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Hot corrosion behavior of Ni-base alloys in a molten salt under an oxidizing atmosphere
Journal of Alloys and Compounds 468 (2009) 263–269
Hot corrosion behavior of Ni-base alloys in a molten salt under an oxidizing atmosphere Soo-Haeng Cho ∗ , Jin-Mok Hur, Chung-Seok Seo, Ji-Sup Yoon, Seong-Won Park Korea Atomic Energy Research Institute, 305-353 Daejeon, Republic of Korea Received 20 November 2006; accepted 20 December 2007 Available online 6 February 2008
Abstract The electrolytic reduction of a spent oxide fuel involves liberation of the oxygen in a molten LiCl electrolyte, which is a chemically aggressive environment that is too corrosive for typical structural materials. So, it is essential to choose the optimum material for the process equipment for handling a molten salt. In this study, corrosion behaviors of Inconel 713LC, Inconel MA 754, Haynes 214 and Haynes HR-160 in a molten LiCl–Li2 O salt under an oxidizing atmosphere were investigated at 650 ◦ C for 72–216 h. Inconel 713LC alloy showed the highest corrosion resistance among the examined alloys. Corrosion products of Inconel 713LC were Cr2 O3 , NiCr2 O4 and NiO, and those of Inconel MA 754 were Cr2 O3 and Li2 Ni8 O10 while Cr2 O3 , LiNiO2 , (Al,Cr)2 O3 and LiFeO2 were identified as the corrosion products of Haynes 214. Haynes HR-160 showed LiCoO2, CoCr2 O4 Li(Ni,Co)O2 and (Cr,Ti)2 O3 as its corrosion products. Inconel 713LC showed a localized corrosion behavior while Inconel MA 754 and Haynes 214 showed a uniform corrosion behavior. In the case of Haynes HR-160, an intergranular corrosion was observed. © 2008 Elsevier B.V. All rights reserved. Keywords: High temperature corrosion; Lithium molten salt; Electrolytic reduction
1. Introduction Molten salt technology has been widely applied in the industrial world because of its physical and chemical characteristics, especially its high electrical conductivity, high processing rate, fluid features, etc. And recently, it has attracted much attention in the fields of jet engines, fuel cells, catalysts, solar energy, and metal refinement. Therefore, studies on the corrosion of the equipment and the structural materials for handling high temperature molten salts have also been continuously carried out. For example, the corrosion of a gas turbine by a molten Na2 SO4 [1–3] and the corrosion related to a molten carbonate fuel cell [4,5] have been extensively investigated. Also there are many reports on an accelerated oxidation in chloride molten salts [6–10]. However, they are mainly short time electrochemical experiments and few reports are found on long-term experiments at a high temperature for the corrosion of commercial alloys in
a chloride molten salt by considering their corrosion products, rate and behaviors. The electrochemical reduction process for spent oxide nuclear fuel is carried out in a LiCl–Li2 O molten salt at 650 ◦ C. The liberation of oxygen on an anode and the high temperature molten salts of the process cause a chemically aggressive environment that is too corrosive for ordinary structural materials. Therefore, for an implementation of the electrochemical reduction technology, corrosion resistance materials should be developed. Guided by their excellent mechanical property and corrosion resistance at a high temperature atmosphere, we selected Inconel 713LC, Inconel MA 754, Haynes 214 and Haynes HR-160 as the candidate materials for service in the electrochemical reduction process and investigated their corrosion behaviors under simulated electrochemical reduction conditions. 2. Experimental
∗
Corresponding author. Tel.: +82 42 868 2584; fax: +82 42 868 2042. E-mail address: [email protected] (S.-H. Cho).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.094
The chemical compositions of the used superalloys are shown in Table 1. After the specimens were heat treated at 1050 ◦ C for 1 h and water-quenched, they were prepared with dimensions of 70 mm × 15 mm × 2 mm were ground
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Table 1 The chemical compositions of the tested alloys (wt.%) Alloy
Ni
Cr
Fe
C
Si
Mn
Mo
Nb
Al
Co
Ti
Inconel 713LC Inconel MA 754 Haynes 214 Haynes HR-160
74.0 77.8 ◦ C 75.0 36.8
11.57 20.21 16.03 28.3
0.10 0.27 3.71 <0.1
0.05 0.05 0.03 0.05
0.02 – 0.1 2.67
<0.01 – 0.2 0.45
4.15 – <0.1 <0.05
– – <0.05 <0.05
6.05 0.32 4.46 0.09
0.08 – <0.05 30.8
0.76 0.44 <0.1 0.53
Fig. 1. Schematic diagram of the apparatus for the corrosion test. with SiC paper, polished with a diamond paste and cleaned in deionized water and acetone. The experimental apparatus is shown in Fig. 1. A LiCl–Li2 O molten salt was introduced into a dense MgO crucible and then heated at 300 ◦ C for 3 h in an argon atmosphere to remove any possible moisture pick-up. After reaching the set conditions, the specimens and alumina tube were immersed into the molten salt, and mixed gas (Ar–10%O2 ) was supplied through the alumina tube. The corrosion tests were carried out at 650 ◦ C and for 72–216 h. The Li2 O concentration in LiCl was 3 wt.%. Following the corrosion test, the specimens were withdrawn from the salt and kept under argon gas atmosphere while the furnace was cooled to room temperature. All the specimens were visually inspected after the corrosion test and the salt attached to the test specimens was removed by washing them with deionized water in an ultrasonic cleaner. The insoluble part of the corrosion scale removed from the specimens was analyzed by XRD (X-Ray Diffractometer, Rigaku, DMAX/1200), All the clean specimens were later checked for changes in their weight and subsequentlt examined by SEM (Scanning Electron Microscope, JEOL, JSM-6300). Selected spots on the corroded specimens were analyzed by EDS (Energy Dispersive X-ray Spectroscope, JEOL, JSM-6300).
