Characterization of Ta–W alloy films deposited by molten salt Multi-Anode Reactive alloy Coating (MARC) method

Int. Journal of Refractory Metals and Hard Materials 53 (2015) 23–31

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Characterization of Ta–W alloy films deposited by molten salt Multi-Anode Reactive alloy Coating (MARC) method Young-Jun Lee a, Tae-Hyuk Lee b, Hayk H. Nersisyan b, Kap-Ho Lee b, Seong-Uk Jeong c, Kyoung-Soo Kang c, Ki-Kwang Bae c, Kyoung-Tae Park d, Jong-Hyeon Lee a,b,⁎ a

Graduate School of Green Energy Technology, Chungnam National University, Daejeon 305-764, Republic of Korea Graduate School of Department of Nanomaterials Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea d Korean Institute of Industrial Technology, 7-47 Songdo-dong Yeonsoo-gu Incheon 406-840, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 29 December 2014 Received in revised form 7 April 2015 Accepted 18 April 2015 Available online 22 April 2015 Keywords: Tantalum Tantalum alloy Tungsten alloy Electroplating Molten salt

a b s t r a c t The harsh atmosphere of the sulfur–iodine process used for producing hydrogen requires better corrosion resistance and mechanical properties than are possible to obtain with pure tantalum. Ta–W alloy is superior to pure tantalum but is difficult to alloy due to its high melting temperature. In this study, substrate samples were coated with Ta–W (Ta–7W, Ta–4W and Ta–1W) using the Multi-Anode Reactive alloy Coating (MARC) process in molten salt (LiF–NaF–K2TaF7), with varying distances between cathode and anode. In the case of Ta–4W, a corrosion rate of less than 0.011 mm/year was attained in hydriodic acid at 160 °C, and thickness uniformity was 80.3 ± 2.15 μm. Also, the resulting coated films had hardnesses up to 12.9% stronger than pure tantalum coated film. The alloy coating films also contributed to significant enhancement of corrosion resistance. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction As an energy source, hydrogen has the biggest chemical energy per weight, without environmental pollution emissions. In the move from hydrocarbon to non-hydrocarbon energy economies, hydrogen will be one of the fuels increasingly used. Many countries have committed to accelerating the development of hydrogen production technologies to improve their energy, environmental and economic security. The sulfur–iodine (SI) cycle combined with a very high temperature gas-cooled reactor (VHTR), is a well-known, feasible technology for hydrogen production [1–4]. However, there is a corrosion problem involving structures in the SI process environment, where a chemical reaction with sulfuric acid (H2SO4) and hydriodic acid (HI) happens at temperatures greater than 300 °C (570 °F) and at pressures ranging between 20 and 30 bar [5,6]. Tantalum is known to be the only corrosion resistant metal for use in this environment. However, the relatively high cost and melting temperature of tantalum are obstacles to its wide application as a structural material in SI processing.

Abbreviations: EDX, energy-dispersive X-ray spectroscopic analysis; FESEM, field emission scanning electron microscopy; TEM, transmission electron microscopy; XRD, X-ray diffractometer. ⁎ Corresponding author at: Graduate School of Green Energy Technology, Chungnam National University, Daejeon 305-764, Republic of Korea. E-mail address: [email protected] (J.-H. Lee).

http://dx.doi.org/10.1016/j.ijrmhm.2015.04.022 0263-4368/© 2015 Elsevier Ltd. All rights reserved.

The most interesting characteristics of tantalum are its surface properties. These can be used to advantage by employing a tantalum coating on a base metal or alloy that best fits the engineering requirements in a cost-effective manner. It has already been reported by Senderoff et al. [7] that electrodeposition in molten salt facilitates the coating of metallic tantalum. Molten salt electrodeposition (MSE) processes for tantalum were reviewed by Cardarelli et al. and compared with other coating techniques [8]. Previous research results on the corrosion properties of a tantalum coating layer fabricated by MSE showed that the harsh atmosphere of the SI process requires better corrosion resistance and mechanical properties than those of the pure tantalum [9–12]. The Ta–W alloy has superior corrosion resistance and mechanical properties compared to pure tantalum, but are difficult to alloy due to its high melting temperature [13–15]. Hence, the present authors have developed a molten salt multi anode reactive alloy coating (MARC) process which enables the fabrication of a compact and uniform Ta alloy coating film. In this paper, the coating conditions of the Ta–W alloy were investigated via the MARC process using pure tantalum and tungsten feedstocks. This investigation was initiated because metallic materials like tantalum and tungsten are dissolved at an anode above the standard redox potential and they are simultaneously reduced at a cathode, provided that the cathodic potential is controlled to be below the redox potential of the base metal. The coating of a stainless steel tube with a Ta–W alloy via MARC process was investigated, and the methodology for regulating the alloy

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Fig. 1. Design of the MARC process.

