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Corrosion behavior and surface film characterization of TaNbHfZrTi high entropy alloy in aggressive nitric acid medium
Intermetallics 89 (2017) 123–132
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Corrosion behavior and surface film characterization of TaNbHfZrTi high entropy alloy in aggressive nitric acid medium
MARK
J. Jayaraja,∗, C. Thinaharana, S. Ningshena, C. Mallikaa, U. Kamachi Mudalib a b
Corrosion Science and Technology Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, 603 102, India Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, 603 102, India
A R T I C L E I N F O
A B S T R A C T
Keywords: A. High-entropy alloys B. Corrosion C. Casting D. Microstructure F. Scanning electron microscopy Spectroscopic methods
Corrosion behavior of TaNbHfZrTi high-entropy alloy (HEA) was investigated in nitric and fluorinated nitric acid at ambient (27 °C) and boiling (120 °C) conditions. The alloy passivated spontaneously during potentiodynamic polarization in 11.5 M HNO3 at ambient condition. The corrosion rate was negligible in boiling 11.5 M HNO3, exposed for 240 h. Scanning electron microscopic (SEM) studies did not show any significant corrosion attack. The high corrosion resistance of TaNbHfZrTi HEA was attributed to its single phase bcc structure. X-ray photoelectron spectroscopic (XPS) analysis revealed that the protective passive film formed in boiling nitric acid was predominantly composed of Ta2O5, in contrast to the presence of ZrO2 and HfO2 in air-formed native film. Potentiodynamic polarization studies indicated a pseudo-passivation behavior of the HEA in 11.5 M HNO3 + 0.05 M NaF at ambient condition. In boiling fluorinated nitric acid, SEM images of TaNbHfZrTi HEA displayed a severely corroded morphology indicating the instability of the metal-oxides of the alloying elements. XPS investigations confirmed the presence of ZrF4, ZrOF2 and HfF4 along with un-protective oxides of Ta, Nb and Ti on the film, resulting in decreased corrosion resistance of TaNbHfZrTi HEA in fluorinated nitric acid.
1. Introduction Recently, a novel class of advanced structural materials, called High-Entropy Alloys (HEAs) is gaining significant importance in the field of Materials Science [1,2]. Compared to conventional alloys containing one principal element, HEAs are usually composed of multiple elements with equimolar or near equimolar elemental fractions, which form predominantly single solid solution phase [1,2]. The four core effects of HEAs are high entropy effect, distorted lattice, sluggish diffusion, and cocktail effect [1,2]. Owing to these effects, HEAs exhibit solid solution strengthening with better mechanical, wear, corrosion resistance properties and they also possess excellent microstructural stability at high temperatures [1,2]. Initially, the development of HEAs was focused mainly on alloying elements containing Fe, Ni, Co, Cr, Mn, Cu, Ti, V and Al [3–6]. Senkov and co-workers [7,8] developed refractory WTaNbMo and WTaNbMoV HEAs that exhibited high yield strength even at an elevated temperature of 1600 °C. Subsequently, Senkov et al. [9] decreased the density of the HEA by replacing W and Mo with Ti, Hf and Zr and developed TaNbHfZrTi HEA with improved specific strength. This TaNbHfZrTi HEA exhibited homogeneous plastic flow and up to 40% compression strain with marked strain hardening at
∗
room temperature. The excellent ductility of TaNbHfZrTi alloy in compression allowed the material to be produced in thin sheet by cold rolling process [10]. The as-cast microstructure of TaNbHfZrTi HEA consisted of dendritic colonies whereas the cold rolled and annealed sample consisted of an equiaxed structure [9,10]. Both the as-cast and cold-rolled TaNbHfZrTi HEA exhibited a single-phase with bcc structure [9,10]. Consequently, researchers concentrated on developing several refractory elements based high-entropy alloys such as NbCrMo0.5Ta0.5TiZr [11], TiZrNbMoV [12], Al(V)NbTaTiZr [13], CrNbTiVZr [14], AlHfNbTaTiZr [15], HfMoNbTaTiZr [16] and MoWAlCrTi [17]. Most of the studies on these HEAs were focused on the phase formation, microstructure and mechanical properties [11–16] and oxidation behavior [17,18]. The advantage of the mechanical properties of the HEAs could not be fully exploited since their corrosion resistance was not understood properly. Exhaustive reviews on the corrosion resistant HEAs based on non-refractory elements are available in literature, where the effect of alloying elements, processing methods, heat-treatment and corrosion behavior, mainly in H2SO4 and NaCl environments were discussed and compared with the conventional engineering alloys [19–21]. Only limited studies have been reported for the corrosion behavior of refractory HEAs such as TiZr0.5NbCr0.5,
Corresponding author. E-mail addresses: [email protected], [email protected] (J. Jayaraj), [email protected] (C. Thinaharan), [email protected] (S. Ningshen), [email protected] (C. Mallika), [email protected] (U. