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Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production Bharat Bhushan a,b, Nitesh Goswami a, S.C. Parida b,c, B.N. Rath d, Sanjukta A. Kumar e, V. Karki f, R.C. Bindal a,b, Soumitra Kar a,b,* a Membrane Development Section, Chemical Engineering Group, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India b Homi Bhabha National Institute, Anushakti Nagar, Mumbai, 400094, India c Product Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India d Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India e Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India f Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India
article info
abstract
Article history:
HI decomposition in Iodine-Sulfur (IS) thermochemical process for hydrogen produc-
Received 23 February 2018
tion is one of the critical steps, which suffers from low equilibrium conversion as well
Received in revised form
as highly corrosive environment. Corrosion-resistant metal membrane reactor is pro-
10 April 2018
posed to be a process intensification tool, which can enable efficient HI decomposition
Accepted 25 April 2018
by enhancing the equilibrium conversion value. Here we report corrosion resistance
Available online xxx
studies on tantalum, niobium and palladium membranes, along with their comparative evaluation. Thin layer each of tantalum, palladium and niobium was coated on
Keywords:
tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition
Metallic membrane
technique. Small pieces of the coated tubes were subject to immersion coupon tests in
Corrosion
HI-water environment (57 wt% HI in water) at a temperature of 125e130 C under
Membrane reactor
reflux environment, and simulated HI decomposition environment at 450 C. The un-
Hydrogen
exposed and exposed cut pieces were analyzed using scanning electron microscope
IS thermochemical cycle
(SEM), energy dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS). The
HI decomposition
extent of leaching of metal into liquid HI was quantified using inductively coupled plasma-mass spectrometer (ICP-MS). Findings confirmed that tantalum is the most resistant membrane material in HI environment (liquid and gas) followed by niobium and palladium. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Membrane Development Section, Chemical Engineering Group, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India. E-mail addresses: [email protected], [email protected] (S. Kar). https://doi.org/10.1016/j.ijhydene.2018.04.212 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Introduction The increasing demand of clean energy and long term solution for energy crisis has resulted in the faster evolution of Hydrogen economy [1]. There are various methods available for hydrogen production [2e4], but thermochemical water splitting processes are the best options where heat energy from nuclear power plants is available [5e8] for utilisation. Thermochemical water splitting from IodineeSulfur (IS) thermochemical cycle offers a clean and economical route for large scale hydrogen production [9e14]. This water splitting process consists of three coupled reactions [15]. The first reaction is Bunsen reaction in which sulfur dioxide (SO2), water, and iodine (I2) react to form sulfuric acid (H2SO4) and hydrogen iodide (HI). Sulfuric acid and hydrogen iodide are separated and decomposed separately at ~850 C and ~300e450 C respectively. HI decomposition section consists of acid mixtures of HI, H2O and I2 commonly known as HIX. Hydrogen is produced in the HI decomposition section of the cycle. A critical challenge with this step is the low equilibrium conversion of HI decomposition reaction (~22% at 700 K) which increases the amount of recycled HI, affecting the overall thermal efficiency of the cycle. Moreover, the reaction environment of HIx is extremely corrosive in nature. It is, therefore, important that the material used in IS thermochemical cycle should be resistant to corrosive environment at 450 C. Hydrogen permselective membrane reactors can be used to remove hydrogen in-situ from the reaction mixture and push the reaction forward, enhancing the conversion of HI decomposition reaction [16,17], and the overall efficiency of the IS process. Though palladium and its alloys are widely used and studied for application of metal membranes in hydrogen separation [18e21], they cannot withstand the corrosive environment of HIx processing section [22]. Very few literatures are available on corrosion resistance studies of potential materials for application in IS cycle [23e25]. Corrosion behavior of niobium and tantalum has been studied in acidic [26e28], chemical [29] and electrolyser (fuel cell) environments [30] along with that of tantalum base alloys [31,32]. Apart from tantalum, corrosion resistance of palladium films has also been widely studied. Role of palladium films in enhancing corrosion resistance of stainless steel [33,34] and titanium [35] have also been highlighted. Corrosion behavior of niobium in sulfuric and hydrochloric acid solutions [36], corrosion of niobium and tantalum in alkaline media [37], and effect of niobium on the corrosion resistance of nickel-base alloys [38] have also been reported. Though refractory materials like tantalum and niobium are envisaged as corrosion resistant materials with higher hydrogen permeabilities compared to palladium and its alloys [22], to the best of our knowledge, the corrosion resistance analysis of metals in view of membrane applications is rare in literature. In the present work, corrosion resistance studies of tantalum, niobium and palladium membranes have been carried out in HI liquid and gas environment, along with their comparative evaluation. The study includes two important analytical domains, namely the leaching studies of metal into liquid HI environment using inductively coupled plasma-mass spectrometer (ICP-MS)
studies, and iodine percolation behavior analysis using secondary ion mass spectrometer (SIMS) studies. Moreover, to the best of our knowledge, the said two studies (metal leaching and iodine percolation into the metals), and an in-depth analysis (HI in liquid and gas environment) of corrosion behavior of potential metal membrane materials for application in HIx processing section have not been carried out in the literature.
