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Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl2 and the effects of alloy element Sc on its corrosion resistance
Journal Pre-proof Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl2 and the effects of alloy element Sc on its corrosion resistance
Jun Wei Wang, Zelong Bao, Rong Qiao Xiang, Gao Hao Hu, Yong Kang Hu, Qing Ma, Bin Li, Hong Guang Jia PII:
S0254-0584(19)31207-6
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122392
Reference:
MAC 122392
To appear in:
Materials Chemistry and Physics
Received Date:
02 May 2018
Accepted Date:
01 November 2019
Please cite this article as: Jun Wei Wang, Zelong Bao, Rong Qiao Xiang, Gao Hao Hu, Yong Kang Hu, Qing Ma, Bin Li, Hong Guang Jia, Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl 2 and the effects of alloy element Sc on its corrosion resistance, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122392
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Journal Pre-proof
Journal Pre-proof
Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl2 and the effects of alloy element Sc on its corrosion resistance Jun Wei Wang a, Zelong Baoa, Rong Qiao Xiang a, Gao Hao Hu a, Yong Kang Hu a, Qing Ma b, Bin Li a, Hong Guang Jia a,* a
Engineering Research Center of High Performance Light Metal Alloys and Forming, Department of Mechanical Engineering, Qinghai University, Xining 810016, PR China; b State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, PR China
Abstract: As a solar energy storage medium or heat transfer medium at moderate-high temperature, the molten NaCl-MgCl2 has the advantages of high stability and low cost. But a pipe and container material with Cr will be corroded badly in a molten chloride. Based on the founded corrosion resistance phase (Fe0.64Ni0.36) in previous experiments, the corrosion behavior of a new alloy of Fe-37 wt% Ni with rare earth alloy element Sc in the molten eutectic NaCl-MgCl2 at 520 ℃ was studied. The corrosion rate of the Fe-37 wt% Ni alloy is lower than that of the researched Fe-based superalloys (GH1035 and GH1140). After corrosion for 15 h, a porous structure with Ni-rich and Fe-depletion is formed at the local location on the Fe-37 wt% Ni surface. For the specimens with Sc (0.3 and 0.5 wt %), a compact protective MgO layer covers on the porous structure, which prevents further corrosion in the molten salt. It is speculated that the Sc is oxidized to Sc2O3 directly or by the chlorination-oxidation reactions, which improves the adhesion strength between the MgO layer and the alloy matrix by the effects of inhomogeneous nucleation and pinning. Key words: Molten salt corrosion, Chloride salt, Scandium, Fe-Ni alloy
1. Introduction Concentrating solar power (CSP) is an important clean and renewable energy technology. In order to prolong the equipment running time after sunset and improve the efficiency of lightthermal-electric conversion, the method of phase change thermal energy storage by molten salt at moderate-high temperature is study spot now [1-2]. Molten chloride is concerned for many advantages, such as high stability and thermal energy storage density, low cost and so on [3-5]. The Oak Ridge National Laboratory [6] has chosen a molten chloride as a candidate heat transfer
* Corresponding author. Engineering Research Center of High Performance Light Metal Alloys and Forming, Department of Mechanical Engineering, Qinghai University, Xining 810016, PR China; E-mail address: [email protected] (H.G. Jia).
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Journal Pre-proof medium in the next generation nuclear reactors, too. It is reported that the fusion latent heat of NaCl-52 wt% MgCl2 per unit mass is higher than it of other compared chlorides [7]. The chloride has broad application prospects. However, a metal pipe and container material will be corroded by the molten chloride severely. At present, many studies have shown that the corrosion rate of many kinds of commercially available stainless steels and super alloys in a molten chloride is hundreds micrometer to millimeter per year. The corrosion rate can’t been accepted for the CSP plants expected rate is lower than 50 μm/y [8-10]. A pre-oxidized film of Al2O3 was made on three kinds of nickel base super alloys by J. C. Gomez-Vidal et al. [11-12], but the film can’t protect these alloys matrix from corrosion in molten KCl-MgCl2. The corrosion behavior of ten kinds of superalloys in 850℃ molten KCl-32 at% MgCl2 for 100 h has been studied by A. J. Wallace [13]. Pure Ni and its alloy have the minimum corrosion depth. Inc625 and Inc718 were corroded uniformly. And there is a uniform Cr-depletion layer near these sample surfaces. Other specimens have a depletion of Cr along a grain boundary. After corrosion for 12 h of TP347H steel and a Ni based super alloy C22 in molten KCl-NaCl at 700℃, skeleton structure with Ni-rich and Cr-poor formed on these samples surface [14]. In our experiments, we also have found that a specimen with Cr has a depletion Cr in molten NaCl-MgCl2. The residual phase on sample surface are mainly (Ni, Fe) and Fe0.64Ni0.36 [9, 15-16]. There are two reasons: (1) Chlorination- oxidation reactions: Like corrosion in chloride gas or hot corrosion by chloride film, a Cr2O3 protected film on a sample surface will be destroyed in the way of acidic or basic dissolution. Then an element Cr in matrix reacts preferentially with corrosion product Cl2 and dissolved O2 in the molten chloride salt. Produced a chromium chloride volatilizes at experiment temperature. The chromium chloride also can react with O2 at high oxygen partial pressure zone, producing Cl2 and unprotected Cr2O3. The Cl2 dissolves in the molten salt and reacts with element Cr again [17-18]. (2) Because the molten chloride is a electrolyte, and the oxygen partial pressure is low in it, the protective Cr2O3 film can’t be formed. Consequently, the exposed Cr atom on a sample surface will dissolves in the molten salt preferentially as anode [9, 19]. However, the corrosion behavior of a Ni-Fe alloy without Cr in molten chloride salt has not been studied in detail. In addition, in a molten salt containing the MgCl2, preferentially oxidized MgO deposits on a sample surface, which can protect the sample matrix from corrosion in some degree [7, 20]. But -2-
Journal Pre-proof the adhesion strength between the MgO layer and the alloy matrix is weak. The MgO layer spalls off easily and losses protection. It is well know that the rear earth alloy elements of can refine grain size and improve adhesion strength of oxide layer. In a molten chloride, ScCl3 and Sc2O3 are sufficient stable at high temperature [21]. After adding 0.1 wt%~ 0.5 wt% Sc to Al-5.3 Cu-0.8 Mg alloy, their corrosion resistant is improved, a grain size became smaller, and the dendritic crystal changed to isometric crystal [22]. F. Rosalbino et al. [23] also reported that the corrosion rate of an aluminum alloy decreased with the increasing of the element Sc content. For the oxidized alloys of Fe-25Cr-4Al, some containing small addition of Sc strongly promotes oxide adherence for refining grain size, which providing rapid diffusion paths for the elements O and Al, and eliminating voids at oxide-matrix interface [24]. But how does the effects of the Sc on the corrosion behavior of the Fe-Ni alloy in the molten NaCl-MgCl2? Therefore, three kinds of Fe-36 at% Ni based alloys with different content of Sc and without Cr were made (Ni mass fraction is 37.15 wt%, following Fe-37Ni-xSc alloy for short). The corrosion behavior of these alloys in molten NaCl-52 wt% MgCl2 (Tm = 445℃ ) at 520℃ was studied. Combining the corrosion characteristic and thermodynamic calculation, the corrosion mechanism was discussed at last.
2. Experiment Electrolytic iron and nickel, and intermediate alloys Ni-Sc were used as raw materials, three kinds of alloy ingots with different content of Sc were made by a medium frequency induction melting furnace (Type: SP-25TC) under the protection of Ar atmosphere. The samples with the content of Sc (wt. %) 0.00, 0.30 and 0.50 were labeled as Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc, respectively. These ingots were cut into a cubes with the size of 10 mm×10 mm × 3 mm by a water cooled wire electric discharge machining (Type: DK7720). Then every specimen was polished by a metallographic abrasive paper of 800#, 1200#, 1500# and 2000#, successively. At last, all samples were washed in the distilled water and alcohol successively by the high frequency ultrasonic cleaning machine (Type: Shumei ultrasonic cleaners, KQ-200TDE), and then dried. The composition of specimens were detected by X Ray fluorescence (XRF), which is showed in Table 1. Because of the melting errors, the tested contents of the Ni in the specimens (36.1, 39.6 and 36.7 wt.%) are difference from the nominal composition (37 wt.% Ni). As -3-
Journal Pre-proof described below, a trace of Sc can be found at a grain boundary. Table 1. Detected composition of the specimens of Fe-37Ni-xSc by XRF (wt%) No. Ni Fe Sc Fe-37Ni-0.0Sc 36.10 Bal. 0.00 Fe-37Ni-0.3Sc 39.60 Bal. 0.30a Fe-37Ni-0.5Sc 36.70 Bal. 0.50a a: Nominal composition. The Sc can’t be detected here for too low content.