2. Experimental

Table 1 Calculated electrodeposition times for various concentrations of tungsten.

1 2 3 4 5

Concentration of tungsten (wt.%)

Coating layer thickness (μm)

Deposition time, T(s) (h)

1 2 4 6 8

50 50 50 50 50

6.16 6.23 6.37 6.51 6.65

2.1. Sample preparation The substrate specimens used were 40 mm × 20 mm × 2 mm commercial stainless steel plates. Their chemical composition was as follows: Cr 18%, Ni 2.5%, Mo 3.5%, C 0.080% and Fe balance. These plates were polished with 800-grit SiC paper and then cleaned ultrasonically in acetone. Also, tantalum and tungsten plates were used to maintain a constant concentration of coating target ions (Ta3+, W5+) in the electrolyte solution surrounding the electrodes.

composition using the arrangement of electrodes is also reported. The properties of the Ta–W alloy coating layer were studied by using hardness test, electron microscopy, and corrosion testing in a high temperature HI solution.

2.2. Design of MARC method We designed the multi anode reactive alloy coating (MARC) process to obtain better corrosion resistance and mechanical properties than

A

10µm

A

B

B 10µm

C C MARC I 10µm Fig. 2. Surface morphology and SEM micrographs of the Ta–W alloy coating layers, fabricated by the MARC 1 method.

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0.81

Ta(wt%) W(wt%) 4.36 9.24

99.19

95.64

90.76

Area A

Area B

Area C

Fig. 3. Analysis of W concentration using energy-dispersive X-ray spectroscopy.

those of pure tantalum. The MARC process in molten salt is designed to enable deposition of a compact and uniform Ta alloy coating film using electrode distance and a potential difference. MARC 1 utilizes an arrangement of two sacrificial anodes for coating vertically, as shown in Fig. 1a. This electrode configuration allows electro-deposition of two species on the substrate (SUS316L) with a gradual variation in composition along the height of the substrate. The advantage of this method is that it can reduce the test time needed to choose the optimum composition, as shown in Fig. 1c. MARC 2 is designed to produce a coating film of a required concentration by adjusting the distance between the anode (Ta–W plate) and cathode (SUS316L) as shown in Fig. 1b. A vertical Inconel 625 tube was used as the reaction chamber. The temperature of the chamber was measured using an R-type thermocouple. The temperature was controlled within 0.5 K by a thermostat. LiF, NaF, and K2TaF7 were mixed by direct weighing in a molar ratio of 60.4:38.6:1. The crucible containing the salt mixture was placed in the chamber, and then, the chamber temperature was raised to 800 °C. 2.3. Analysis of Ta–W alloy coating film characteristics The electrochemical measurements were performed with the help of a potentiostat (AUT70530, Utrecht, Netherlands). The Ta–W coated films were characterized using an X-ray diffractometer with a Cu Kα radiation source field-emission (XRD, Siemens D5000, Germany), scanning electron microscopy (FE-SEM, JSM 6330F, Japan), energydispersive X-ray (EDX, JSM 5410, Japan) analysis, orientation imaging microscopy (OIM, JSM 5410, Japan), scanning transmission electron

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Table 2 The XRD peaks position for MARC 1 deposited Ta–W alloy coating layers fabricated at different W contents. Contents of W (wt.%)

Ta (110)

W (110)

Ta (200)

W (200)

Ta (211)

W (211)

0.81 4.36 9.24

38.6° 38.7° 38.85°

– 40.25° 40.45°

55.45° 55.6° 55.8°

– 57.2° 57.55°

69.6° 69.7° 69.81°

– 72.4° 72.6°

microscopy (STEM, JEOL, JEM-1200, Japan) and micro Vickers hardness testing (FM-7, Future Tech Co., Ltd., Japan). Further, we carried out an accelerated corrosion test in an autoclave (100 h, 150 °C, 2 MPa, HI = 26.1, I2 = 51.8, H2O = 22.0 wt.%). The deposition time needed to form a 50 μm thick coating film on the base metal (SUS316L) substrate can be estimated by using Eq. (1):   TðsÞ ¼ Cg = Weq  CA ;