to its spallation from the base metal surface. Therefore, it was thought that the weight variations of the specimen would be greatly affected by the adhesive strength between the corrosion layer and the base metal considering the formation and spallation of the corrosion layer as well as the stability of the growth of the corrosion layer. 3.2. Corrosion products Fig. 3 shows the XRD patterns of the corrosion products after the corrosion tests in a LiCl–3% Li2 O molten salt at 650 ◦ C dur-
3. Results and discussion 3.1. Corrosion rate The weight losses of Inconel 713LC, Inconel MA 754, Haynes 214 and Haynes HR-160 after the corrosion tests in a LiCl–3% Li2 O molten salt at 650 ◦ C as a function of the time are presented in Fig. 2. Inconel 713LC showed the lowest corrosion rate while Haynes HR-160 showed the highest corrosion rate. Under a high temperature oxidative molten salt environment, the reason for the weight loss of the specimens with time was attributed to a cracking of the protective layer, which led
Fig. 2. Weight loss of the alloys corroded at 650 ◦ C, as a function of the time.
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Fig. 3. XRD patterns of the corrosion products of (a) Inconel 713LC, (b) Inconel MA 754, (c) Haynes 214 and (d) Haynes HR-160 corroded at 650 ◦ C for 72 and 216 h.
ing 72–216 h. For Inconel 713LC corroded for 72 and 216 h, the peaks attributed to Cr2 O3 , NiCr2 O4 and NiO were observed (Fig. 3(a)). The corrosion products of Inconel MA 754 were Cr2 O3 for 72 h and Li2 Ni8 O10 and Cr2 O3 for 216 h, respectively. From a thermodynamic point of view, Cr2 O3 is the most stable oxide in the Ni-Cr based alloys [11], therefore, Cr2 O3 was initially formed as a corrosion product. With the progress of the corrosion, Li2 Ni8 O10 in addition to Cr2 O3 was observed and the reason was attributed to an outward diffusion of Ni ions through the initially formed Cr oxide layer and its reaction with the Li based molten salt (Fig. 3(b)). Fig. 3(c) shows the corrosion products of Haynes 214, an Al-rich alloy. After the corrosion test for 72 h, Cr2 O3 , LiNiO2 and (Al,Cr)2 O3 were observed as the corrosion products and the reason was attributed to the inward diffusion of the Li ion and the outward diffusion of Ni and Al. As the diffusion coefficient of a metal ion in an oxide is Fe3+ > Fe2+ > Ni2+ > Cr3+ , Fe3+ has the highest diffusion rate through the Cr oxides [12]. Therefore, the outward diffusion of the Fe ion through the initially formed Cr2 O3 layer and its reaction with the Li based molten salt resulted in LiFeO2 . The corrosion products of Haynes HR-160 after the 72 h corrosion test were Co based and Co–Cr based oxides (LiCoO2 , CoCr2 O4 ) due to the preferential oxidations of Cr and Co. Since the diffusion coefficient of a metal ion in an oxide is Fe3+ > Fe2+ > Ti3+ > Ni2+ > Cr3+ [12], the corrosion products, Li(Ni,Co)O2 and (Cr,Ti)2 O3 , might be formed by an inward diffusion of the Li ion and an outward diffusion of Ni and Ti