Experimental details Fabrication of metal membranes Thin layers each of tantalum, niobium and palladium, of thickness ~2.5 mm, was coated on tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition technique. The base pressure of sputtering chamber was 5.5 106 torr and deposition pressure was kept at 3.2 102 torr. The flow of ultra-pure Ar gas to the chamber was kept at 70 sccm and was regulated by a mass flow controller. DC power was kept at 200 W and substrate temperature was kept at 500 C. Membranes were annealed for 2 h after sputter coating. The morphological analysis of coated surface was carried out using scanning electron microscope (SEM). The surface layer images were recorded at an acceleration voltage of 30 kV and 1000 magnification when operated in secondary electron mode. The quantitative surface elemental analysis and mapping of membrane were carried out by an energy dispersive X-ray spectrometer (EDX) coupled with the SEM and a micro analysis system (INCA Oxford Instrument, UK).
Corrosion resistance studies Corrosion resistance studies of metallic membranes were carried out using immersion coupon tests in liquid HIeH2O environment, and simulated HI decomposition environment at 450 C, both upto 200 h of exposure. The coated tubes were cut into small pieces of 2 cm length and immersed in HI-water azeotrope (57 wt% HI in water) at a temperature range of 125e130 C under reflux environment as shown in Fig. 1. The tubes were taken out after every 50 h of exposure in HI medium and were analyzed using SEM, EDX and SIMS. The extent of leaching of metal into liquid HI was analyzed by ICP-MS (VG PQ Ex Cell, Thermo Elemental). The elemental depth distribution analyses of iodine by SIMS in all the samples were carried out using magnetic sector based Cameca IMS-7f in strument equipped with both oxygen (Oþ 2 and O ) and cesium þ (Cs ) primary ion beams. All the analyses were carried out using negative secondary ion detection mode, since iodine as negative ion (I) is more sensitive as compared to positive ion (Iþ). Since, the samples were non-conducting, a thin film (thickness ~ 50 nm) of Au were deposited in all the samples using sputter deposition technique. Csþ primary ion beam at an impact energy of 15 keV (sample potential 5 keV) with negative secondary ion detection mode was opted for all the analyses. Primary ion beam current of 62 ± 1 nA was raster over an area of 250 mm 250 mm and secondary ions were collected from an analysis region of 63 mm in diameter by
Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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azeotropic vapors) at 450 C as shown in Fig. 2. The terminology “simulated” has been used throughout the manuscript in order to specify that no catalyst has been used in the membrane assembly to decompose HI. The cut tubes were taken out after every 50 h of exposure and were analyzed using SEM, EDX and SIMS.
Results and discussion Corrosion resistance studies: immersion coupon tests in liquid HI at 125e130 C
Fig. 1 e Experimental set-up for immersion coupon test of metal membranes in liquid HI environment at 125e130 C.
