 d, respectively, that could be characterized by rapid kinetics followed by a stage of parabolic-rate
Fig. 1. Schematic diagram of the apparatus used for the corrosion tests (inset shows the layout of the specimen in liquid silver). 2
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 2. Cross-sectional micrographs of Kanthal alloy corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a, e) 1, (b, f) 3, (c, g) 5, and (d, h) 7 d ((a–d) in the gas section and (e–h) in the liquid silver section).
Fig. 3. XRD patterns of the gas sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
behavior as presented by the internal attack depth with exposure time. For a liquid silver section, an Ag-rich layer retards the progress of the corrosion reactions owing to the formation of a cohesive Ag-rich layer beneath an oxide layer acting as an effective barrier toward the inward diffusion of oxygen and outward diffusion of alloying elements in the Kanthal alloy. This phenomenon was reported in Fe-20Cr-4Al alloyed with noble metals (Pd, Pt, and Au) and yttrium, which contributed through the adherence of the Al2O3 passivation layer [25]. Hence, silver effectively protects the specimens from continuous oxidation in a corrosion environment by enhancing the bonding strength of the Al2O3
corrosion layer. In general, under an oxidative environment with an elevated temperature, high-temperature corrosion phenomena can be significantly affected by the adhesive strength between the corrosion layer and base metal, considering the formation, maintenance, dissolution, and spallation of the corrosion layer, as well as the stability of the growth of the corrosion layer. 3.2. High-temperature corrosion products Fig. 3 shows the XRD patterns of the high-temperature corrosion 3
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 4. XRD patterns of the liquid silver sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Fig. 5. Surface SEM images and EDS point analysis results for Kanthal alloy corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a, c) 1 and (b, d) 7 d ((a, b) in the gas section and (c, d) in liquid silver section).
peaks of Ag, Al2O3, and FeAl2O4 for 3, 5, and 7 d. The surface SEM images and EDS analysis results of the Kanthal alloys corroded at 1150 °C in molten silver under an oxidizing atmosphere for 1 and 7 d in the gas section are shown in Fig. 5a and b, respectively. As shown in Fig. 5a and b, the localized EDS results indicate that the alloys contained 91.71–99.22 wt.% Ag from S1 and S2; and 47.21–50.06 wt.% Al and 42.31–49.00 wt.% O from S3 to S6 for 1 d; and 97.16 wt.% Ag from S6; 38.24–43.85 wt.% Al and 47.40–50.38 wt.% O from S1, S3, and S5;
products on the top surface of the specimen after the high-temperature corrosion tests at 1150 °C in molten silver under an oxidizing atmosphere for 1, 3, 5, and 7 d in the gas section. Fig. 4 shows the XRD patterns of the high-temperature corrosion products on the bottom surface of the specimen after the high-temperature corrosion tests at 1150 °C in molten silver under an oxidizing atmosphere for 1, 3, 5, and 7 d in the liquid silver section. In the case of a gas atmosphere, as shown in Fig. 3, the XRD patterns showed peaks of Ag and Al2O3 for 1 d, and 4
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Chemistry software version 7.11 (Outotec). The initial element composition is 66.44 mol% Fe, 22.43 mol% Cr, and 11.13 mol% Al according to the Kanthal alloy composition. The concentration of metallic Al rapidly decreases with an increase in oxygen, which causes a rapid increase in Al2O3. From a thermodynamic perspective, Al2O3 is an extremely stable oxide in Fe-Cr-Al based alloys at above 1000 °C. Hence, for application in an oxygen-rich environment at higher temperatures, namely, up to 1200 or 1300 °C, alloys and alloy coatings are often designed to develop the surface layers of Al2O3 that do not form higher volatile oxides [26]. Hence, the corrosion product of the specimens immersed in liquid silver with a limited oxygen concentration is only Al2O3 without a spinel formation, as confirmed in Fig. 4. With an increase in oxygen concentration, metallic Al and Cr are completely converted into FeAl2O4 and FeCr2O4, respectively, as shown in Fig. 6. In order to form FeCr2O4 phase, it is necessary to continuously supply oxygen, but due to the formation of the dense FeAl2O4 phase, sufficient oxygen is not supplied and then only FeAl2O4 phase was observed in the corrosion products, as shown in Fig. 3b–d. Consequently, in the tested alloys exposed to a gas section, Al2O3 was initially formed under a hot corrosion environment followed by the formation of FeAl2O4 spinel through a solid-phase reaction between FeO and Al2O3 [27].