The mixture salt of NaCl-52 wt% MgCl2 was prepared by mixing dried analytically pure grade NaCl and MgCl2 (NaCl manufacturer: Yantai Shuangshuang Chemical Industry Co. Ltd., the purity ≥ 99.5%; MgCl2: Aladdin Industrial Corporation, the purity ≥ 99%). Then the mixture salt was put in a corundum crucibles and heated to 520℃ in a muffle furnace (heating rate 5℃ /min). When the mixture salt melts thoroughly, four samples of each kind of alloy were immersed in the molten salt in one crucible completely. Three parallel samples of them were weighed by an electronic balance with a accuracy 0.1 mg (SOPTOP-FA2004, Shanghai Sunny Hengping Scientific Instrument Co., Ltd.), and the surface areas were measured by a vernier caliper before immersion. After corrosion for 15 h and 30 h at 520℃ under air atmosphere, the crucible were took out from the furnace and cooled to room temperature. Then the residual salt on the surface of weighted specimens was dissolved in distilled water, then were washed under same conditions for all samples in distilled water and alcohol respectively for 10 minutes by the high frequency ultrasonic cleaning machine at room temperature (KQ-200TDE) (Total time: 20 minutes, Temperature: 23 ~ 26 ℃, Ultrasonic frequency: 80KHz, Power: 180W). Another specimen was immersed in distilled water statically until the residual salt on its surface was dissolved thoroughly. Then remove the specimen into alcohol. After immersed for several minutes, the sample was put out and dried, then their surface morphology and elements were analyzed by a scanning electron microscope (SEM, JSM-6610 LV) with energy dispersive spectrum (EDS) parts, and the composition on the sample surface was analyzed by a X-ray diffractometer (XRD, Rigaku, D/MAX2500X). After corrosion for 15 h and 30 h, cross sections of the three kinds of specimens were also analyzed by EDS.
3. Results and Analysis 3.1. Results After corroded by a corrosive of aqua regia, metallography of the specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc is showed in Fig. 1(a), (b) and (c), respectively. It is clear that -4-
Journal Pre-proof the grain size of the specimen with Sc is smaller than it without Sc. Fig. 1(d) and (e) is backscattered electron images (Signal: AsB) and EDS results of Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc, respectively. It is found that the element of Fe and Ni distribute evenly in all specimens. In Fig. 1(d) (specimen Fe-37Ni-0.3Sc), the element Sc distributes along the grain boundaries except enrichment in several small zones (also rich-Ni). In Fig. 1(e) (specimen Fe-37Ni-0.5Sc), the element Sc distributes more evenly than it in the specimen Fe-37Ni-0.3Sc, which is almost results of the small grain size in the specimen Fe-37Ni-0.5Sc. (a)
(c)
(b) 190 μm
150 μm
580 μm
(d)
(e)
Fig. 1 Metallography ((a), (b) and (c)), backscattered electron images and EDS results((d) and (e)) of the three kinds of specimens; (a) Fe-37Ni-0.0Sc; (b), (d) Fe-37Ni-0.3Sc; (c), (e) Fe-37Ni-0.5Sc.
After corrosion for 15 h in the molten salt, for all samples, corrosion products on the sample surface were cleaned by ultrasonic cleaning in the distilled water and alcohol under same conditions. The average mass change per unit area per hour is showed in Fig. 2. The dates of GH1140 and GH1035 were reported in the reference [25], which under the same corrosion conditions as in this paper. The specimens of GH1140 and GH1035 are iron based super-alloys. The Ni content in the GH1140 and GH1035 is (35 ~ 40) wt%, which is similar to the alloy studied here. But there are (20 ~ 23) wt% Cr and small amount of other alloy elements in the GH1140 and GH1035. After corrosion for 15 h, the mass of GH1140, GH1035 and Fe-37Ni-0.0Sc decrease, but Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc increase. In addition, the reduce amount of Fe-37Ni-0.0Sc is less than GH1140 and GH1035. And the increase amount of the samples Fe-37Ni-0.3Sc and -5-
Journal Pre-proof Fe-37Ni-0.5Sc increases with the increasing of the Sc content.
Average mass change 0.10
Average mass change (mg/(cm2·h))
0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10
GH1140*
GH1035*
*Reported in reference [25]
1# Sample
2#
3#
Fig. 2 Average mass change of the alloys in molten NaCl-53.2 wt% MgCl2 at 520℃
After the residual salt is dissolved in the distilled water, the surface morphology and local EDS results (at%) of the specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc are showed in Fig. 3(a), 3(b) and 3(c), respectively. It is found that a porous structure is formed locally on the three kinds of sample surface. For the samples Fe-37Ni-0.0Sc and Fe-37Ni-0.3Sc, the content of Fe at the porous structure (zone B) is lower than that on the uncorrected zone (zone A), but the content of Ni increase accordingly. And a little O and Mg is found. The sample Fe-37Ni-0.5Sc surface is mostly covered by a shell, which is showed in Fig.4 (d) later. The bare zone shown in Fig. 3(c) is the only place that don’t been covered by the shell. In order to reflect the sample surface morphology roundly, a low magnification (×50) picture is used here. It is speculated from the shell edge morphology that the bare zone was formed because of insufficient developing of the shell, not peeling off. Fig. 3(d) is magnification of zone D in Fig. 3(c). A porous structure (marked up with dashed line) is also formed slightly on the sample Fe-37Ni-0.5Sc surface, where the content of Fe decreases slightly, too, but Ni change lightly.
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Journal Pre-proof (a)
Pos. A B C
Fe 60.2 19.8 2.6
Ni 37.8 48.4 1.8
Mg 2.0 13.5 49.2
O 18.4 46.5
(b)
Pos. A B C
Fe 62.9 23.5 0.3
Ni 37.1 55.7 -
Mg 4.9 46.8
O 15.9 52.9
B
C
C B
A
A (d)
(c) D
A
B
A
B
C Pos. A B C
Fe 63.0 61.9 0.3
Ni 37.0 36.5 -
Mg 1.7 45.8
O 53.9
Pos. A B C
Fe 1.4 61.9 16.3
Ni 0.9 36.5 34.2 ×200
Mg 46.1 1.7 25.5
O 51.5 24.0
C
100 μm
Fig. 3 Surface morphology and local EDS results (at%) of the three kinds of sample surface after corroded in the molten NaCl-53.2 wt% MgCl2 at 520℃ for 15 h; (a) Fe-37 Ni; (b) Fe-37Ni-0.3 Sc; (c) Fe-37Ni-0.5 Sc; (d) magnification of zone D in Fig. 2(c).
It is also found from the Fig. 3(a), (b) and (c) that all the three kinds of samples are covered by a shell, which is rich in Mg and O. Coupled with XRD results of these sample surface in Fig. 4(a), (b) and (c), these shells are mainly MgO, and a little nickel oxide. For the samples Fe-37Ni-0.0Sc and Fe-37Ni-0.3Sc, the MgO layer mainly coves on the top of porous structure. For the sample Fe-37Ni-0.5Sc, as shown in Fig. 3(c) and (d), not only the slightly corroded place (porous structure) is covered by the MgO layer, but also the most surface is covered, which is showed in Fig. 4(d).
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Journal Pre-proof Fe-37 Ni-0.5 Sc
(a)
a: MgO b: Mg0.4Ni0.6O c: (Ni, Fe) d: Fe0.64Ni0.36
d
(b)
d dc ab
30
a
d c
40
50
a b b a c
b
60
70 80 2θ (°)
b cd
ab
b
90
b
100
a: MgO b: Mg0.4Ni0.6O c: (Ni, Fe) d: Ni3Fe
a
Intensity
b
Intensity
a
Fe-37 Ni-0.3 Sc
a
b
cd
abcd
b
110
120
30
40
a
50
60
b
70 80 2θ (°)
c
90
c
100
b
110
120
Fe-37 Ni
(c)
Intensity
b
a b
30
40
c
(d)
a: MgO b: Mg0.4Ni0.6O c: (Ni, Fe)
a
a
b c
50
60
a
a b b c
70 80 2θ (°)
b b
c b 90
100
110
120
Fig. 4 XRD results of the three kinds of specimen surface and surface morphology of Fe-37Ni-0.5Sc specimen after corrosion in molten NaCl-53.2 wt% MgCl2 at 520℃ for 15 h; (a) Fe-37 Ni; (b) Fe-37Ni-0.3 Sc; (c) Fe-37Ni-0.5 Sc; (d) surface morphology of Fe-37Ni-0.5Sc.