selecting the field aperture of 750 mm. Charge compensation during primary ion beam sputtering process was carried out by using normal incidence electron gun. Pressure in the analysis chamber was maintained ~4 109 mbar and mass resolution (m/dm) of 400 was selected in order to minimize the isobaric mass interference arising due to the gases present in the analysis chamber and undesirable species present on the sample surface. The same sets of tubes were exposed to simulated HI decomposition environment (57% w/w HI
The unexposed membrane samples as well as those exposed to liquid HI for different durations of time is shown in Fig. 3. The visual observations of the unexposed and exposed membrane samples of Pd/Ta/Nb clearly indicate that the Pd coating over alumina was peeled off after 50 h of exposure to HI. Nb, on the other hand, was found to exhibit better resistance than Pd, but its coating started peeling off before 100 h of exposure. However, the Ta coating remained intact for up to 200 h of exposure. Fig. 4 shows the SEM images of metal membrane samples for different exposure durations. SEM images do not show any distinct changes in morphology of exposed membrane samples. The EDX studies of metal membrane samples for different exposure durations were carried out to find out the elemental analyses of the samples and their comparative evaluation. Fig. 5 shows the EDX analysis of the samples and it is evident from the chart that Ta layer remains intact even after 200 h of exposure, whereas Pd layer gets removed before 100 h of exposure. Nb membrane sample shows some percentage of Nb at 100 h which finally disappears at 200 h, confirming better corrosion resistance than Pd, but less than that of Ta in liquid HI environment. The values in the histograms represent relative weight percentage of the elements in the samples for various durations. Referring to Fig. 5 (for liquid HI
Fig. 2 e Experimental setup for corrosion resistance studies of metal membrane under simulated HI decomposition environment at 450 C. Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 3 e Pictures of unexposed and exposed membrane samples in liquid HI environment at 125e130 C at different durations of time.
Fig. 4 e SEM images of metal membrane samples for different exposure durations in liquid HI environment.
environment), in the case of tantalum, the iodine deposition has increased with time, but since the Ta layer is intact up till 200 Hrs of exposure, it doesn't allow the iodine to get into the bulk pores of the alumina. Whereas, in the case of Nb, the
layer gets peeled off at 200 h of exposure (as clearly evident in Fig. 5), it therefore allows the alumina to get exposed to iodine and hence bulk absorption of iodine increases. Due to this phenomenon, relatively more iodine is seen in case of Nb at
Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 5 e Elemental analysis of unexposed and exposed metal membrane samples in liquid HI environment.
200 Hrs of exposure and almost nil iodine in case of Ta, though in both the cases surface as well as bulk absorption of iodine has increased with time. Also, since weight loss is higher in case of Nb (layer gets peeled off), the relative weight of iodine and hence wt% increases with time. But weight loss in Ta is less (layer remains intact). Owing to both the effects - less bulk absorption of iodine and lower weight loss of Ta, the weight % of iodine (relative to other elements) seems to be too low for 200 h, and hence practically not detectable.
Another interesting finding that can be obtained from the above study is the leaching behavior of alumina. The alumina leaching is minimal when it is coated with tantalum, which is not the case for Pd and Nb coated tantalum. This further reconfirmed the superior corrosion resistance property of Ta compared to Nb and Pd membranes. Elemental depth profile of 127I in the Ta, Nb and Pd coated alumina samples (blank as well as those exposed to 50, 100, and 200 h at 127 C in HI solution) is shown in Fig. 6 (a), (b) and
Fig. 6 e (Color online) SIMS depth profile of Iodine (I¡) in blank (unexposed), 50 h, 100 h, and 200 h exposure samples at 127 C in (a) Tantalum (b) Niobium and (c) Palladium membrane samples. Experimental parameters; primary ion beam: Csþ; primary ion current: 62±1 nA; impact energy: 15 keV; raster area: 250 mm £ 250 mm: diameter of analysis area: 63 mm; Secondary ion polarity: negative, Electron flooding. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 7 e Pictures of unexposed and exposed membrane samples in simulated HI decomposition environment at 450 C at different durations of time.
(c) respectively. It is observed that with increase in the exposure time, the iodine concentration at the surface of the metal increases. It is also evident from the figure that the concentration of iodine at the surface of the Pd and Nb membrane samples which have been exposed to 100 and 200 h, saturates at a certain point, which is not the case for Ta membranes. The extent of iodine percolation in case of Ta membrane is increased by one order of magnitude, whereas it amounts to about 3 orders of magnitude in case of Pd and Nb membrane samples. Leaching studies of metal in liquid HI environment was carried out through ICP-MS analysis. Sampling was done from the liquid HI used for immersion coupon tests to find out metal concentration in it after different intervals of time. The results are shown in Table 1. The leaching of Pd is maximum in first 100 h with concentration of Pd to be 2625 ng/ml, whereas the leaching of Ta is minimum i.e. 59 ng/ml. Leaching of Nb, as expected lied in between Ta and Pd, that is, 563 ng/ml. It can also be seen from Table- 1 that the leaching of metal into liquid HI increases with increase in
Table 1 e Concentration of metal leached into the liquid HI environment at 125e130 C at different exposure period. Membrane sample
Tantalum Palladium Niobium
Concentration (ng/ml) Pure HI
100 h
150 h
200 h
0.2 17 0.4
59 2625 563
86 2578 582
163 2243 620
exposure time, except that of Pd, which is due to the fact that most of palladium has leached out into liquid HI before 100 h of exposure. The study clearly indicates that Ta is the most corrosion resistant membrane material in HI environment, while Palladium being the least resistant. It is important to note that the performance (permeability and selectivity) evaluation of the membrane coupons post exposure was not carried out, because it does not mimic the actual HI decomposition environment.