through the oxide layer. Consequently, the formation of corrosion products in an oxidative Li based molten salt depends on the thermodynamic stability of the oxide formation along with the diffusion of constituent elements and the Li ion. 3.3. Corrosion behaviors Fig. 4 shows the cross-sectional SEM images of the superalloys corroded for 72 h at 650 ◦ C in a LiCl–3% Li2 O molten salt. For Inconel 713LC and Inconel MA 754, a spallation of the outer corrosion layer was not observed. The inner corrosion layers of Inconel 713LC and Inconel MA 754 showed a localized and a uniform corrosion behavior, respectively However, Haynes 214 and Haynes HR-160 spalled off their outer corrosion layers and their corrosion behaviors were a uniform corrosion for Haynes 214 and an Intergranular corrosion for Haynes HR-160. Fig. 5 shows the cross-sectional SEM image and elemental maps of Inconel 713LC corroded for 72 h. The corrosion layer was continuous, dense and adherent. The outer corrosion layer mainly consisted of Cr, Ni based oxides. A solid state reaction between the surface Ni based oxide(NiO) and Cr2 O3 might cause the formation of the NiCr2 O4 spinel oxides observed in Fig. 3(a) [12]. Beneath the outer corrosion layer, the oxygen active elements such as Al and Ti were preferentially oxidized and participated in the corrosion layer [11]. It was reported that the addition of 4–5 wt.% Al enhanced the oxidation resistance of alloys due to the formation of an Al2 O3 protective layer at a high
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Fig. 4. Cross-sectional SEM images of (a) Inconel 713LC, (b) Inconel MA 754, (c) Haynes 214 and (d) Haynes HR-160 corroded at 650 ◦ C for 72 h.
temperature environment [13]. Ni is an element which prevents the inward diffusion of an oxygen ion by its accumulation on the interface of oxide/oxide or beneath the oxide layers [14,15]. In this study, an enrichment of Ni in an outer corrosion layer was identified and thus Ni was considered as a useful element for an enhancement of the corrosion resistance in a hot molten salt under an oxidizing atmosphere. Fig. 6 shows the cross-sectional SEM image and elemental maps of Inconel MA 754 corroded for 72 h. Inconel MA 754 is an oxide dispersion strengthened, Ni-Cr superalloy, produced by a mechanical alloying. Its Y based oxide (0.6 wt.%) dispersion imparts an exceptionally high temperature strength and creep resistance to the highly corrosion-resistant Ni–Cr matrix. The oxygen active elements were primarily added for an increase of the adhesive strength between the metal oxides and a base metal.
The outer corrosion layer consisted of dense and continuous Cr based oxides in consistent with the results of the XRD analysis shown in Fig. 3(b). The accumulation of Y2 O3 particles at the interface between a Cr2 O3 layer and a base metal is shown in Fig. 6. The inward diffusion rate of atomic oxygen in the Y2 O3 particles is faster than the outward diffusion rate of a metal ion. Therefore, Cr showing a greater affinity for oxygen was preferentially oxidized resulting in the formation of much more Cr2 O3 [12]. The crack in the corrosion layer interface was attributed to the stress generated by the oxide formation in an oxide layer [13]. The cross-sectional SEM image and elemental maps of Haynes 214 corroded for 72 h are shown in Fig. 7. The surface of the outer corrosion layer mainly consisted of a Fe based oxide. The reason can be explained by the faster outward diffu-
Fig. 5. Cross-sectional SEM image and the elemental distribution of Inconel 713LC corroded at 650 ◦ C for 72 h.
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Fig. 6. Cross-sectional SEM image and the elemental distribution of Inconel MA 754 corroded at 650 ◦ C for 72 h.