After stripped away the MgO layer on the specimens surface shown in Fig. 3 by a steel brush, back scattered electron images with EDS results of the specimens Fe-37Ni-0.3Sc and Fe-37Ni0.5Sc are showed in Fig. 5(a) and Fig. 6(a), respectively. There is a skeletal microstructure which is mainly Ni and Fe on the two kinds of specimen surface. The element Sc is found obviously at local region. In the figures group of Fig. 5 and Fig. 6: the figure (b) is magnification back scattered electron images of zone “A”, where is enrichment Sc in figure (a). the figure (c) is EDS results and relevant elements content (wt%) of zone “B” in the figure (b). Compared the Fig. 5(c) with Fig. 6(c), there are mainly elements O and Sc except Fe and Ni in the zone “B”. Consequently, it is speculate that a scandium oxide (Sc2O3) is formed here. The conclusion also can be proved by the Sc and O corresponding distribution results (Dotted line circle) in Fig. 5(a) and Fig. 6(a). However, the contents of O and Sc in zone “B” of the specimen Fe-37Ni-0.5Sc are higher than it at similar zone of the specimen Fe-37Ni-0.3Sc. On the contrary, the contents of Fe -8-
Journal Pre-proof and Ni in zone “B” of Fe-37Ni-0.5Sc are one time lower than it of the specimen Fe-37Ni-0.3Sc. In addition, it is found from Fig. 6(a) that the microstructure with Sc-rich is more compact than it around area. (a) Fig. 5(b)
A
Cps 5000
(b)
(c)
Sc, 31.4
4000
B
Cps (eV)
3000 O, 28.3 2000 Fe, 13.6
Ni, 24.1
Sc C Ni Mg,2.2
1000
Cl,0.4
Ni
0 0
1
2
3
4
5 6 E (keV)
7
8
9
10
Fig. 5 Back scattered electron images and EDS results of Fe-37Ni-0.3Sc after corrosion for 15 h then stripping away MgO layer; (a) back scattered electron images and EDS results; (b) magnification image of zone “A” in Fig. 5(a); (c) EDS results and elements content (wt%) of zone “B” in Fig. 5(b). (a)
A Fig. 6(b)
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Journal Pre-proof Cps 5000
(c)
(b)
4000 Ni, 50.3 Fe: 26.3
B
Cps (eV)
3000 Sc: 10.9 2000
Ni O: 9.4 Fe Mg:3.0
1000
Ni
Cl: 0.2
C 0 0
1
2
3
4
5 6 E (keV)
7
8
9
10
Fig. 6 Back scattered electron images and EDS results of Fe-37Ni-0.5Sc after corrosion for 15 h then stripping away MgO layer; (a) back scattered electron images and EDS results; (b) magnification image of zone “A” in Fig. 6(a); (c) EDS results and elements content (wt%) of zone “B” in Fig. 6(b).
After corrosion for 15 h, cross section back scattered electron images and EDS results of specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc are showed in Fig. 7(a), (b) and (c), respectively. The surface of the specimen Fe-37Ni-0.0Sc is covered locally by the MgO layer. An evenly MgO layer formed on the surface of Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc specimens surface, too. On the matrix of the three kinds of specimen, a porous microstructure under the MgO layer is found obviously. On the porous microstructure, it is enrichment Ni and depletion Fe. A plenty of Mg and O elements are also found here. Consequently, it is speculated that the molten salt penetrate in the porous microstructure, and then the MgO is formed in the holes. After corrosion for 30 h, cross section back scattered electron images and EDS results of the specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc are showed in Fig. 8(a), (b) and (c), respectively. A porous microstructure layer about 50 μm is formed on the matrix near surface of the specimen Fe-37Ni-0.0Sc, where enrichment Ni and depletion Fe. A plenty of Mg and O also are found on the porous microstructure. A thin evenly MgO layer formed on the surface of the specimens Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc. But the matrix microstructure of the two specimens is almost dense, except only a small region with Ni-rich and Fe-poor near the surface of specimen Fe-37Ni-0.5Sc, as shown in Fig. 8(c). Whatever corrosion for 15 h or 30 h, a local enrichment Sc on the cross section of the specimens Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc can be found obviously.
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Journal Pre-proof (a)
(b)
(c)
Fig.7 Cross section back-scattered electron images and EDS results of the three kinds of specimen after corrosion for 15 h; (a) Fe-37Ni-0.0Sc; (b) Fe-37Ni-0.3Sc; (c) Fe-37Ni-0.5Sc.
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Journal Pre-proof (a)
(b)
(c)
Fig. 8 Cross section back-scattered electron images and EDS results of the three kinds of specimen after corrosion for 30 h; (a) Fe-37Ni-0.0Sc; (b) Fe-37Ni-0.3Sc; (c) Fe-37Ni-0.5Sc.
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Journal Pre-proof 3.2. Analysis and discussion Like the corrosion mechanism of hot corrosion by chloride film, although the oxygen partial is low in molten chloride salt, but the chlorination-oxidation reactions are mainly reasons of a metal corrosion in it 0 0. However, in the molten MgCl2, the process of chlorination- oxidation is a little difference with in the molten KCl or NaCl. Because the MgCl2 very easily absorb moisture, during the heating process (before the mixed salt melting) under air atmosphere, MgCl2·6H2O forms and then decomposes into MgO, HCl and H2O. The reaction is showed as follow [20]: MgCl2·6H2O = MgO + 2HCl (g) + 5H2O (g)
0 ΔG793k = -251.911 kJ/mol
(1)
The HCl(g) that dissolved in the molten salt will react with the alloy elements (M, M = Fe, Cr, and Ni) directly as follow[15]:
2M + 2nHCl g = 2MCl n + nH 2 g
(2)
The HCl(g) also will react with O2, then producing Cl2. The reaction as follow: 4HCl g O2 g 2Cl 2 g 2H 2 O g
0 ΔG793k = -10.16 kJ/mol
(3)
The produced Cl2 dissolves in the molten salt and reacts with alloy elements. Then a metal chloride MCln (M = Fe, Ni, Sc) is produced, which is also called chlorination reaction here. The Standard Gibbs Free Energy change of the chlorination reactions (ΔGcl0) of alloy elements at different temperatures are showed in Fig. 9. The melting point of the produced chloride and its T4 temperature are also showed in the figure. The T4 temperature is a temperature value of a material start volatilizing obviously (material’s party pressure up to 1E-4 atm) [26]. Compared with Ni, the ΔGcl0 of Fe is more negative than it of Ni. And FeCl2 is produced preferentially. The T4 temperature of FeCl2 (536℃) is close to the experiment temperature (520℃). Iron chloride will volatilize during corrosion process. The element Ni is relatively stable in the molten salt. After the element Fe runs off from the specimens surface, the content of the Ni increase on the specimens surface layers. Consequently, as shown in Fig. 3, Fig. 7 and Fig. 8(a), these specimens are corroded to varying degrees. A porous structure with Fe-poor and Ni-rich is formed on their surface. But as shown in Fig. 4 (a), (b) and (c), iron oxide didn’t been detected obviously. It is also found from Fig. 9 that, for the alloy Fe-37Ni-Sc, ScCl3 is more easily formed because the ΔGcl0 of Sc is more negative than that of Fe and Ni. Because ScCl3 is stable in the - 13 -
Journal Pre-proof molten salt for its high melting point (960℃) [21]. It will react with the dissolved O2, which is showed as follow: (4/3) ScCl3 + O2 → (2/3) Sc2O3 + 2Cl2
(4)
0 -100
(1001℃ , 607℃ )
×
(306℃ , 167℃ )
×
ΔG0Cl (kJ/mol)
-200
×
(670℃ , 536℃ )
-300 Experiment temperature
-400
(960℃ , -)
×
Fe + Cl2 = FeCl2 (2/3)Fe + Cl2= (2/3)FeCl3 Ni + Cl2 = NiCl2 (2/3)Sc + Cl2= (2/3)ScCl3
-500 -600
0
200
400
600 800 1000 Temperature (℃ )
1200
1400
1600
Fig. 9 The Standard Gibbs Free Energy change of alloy elements react with 1 mol Cl2 at different temperatures, and the melting point and T4 temperature of chloride (Tm, T4)
The Standard Gibbs Free Energy change of oxygenation reactions (ΔGO0) of alloy element chloride at different temperature are showed in Fig. 10. The ScCl3 is oxidized preferentially than the chloride of Fe and Ni in a wide temperature range. Therefore, for the alloys with Sc, alloy element Sc will not only be oxidized to Sc2O3 by directly reacts with O2, but also in the form of chlorination-oxidation reactions. Because the Sc2O3 (melting point is 2314℃) is stable in molten chloride at the experiment temperature, it will adhere on the sample surface. In addition, the Sc2O3, MgO and Sc have same crystal texture (cubic structure); And the lattice parameter of Sc and MgO are also same (Sc: 4.52 ×4.52×4.52 angstrom, 90° × 90° × 90°；MgO: 4.21×4.52×4.52 angstrom, 90° × 90° × 90°). The Sc and MgO have same lattice group (Fm-3m), too. Therefore, it is think that the active element Sc will be oxidized preferentially to Sc2O3. Then the MgO deposits on the Sc2O3 and grows up little by little. The Sc2O3 as heterogeneous nucleation base for the MgO, which helps to form a MgO layer. As shown in Fig. 5 and Fig. 6, scandic oxide is found under MgO layer.