Corrosion resistance studies: in simulated HI decomposition environment at 450 C At a temperature of 450 C, HI gets decomposed to hydrogen and iodine. In the corrosion resistance test set-up, no carrier gas was used to sweep away the HI gas. Thus, as seen in Fig. 7, all the membrane coupons have turned brown due to deposit of iodine onto the surface of Pd/Ta/Nb, and it is indeed difficult to conclude anything from the visual studies, unlike the membrane samples obtained from immersion coupon studies in liquid HI environment. Fig. 8 shows the SEM images of metal membranes at different exposure durations in simulated HI decomposition environment. The sooty deposits are clearly observed on all the membrane surfaces, which are due to iodine deposits, which was not the case in immersion coupon studies. Fig. 9 shows the elemental analysis of unexposed and exposed metal membranes. As evident from the chart, the Ta layer is distinct even after 200 h of exposure, whereas Pd layer is not detectable before 100 h of exposure. The restoration of Nb is better than that of Pd, though worse than Ta.
Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 8 e SEM images of metal membrane samples at different exposure durations in simulated HI decomposition environment at 450 C.
Fig. 9 e Elemental analysis of unexposed and exposed metal membranes at different exposure durations in simulated HI decomposition environment at 450 C.
Elemental depth profile of 127I in the Ta, Nb and Pd deposited alumina samples exposed to HI vapors for 50, 100, and 200 h at 450 C is shown in Fig. 10 (a), (b) and (c) respectively. In all the samples, depth profile analysis clearly indicates that with increase in the exposure time, the iodine concentration at the surface as well as in the bulk increases. However, as evident from Fig. 10, the extent of percolation of iodine into the metal in gaseous HI environment is found to be
about 2 orders of magnitude less than that of liquid HI, which indicates very low percolation of iodine into the bulk of the membrane, though surface is visually tanned. This could be due to that fact that iodine, being in gaseous phase at 450 C, gets low contact time to interact with and percolate into the bulk of the membranes. Whereas, in liquid HI environment, though evolution of iodine is expected to be less, its percolation could be more owing to higher residence time. The
Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 10 e (Color online) SIMS depth profile of Iodine (I¡) in 50 h, 100 h, and 200 h exposure samples at 450 C in (a) Tantalum coated zeolite (b) Niobium coated zeolite and (c) Palladium coated zeolite samples. Experimental parameters; primary ion beam: Csþ; primary ion current: 62±1 nA; impact energy: 15 keV; raster area: 250 mm £ 250 mm: diameter of analysis area: 63 mm; Secondary ion polarity: negative, Electron flooding. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
findings corroborate well with the membrane performance data (as reported in our previous work [17]), wherein the performance of Ta membrane reactor has been found intact even upto more than 200 h of exposure in real HI decomposition environment.