Cg ¼ A  ρ  Ct

ð1Þ

where T(s) is the deposition time for obtaining the target coating thickness, Weq is the electrochemical equivalent of tantalum, CA is the current applied to the electrode, A is the area of SUS316L, is the density of tantalum, Ct is the target coating thickness, and Cg is the surface coating mass of tantalum for SUS316 electrodeposition. The calculated operating times are shown in Table 1. 3. Results and discussion 3.1. Characteristics of Ta–W alloy coatings film prepared using the MARC 1 method Fig. 2 shows the morphology of the Ta–W alloy coating films and the SEM micrographs obtained for different distances between anode (Ta, W) and cathode (SUS 316L substrate) using the MARC 1 process. It was found that a dense coating layer was formed on the SUS substrate throughout the vertical direction, but there was a difference in the thickness of the coating layer. The thickness of the coating layer at area A, where the Ta anode was closely located, is thicker than at area B where the W anode was closely located, as shown in Fig. 2. W (− 0.847 V vs Cl) has a higher redox potential than Ta (− 1.226 V vs Cl), so it is harder to dissolve W at the anode. The higher density of W

300

Ta-W alloy coating layer [17] Pure Tantalum

Vickers Hardness (Hv)

280 260 240 220 200 180 160 140 120 -1

0

1

2

3

4

5

6

7

8

9

10

Tungsten wt% Fig. 4. Typical XRD patterns obtained from Ta–W alloy coatings prepared in LiF–NaF melts containing 1 mol% K2TaF7 at different W contents.

Fig. 5. Variation in micro Vickers hardness of Ta–W alloy coatings and SUS316L substrates with W contents.

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Fig. 6. OIM mapping of the cross section of the Ta–W alloy coating layer and average grain size change for different W contents: a) Ta–0.81W; b) Ta–4.36W; c) Ta–9.24W.

relative to Ta can also be the reason for why the coating layer thickness of area B is lower than that of area A. In Fig. 3, at area A nearest the tantalum anode, the concentration of tungsten was 0.81 wt.%. The Ta–0.81W coated film thickness ranged from 39.25 to 45.91 μm, close to the design thickness of 50 μm. However, with the Ta–4.36W alloy film (Fig. 3 Area B) in the middle distance between tantalum and tungsten, the coated film thickness ranged from 24.43 μm to 29.91 μm. At the Ta–4.36W alloy film nearest the tungsten (Fig. 3 Area C), the coated film thickness ranged from 19.54 μm to 23.71 μm. The thicknesses of the Ta–9.24W and Ta–4.36W alloy coated films were still far from the expected value of 50 μm. This was due to the difference in redox potential of the coating target (tantalum and tungsten). Diffraction patterns of Ta–W alloy coated film prepared in molten salt (LiF–NaF) containing 1 mol% K2TaF7 observed for different W contents are shown in Fig. 4. The weight fraction of W element in the alloy coated film was analyzed by using EDX analysis. The XRD peaks detected α-Ta and W, which are shown in Table 2 at 38.72°, 56.21°, and 69.7° related to α-Ta (1 1 0), (2 0 0), and (2 1 1), respectively. Also, the XRD peaks detected α-W (1 1 0), (2 0 0), and (2 1 1). The peak reflexes (111), (200), and (220) are displaced depending on the element content in the alloy. The observed peak-shift to smaller angles

corresponds to increasing lattice constants, i.e. the formation of two phases with the same structure (BCC) but with different chemical composition, which corresponds to a Ta–W solid solution with a tantalum rich phase, and single phase tungsten [16]. In the following results, the amount of uniformly distributed tungsten particles in the Ta matrix increases and this makes plastic deformation more difficult. In addition, grain refining and dispersive strengthening contributes to the hardness increase as shown in Fig. 5. A hardness test was carried out to measure the hardness of the Ta–W coated film. The bars indicate the variation in hardness across the

Table 3 Corrosion rate for various contents of tungsten.

Before corrosion test (g) After corrosion test (g) Weight loss (mg) Metal density (g/cm3) Area of the sample (cm2) Test hours (h) Corrosion rate (mm/year)