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Journal Pre-proof Furthermore, because the molten chloride is strong electrolyte, the electro-chemical corrosion is another corrosion form of an alloy in it. A metal atom (M = Fe, Ni) with low potential dissolves in the molten salt as an anode after losing electrons. An MgO particle is formed by the reaction between Mg2+ and O2-. The O2- is formed by dissolved oxygen atom captures the loosed electrons. These reactions as follow [9, 18]: M → Mn+ + ne
(5)
O 2 2e- O 2-
(6)
O2- + Mg2+ = MgO
(7)
100
FeCl3→FeO
50
FeCl2→FeO
ΔG0O (kJ/mol)
NiCl2→NiO
0 FeCl3→Fe3O4
FeCl2→Fe3O4
-50
FeCl3→Fe2O3
-100
FeCl2→Fe2O3
-150
Experiment temperature ScCl3→Sc2O3
-200
0
100
200 300 400 Temperature (℃ )
500
600
Fig. 10 The Standard Gibbs Free Energy change of oxidation reaction of alloy chlorides with 1 mol O2 at different temperatures
After exposure for a certain time in the molten salt, many corrosion pits are formed on the specimen surface for the reaction (5). At the same time, the MgO will deposit near these corrosion pits not only because O2-content is high near these corrosion pits, but also these corrosion pits are more conducive to the MgO nucleation. Consequently, as showed in Fig. 3, Fig.7 and Fig. 8, the MgO layer mostly covered on the top of the porous structure. However, by compared Fig. 7(a) with Fig. 8(a), the MgO layer on the Fe-37Ni-0.0Sc specimen (Fe-37Ni) surface can’t protect it from corrosion in the molten salt. It is maybe for the MgO particle on the sample surface is loose and incomplete, which can’t prevent the molten salt contacting with the specimen matrix. For the specimens with Sc, as analyzed above, the scandium oxide on sample surface will as
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Journal Pre-proof a base for MgO nucleation. In addition, it is surmise that when a corrosion pits penetrates into Sc atom along grain boundary, the Sc atom will be oxidized to Sc2O3 particle, too. Then a MgO takes the Sc2O3 particle as a nucleation substrate, and grows up in the corrosion pits. When the corrosion pit is filled by MgO, the corrosion process in this hole stop. The MgO layer in the corrosion pit grows continually by the deposition of MgO particles (produced in the form of reactions (6) and (7)) which diffused from other corrosion place in molten salt. After exposure for a sufficient time, these small MgO pieces on sample surface will connect each other and act as a solid barrier against further corrosion in contact with the molten salt. The MgO in the corrosion pit penetrates in alloy matrix likes a “nail”, which plays a pinning effect for the oxide film, and improves the adhesion strength between the oxide film and the alloy substrate. Moreover, the higher content of Sc in the alloy, the more number of "nail", and the stronger adhesion strength between the oxide layer and alloy matrix. Consequently, the Sc2O3 acts as MgO nucleation substrate and nail for MgO layer on samples surface. The development process is showed in Fig. 8. What’ more, there are more porous structures on the surface of specimen with Sc, which helps to increase the adherence strength between the MgO shell and the specimen matrix.
O2 + 2e- = O2O2- + Mg2+ = MgO O2 Molten Salt Oxide
Sc2O3 Sc atom
MgO layer
“Nail”
Alloy
Fig. 11 The schematic diagram of the development process of Sc2O3 acts as MgO nucleation substrate and nail for MgO layer on samples surface
Therefore, a porous structure is formed on the three kinds of sample surface during initial corrosion for 15 h, as shown in Fig. 7. However, with the extension of exposure time, a compact, intact MgO layer is pegged by Sc2O3 on the surface of specimen with Sc. The MgO layer as a solid barrier protects the specimen substrate from corrosion from molten salt. Consequently, a compact structure is found near Sc-rich zone under MgO layer, as shown in Fig. 6(a). A porous structure also can’t be found on Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc specimens surface after corrosion for 30 h, as shown in Fig. 8(b) and (c). - 16 -
Journal Pre-proof Comparing with the specimen Fe-37Ni-0.3Sc, because the Sc in the specimen Fe-37Ni-0.5Sc distributes more evenly, the pin effect is better, and the MgO layer coverage rate is higher. In addition, as shown in Fig.5(c) and Fig.6(c), the content of Sc and O on the Sc-rich zone in the specimen Fe-37Ni-0.5Sc are low than it in the specimen Fe-37Ni-0.3Sc, but the Fe and Ni are higher. It is clean that the mass gain of specimens with Sc (Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc) in Fig. 1 for deposited MgO layer on their surface. In addition, because these mass change values were measured after these samples ultrasonic cleaning for 10 minutes in distilled water and alcohol respectively, it is reveal that the adherence strength of MgO layer is strong enough.
4. Conclusion The corrosion behavior of Fe-37Ni alloy with different content of Sc (0.0, 0.3, and 0.5 wt%) was studied in molten NaCl-MgCl2 at 520℃. The corrosion resistance mechanism was discussed, too. (1) The corrosion rate of the advance alloy Fe-37wt% Ni is lower than that of iron based superalloys GH1140 and GH1035 under the same corrosion conditions. After corrosion for 15 h, the mass of specimens with Sc are increases for the formation of MgO protecting shell. (2) After corrosion for 15 h, there is a local corrosion with porous microstructure on all studied specimens’ surface. The porous structure with Fe-poor and Ni-rich is covered by the MgO shell. But the local corrosion is slightly for specimens with Sc. And a lot of compact MgO layer is covered on the sample Fe-37wt%Ni- 0.5Sc surface, which as a solid barrier against further corrosion in contact with the molten salt. (3) It is surmised that a Sc2O3 is formed after Sc is oxidized directly or by chlorinationoxidation reactions during corrosion process, which acts as heterogeneous nucleation base and a nail for the MgO layer.
Acknowledgement This work was been supported by Qinghai Natural Science Foundation [grant numbers 2016ZJ-759].
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Journal Pre-proof Reference [1] K.Vignarooban, X.H. Xu, A. Arvay, K. Hsu, A.M. Kannan, Heat transfer fluids for concentrating solar power systems - A review, Appl. Energy. 146 (2015) 383-396. [2] S.A. Mohamed, F.A. Al-Sulaiman, N.I. Ibrahim, Md. Hasan Zahir, A. Al-Ahmed, R. Saidur, B.S.Yılbaş, A.Z.Sahin, A review on current status and challenges of inorganic phase change materials for thermal energy storage systems, Renew. Sust. Energ. Rev. 70 (2017) 1072-1089. [3] P.D. Myers Jr, D.Y. Goswami, Thermal energy storage using chloride salts and their eutectics, Appl. Therm. Eng. 109B (2015) 889-900. [4] Y.Y. Li, X.K. Xu, X.X. Wang, P.W. Li, Q.Hao, B.X. Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP, Sol. Energy. 152 (2017) 57-79. [5] X.L. Wei, M. Song, W.L. Wang, J. Ding, J. P. Yang, Design and thermal properties of a novel ternary chloride eutectics for high- temperature solar energy storage, Appl. Energy. 156 (2015) 306-310. [6] Oak Ridge National Laboratory, Assessment of candidate molten salt coolants for the NGNP/NHI heat-transfer loop, Washington: U.S. Department of Energy, 2006. [7] M.M. Kenisarin, High-temperature phase change materials for thermal energy storage, Renew. Sust. Energ. Rev. 14 (2010) 955-970. [8] B. Liu, X.L. Wei, W.L. Wang, J.F. Lu, J. Ding, Corrosion behavior of Ni-based alloys in molten NaCl- CaCl2- MgCl2 eutectic salt for concentrating solar power, Sol. Energy Mater. Sol. Cells.170 (2017) 77-86. [9] J.W. Wang, C.Z. Zhang, Z.H. Li, H.X. Zhou, J.X. He, J.C. Yu, Corrosion behavior of nickelbased superalloys in thermal storage medium of molten eutectic NaCl-MgCl2 in atmosphere, Sol. Energy Mater. Sol. Cells. 164 (2017) 146-155. [10] M. Hofmeister, L. Klein, H. Miran, R. Rettig, S. Virtanen, R. F. Singer, Corrosion behavior of stainless steels and a single crystal superalloy in a ternary LiCl-KCl-CsCl molten salt, Corros. Sci. 90 (2015) 46-53. [11] J.C. Gomez-Vidal, A.G. Fernandez, R. Tirawat, C. Turchi, W. Huddleston, Corrosion resistance of alumina forming alloys against molten chlorides for energy production. II: electrochemical impedance spectroscopy under thermal cycling conditions, Sol. Energy Mater. Sol. Cells. 166 (2017) 234-245. [12] J.C. Gomez-Vidal, A.G. Fernandez, R. Tirawat, C. Turchi, W. Huddleston, Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests, Sol. Energy Mater. Sol. Cells. 166 (2017) 222-233. [13] A.J. Wallace, Molten Chloride Salts for Heat Transfer in Nuclear Systems. The University of Wisconsin-Madison, 2011. [14] S.N. Liu, Z.D. Liu, Y.T. Wang, J. Tang, A comparative study on the high temperature - 18 -
Journal Pre-proof corrosion of TP347H stainless steel, C22 alloy and laser-cladding C22 coating in molten chloride salts, Corros. Sci. 83 (2014) 396-408. [15] H.X. Zhou, J.W. Wang, Corrosion mechanism of GH4033 and GH4169 alloys in molten NaCl-MgCl2 at 520℃, Transactions of Materials and Heat Treatment. 38 (2017) 164-169. [16] Y. Lu, B. Cheng, J.W. Wang, Z.P. Wang, W.S. Li, H.F. Dong, Electrochemical⁃chemical⁃ dilution corrosion model of stainless steel in molten NaCl, Materials Sci. Tech-lond. 23 (2015) 75-80. [17] R.A. Rapp, Chemistry and electrochemistry of the hot corrosion of metals, Corrosion. 42 (1986) 568-577. [18] Y.S. Li, Y. Niu, G. Liu, W.T. Wu. Corrosion of pure Fe and 310SS beneath ZnCl2-KCl salt film at 450℃, Scta. Metallurgica Sinica. 36 (2000) 1183-1186. [19] A. Nishikata, Y. Shimatani, S. Haruyama. Electrochemical studies of the corrosion of Ni and Fe in molten chloride, J. Japan Inst. Metals. 48 (1984) 705-712. [20] J.W. Wang, H.X. Zhou, C.Z. Zhang, W.N. Liu, B.Y. Zhao, Influence of MgCl2 content on corrosion behavior of GH1140 in molten NaCl-MgCl2 as thermal storage medium, Sol. Energy Mater. Sol. Cells. 179 (2018): 194-201. [21] H.J. Seifert, Melting points of lanthanide trichlorides, J. Therm. Anal. Calorim. 82 (2005) 575 -580. [22] D.H. Xiao, B.Y. Huang, K.H. Chen, Microstructure and corrosion properties of Al- Cu- MgAg- (Sc) alloys. Journal of Central South University: Science and Technology, 40 (2009) 1252-1258. [23] F. Rosalbino, S. Delsante, G. Borzone, G. Scavino. Assessing the corrosion resistance of binary Al-Sc alloys in chloride-containing environment. Mater. and Corros. 68 (2017) 444449. [24] J.K. Tien, F.S. Pettit. Mechanism of oxide adherence on Fe-25Cr-4Al(Y or Sc) alloys. Metallurgical Transactions. 3 (1972) 1587- 1599. (https://doi.org/10.1007/BF02643050) [25] H.X. Zhou, J.W. Wang, X.H. Meng, G. Liu. The corrosion mechanism of Fe based superalloy in molten NaCl - 52 wt.% MgCl2 at 520℃, Corros. Eng. Sci. Techn. 52 (2017) 261-266 [26] R. Bender, The role of alloying elements in commercial alloys for corrosion resistance in oxidizing-chloridizing atmospheres Part I: Literature evaluation and thermodynamic calculations on phase stabilities, Corrosion resistance in oxidizing- chloridizing atmospheres, Mater. Corros. 54 (2003) 567-586.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
No.