Conclusions Corrosion resistance studies of tantalum, niobium and palladium membranes were carried out in HI liquid and gas environment, along with their comparative evaluation. Sputter deposited thin layers of tantalum, palladium and niobium were used for immersion coupon tests in liquid HI (57 wt% HI in water) at a temperature range of 125e130 C under reflux environment. The same sets of as-grown tubes were exposed to simulated HI decomposition environment at 450 C. The tubes after exposure were analyzed using SEM, EDX and SIMS. The depth profile analysis through SIMS showed that with increase in the exposure time, the iodine concentration at the surface as well as in the bulk increases. Leaching studies of metal in HI environment was carried out by ICPMS analysis, which showed that palladium has got the highest leachability in liquid HI environment followed by niobium and tantalum. Results confirmed that tantalum has the highest corrosion
resistance ability in HI environment (both liquid and gas) followed by niobium and palladium.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production Bharat Bhushan a,b, Nitesh Goswami a, S.C. Parida b,c, B.N. Rath d, Sanjukta A. Kumar e, V. Karki f, R.C. Bindal a,b, Soumitra Kar a,b,* a Membrane Development Section, Chemical Engineering Group, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India b Homi Bhabha National Institute, Anushakti Nagar, Mumbai, 400094, India c Product Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India d Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India e Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India f Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India
article info
abstract
Article history:
HI decomposition in Iodine-Sulfur (IS) thermochemical process for hydrogen produc-
Received 23 February 2018
tion is one of the critical steps, which suffers from low equilibrium conversion as well
Received in revised form
as highly corrosive environment. Corrosion-resistant metal membrane reactor is pro-
10 April 2018
posed to be a process intensification tool, which can enable efficient HI decomposition
Accepted 25 April 2018
by enhancing the equilibrium conversion value. Here we report corrosion resistance
Available online xxx
studies on tantalum, niobium and palladium membranes, along with their comparative evaluation. Thin layer each of tantalum, palladium and niobium was coated on
Keywords:
tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition
Metallic membrane
technique. Small pieces of the coated tubes were subject to immersion coupon tests in
Corrosion
HI-water environment (57 wt% HI in water) at a temperature of 125e130 C under
Membrane reactor
reflux environment, and simulated HI decomposition environment at 450 C. The un-
Hydrogen
exposed and exposed cut pieces were analyzed using scanning electron microscope
IS thermochemical cycle
(SEM), energy dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS). The
HI decomposition
extent of leaching of metal into liquid HI was quantified using inductively coupled plasma-mass spectrometer (ICP-MS). Findings confirmed that tantalum is the most resistant membrane material in HI environment (liquid and gas) followed by niobium and palladium. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Membrane Development Section, Chemical Engineering Group, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India. E-mail addresses: [email protected], [email protected] (S. Kar). https://doi.org/10.1016/j.ijhydene.2018.04.212 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Introduction The increasing demand of clean energy and long term solution for energy crisis has resulted in the faster evolution of Hydrogen economy [1]. There are various methods available for hydrogen production [2e4], but thermochemical water splitting processes are the best options where heat energy from nuclear power plants is available [5e8] for utilisation. Thermochemical water splitting from IodineeSulfur (IS) thermochemical cycle offers a clean and economical route for large scale hydrogen production [9e14]. This water splitting process consists of three coupled reactions [15]. The first reaction is Bunsen reaction in which sulfur dioxide (SO2), water, and iodine (I2) react to form sulfuric acid (H2SO4) and hydrogen iodide (HI). Sulfuric acid and hydrogen iodide are separated and decomposed separately at ~850 C and ~300e450 C respectively. HI decomposition section consists of acid mixtures of HI, H2O and I2 commonly known as HIX. Hydrogen is produced in the HI decomposition section of the cycle. A critical challenge with this step is the low equilibrium conversion of HI decomposition reaction (~22% at 700 K) which increases the amount of recycled HI, affecting the overall thermal efficiency of the cycle. Moreover, the reaction environment of HIx is extremely corrosive in nature. It is, therefore, important that the material used in IS thermochemical cycle should be resistant to corrosive environment at 450 C. Hydrogen permselective membrane reactors can be used to remove hydrogen in-situ from the reaction mixture and push the reaction forward, enhancing the conversion of HI decomposition reaction [16,17], and the overall efficiency of the IS process. Though palladium and its alloys are widely used and studied for application of metal membranes in hydrogen separation [18e21], they cannot withstand the corrosive environment of HIx processing section [22]. Very few literatures are available on corrosion resistance studies of potential materials for application in IS cycle [23e25]. Corrosion behavior of niobium and tantalum has been studied in acidic [26e28], chemical [29] and electrolyser (fuel cell) environments [30] along with that of tantalum base alloys [31,32]. Apart from tantalum, corrosion resistance of palladium films has also been widely studied. Role of palladium films in enhancing corrosion resistance of stainless steel [33,34] and titanium [35] have also been highlighted. Corrosion behavior of niobium in sulfuric and hydrochloric acid solutions [36], corrosion of niobium and tantalum in alkaline media [37], and effect of niobium on the corrosion resistance of nickel-base alloys [38] have also been reported. Though refractory materials like tantalum and niobium are envisaged as corrosion resistant materials with higher hydrogen permeabilities compared to palladium and its alloys [22], to the best of our knowledge, the corrosion resistance analysis of metals in view of membrane applications is rare in literature. In the present work, corrosion resistance studies of tantalum, niobium and palladium membranes have been carried out in HI liquid and gas environment, along with their comparative evaluation. The study includes two important analytical domains, namely the leaching studies of metal into liquid HI environment using inductively coupled plasma-mass spectrometer (ICP-MS)
studies, and iodine percolation behavior analysis using secondary ion mass spectrometer (SIMS) studies. Moreover, to the best of our knowledge, the said two studies (metal leaching and iodine percolation into the metals), and an in-depth analysis (HI in liquid and gas environment) of corrosion behavior of potential metal membrane materials for application in HIx processing section have not been carried out in the literature.