sion rate of the Fe ion than the Cr ion and its diffusion through the initially formed Cr based oxide layer and its reaction with the oxygen ion. These results are in coincident with the analysis of the corrosion products (Fig. 3(c)). The corrosion layer in Fig. 7 shows the depletion of Cr and Al and the accumulation of Ni. The depletion of Cr and Al was attributed to a spallation of the outer corrosion layer due to their surface diffusion and preferential oxidation for the formation of an outer corrosion layer. An accumulation of the Fe content in the corrosion layer was observed. The fast outward diffusion of Fe in the base metal caused the formation of Fe oxide during the oxidation reaction and a spallation of the outer corrosion layer. Fe based oxides are less dense than Ni based and Cr based oxides which can lead to many cation defects [16], a void formation, internal stress and a consequent spallation in the interface of an oxide layer/oxide layer or the interface of an oxide layer/base metal. The depletion of Cr and a Ni-rich region were observed in the corrosion layer. The cross-sectional SEM image and elemental maps of Haynes HR-160 corroded for 72 h are shown in Fig. 8. The corrosion showed an intergranular corrosion behavior, and the
depletion of Cr and Co and the formation of a Ni, Si-rich region were observed at the grain boundaries. As a result, the grain boundary or adjacent regions are often less corrosion resistant, and a preferential corrosion at a grain boundary may be severe enough to drop grains out of the surface [17]. Fig. 9 shows the cross-sectional SEM image and EDS results of Haynes HR-160 corroded for 72 h. The depletion of Cr and Co and the formation of a Ni, Si-rich region were observed at the grain boundaries and these results are coincident with the results shown in Fig. 8. The initially formed corrosion products were Cr, Co based oxides caused by an outer diffusion of Co which is in agreement with the results shown in Fig. 3(d). Fig. 10 shows the cross-sectional SEM images of the oxide scale detached Haynes HR-160 which was corroded for 168 h. The reason for the observation of the base metal surface after a corrosion of multi-component alloys is generally attributed to an accompanying spallation of their grains and their corrosion layer when an intergranular corrosion proceeds. Joshi and Stein [18] suggested that the electrochemical potential differences owing to the more noble character of carbides formed at
Fig. 7. Cross-sectional SEM image and the elemental distribution of Haynes 214 corroded at 650 ◦ C for 72 h.
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Fig. 8. Cross-sectional SEM image and the elemental distribution of Haynes HR-160 corroded at 650 ◦ C for 72 h.
Fig. 9. Cross-sectional SEM image and EDS results of Haynes HR-160 corroded at 650 ◦ C for 72 h.
Fig. 10. Cross-sectional SEM images of the oxide scale detached Haynes HR-160 corroded at 650 ◦ C for 168 h.
the grain boundaries than a base metal caused corrosion at the interface. Also Haynes HR-160 might be very susceptible to an intergranular corrosion at this corrosion temperature condition which is equal to the sensitizing range. The main reason for the highest corrosion rate of Haynes HR-160 shown in Fig. 2 was also attributed to a spallation of the grains due to an intergranular corrosion. 4. Conclusions The corrosion behaviors of Inconel alloys in an oxidative LiCl–Li2 O molten salt were investigated in association with their electrochemical reduction process. The corrosion rate was in the order of Inconel 713LC < Inconel MA 754 < Haynes 214 < Haynes HR-160 showing an increase of the corrosion rate
with the corrosion time under a high temperature and oxidative Li molten salt environment. The corrosion products of Inconel 713LC were Cr2 O3 , NiCr2 O4 and NiO, and those of Inconel MA 754 were Cr2 O3 and Li2 Ni8 O10 , while Cr2 O3 , LiNiO2 , (Al,Cr)2 O3 and LiFeO2 were identified as the corrosion products of Haynes 214. Haynes HR-160 showed LiCoO2 , CoCr2 O4 , Li(Ni,Co)O2 and (Cr,Ti)2 O3 as its corrosion products. Inconel 713LC showed a localized corrosion behavior while Inconel MA 754 and Haynes 214 showed a uniform corrosion behavior. In the case of Haynes HR-160, an Intergranular corrosion was observed. Ni which had accumulated around the inner oxides delayed the corrosion rate. As Fe-rich oxides were not dense, the oxygen ions which had penetrated through the oxides accelerated their internal oxidation. Therefore, an excess Fe appeared to be deleterious to their corrosion resistance. Al and Ti, oxygen
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active elements, formed oxides by their preferential oxidation through their external diffusion, and prevented an internal oxidation by their accumulation in an interface between the oxide and the base metal. Acknowledgement This work was funded by the National Mid- and Long-term Atomic Energy R&D Program supported by the Ministry of Science and Technology of Korea. References [1] R.A. Rapp, Corros. Sci. 44 (2002) 209–221. [2] M.A. Uusitalo, P.M.J. Vuoristo, T.A. Mantyla, Corros. Sci. 46 (2004) 1311–1331. [3] J.G. Gonzalez, S. Haro, A. Martinez-Villafane, V.M. Salinas-Bravo, J. Porcayo-Calderon, Mater. Sci. Eng. A 435–436 (2006) 258–265.