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1. Corrosion behavior of a new designed Fe-Ni alloy in molten NaCl-MgCl2 was studied. 2. The effects of the alloy element Sc on the corrosion behavior of the alloy were analyzed. 3. The mechanism of the corrosion resistant of Fe-37Ni-xSc alloy in the molten NaCl-MgCl2 was discussed.
Jun Wei Wang, Zelong Bao, Rong Qiao Xiang, Gao Hao Hu, Yong Kang Hu, Qing Ma, Bin Li, Hong Guang Jia PII:
S0254-0584(19)31207-6
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122392
Reference:
MAC 122392
To appear in:
Materials Chemistry and Physics
Received Date:
02 May 2018
Accepted Date:
01 November 2019
Please cite this article as: Jun Wei Wang, Zelong Bao, Rong Qiao Xiang, Gao Hao Hu, Yong Kang Hu, Qing Ma, Bin Li, Hong Guang Jia, Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl 2 and the effects of alloy element Sc on its corrosion resistance, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122392
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Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl2 and the effects of alloy element Sc on its corrosion resistance Jun Wei Wang a, Zelong Baoa, Rong Qiao Xiang a, Gao Hao Hu a, Yong Kang Hu a, Qing Ma b, Bin Li a, Hong Guang Jia a,* a
Engineering Research Center of High Performance Light Metal Alloys and Forming, Department of Mechanical Engineering, Qinghai University, Xining 810016, PR China; b State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, PR China
Abstract: As a solar energy storage medium or heat transfer medium at moderate-high temperature, the molten NaCl-MgCl2 has the advantages of high stability and low cost. But a pipe and container material with Cr will be corroded badly in a molten chloride. Based on the founded corrosion resistance phase (Fe0.64Ni0.36) in previous experiments, the corrosion behavior of a new alloy of Fe-37 wt% Ni with rare earth alloy element Sc in the molten eutectic NaCl-MgCl2 at 520 ℃ was studied. The corrosion rate of the Fe-37 wt% Ni alloy is lower than that of the researched Fe-based superalloys (GH1035 and GH1140). After corrosion for 15 h, a porous structure with Ni-rich and Fe-depletion is formed at the local location on the Fe-37 wt% Ni surface. For the specimens with Sc (0.3 and 0.5 wt %), a compact protective MgO layer covers on the porous structure, which prevents further corrosion in the molten salt. It is speculated that the Sc is oxidized to Sc2O3 directly or by the chlorination-oxidation reactions, which improves the adhesion strength between the MgO layer and the alloy matrix by the effects of inhomogeneous nucleation and pinning. Key words: Molten salt corrosion, Chloride salt, Scandium, Fe-Ni alloy
1. Introduction Concentrating solar power (CSP) is an important clean and renewable energy technology. In order to prolong the equipment running time after sunset and improve the efficiency of lightthermal-electric conversion, the method of phase change thermal energy storage by molten salt at moderate-high temperature is study spot now [1-2]. Molten chloride is concerned for many advantages, such as high stability and thermal energy storage density, low cost and so on [3-5]. The Oak Ridge National Laboratory [6] has chosen a molten chloride as a candidate heat transfer
* Corresponding author. Engineering Research Center of High Performance Light Metal Alloys and Forming, Department of Mechanical Engineering, Qinghai University, Xining 810016, PR China; E-mail address: [email protected] (H.G. Jia).
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Journal Pre-proof medium in the next generation nuclear reactors, too. It is reported that the fusion latent heat of NaCl-52 wt% MgCl2 per unit mass is higher than it of other compared chlorides [7]. The chloride has broad application prospects. However, a metal pipe and container material will be corroded by the molten chloride severely. At present, many studies have shown that the corrosion rate of many kinds of commercially available stainless steels and super alloys in a molten chloride is hundreds micrometer to millimeter per year. The corrosion rate can’t been accepted for the CSP plants expected rate is lower than 50 μm/y [8-10]. A pre-oxidized film of Al2O3 was made on three kinds of nickel base super alloys by J. C. Gomez-Vidal et al. [11-12], but the film can’t protect these alloys matrix from corrosion in molten KCl-MgCl2. The corrosion behavior of ten kinds of superalloys in 850℃ molten KCl-32 at% MgCl2 for 100 h has been studied by A. J. Wallace [13]. Pure Ni and its alloy have the minimum corrosion depth. Inc625 and Inc718 were corroded uniformly. And there is a uniform Cr-depletion layer near these sample surfaces. Other specimens have a depletion of Cr along a grain boundary. After corrosion for 12 h of TP347H steel and a Ni based super alloy C22 in molten KCl-NaCl at 700℃, skeleton structure with Ni-rich and Cr-poor formed on these samples surface [14]. In our experiments, we also have found that a specimen with Cr has a depletion Cr in molten NaCl-MgCl2. The residual phase on sample surface are mainly (Ni, Fe) and Fe0.64Ni0.36 [9, 15-16]. There are two reasons: (1) Chlorination- oxidation reactions: Like corrosion in chloride gas or hot corrosion by chloride film, a Cr2O3 protected film on a sample surface will be destroyed in the way of acidic or basic dissolution. Then an element Cr in matrix reacts preferentially with corrosion product Cl2 and dissolved O2 in the molten chloride salt. Produced a chromium chloride volatilizes at experiment temperature. The chromium chloride also can react with O2 at high oxygen partial pressure zone, producing Cl2 and unprotected Cr2O3. The Cl2 dissolves in the molten salt and reacts with element Cr again [17-18]. (2) Because the molten chloride is a electrolyte, and the oxygen partial pressure is low in it, the protective Cr2O3 film can’t be formed. Consequently, the exposed Cr atom on a sample surface will dissolves in the molten salt preferentially as anode [9, 19]. However, the corrosion behavior of a Ni-Fe alloy without Cr in molten chloride salt has not been studied in detail. In addition, in a molten salt containing the MgCl2, preferentially oxidized MgO deposits on a sample surface, which can protect the sample matrix from corrosion in some degree [7, 20]. But -2-
Journal Pre-proof the adhesion strength between the MgO layer and the alloy matrix is weak. The MgO layer spalls off easily and losses protection. It is well know that the rear earth alloy elements of can refine grain size and improve adhesion strength of oxide layer. In a molten chloride, ScCl3 and Sc2O3 are sufficient stable at high temperature [21]. After adding 0.1 wt%~ 0.5 wt% Sc to Al-5.3 Cu-0.8 Mg alloy, their corrosion resistant is improved, a grain size became smaller, and the dendritic crystal changed to isometric crystal [22]. F. Rosalbino et al. [23] also reported that the corrosion rate of an aluminum alloy decreased with the increasing of the element Sc content. For the oxidized alloys of Fe-25Cr-4Al, some containing small addition of Sc strongly promotes oxide adherence for refining grain size, which providing rapid diffusion paths for the elements O and Al, and eliminating voids at oxide-matrix interface [24]. But how does the effects of the Sc on the corrosion behavior of the Fe-Ni alloy in the molten NaCl-MgCl2? Therefore, three kinds of Fe-36 at% Ni based alloys with different content of Sc and without Cr were made (Ni mass fraction is 37.15 wt%, following Fe-37Ni-xSc alloy for short). The corrosion behavior of these alloys in molten NaCl-52 wt% MgCl2 (Tm = 445℃ ) at 520℃ was studied. Combining the corrosion characteristic and thermodynamic calculation, the corrosion mechanism was discussed at last.