Experimental details Fabrication of metal membranes Thin layers each of tantalum, niobium and palladium, of thickness ~2.5 mm, was coated on tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition technique. The base pressure of sputtering chamber was 5.5 106 torr and deposition pressure was kept at 3.2 102 torr. The flow of ultra-pure Ar gas to the chamber was kept at 70 sccm and was regulated by a mass flow controller. DC power was kept at 200 W and substrate temperature was kept at 500 C. Membranes were annealed for 2 h after sputter coating. The morphological analysis of coated surface was carried out using scanning electron microscope (SEM). The surface layer images were recorded at an acceleration voltage of 30 kV and 1000 magnification when operated in secondary electron mode. The quantitative surface elemental analysis and mapping of membrane were carried out by an energy dispersive X-ray spectrometer (EDX) coupled with the SEM and a micro analysis system (INCA Oxford Instrument, UK).
Corrosion resistance studies Corrosion resistance studies of metallic membranes were carried out using immersion coupon tests in liquid HIeH2O environment, and simulated HI decomposition environment at 450 C, both upto 200 h of exposure. The coated tubes were cut into small pieces of 2 cm length and immersed in HI-water azeotrope (57 wt% HI in water) at a temperature range of 125e130 C under reflux environment as shown in Fig. 1. The tubes were taken out after every 50 h of exposure in HI medium and were analyzed using SEM, EDX and SIMS. The extent of leaching of metal into liquid HI was analyzed by ICP-MS (VG PQ Ex Cell, Thermo Elemental). The elemental depth distribution analyses of iodine by SIMS in all the samples were carried out using magnetic sector based Cameca IMS-7f in strument equipped with both oxygen (Oþ 2 and O ) and cesium þ (Cs ) primary ion beams. All the analyses were carried out using negative secondary ion detection mode, since iodine as negative ion (I) is more sensitive as compared to positive ion (Iþ). Since, the samples were non-conducting, a thin film (thickness ~ 50 nm) of Au were deposited in all the samples using sputter deposition technique. Csþ primary ion beam at an impact energy of 15 keV (sample potential 5 keV) with negative secondary ion detection mode was opted for all the analyses. Primary ion beam current of 62 ± 1 nA was raster over an area of 250 mm 250 mm and secondary ions were collected from an analysis region of 63 mm in diameter by
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azeotropic vapors) at 450 C as shown in Fig. 2. The terminology “simulated” has been used throughout the manuscript in order to specify that no catalyst has been used in the membrane assembly to decompose HI. The cut tubes were taken out after every 50 h of exposure and were analyzed using SEM, EDX and SIMS.
Results and discussion Corrosion resistance studies: immersion coupon tests in liquid HI at 125e130 C
Fig. 1 e Experimental set-up for immersion coupon test of metal membranes in liquid HI environment at 125e130 C.