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[4] B. Zhu, G. Lindbergh, Electrochim. Acta 46 (2001) 2593–2604. [5] S. Mitsushima, N. Kamiya, K.I. Ota, J. Electrochem. Soc. 137 (1990) 2713–2716. [6] T. Ishitsuka, K. Nose, Corros. Sci. 44 (2002) 247–263. [7] B.P. Mohanty, D.A. Shores, Corros. Sci. 46 (2004) 2893–2907. [8] F. Colom, A. Bodalo, Corros. Sci. 12 (1972) 731–738. [9] A. Ruh, M. Spiegel, Corros. Sci. 48 (2006) 679–695. [10] Tz. Tzvetkoff, J. Kolchakov, Mater. Chem. Phys. 87 (2004) 201– 211. [11] E.T. Turkdogan, Physical Chemistry of High Temperature Technology, Academic Press, New York, 1980. [12] H. Izuta, Y. Komura, J. Jpn. Inst. Met. 58 (1994) 1196–1205. [13] Y. Harada, Jpn. Therm. Spraying Soc. 33 (1996) 128–134. [14] G.C. Wood, B. Chattopadhyay, Corros. Sci. 10 (1970) 471–480. [15] F.H. Stott, G.C. Wood, J. Stringer, Oxid. Met. 32 (1989) 113. [16] S.H. Park, Y.D. Lee, Y.Y. Lee, J. Kor. Inst. Met. Mater. 33 (1995) 1323–1329. [17] D.A. Jones, in: D. Johnstone (Ed.), Principles and Prevention of Corrosion, Macmillan Publishing Company, New York, 1992, p. 19. [18] A. Joshi, D.F. Stein, Corrosion 28 (1972) 321–330.
Hot corrosion behavior of Ni-base alloys in a molten salt under an oxidizing atmosphere Soo-Haeng Cho ∗ , Jin-Mok Hur, Chung-Seok Seo, Ji-Sup Yoon, Seong-Won Park Korea Atomic Energy Research Institute, 305-353 Daejeon, Republic of Korea Received 20 November 2006; accepted 20 December 2007 Available online 6 February 2008
Abstract The electrolytic reduction of a spent oxide fuel involves liberation of the oxygen in a molten LiCl electrolyte, which is a chemically aggressive environment that is too corrosive for typical structural materials. So, it is essential to choose the optimum material for the process equipment for handling a molten salt. In this study, corrosion behaviors of Inconel 713LC, Inconel MA 754, Haynes 214 and Haynes HR-160 in a molten LiCl–Li2 O salt under an oxidizing atmosphere were investigated at 650 ◦ C for 72–216 h. Inconel 713LC alloy showed the highest corrosion resistance among the examined alloys. Corrosion products of Inconel 713LC were Cr2 O3 , NiCr2 O4 and NiO, and those of Inconel MA 754 were Cr2 O3 and Li2 Ni8 O10 while Cr2 O3 , LiNiO2 , (Al,Cr)2 O3 and LiFeO2 were identified as the corrosion products of Haynes 214. Haynes HR-160 showed LiCoO2, CoCr2 O4 Li(Ni,Co)O2 and (Cr,Ti)2 O3 as its corrosion products. Inconel 713LC showed a localized corrosion behavior while Inconel MA 754 and Haynes 214 showed a uniform corrosion behavior. In the case of Haynes HR-160, an intergranular corrosion was observed. © 2008 Elsevier B.V. All rights reserved. Keywords: High temperature corrosion; Lithium molten salt; Electrolytic reduction
1. Introduction Molten salt technology has been widely applied in the industrial world because of its physical and chemical characteristics, especially its high electrical conductivity, high processing rate, fluid features, etc. And recently, it has attracted much attention in the fields of jet engines, fuel cells, catalysts, solar energy, and metal refinement. Therefore, studies on the corrosion of the equipment and the structural materials for handling high temperature molten salts have also been continuously carried out. For example, the corrosion of a gas turbine by a molten Na2 SO4 [1–3] and the corrosion related to a molten carbonate fuel cell [4,5] have been extensively investigated. Also there are many reports on an accelerated oxidation in chloride molten salts [6–10]. However, they are mainly short time electrochemical experiments and few reports are found on long-term experiments at a high temperature for the corrosion of commercial alloys in