2. Experiment Electrolytic iron and nickel, and intermediate alloys Ni-Sc were used as raw materials, three kinds of alloy ingots with different content of Sc were made by a medium frequency induction melting furnace (Type: SP-25TC) under the protection of Ar atmosphere. The samples with the content of Sc (wt. %) 0.00, 0.30 and 0.50 were labeled as Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc, respectively. These ingots were cut into a cubes with the size of 10 mm×10 mm × 3 mm by a water cooled wire electric discharge machining (Type: DK7720). Then every specimen was polished by a metallographic abrasive paper of 800#, 1200#, 1500# and 2000#, successively. At last, all samples were washed in the distilled water and alcohol successively by the high frequency ultrasonic cleaning machine (Type: Shumei ultrasonic cleaners, KQ-200TDE), and then dried. The composition of specimens were detected by X Ray fluorescence (XRF), which is showed in Table 1. Because of the melting errors, the tested contents of the Ni in the specimens (36.1, 39.6 and 36.7 wt.%) are difference from the nominal composition (37 wt.% Ni). As -3-
Journal Pre-proof described below, a trace of Sc can be found at a grain boundary. Table 1. Detected composition of the specimens of Fe-37Ni-xSc by XRF (wt%) No. Ni Fe Sc Fe-37Ni-0.0Sc 36.10 Bal. 0.00 Fe-37Ni-0.3Sc 39.60 Bal. 0.30a Fe-37Ni-0.5Sc 36.70 Bal. 0.50a a: Nominal composition. The Sc can’t be detected here for too low content.
The mixture salt of NaCl-52 wt% MgCl2 was prepared by mixing dried analytically pure grade NaCl and MgCl2 (NaCl manufacturer: Yantai Shuangshuang Chemical Industry Co. Ltd., the purity ≥ 99.5%; MgCl2: Aladdin Industrial Corporation, the purity ≥ 99%). Then the mixture salt was put in a corundum crucibles and heated to 520℃ in a muffle furnace (heating rate 5℃ /min). When the mixture salt melts thoroughly, four samples of each kind of alloy were immersed in the molten salt in one crucible completely. Three parallel samples of them were weighed by an electronic balance with a accuracy 0.1 mg (SOPTOP-FA2004, Shanghai Sunny Hengping Scientific Instrument Co., Ltd.), and the surface areas were measured by a vernier caliper before immersion. After corrosion for 15 h and 30 h at 520℃ under air atmosphere, the crucible were took out from the furnace and cooled to room temperature. Then the residual salt on the surface of weighted specimens was dissolved in distilled water, then were washed under same conditions for all samples in distilled water and alcohol respectively for 10 minutes by the high frequency ultrasonic cleaning machine at room temperature (KQ-200TDE) (Total time: 20 minutes, Temperature: 23 ~ 26 ℃, Ultrasonic frequency: 80KHz, Power: 180W). Another specimen was immersed in distilled water statically until the residual salt on its surface was dissolved thoroughly. Then remove the specimen into alcohol. After immersed for several minutes, the sample was put out and dried, then their surface morphology and elements were analyzed by a scanning electron microscope (SEM, JSM-6610 LV) with energy dispersive spectrum (EDS) parts, and the composition on the sample surface was analyzed by a X-ray diffractometer (XRD, Rigaku, D/MAX2500X). After corrosion for 15 h and 30 h, cross sections of the three kinds of specimens were also analyzed by EDS.
3. Results and Analysis 3.1. Results After corroded by a corrosive of aqua regia, metallography of the specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc is showed in Fig. 1(a), (b) and (c), respectively. It is clear that -4-
Journal Pre-proof the grain size of the specimen with Sc is smaller than it without Sc. Fig. 1(d) and (e) is backscattered electron images (Signal: AsB) and EDS results of Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc, respectively. It is found that the element of Fe and Ni distribute evenly in all specimens. In Fig. 1(d) (specimen Fe-37Ni-0.3Sc), the element Sc distributes along the grain boundaries except enrichment in several small zones (also rich-Ni). In Fig. 1(e) (specimen Fe-37Ni-0.5Sc), the element Sc distributes more evenly than it in the specimen Fe-37Ni-0.3Sc, which is almost results of the small grain size in the specimen Fe-37Ni-0.5Sc. (a)
(c)
(b) 190 μm
150 μm
580 μm
(d)
(e)
Fig. 1 Metallography ((a), (b) and (c)), backscattered electron images and EDS results((d) and (e)) of the three kinds of specimens; (a) Fe-37Ni-0.0Sc; (b), (d) Fe-37Ni-0.3Sc; (c), (e) Fe-37Ni-0.5Sc.
After corrosion for 15 h in the molten salt, for all samples, corrosion products on the sample surface were cleaned by ultrasonic cleaning in the distilled water and alcohol under same conditions. The average mass change per unit area per hour is showed in Fig. 2. The dates of GH1140 and GH1035 were reported in the reference [25], which under the same corrosion conditions as in this paper. The specimens of GH1140 and GH1035 are iron based super-alloys. The Ni content in the GH1140 and GH1035 is (35 ~ 40) wt%, which is similar to the alloy studied here. But there are (20 ~ 23) wt% Cr and small amount of other alloy elements in the GH1140 and GH1035. After corrosion for 15 h, the mass of GH1140, GH1035 and Fe-37Ni-0.0Sc decrease, but Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc increase. In addition, the reduce amount of Fe-37Ni-0.0Sc is less than GH1140 and GH1035. And the increase amount of the samples Fe-37Ni-0.3Sc and -5-
Journal Pre-proof Fe-37Ni-0.5Sc increases with the increasing of the Sc content.
Average mass change 0.10
Average mass change (mg/(cm2·h))
0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10
GH1140*
GH1035*
*Reported in reference [25]
1# Sample
2#
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Fig. 2 Average mass change of the alloys in molten NaCl-53.2 wt% MgCl2 at 520℃
After the residual salt is dissolved in the distilled water, the surface morphology and local EDS results (at%) of the specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc are showed in Fig. 3(a), 3(b) and 3(c), respectively. It is found that a porous structure is formed locally on the three kinds of sample surface. For the samples Fe-37Ni-0.0Sc and Fe-37Ni-0.3Sc, the content of Fe at the porous structure (zone B) is lower than that on the uncorrected zone (zone A), but the content of Ni increase accordingly. And a little O and Mg is found. The sample Fe-37Ni-0.5Sc surface is mostly covered by a shell, which is showed in Fig.4 (d) later. The bare zone shown in Fig. 3(c) is the only place that don’t been covered by the shell. In order to reflect the sample surface morphology roundly, a low magnification (×50) picture is used here. It is speculated from the shell edge morphology that the bare zone was formed because of insufficient developing of the shell, not peeling off. Fig. 3(d) is magnification of zone D in Fig. 3(c). A porous structure (marked up with dashed line) is also formed slightly on the sample Fe-37Ni-0.5Sc surface, where the content of Fe decreases slightly, too, but Ni change lightly.
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Pos. A B C
Fe 60.2 19.8 2.6
Ni 37.8 48.4 1.8
Mg 2.0 13.5 49.2
O 18.4 46.5
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Fe 1.4 61.9 16.3
Ni 0.9 36.5 34.2 ×200
Mg 46.1 1.7 25.5
O 51.5 24.0
C
100 μm
Fig. 3 Surface morphology and local EDS results (at%) of the three kinds of sample surface after corroded in the molten NaCl-53.2 wt% MgCl2 at 520℃ for 15 h; (a) Fe-37 Ni; (b) Fe-37Ni-0.3 Sc; (c) Fe-37Ni-0.5 Sc; (d) magnification of zone D in Fig. 2(c).
It is also found from the Fig. 3(a), (b) and (c) that all the three kinds of samples are covered by a shell, which is rich in Mg and O. Coupled with XRD results of these sample surface in Fig. 4(a), (b) and (c), these shells are mainly MgO, and a little nickel oxide. For the samples Fe-37Ni-0.0Sc and Fe-37Ni-0.3Sc, the MgO layer mainly coves on the top of porous structure. For the sample Fe-37Ni-0.5Sc, as shown in Fig. 3(c) and (d), not only the slightly corroded place (porous structure) is covered by the MgO layer, but also the most surface is covered, which is showed in Fig. 4(d).
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Journal Pre-proof Fe-37 Ni-0.5 Sc
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a: MgO b: Mg0.4Ni0.6O c: (Ni, Fe) d: Fe0.64Ni0.36
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Fig. 4 XRD results of the three kinds of specimen surface and surface morphology of Fe-37Ni-0.5Sc specimen after corrosion in molten NaCl-53.2 wt% MgCl2 at 520℃ for 15 h; (a) Fe-37 Ni; (b) Fe-37Ni-0.3 Sc; (c) Fe-37Ni-0.5 Sc; (d) surface morphology of Fe-37Ni-0.5Sc.