Pure Ta

0.81

4.36

9.24

15.0744 15.0643 10.1 16.91 14.1 100 0.037108

15.3213 15.3145 6.8 16.91 12.12 100 0.029065

15.7583 15.7559 2.4 16.91 11.27 100 0.011032

15.3471 15.1054 241.7 16.91 13.16 100 0.951440

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specimens. It is apparent from this variation that the density of the Ta– W films increases with increasing concentration of tungsten. The mechanical properties of the coated film also seem to be closely related not only to the intrinsic hard material properties of tungsten, but also to microstructural changes associated with the solutionization of tungsten. These are expected, since the substitution of tungsten with high hardness (247.4 Hv) into tantalum lattice occurred. Given the fact that the hardness of pure tantalum coating film was 153.7 Hv [17], the increase in tungsten concentration and resulting increase in hardness, ranging from 5.98% (165.2 Hv) to 12.9% (244.5 Hv) will contribute to its application in a wide number of harsh engineering processes. In order to investigate the effect of the concentration of tungsten on the microstructure of the electroplated Ta–W coating film, OIM analyses were carried out to obtain information on the orientation of the coated films. The OIM results were correlated with the surface morphology by comparing them with top-view SEM images. The distribution of the crystallographic orientation may be observed in the corresponding OIM map of Fig. 6. Different gray scales indicate the degree of orientations for different tantalum planes. Sub-micron-sized Ta–W grains were formed at the surface of the SUS316L substrate, and larger grains started to form just after the intermediate film. This result can be explained by the fact that the Ta–W alloy deposit in the initial coated film has smaller grains (probably due to more suitable nucleation sites on the SUS316L substrate interface) which is the dominant parameter [18]. The average grain sizes of the Ta–W crystals were 8.64 μm2 for tungsten of 0.81 wt.%, 7.77 μm2 for tungsten of 4.36 wt.%, and 6.01 μm2 for tungsten of 9.24 wt.%. A corrosion test was carried out to measure the corrosion resistance of the Ta-W alloy coated films formed at different W contents, using an autoclave at 160 °C and 20 MPa, in an HI (HI = 26.1, I2 = 51.8, H2O =

27

22.0 wt.%) environment. The corrosion rate from metal loss can be calculated using the following Eq. (2): mm=year ¼ 87:6  ðW=ðD  A  TÞÞ:

ð2Þ

In this equation, W represents the weight loss in milligrams, D represents the metal density, A represents the area of the sample, and T represents the exposure time of the metal sample in hours. The corrosion rates of the Ta–W alloy coating films are summarized in Table 3. Figs. 7 and 8 show the microstructural change of the Ta–W coated film after the corrosion test. In the case of Ta–0.81W and Ta–4.36W, the thickness of the coated film was reduced from 44.15 ± 4.92 μm to 42.32 ± 2.35 μm and 26.41 ± 3.46 μm to 25.17 ± 1.62 μm respectively. However, in the case of Ta–9.24W, the corrosion occurred intensively, with the porous coated film falling away during the corrosion test, and most of the Ta–W film was damaged. This is because a porous coated film with W rich phase was formed when W was precipitated with increasing W concentration, which was greater than its solubility in the Ta phase. The morphology of the W rich phase, with cubic particles of 10–20 μm in size is completely different from the other phase, as shown in Fig. 7c. The reduction ratio of coating layer thickness of the specimens, seen in Fig. 8, can be clearly compared according to W concentration. This is in agreement with the corrosion rate calculated in Table 3. The surface morphologies of the pure Ta and Ta–4.36W alloy coated films investigated by AFM were consistent with the results of the corrosion test. AFM images were obtained on a Digital Instruments Nanoscope IIIa system with a Si3N4 cantilever operating in tapping mode. Fig. 9 shows the AFM surface morphologies of nanostructured

Fig. 7. Surface morphology of Ta–W alloy coated samples after corrosion testing of varying W contents: a) Ta–0.81W; b) Ta–4.36W; c) Ta–9.24W.

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Fig. 8. Cross section of Ta–W alloy coated samples after corrosion test with W contents: a) Ta–0.81W; b) Ta–4.36W; c) Ta–9.24W.

pure Ta and Ta–4.36W alloy films after the corrosion test. The AFM results of the Ta and Ta–W alloy coating films are summarized in Table 4. The change in surface roughness (Ra) of the pure Ta coated film was barely visible in the AFM analysis, as shown in Fig. 9a and b. However, in the case of Ta–4.36W, the surface roughness (Ra) values of the Ta–4.36W alloy film after the corrosion test indicate a smoother surface than before the corrosion test, as shown in Fig. 9c and d. This is because the outer-most part of the porous Ta–W alloy films, a single phase of tungsten from the precipitation phase, was exfoliated during the corrosion test in HI acid mentioned above. 3.2. Characteristics of Ta–W alloy coating film prepared by the MARC 2 method In this study, MARC 2 was designed to obtain a coated film of required content (Ta–4.2W), which is based on the lowest corrosion rate obtained in the MARC 1 results, by adjusting the distance (1.5 cm, 2.5 cm and 3.5 cm) between the anode and cathode, as shown in Fig. 10. Fig. 11 shows the morphology of the Ta–W coating films obtained for different distances between cathode and anode. At an inter-electrode (tungsten) distance of 1.5 cm, coatings that were thinner than the theoretical value were obtained even though the same amount of electric current was applied, as shown in Fig. 11a. As the inter-electrode distance was increased to 2.5 cm and 3.5 cm, the thickness of the Ta–W films increased ranging from 78.82 to 81.63 μm and 82.13 to 85.22 μm, respectively, which was close to the design thickness of 80 μm (Fig. 11b and c). The concentration of tungsten was 7.31 wt.% at an inter-electrode distance of 1.5 cm, 4.12 wt.% at 2.5 cm and