a chloride molten salt by considering their corrosion products, rate and behaviors. The electrochemical reduction process for spent oxide nuclear fuel is carried out in a LiCl–Li2 O molten salt at 650 ◦ C. The liberation of oxygen on an anode and the high temperature molten salts of the process cause a chemically aggressive environment that is too corrosive for ordinary structural materials. Therefore, for an implementation of the electrochemical reduction technology, corrosion resistance materials should be developed. Guided by their excellent mechanical property and corrosion resistance at a high temperature atmosphere, we selected Inconel 713LC, Inconel MA 754, Haynes 214 and Haynes HR-160 as the candidate materials for service in the electrochemical reduction process and investigated their corrosion behaviors under simulated electrochemical reduction conditions. 2. Experimental
∗
Corresponding author. Tel.: +82 42 868 2584; fax: +82 42 868 2042. E-mail address: [email protected] (S.-H. Cho).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.094
The chemical compositions of the used superalloys are shown in Table 1. After the specimens were heat treated at 1050 ◦ C for 1 h and water-quenched, they were prepared with dimensions of 70 mm × 15 mm × 2 mm were ground
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Table 1 The chemical compositions of the tested alloys (wt.%) Alloy
Ni
Cr
Fe
C
Si
Mn
Mo
Nb
Al
Co
Ti
Inconel 713LC Inconel MA 754 Haynes 214 Haynes HR-160
74.0 77.8 ◦ C 75.0 36.8
11.57 20.21 16.03 28.3
0.10 0.27 3.71 <0.1
0.05 0.05 0.03 0.05
0.02 – 0.1 2.67
<0.01 – 0.2 0.45
4.15 – <0.1 <0.05
– – <0.05 <0.05
6.05 0.32 4.46 0.09
0.08 – <0.05 30.8
0.76 0.44 <0.1 0.53
Fig. 1. Schematic diagram of the apparatus for the corrosion test. with SiC paper, polished with a diamond paste and cleaned in deionized water and acetone. The experimental apparatus is shown in Fig. 1. A LiCl–Li2 O molten salt was introduced into a dense MgO crucible and then heated at 300 ◦ C for 3 h in an argon atmosphere to remove any possible moisture pick-up. After reaching the set conditions, the specimens and alumina tube were immersed into the molten salt, and mixed gas (Ar–10%O2 ) was supplied through the alumina tube. The corrosion tests were carried out at 650 ◦ C and for 72–216 h. The Li2 O concentration in LiCl was 3 wt.%. Following the corrosion test, the specimens were withdrawn from the salt and kept under argon gas atmosphere while the furnace was cooled to room temperature. All the specimens were visually inspected after the corrosion test and the salt attached to the test specimens was removed by washing them with deionized water in an ultrasonic cleaner. The insoluble part of the corrosion scale removed from the specimens was analyzed by XRD (X-Ray Diffractometer, Rigaku, DMAX/1200), All the clean specimens were later checked for changes in their weight and subsequentlt examined by SEM (Scanning Electron Microscope, JEOL, JSM-6300). Selected spots on the corroded specimens were analyzed by EDS (Energy Dispersive X-ray Spectroscope, JEOL, JSM-6300).
to its spallation from the base metal surface. Therefore, it was thought that the weight variations of the specimen would be greatly affected by the adhesive strength between the corrosion layer and the base metal considering the formation and spallation of the corrosion layer as well as the stability of the growth of the corrosion layer. 3.2. Corrosion products Fig. 3 shows the XRD patterns of the corrosion products after the corrosion tests in a LiCl–3% Li2 O molten salt at 650 ◦ C dur-
3. Results and discussion 3.1. Corrosion rate The weight losses of Inconel 713LC, Inconel MA 754, Haynes 214 and Haynes HR-160 after the corrosion tests in a LiCl–3% Li2 O molten salt at 650 ◦ C as a function of the time are presented in Fig. 2. Inconel 713LC showed the lowest corrosion rate while Haynes HR-160 showed the highest corrosion rate. Under a high temperature oxidative molten salt environment, the reason for the weight loss of the specimens with time was attributed to a cracking of the protective layer, which led
Fig. 2. Weight loss of the alloys corroded at 650 ◦ C, as a function of the time.
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Fig. 3. XRD patterns of the corrosion products of (a) Inconel 713LC, (b) Inconel MA 754, (c) Haynes 214 and (d) Haynes HR-160 corroded at 650 ◦ C for 72 and 216 h.