After stripped away the MgO layer on the specimens surface shown in Fig. 3 by a steel brush, back scattered electron images with EDS results of the specimens Fe-37Ni-0.3Sc and Fe-37Ni0.5Sc are showed in Fig. 5(a) and Fig. 6(a), respectively. There is a skeletal microstructure which is mainly Ni and Fe on the two kinds of specimen surface. The element Sc is found obviously at local region. In the figures group of Fig. 5 and Fig. 6: the figure (b) is magnification back scattered electron images of zone “A”, where is enrichment Sc in figure (a). the figure (c) is EDS results and relevant elements content (wt%) of zone “B” in the figure (b). Compared the Fig. 5(c) with Fig. 6(c), there are mainly elements O and Sc except Fe and Ni in the zone “B”. Consequently, it is speculate that a scandium oxide (Sc2O3) is formed here. The conclusion also can be proved by the Sc and O corresponding distribution results (Dotted line circle) in Fig. 5(a) and Fig. 6(a). However, the contents of O and Sc in zone “B” of the specimen Fe-37Ni-0.5Sc are higher than it at similar zone of the specimen Fe-37Ni-0.3Sc. On the contrary, the contents of Fe -8-
Journal Pre-proof and Ni in zone “B” of Fe-37Ni-0.5Sc are one time lower than it of the specimen Fe-37Ni-0.3Sc. In addition, it is found from Fig. 6(a) that the microstructure with Sc-rich is more compact than it around area. (a) Fig. 5(b)
A
Cps 5000
(b)
(c)
Sc, 31.4
4000
B
Cps (eV)
3000 O, 28.3 2000 Fe, 13.6
Ni, 24.1
Sc C Ni Mg,2.2
1000
Cl,0.4
Ni
0 0
1
2
3
4
5 6 E (keV)
7
8
9
10
Fig. 5 Back scattered electron images and EDS results of Fe-37Ni-0.3Sc after corrosion for 15 h then stripping away MgO layer; (a) back scattered electron images and EDS results; (b) magnification image of zone “A” in Fig. 5(a); (c) EDS results and elements content (wt%) of zone “B” in Fig. 5(b). (a)
A Fig. 6(b)
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Journal Pre-proof Cps 5000
(c)
(b)
4000 Ni, 50.3 Fe: 26.3
B
Cps (eV)
3000 Sc: 10.9 2000
Ni O: 9.4 Fe Mg:3.0
1000
Ni
Cl: 0.2
C 0 0
1
2
3
4
5 6 E (keV)
7
8
9
10
Fig. 6 Back scattered electron images and EDS results of Fe-37Ni-0.5Sc after corrosion for 15 h then stripping away MgO layer; (a) back scattered electron images and EDS results; (b) magnification image of zone “A” in Fig. 6(a); (c) EDS results and elements content (wt%) of zone “B” in Fig. 6(b).
After corrosion for 15 h, cross section back scattered electron images and EDS results of specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc are showed in Fig. 7(a), (b) and (c), respectively. The surface of the specimen Fe-37Ni-0.0Sc is covered locally by the MgO layer. An evenly MgO layer formed on the surface of Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc specimens surface, too. On the matrix of the three kinds of specimen, a porous microstructure under the MgO layer is found obviously. On the porous microstructure, it is enrichment Ni and depletion Fe. A plenty of Mg and O elements are also found here. Consequently, it is speculated that the molten salt penetrate in the porous microstructure, and then the MgO is formed in the holes. After corrosion for 30 h, cross section back scattered electron images and EDS results of the specimens Fe-37Ni-0.0Sc, Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc are showed in Fig. 8(a), (b) and (c), respectively. A porous microstructure layer about 50 μm is formed on the matrix near surface of the specimen Fe-37Ni-0.0Sc, where enrichment Ni and depletion Fe. A plenty of Mg and O also are found on the porous microstructure. A thin evenly MgO layer formed on the surface of the specimens Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc. But the matrix microstructure of the two specimens is almost dense, except only a small region with Ni-rich and Fe-poor near the surface of specimen Fe-37Ni-0.5Sc, as shown in Fig. 8(c). Whatever corrosion for 15 h or 30 h, a local enrichment Sc on the cross section of the specimens Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc can be found obviously.
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Journal Pre-proof (a)
(b)
(c)
Fig.7 Cross section back-scattered electron images and EDS results of the three kinds of specimen after corrosion for 15 h; (a) Fe-37Ni-0.0Sc; (b) Fe-37Ni-0.3Sc; (c) Fe-37Ni-0.5Sc.
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Journal Pre-proof (a)
(b)
(c)
Fig. 8 Cross section back-scattered electron images and EDS results of the three kinds of specimen after corrosion for 30 h; (a) Fe-37Ni-0.0Sc; (b) Fe-37Ni-0.3Sc; (c) Fe-37Ni-0.5Sc.
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Journal Pre-proof 3.2. Analysis and discussion Like the corrosion mechanism of hot corrosion by chloride film, although the oxygen partial is low in molten chloride salt, but the chlorination-oxidation reactions are mainly reasons of a metal corrosion in it 0 0. However, in the molten MgCl2, the process of chlorination- oxidation is a little difference with in the molten KCl or NaCl. Because the MgCl2 very easily absorb moisture, during the heating process (before the mixed salt melting) under air atmosphere, MgCl2·6H2O forms and then decomposes into MgO, HCl and H2O. The reaction is showed as follow [20]: MgCl2·6H2O = MgO + 2HCl (g) + 5H2O (g)
0 ΔG793k = -251.911 kJ/mol
(1)
The HCl(g) that dissolved in the molten salt will react with the alloy elements (M, M = Fe, Cr, and Ni) directly as follow[15]:
2M + 2nHCl g = 2MCl n + nH 2 g
(2)
The HCl(g) also will react with O2, then producing Cl2. The reaction as follow: 4HCl g O2 g 2Cl 2 g 2H 2 O g
0 ΔG793k = -10.16 kJ/mol
(3)
The produced Cl2 dissolves in the molten salt and reacts with alloy elements. Then a metal chloride MCln (M = Fe, Ni, Sc) is produced, which is also called chlorination reaction here. The Standard Gibbs Free Energy change of the chlorination reactions (ΔGcl0) of alloy elements at different temperatures are showed in Fig. 9. The melting point of the produced chloride and its T4 temperature are also showed in the figure. The T4 temperature is a temperature value of a material start volatilizing obviously (material’s party pressure up to 1E-4 atm) [26]. Compared with Ni, the ΔGcl0 of Fe is more negative than it of Ni. And FeCl2 is produced preferentially. The T4 temperature of FeCl2 (536℃) is close to the experiment temperature (520℃). Iron chloride will volatilize during corrosion process. The element Ni is relatively stable in the molten salt. After the element Fe runs off from the specimens surface, the content of the Ni increase on the specimens surface layers. Consequently, as shown in Fig. 3, Fig. 7 and Fig. 8(a), these specimens are corroded to varying degrees. A porous structure with Fe-poor and Ni-rich is formed on their surface. But as shown in Fig. 4 (a), (b) and (c), iron oxide didn’t been detected obviously. It is also found from Fig. 9 that, for the alloy Fe-37Ni-Sc, ScCl3 is more easily formed because the ΔGcl0 of Sc is more negative than that of Fe and Ni. Because ScCl3 is stable in the - 13 -
Journal Pre-proof molten salt for its high melting point (960℃) [21]. It will react with the dissolved O2, which is showed as follow: (4/3) ScCl3 + O2 → (2/3) Sc2O3 + 2Cl2
(4)
0 -100
(1001℃ , 607℃ )
×
(306℃ , 167℃ )
×
ΔG0Cl (kJ/mol)
-200
×
(670℃ , 536℃ )
-300 Experiment temperature
-400
(960℃ , -)
×
Fe + Cl2 = FeCl2 (2/3)Fe + Cl2= (2/3)FeCl3 Ni + Cl2 = NiCl2 (2/3)Sc + Cl2= (2/3)ScCl3
-500 -600
0
200
400
600 800 1000 Temperature (℃ )
1200
1400
1600
Fig. 9 The Standard Gibbs Free Energy change of alloy elements react with 1 mol Cl2 at different temperatures, and the melting point and T4 temperature of chloride (Tm, T4)
The Standard Gibbs Free Energy change of oxygenation reactions (ΔGO0) of alloy element chloride at different temperature are showed in Fig. 10. The ScCl3 is oxidized preferentially than the chloride of Fe and Ni in a wide temperature range. Therefore, for the alloys with Sc, alloy element Sc will not only be oxidized to Sc2O3 by directly reacts with O2, but also in the form of chlorination-oxidation reactions. Because the Sc2O3 (melting point is 2314℃) is stable in molten chloride at the experiment temperature, it will adhere on the sample surface. In addition, the Sc2O3, MgO and Sc have same crystal texture (cubic structure); And the lattice parameter of Sc and MgO are also same (Sc: 4.52 ×4.52×4.52 angstrom, 90° × 90° × 90°；MgO: 4.21×4.52×4.52 angstrom, 90° × 90° × 90°). The Sc and MgO have same lattice group (Fm-3m), too. Therefore, it is think that the active element Sc will be oxidized preferentially to Sc2O3. Then the MgO deposits on the Sc2O3 and grows up little by little. The Sc2O3 as heterogeneous nucleation base for the MgO, which helps to form a MgO layer. As shown in Fig. 5 and Fig. 6, scandic oxide is found under MgO layer.