1.92 wt.% at 3.5 cm. However, the surface density and coating thickness decreased with increasing W content. Scanning transmission electron microscopy (STEM) and selected area electron diffraction pattern (SAED) analyses were carried out for precise analysis of the W content in the Ta–4.12W coated film, as shown in Fig. 12. The STEM sample was prepared by removing the outer-most porous Ta–W alloy films in order to analyze the W content in the alloy film inside. It is difficult to differentiate between the diffraction pattern of tungsten and tantalum using with SAED analysis because the lattice constants mismatch between bcc tantalum (0.3298 nm) and bcc tungsten (0.3165 nm) is less than 0.01% (inset of Fig. 12a). Based on the STEM-EDX result shown in Fig. 12b, it is clear that the tungsten atoms of 3.4 wt.% were uniformly dispersed in the tantalum lattice. 4. Conclusions Ta–W coated samples were prepared by electrodeposition in molten salt (LiF–NaF–K2TaF7) by varying the distances between the cathode and anode. A deposit of tantalum and tungsten was formed on SUS316, and the conclusions are as follows. (1) The molten salt multi-anode reactive alloy coating (MARC) process enables compact and uniform tantalum alloy coating. (2) The observed XRD peak-shift to smaller angles corresponds to increasing lattice constants i.e. the formation of two phases with the same structure (BCC) but with different chemical composition, which corresponds to a Ta–W solid solution of tantalum rich phase and single phase tungsten.

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a)

b)

c)

d)

Fig. 9. AFM image of the surface morphologies of Ta–4.36W alloy coated samples after corrosion test: a) pure Ta before corrosion test; b) pure Ta after corrosion test; c) Ta–4.36W before corrosion test; d) Ta–4.36W after corrosion test.

(3) The Ta–W alloy deposit in the initial coated film had smaller grains (probably due to more suitable nucleation sites at the SUS316L substrate interface), which are the dominant parameters. (4) The hardness of pure tantalum coating film was 153.7 Hv, so the increase in hardness resulting from the concentration of tungsten, ranging from 5.98% (165.2 Hv) to 12.9% (244.5 Hv), will contribute to its use in a wide variety of harsh engineering process environments. (5) Based on the results of corrosion tests in HI solution at temperatures up to 160 °C and pressures of 20 MPa, it was confirmed that the Ta–4.36W alloy coating film has a higher corrosion resistance (0.011032 mm/year) than the Ta–0.81W and Ta–9.24W alloy coating films. (6) The mechanical properties of the coating film also seem to be closely related to the microstructural changes associated with W contents.

The excellent corrosion resistance and mechanical properties of the Ta–W alloy coating films contributed to the significant enhancement of corrosion resistance.

Table 4 Surface roughness data on Ta and Ta–W coatings determined from AFM scans before and after corrosion testing. Condition

Ra (nm)

Rq (nm)

Z range (nm)

Scan size (μm)

Pure Ta before corrosion test Pure Ta after corrosion test Before corrosion test (Ta–4W) After corrosion test (Ta–4W)

33.744 19.184 110.67 23.041

43.401 30.114 71.022 31.759

409.89 371.84 578.56 425.56

10 10 10 10

Fig. 10. Design of the MARC 2 method, which permits adjusting the distance between the anode and cathode.

30

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Fig. 11. SEM micrographs of the surfaces and cross sections of Ta–W deposits on SUS 316L according to different distances between the cathode and anode: a) 1.5 cm; b) 2.5 cm; c) 3.5 cm.

Fig. 12. STEM micrographs of the surfaces of Ta–W deposits on SUS 316L with an inter-electrode distance of 2.5 cm: a) SAED analysis of Ta–4.12W; b) STEM-EDX result of Ta–4.12W.

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Acknowledgment This research was supported by the Korea Institute of Energy Research (No. KIER B2-2144-03) and was also partially supported by National Research Foundation of Korea (No. NRF-2011-0031839).

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