Fig. 6. Calculated equilibrium composition of the reaction system based on the amount of oxygen at 1150 °C (initial composition of 66.44 mol% Fe, 22.43 mol % Cr, and 11.13 mol% Al).
and 34.99–36.51 wt.% Fe, 24.79–33.67 wt.% Al, and 24.95–25.46 wt.% O from S2 and S4 for 7 d, respectively. By combining the XRD (Fig. 3a and d) and EDS analysis results, these were confirmed as Ag and Al2O3 for 1 d, and Ag, Al2O3, and FeAl2O4 for 7 d. Fig. 5c and d show the surface SEM images and EDS analysis results, respectively, of the liquid silver section of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for 1 and 7 d. As shown in Fig. 5c and d, the localized EDS results indicate that they contained 100 wt.% Ag from S1 and S4; 45.36–49.12 wt.% Al; and 43.53–45.45 wt.% O from S2, S3, S5, and S6 for 1 d, and 93.54–100 wt. % Ag from S2 to S4, and 46.37–49.82 wt.% Al and 43.98–45.26 wt.% O from S1, S5, and S6 for 7 d, respectively. By combining the XRD (Fig. 4a and d) and EDS analysis results, these were confirmed as Ag and Al2O3 for 1 and 7 d, respectively. Fig. 6 shows the equilibrium composition of the reaction system along with the amount of oxygen at 1150 °C as calculated using HSC
3.3. High-temperature corrosion behavior Figs. 7 and 8 show the cross-sectional SEM images and elemental mapping as well as EDS point analysis results for the gas section of the Kanthal alloy specimens exposed to 1150 °C in molten silver under an oxidizing atmosphere for 1 (Figs. 7a and 8 a), 3 (Figs. 7b and 8 b), 5 (Figs. 7c and 8 c), and 7 d (Figs. 7d and 8 d). As shown in Figs. 7 and 8, the dark area occurred beneath the surface of the specimens, which caused an effect of the internal corrosion based on the different thermal-expansion characteristics between the dark area and the base metal. For example, the localized EDS results obtained from S6 (Fig. 8a) indicate that it contained 14.59 wt.% Al, 19.16 wt.% Cr, and 66.25 wt. % Fe, the depletions of which would increase the chemical potential, and thus lead to a higher corrosion rate [28]. Figs. 7b and 8 b show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the gas section of
Fig. 7. Cross-sectional SEM images and elemental mapping for the gas sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d. 5
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 8. Cross-sectional SEM images and EDS point analysis results for the gas sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Fig. 9. Cross-sectional SEM images and elemental mapping for the liquid silver sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Figs. 9a and 10 a show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the liquid silver section of the Kanthal alloy specimen exposed to 1150 °C in molten silver under an oxidizing atmosphere for 1 d. As can be observed from Fig. 9a, there was no evidence of dark area beneath the outer oxide layer and/or silver layer (Fig. 7a). As mentioned above, a cohesive Agrich layer beneath the oxide layer retards the corrosion progress by acting as an effective barrier. Figs. 9b–d and 10 b–d show the cross-sectional SEM images and elemental mapping as well as the EDS point analysis results for the liquid silver sections of the Kanthal alloy specimens exposed to 1150 °C in molten silver under an oxidizing atmosphere for 3 (Figs. 9b and 10 b), 5 Figs. 9c and 10 c), and 7 d (Figs. 9d and 10 d). As shown in Figs. 9 and 10, there was no evidence of dark area beneath the outer oxide layer and/or silver layer. The SOM anode simulation experiments showed that the Kanthal
the Kanthal alloy exposed to 1150 °C in molten silver under an oxidizing atmosphere for 3 d. As shown in Figs. 7b and 8b, much dark area occurred than those for 1 d (Figs. 7a and 8 a), and the amount of porous corrosion composed of Al and O was greater. Figs. 7c and 8 c show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the gas section of the Kanthal alloy specimen exposed to 1150 °C for 5 d. This specimen showed more dark area than that exposed for 3 d (Fig. 7b). Figs. 7d and 8 d show the cross-sectional SEM image and elemental mapping as well as the EDS point analysis results for the gas section of the Kanthal alloy specimen exposed to 1150 °C in molten silver under an oxidizing atmosphere for 7 d. As can be observed from Fig. 7d, this specimen showed more dark area than that exposed for 5 d (Fig. 7c). As shown in Fig. 7a–c, the dark area comprised an Al-rich phase; however, it subsequently developed into an oxide phase, Al2O3, with an increase in the exposure time as shown in Fig. 7d. 6
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
Fig. 10. Cross-sectional SEM images and EDS point analysis results for the liquid silver sections of the Kanthal alloy specimens corroded at 1150 °C in molten silver under an oxidizing atmosphere for (a) 1, (b) 3, (c) 5, and (d) 7 d.