selecting the field aperture of 750 mm. Charge compensation during primary ion beam sputtering process was carried out by using normal incidence electron gun. Pressure in the analysis chamber was maintained ~4 109 mbar and mass resolution (m/dm) of 400 was selected in order to minimize the isobaric mass interference arising due to the gases present in the analysis chamber and undesirable species present on the sample surface. The same sets of tubes were exposed to simulated HI decomposition environment (57% w/w HI
The unexposed membrane samples as well as those exposed to liquid HI for different durations of time is shown in Fig. 3. The visual observations of the unexposed and exposed membrane samples of Pd/Ta/Nb clearly indicate that the Pd coating over alumina was peeled off after 50 h of exposure to HI. Nb, on the other hand, was found to exhibit better resistance than Pd, but its coating started peeling off before 100 h of exposure. However, the Ta coating remained intact for up to 200 h of exposure. Fig. 4 shows the SEM images of metal membrane samples for different exposure durations. SEM images do not show any distinct changes in morphology of exposed membrane samples. The EDX studies of metal membrane samples for different exposure durations were carried out to find out the elemental analyses of the samples and their comparative evaluation. Fig. 5 shows the EDX analysis of the samples and it is evident from the chart that Ta layer remains intact even after 200 h of exposure, whereas Pd layer gets removed before 100 h of exposure. Nb membrane sample shows some percentage of Nb at 100 h which finally disappears at 200 h, confirming better corrosion resistance than Pd, but less than that of Ta in liquid HI environment. The values in the histograms represent relative weight percentage of the elements in the samples for various durations. Referring to Fig. 5 (for liquid HI
Fig. 2 e Experimental setup for corrosion resistance studies of metal membrane under simulated HI decomposition environment at 450 C. Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 3 e Pictures of unexposed and exposed membrane samples in liquid HI environment at 125e130 C at different durations of time.
Fig. 4 e SEM images of metal membrane samples for different exposure durations in liquid HI environment.
environment), in the case of tantalum, the iodine deposition has increased with time, but since the Ta layer is intact up till 200 Hrs of exposure, it doesn't allow the iodine to get into the bulk pores of the alumina. Whereas, in the case of Nb, the
layer gets peeled off at 200 h of exposure (as clearly evident in Fig. 5), it therefore allows the alumina to get exposed to iodine and hence bulk absorption of iodine increases. Due to this phenomenon, relatively more iodine is seen in case of Nb at
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Fig. 5 e Elemental analysis of unexposed and exposed metal membrane samples in liquid HI environment.
200 Hrs of exposure and almost nil iodine in case of Ta, though in both the cases surface as well as bulk absorption of iodine has increased with time. Also, since weight loss is higher in case of Nb (layer gets peeled off), the relative weight of iodine and hence wt% increases with time. But weight loss in Ta is less (layer remains intact). Owing to both the effects - less bulk absorption of iodine and lower weight loss of Ta, the weight % of iodine (relative to other elements) seems to be too low for 200 h, and hence practically not detectable.
Another interesting finding that can be obtained from the above study is the leaching behavior of alumina. The alumina leaching is minimal when it is coated with tantalum, which is not the case for Pd and Nb coated tantalum. This further reconfirmed the superior corrosion resistance property of Ta compared to Nb and Pd membranes. Elemental depth profile of 127I in the Ta, Nb and Pd coated alumina samples (blank as well as those exposed to 50, 100, and 200 h at 127 C in HI solution) is shown in Fig. 6 (a), (b) and
Fig. 6 e (Color online) SIMS depth profile of Iodine (I¡) in blank (unexposed), 50 h, 100 h, and 200 h exposure samples at 127 C in (a) Tantalum (b) Niobium and (c) Palladium membrane samples. Experimental parameters; primary ion beam: Csþ; primary ion current: 62±1 nA; impact energy: 15 keV; raster area: 250 mm £ 250 mm: diameter of analysis area: 63 mm; Secondary ion polarity: negative, Electron flooding. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article in press as: Bhushan B, et al., Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.212
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Fig. 7 e Pictures of unexposed and exposed membrane samples in simulated HI decomposition environment at 450 C at different durations of time.
(c) respectively. It is observed that with increase in the exposure time, the iodine concentration at the surface of the metal increases. It is also evident from the figure that the concentration of iodine at the surface of the Pd and Nb membrane samples which have been exposed to 100 and 200 h, saturates at a certain point, which is not the case for Ta membranes. The extent of iodine percolation in case of Ta membrane is increased by one order of magnitude, whereas it amounts to about 3 orders of magnitude in case of Pd and Nb membrane samples. Leaching studies of metal in liquid HI environment was carried out through ICP-MS analysis. Sampling was done from the liquid HI used for immersion coupon tests to find out metal concentration in it after different intervals of time. The results are shown in Table 1. The leaching of Pd is maximum in first 100 h with concentration of Pd to be 2625 ng/ml, whereas the leaching of Ta is minimum i.e. 59 ng/ml. Leaching of Nb, as expected lied in between Ta and Pd, that is, 563 ng/ml. It can also be seen from Table- 1 that the leaching of metal into liquid HI increases with increase in
Table 1 e Concentration of metal leached into the liquid HI environment at 125e130 C at different exposure period. Membrane sample
Tantalum Palladium Niobium
Concentration (ng/ml) Pure HI
100 h
150 h
200 h
0.2 17 0.4
59 2625 563
86 2578 582
163 2243 620
exposure time, except that of Pd, which is due to the fact that most of palladium has leached out into liquid HI before 100 h of exposure. The study clearly indicates that Ta is the most corrosion resistant membrane material in HI environment, while Palladium being the least resistant. It is important to note that the performance (permeability and selectivity) evaluation of the membrane coupons post exposure was not carried out, because it does not mimic the actual HI decomposition environment.