ing 72–216 h. For Inconel 713LC corroded for 72 and 216 h, the peaks attributed to Cr2 O3 , NiCr2 O4 and NiO were observed (Fig. 3(a)). The corrosion products of Inconel MA 754 were Cr2 O3 for 72 h and Li2 Ni8 O10 and Cr2 O3 for 216 h, respectively. From a thermodynamic point of view, Cr2 O3 is the most stable oxide in the Ni-Cr based alloys [11], therefore, Cr2 O3 was initially formed as a corrosion product. With the progress of the corrosion, Li2 Ni8 O10 in addition to Cr2 O3 was observed and the reason was attributed to an outward diffusion of Ni ions through the initially formed Cr oxide layer and its reaction with the Li based molten salt (Fig. 3(b)). Fig. 3(c) shows the corrosion products of Haynes 214, an Al-rich alloy. After the corrosion test for 72 h, Cr2 O3 , LiNiO2 and (Al,Cr)2 O3 were observed as the corrosion products and the reason was attributed to the inward diffusion of the Li ion and the outward diffusion of Ni and Al. As the diffusion coefficient of a metal ion in an oxide is Fe3+ > Fe2+ > Ni2+ > Cr3+ , Fe3+ has the highest diffusion rate through the Cr oxides [12]. Therefore, the outward diffusion of the Fe ion through the initially formed Cr2 O3 layer and its reaction with the Li based molten salt resulted in LiFeO2 . The corrosion products of Haynes HR-160 after the 72 h corrosion test were Co based and Co–Cr based oxides (LiCoO2 , CoCr2 O4 ) due to the preferential oxidations of Cr and Co. Since the diffusion coefficient of a metal ion in an oxide is Fe3+ > Fe2+ > Ti3+ > Ni2+ > Cr3+ [12], the corrosion products, Li(Ni,Co)O2 and (Cr,Ti)2 O3 , might be formed by an inward diffusion of the Li ion and an outward diffusion of Ni and Ti
through the oxide layer. Consequently, the formation of corrosion products in an oxidative Li based molten salt depends on the thermodynamic stability of the oxide formation along with the diffusion of constituent elements and the Li ion. 3.3. Corrosion behaviors Fig. 4 shows the cross-sectional SEM images of the superalloys corroded for 72 h at 650 ◦ C in a LiCl–3% Li2 O molten salt. For Inconel 713LC and Inconel MA 754, a spallation of the outer corrosion layer was not observed. The inner corrosion layers of Inconel 713LC and Inconel MA 754 showed a localized and a uniform corrosion behavior, respectively However, Haynes 214 and Haynes HR-160 spalled off their outer corrosion layers and their corrosion behaviors were a uniform corrosion for Haynes 214 and an Intergranular corrosion for Haynes HR-160. Fig. 5 shows the cross-sectional SEM image and elemental maps of Inconel 713LC corroded for 72 h. The corrosion layer was continuous, dense and adherent. The outer corrosion layer mainly consisted of Cr, Ni based oxides. A solid state reaction between the surface Ni based oxide(NiO) and Cr2 O3 might cause the formation of the NiCr2 O4 spinel oxides observed in Fig. 3(a) [12]. Beneath the outer corrosion layer, the oxygen active elements such as Al and Ti were preferentially oxidized and participated in the corrosion layer [11]. It was reported that the addition of 4–5 wt.% Al enhanced the oxidation resistance of alloys due to the formation of an Al2 O3 protective layer at a high
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Fig. 4. Cross-sectional SEM images of (a) Inconel 713LC, (b) Inconel MA 754, (c) Haynes 214 and (d) Haynes HR-160 corroded at 650 ◦ C for 72 h.
temperature environment [13]. Ni is an element which prevents the inward diffusion of an oxygen ion by its accumulation on the interface of oxide/oxide or beneath the oxide layers [14,15]. In this study, an enrichment of Ni in an outer corrosion layer was identified and thus Ni was considered as a useful element for an enhancement of the corrosion resistance in a hot molten salt under an oxidizing atmosphere. Fig. 6 shows the cross-sectional SEM image and elemental maps of Inconel MA 754 corroded for 72 h. Inconel MA 754 is an oxide dispersion strengthened, Ni-Cr superalloy, produced by a mechanical alloying. Its Y based oxide (0.6 wt.%) dispersion imparts an exceptionally high temperature strength and creep resistance to the highly corrosion-resistant Ni–Cr matrix. The oxygen active elements were primarily added for an increase of the adhesive strength between the metal oxides and a base metal.
The outer corrosion layer consisted of dense and continuous Cr based oxides in consistent with the results of the XRD analysis shown in Fig. 3(b). The accumulation of Y2 O3 particles at the interface between a Cr2 O3 layer and a base metal is shown in Fig. 6. The inward diffusion rate of atomic oxygen in the Y2 O3 particles is faster than the outward diffusion rate of a metal ion. Therefore, Cr showing a greater affinity for oxygen was preferentially oxidized resulting in the formation of much more Cr2 O3 [12]. The crack in the corrosion layer interface was attributed to the stress generated by the oxide formation in an oxide layer [13]. The cross-sectional SEM image and elemental maps of Haynes 214 corroded for 72 h are shown in Fig. 7. The surface of the outer corrosion layer mainly consisted of a Fe based oxide. The reason can be explained by the faster outward diffu-
Fig. 5. Cross-sectional SEM image and the elemental distribution of Inconel 713LC corroded at 650 ◦ C for 72 h.