Acknowledgments
alloy specimens exhibited completely different corrosion behaviors in the gas and liquid silver sections at 1150 °C in molten silver under an oxidizing atmosphere. Based on the fact that a well-defined protective corrosion layer is formed on the Kanthal alloy immersed in molten silver, a greater corrosion resistance is expected than that achievable with other current collector systems. This simple and durable SOM anode system will enhance the performance of the metal production process in a benign environment.
This work was supported by the Technology Innovation Program (10063427, Development of Eco-Friendly Smelting Technology for the Production of Rare Metal Production for Lowering Manufacturing Costs Using Solid Oxide Membrane) funded by the Ministry of Trade, Industry, and Energy (MOTIE, Korea). References
4. Conclusions
[1] U.B. Pal, D.E. Woolley, G.B. Kenney, Emerging SOM technology for the green synthesis of metals from oxides, JOM 53 (2001) 32–35. [2] Y.M. Gao, B. Wang, S.B. Wang, S. Peng, Study on electrolytic reduction with controlled oxygen flow for iron from molten oxide slag containing FeO, J. Min, Metall. Sect. B-Metall. 49 (1) (2013) 49–55 B. [3] A. Aytimur, I. Uslu, S. Kocyigit, F. Ozcan, Magnesia stabilized zirconia doped with boron, ceria and gadolinia, Ceram. Int. 38 (2012) 3851–3856. [4] S. Das, Primary magnesium production costs for automotive applications, JOM 60 (2008) 63–69. [5] D.J.L. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Intermediate temperature solid oxide fuel cells, Chem. Soc. Rev. 37 (2008) 1568–1578. [6] A. Krishnan, X.G. Lu, U.B. Pal, Solid oxide membrane process for magnesium production directly from magnesium oxide, Metall. Mater. Trans. B 36 (2005) 463–473. [7] S. Selvasekarapandian, M.S. Bhuvaneswari, M. Vijayakumar, C.S. Ramya, P.C. Angelo, A comparative study on ionic conductivity of Sr and Mg stabilized zirconia by impedance spectroscopy, J. Eur. Ceram. Soc. 25 (12) (2005) 2573–2575. [8] U.B. Pal, A.C. Powell IV, The use of solid-oxide-membrane technology for electrometallurgy, JOM 59 (2007) 44–49. [9] S. Li, X. Zou, K. Zheng, X. Lu, Q. Xu, C. Chen, Z. Zhou, Direct production of TiAl3 from Ti/Al-containing oxides precursors by solid oxide membrane (SOM) process, J. Alloys Compd. 727 (2017) 1243–1252. [10] A. Krishnan, X.G. Lu, U.B. Pal, Solid oxide membrane process for magnesium production directly from magnesium oxide, Metall. Mater. Trans. B 36 (2005) 463–473. [11] X. Lu, X. Zou, C. Li, Q. Zhong, W. Ding, Z. Zhou, Green electrochemical process solid-oxide oxygen-ion-conducting membrane (SOM): direct extraction of Ti-Fe alloys from natural ilmenite, Metall. Mater. Trans. B 43 (2012) 503–512. [12] X. Zou, X. Lu, Z. Zhou, C. Li, W. Ding, Direct selective extraction of titanium silicide Ti5Si3 from multi-component Ti-bearing compounds in molten salt by an electrochemical process, Electrochim. Acta 56 (2011) 8430–8437. [13] B. Zhao, X. Lu, Q. Zhong, C. Li, S. Chen, Direct electrochemical preparation of CeNi5 and LaxCe1-xNi5 alloys from mixed oxides by SOM process, Electrochim. Acta 55 (2010) 2996–3001. [14] X. Zou, X. Lu, C. Li, Z. Zhou, A direct electrochemical route from oxides to Ti-Si intermetallics, Electrochim. Acta 55 (2010) 5173–5179. [15] X. Su, X.G. Lu, C.H. Li, W.Z. Ding, X.L. Zou, Y.H. Gao, Q.D. Zhong, Preparation of TiFe based hydrogen storage alloy by SOM method, Int. J. Hydrogen Energy 36 (2011) 4573–4579. [16] U.B. Pal, A lower carbon footprint process for production of metals from their oxide sources, JOM 60 (2008) 43–47. [17] A. Martin, D. Lambertin, J.-C. poignet, M. Allibert, G. Bourges, L. Pescayre, J. Fouletier, The electrochemical deoxidation of metal oxides by calcium using a solid oxide membrane, JOM 55 (2003) 52–54.