Corrosion resistance studies: in simulated HI decomposition environment at 450 C At a temperature of 450 C, HI gets decomposed to hydrogen and iodine. In the corrosion resistance test set-up, no carrier gas was used to sweep away the HI gas. Thus, as seen in Fig. 7, all the membrane coupons have turned brown due to deposit of iodine onto the surface of Pd/Ta/Nb, and it is indeed difficult to conclude anything from the visual studies, unlike the membrane samples obtained from immersion coupon studies in liquid HI environment. Fig. 8 shows the SEM images of metal membranes at different exposure durations in simulated HI decomposition environment. The sooty deposits are clearly observed on all the membrane surfaces, which are due to iodine deposits, which was not the case in immersion coupon studies. Fig. 9 shows the elemental analysis of unexposed and exposed metal membranes. As evident from the chart, the Ta layer is distinct even after 200 h of exposure, whereas Pd layer is not detectable before 100 h of exposure. The restoration of Nb is better than that of Pd, though worse than Ta.
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Fig. 8 e SEM images of metal membrane samples at different exposure durations in simulated HI decomposition environment at 450 C.
Fig. 9 e Elemental analysis of unexposed and exposed metal membranes at different exposure durations in simulated HI decomposition environment at 450 C.
Elemental depth profile of 127I in the Ta, Nb and Pd deposited alumina samples exposed to HI vapors for 50, 100, and 200 h at 450 C is shown in Fig. 10 (a), (b) and (c) respectively. In all the samples, depth profile analysis clearly indicates that with increase in the exposure time, the iodine concentration at the surface as well as in the bulk increases. However, as evident from Fig. 10, the extent of percolation of iodine into the metal in gaseous HI environment is found to be
about 2 orders of magnitude less than that of liquid HI, which indicates very low percolation of iodine into the bulk of the membrane, though surface is visually tanned. This could be due to that fact that iodine, being in gaseous phase at 450 C, gets low contact time to interact with and percolate into the bulk of the membranes. Whereas, in liquid HI environment, though evolution of iodine is expected to be less, its percolation could be more owing to higher residence time. The
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Fig. 10 e (Color online) SIMS depth profile of Iodine (I¡) in 50 h, 100 h, and 200 h exposure samples at 450 C in (a) Tantalum coated zeolite (b) Niobium coated zeolite and (c) Palladium coated zeolite samples. Experimental parameters; primary ion beam: Csþ; primary ion current: 62±1 nA; impact energy: 15 keV; raster area: 250 mm £ 250 mm: diameter of analysis area: 63 mm; Secondary ion polarity: negative, Electron flooding. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
findings corroborate well with the membrane performance data (as reported in our previous work [17]), wherein the performance of Ta membrane reactor has been found intact even upto more than 200 h of exposure in real HI decomposition environment.
Conclusions Corrosion resistance studies of tantalum, niobium and palladium membranes were carried out in HI liquid and gas environment, along with their comparative evaluation. Sputter deposited thin layers of tantalum, palladium and niobium were used for immersion coupon tests in liquid HI (57 wt% HI in water) at a temperature range of 125e130 C under reflux environment. The same sets of as-grown tubes were exposed to simulated HI decomposition environment at 450 C. The tubes after exposure were analyzed using SEM, EDX and SIMS. The depth profile analysis through SIMS showed that with increase in the exposure time, the iodine concentration at the surface as well as in the bulk increases. Leaching studies of metal in HI environment was carried out by ICPMS analysis, which showed that palladium has got the highest leachability in liquid HI environment followed by niobium and tantalum. Results confirmed that tantalum has the highest corrosion
resistance ability in HI environment (both liquid and gas) followed by niobium and palladium.
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