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Journal Pre-proof Furthermore, because the molten chloride is strong electrolyte, the electro-chemical corrosion is another corrosion form of an alloy in it. A metal atom (M = Fe, Ni) with low potential dissolves in the molten salt as an anode after losing electrons. An MgO particle is formed by the reaction between Mg2+ and O2-. The O2- is formed by dissolved oxygen atom captures the loosed electrons. These reactions as follow [9, 18]: M → Mn+ + ne
(5)
O 2 2e- O 2-
(6)
O2- + Mg2+ = MgO
(7)
100
FeCl3→FeO
50
FeCl2→FeO
ΔG0O (kJ/mol)
NiCl2→NiO
0 FeCl3→Fe3O4
FeCl2→Fe3O4
-50
FeCl3→Fe2O3
-100
FeCl2→Fe2O3
-150
Experiment temperature ScCl3→Sc2O3
-200
0
100
200 300 400 Temperature (℃ )
500
600
Fig. 10 The Standard Gibbs Free Energy change of oxidation reaction of alloy chlorides with 1 mol O2 at different temperatures
After exposure for a certain time in the molten salt, many corrosion pits are formed on the specimen surface for the reaction (5). At the same time, the MgO will deposit near these corrosion pits not only because O2-content is high near these corrosion pits, but also these corrosion pits are more conducive to the MgO nucleation. Consequently, as showed in Fig. 3, Fig.7 and Fig. 8, the MgO layer mostly covered on the top of the porous structure. However, by compared Fig. 7(a) with Fig. 8(a), the MgO layer on the Fe-37Ni-0.0Sc specimen (Fe-37Ni) surface can’t protect it from corrosion in the molten salt. It is maybe for the MgO particle on the sample surface is loose and incomplete, which can’t prevent the molten salt contacting with the specimen matrix. For the specimens with Sc, as analyzed above, the scandium oxide on sample surface will as
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Journal Pre-proof a base for MgO nucleation. In addition, it is surmise that when a corrosion pits penetrates into Sc atom along grain boundary, the Sc atom will be oxidized to Sc2O3 particle, too. Then a MgO takes the Sc2O3 particle as a nucleation substrate, and grows up in the corrosion pits. When the corrosion pit is filled by MgO, the corrosion process in this hole stop. The MgO layer in the corrosion pit grows continually by the deposition of MgO particles (produced in the form of reactions (6) and (7)) which diffused from other corrosion place in molten salt. After exposure for a sufficient time, these small MgO pieces on sample surface will connect each other and act as a solid barrier against further corrosion in contact with the molten salt. The MgO in the corrosion pit penetrates in alloy matrix likes a “nail”, which plays a pinning effect for the oxide film, and improves the adhesion strength between the oxide film and the alloy substrate. Moreover, the higher content of Sc in the alloy, the more number of "nail", and the stronger adhesion strength between the oxide layer and alloy matrix. Consequently, the Sc2O3 acts as MgO nucleation substrate and nail for MgO layer on samples surface. The development process is showed in Fig. 8. What’ more, there are more porous structures on the surface of specimen with Sc, which helps to increase the adherence strength between the MgO shell and the specimen matrix.
O2 + 2e- = O2O2- + Mg2+ = MgO O2 Molten Salt Oxide
Sc2O3 Sc atom
MgO layer
“Nail”
Alloy
Fig. 11 The schematic diagram of the development process of Sc2O3 acts as MgO nucleation substrate and nail for MgO layer on samples surface
Therefore, a porous structure is formed on the three kinds of sample surface during initial corrosion for 15 h, as shown in Fig. 7. However, with the extension of exposure time, a compact, intact MgO layer is pegged by Sc2O3 on the surface of specimen with Sc. The MgO layer as a solid barrier protects the specimen substrate from corrosion from molten salt. Consequently, a compact structure is found near Sc-rich zone under MgO layer, as shown in Fig. 6(a). A porous structure also can’t be found on Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc specimens surface after corrosion for 30 h, as shown in Fig. 8(b) and (c). - 16 -
Journal Pre-proof Comparing with the specimen Fe-37Ni-0.3Sc, because the Sc in the specimen Fe-37Ni-0.5Sc distributes more evenly, the pin effect is better, and the MgO layer coverage rate is higher. In addition, as shown in Fig.5(c) and Fig.6(c), the content of Sc and O on the Sc-rich zone in the specimen Fe-37Ni-0.5Sc are low than it in the specimen Fe-37Ni-0.3Sc, but the Fe and Ni are higher. It is clean that the mass gain of specimens with Sc (Fe-37Ni-0.3Sc and Fe-37Ni-0.5Sc) in Fig. 1 for deposited MgO layer on their surface. In addition, because these mass change values were measured after these samples ultrasonic cleaning for 10 minutes in distilled water and alcohol respectively, it is reveal that the adherence strength of MgO layer is strong enough.
4. Conclusion The corrosion behavior of Fe-37Ni alloy with different content of Sc (0.0, 0.3, and 0.5 wt%) was studied in molten NaCl-MgCl2 at 520℃. The corrosion resistance mechanism was discussed, too. (1) The corrosion rate of the advance alloy Fe-37wt% Ni is lower than that of iron based superalloys GH1140 and GH1035 under the same corrosion conditions. After corrosion for 15 h, the mass of specimens with Sc are increases for the formation of MgO protecting shell. (2) After corrosion for 15 h, there is a local corrosion with porous microstructure on all studied specimens’ surface. The porous structure with Fe-poor and Ni-rich is covered by the MgO shell. But the local corrosion is slightly for specimens with Sc. And a lot of compact MgO layer is covered on the sample Fe-37wt%Ni- 0.5Sc surface, which as a solid barrier against further corrosion in contact with the molten salt. (3) It is surmised that a Sc2O3 is formed after Sc is oxidized directly or by chlorinationoxidation reactions during corrosion process, which acts as heterogeneous nucleation base and a nail for the MgO layer.
Acknowledgement This work was been supported by Qinghai Natural Science Foundation [grant numbers 2016ZJ-759].
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Journal Pre-proof Reference [1] K.Vignarooban, X.H. Xu, A. Arvay, K. Hsu, A.M. Kannan, Heat transfer fluids for concentrating solar power systems - A review, Appl. Energy. 146 (2015) 383-396. [2] S.A. Mohamed, F.A. Al-Sulaiman, N.I. Ibrahim, Md. Hasan Zahir, A. Al-Ahmed, R. Saidur, B.S.Yılbaş, A.Z.Sahin, A review on current status and challenges of inorganic phase change materials for thermal energy storage systems, Renew. Sust. Energ. Rev. 70 (2017) 1072-1089. [3] P.D. Myers Jr, D.Y. Goswami, Thermal energy storage using chloride salts and their eutectics, Appl. Therm. Eng. 109B (2015) 889-900. [4] Y.Y. Li, X.K. Xu, X.X. Wang, P.W. Li, Q.Hao, B.X. Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP, Sol. Energy. 152 (2017) 57-79. [5] X.L. Wei, M. Song, W.L. Wang, J. Ding, J. P. Yang, Design and thermal properties of a novel ternary chloride eutectics for high- temperature solar energy storage, Appl. Energy. 156 (2015) 306-310. [6] Oak Ridge National Laboratory, Assessment of candidate molten salt coolants for the NGNP/NHI heat-transfer loop, Washington: U.S. Department of Energy, 2006. [7] M.M. Kenisarin, High-temperature phase change materials for thermal energy storage, Renew. Sust. Energ. Rev. 14 (2010) 955-970. [8] B. Liu, X.L. Wei, W.L. Wang, J.F. Lu, J. Ding, Corrosion behavior of Ni-based alloys in molten NaCl- CaCl2- MgCl2 eutectic salt for concentrating solar power, Sol. Energy Mater. Sol. Cells.170 (2017) 77-86. [9] J.W. Wang, C.Z. Zhang, Z.H. Li, H.X. Zhou, J.X. He, J.C. Yu, Corrosion behavior of nickelbased superalloys in thermal storage medium of molten eutectic NaCl-MgCl2 in atmosphere, Sol. Energy Mater. Sol. Cells. 164 (2017) 146-155. [10] M. Hofmeister, L. Klein, H. Miran, R. Rettig, S. Virtanen, R. F. Singer, Corrosion behavior of stainless steels and a single crystal superalloy in a ternary LiCl-KCl-CsCl molten salt, Corros. Sci. 90 (2015) 46-53. [11] J.C. Gomez-Vidal, A.G. Fernandez, R. Tirawat, C. Turchi, W. Huddleston, Corrosion resistance of alumina forming alloys against molten chlorides for energy production. II: electrochemical impedance spectroscopy under thermal cycling conditions, Sol. Energy Mater. Sol. Cells. 166 (2017) 234-245. [12] J.C. Gomez-Vidal, A.G. Fernandez, R. Tirawat, C. Turchi, W. Huddleston, Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests, Sol. Energy Mater. Sol. Cells. 166 (2017) 222-233. [13] A.J. Wallace, Molten Chloride Salts for Heat Transfer in Nuclear Systems. The University of Wisconsin-Madison, 2011. [14] S.N. Liu, Z.D. Liu, Y.T. Wang, J. Tang, A comparative study on the high temperature - 18 -
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
No.
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1. Corrosion behavior of a new designed Fe-Ni alloy in molten NaCl-MgCl2 was studied. 2. The effects of the alloy element Sc on the corrosion behavior of the alloy were analyzed. 3. The mechanism of the corrosion resistant of Fe-37Ni-xSc alloy in the molten NaCl-MgCl2 was discussed.