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Fig. 6. Cross-sectional SEM image and the elemental distribution of Inconel MA 754 corroded at 650 ◦ C for 72 h.
sion rate of the Fe ion than the Cr ion and its diffusion through the initially formed Cr based oxide layer and its reaction with the oxygen ion. These results are in coincident with the analysis of the corrosion products (Fig. 3(c)). The corrosion layer in Fig. 7 shows the depletion of Cr and Al and the accumulation of Ni. The depletion of Cr and Al was attributed to a spallation of the outer corrosion layer due to their surface diffusion and preferential oxidation for the formation of an outer corrosion layer. An accumulation of the Fe content in the corrosion layer was observed. The fast outward diffusion of Fe in the base metal caused the formation of Fe oxide during the oxidation reaction and a spallation of the outer corrosion layer. Fe based oxides are less dense than Ni based and Cr based oxides which can lead to many cation defects [16], a void formation, internal stress and a consequent spallation in the interface of an oxide layer/oxide layer or the interface of an oxide layer/base metal. The depletion of Cr and a Ni-rich region were observed in the corrosion layer. The cross-sectional SEM image and elemental maps of Haynes HR-160 corroded for 72 h are shown in Fig. 8. The corrosion showed an intergranular corrosion behavior, and the
depletion of Cr and Co and the formation of a Ni, Si-rich region were observed at the grain boundaries. As a result, the grain boundary or adjacent regions are often less corrosion resistant, and a preferential corrosion at a grain boundary may be severe enough to drop grains out of the surface [17]. Fig. 9 shows the cross-sectional SEM image and EDS results of Haynes HR-160 corroded for 72 h. The depletion of Cr and Co and the formation of a Ni, Si-rich region were observed at the grain boundaries and these results are coincident with the results shown in Fig. 8. The initially formed corrosion products were Cr, Co based oxides caused by an outer diffusion of Co which is in agreement with the results shown in Fig. 3(d). Fig. 10 shows the cross-sectional SEM images of the oxide scale detached Haynes HR-160 which was corroded for 168 h. The reason for the observation of the base metal surface after a corrosion of multi-component alloys is generally attributed to an accompanying spallation of their grains and their corrosion layer when an intergranular corrosion proceeds. Joshi and Stein [18] suggested that the electrochemical potential differences owing to the more noble character of carbides formed at
Fig. 7. Cross-sectional SEM image and the elemental distribution of Haynes 214 corroded at 650 ◦ C for 72 h.
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Fig. 8. Cross-sectional SEM image and the elemental distribution of Haynes HR-160 corroded at 650 ◦ C for 72 h.
Fig. 9. Cross-sectional SEM image and EDS results of Haynes HR-160 corroded at 650 ◦ C for 72 h.
Fig. 10. Cross-sectional SEM images of the oxide scale detached Haynes HR-160 corroded at 650 ◦ C for 168 h.
the grain boundaries than a base metal caused corrosion at the interface. Also Haynes HR-160 might be very susceptible to an intergranular corrosion at this corrosion temperature condition which is equal to the sensitizing range. The main reason for the highest corrosion rate of Haynes HR-160 shown in Fig. 2 was also attributed to a spallation of the grains due to an intergranular corrosion. 4. Conclusions The corrosion behaviors of Inconel alloys in an oxidative LiCl–Li2 O molten salt were investigated in association with their electrochemical reduction process. The corrosion rate was in the order of Inconel 713LC < Inconel MA 754 < Haynes 214 < Haynes HR-160 showing an increase of the corrosion rate
with the corrosion time under a high temperature and oxidative Li molten salt environment. The corrosion products of Inconel 713LC were Cr2 O3 , NiCr2 O4 and NiO, and those of Inconel MA 754 were Cr2 O3 and Li2 Ni8 O10 , while Cr2 O3 , LiNiO2 , (Al,Cr)2 O3 and LiFeO2 were identified as the corrosion products of Haynes 214. Haynes HR-160 showed LiCoO2 , CoCr2 O4 , Li(Ni,Co)O2 and (Cr,Ti)2 O3 as its corrosion products. Inconel 713LC showed a localized corrosion behavior while Inconel MA 754 and Haynes 214 showed a uniform corrosion behavior. In the case of Haynes HR-160, an Intergranular corrosion was observed. Ni which had accumulated around the inner oxides delayed the corrosion rate. As Fe-rich oxides were not dense, the oxygen ions which had penetrated through the oxides accelerated their internal oxidation. Therefore, an excess Fe appeared to be deleterious to their corrosion resistance. Al and Ti, oxygen
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