The high-temperature corrosion behaviors of Kanthal alloy were investigated under an oxidizing atmosphere. The high-temperature corrosion behavior of the specimens showed that the gas section of Kanthal alloy undergoes much corrosion than the liquid silver section owing to the localized corrosion in the former. A localized corrosion region is thought to effect the internal corrosion, depending on the dark area formed on the outermost layer of the substrate, and is related to the region and content of the major element depletion. The isothermal corrosion test revealed that the hot corrosion of Kanthal alloy in the gas section resulted in the formation of Al2O3 (1 d) and Al2O3 and FeAl2O4 (3, 5, and 7 d). On the other hand, only Al2O3 was identified as the hot corrosion product in the liquid silver section owing to its limited oxygen solubility. There was dark area beneath the outer oxide layer in the gas section, which cause to effect the internal corrosion by the different thermal-expansion characteristics between the dark area and base metal, whereas there was no evidence of dark area beneath the outer oxide layer in the liquid silver section, and the cohesive Ag-rich layer acting as an effective barrier beneath the oxide layer retards the corrosion progress.
Declaration of Competing Interest ✓ All authors have participated in analysis and interpretation of the data; drafting the article or revising it critically for important intellectual content; and approval of the final version. ✓ This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. ✓ The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript. 7
Corrosion Science xxx (xxxx) xxxx
G.-S. Lim, et al.
[24] J.E. Indacochea, J.L. Smith, K.R. Litko, E.J. Karell, Corrosion performance of ferrous and refractory metals in molten salts under reducing conditions, J. Mater. Res. 14 (1999) 1990–1995. [25] T. Amano, High-temperature oxidation resistance of Al2O3-forming heat-resisting alloys with noble metal and rare earth additions, Mater. Corros. 62 (2011) 659–667. [26] F.H. Stott, G.C. Wood, J. Stringer, The influence of alloying elements on the development and maintenance of protective scales, Oxid. Met. 44 (1995) 113–145. [27] Y. Harada, High temperature oxidation and high temperature corrosion of metallic materials: I high temperature oxidation, Jpn. Therm. Spraying Soc. 33 (1996) 128–134. [28] D.A. Jones, Principles and Prevention of Corrosion, Macmillan Publishing Company, New York, 1992.
[18] X. Guan, U.B. Pal, Design of optimum solid oxide membrane electrolysis cells for metals production, Prog. Nat. Sci. 25 (2015) 591–594. [19] S. Su, U.B. Pal, X. Guan, Solid oxide membrane electrolysis process for aluminum production: experiment and modeling, J. Electrochem. Soc. 164 (2017) F248–F255. [20] X. Guan, U.B. Pal, Y. Jiang, S. Su, Clean metals production by solid oxide membrane electrolysis process, J. Sustain. Metall. 2 (2016) 152–166. [21] A. Allanore, L. Yin, D.R. Sadoway, A new anode material for oxygen evolution in molten oxide electrolysis, Nature 497 (2013) 353–356. [22] G.Z. Chen, D.J. Fray, T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride, Nature 407 (2000) 361–364. [23] J.E. Indacochea, J.L. Smith, K.R. Litko, E.J. Karell, A.G. Raraz, High-temperature oxidation and corrosion of structural materials in molten chlorides, Oxid. Met. 55 (2001) 1–16.
8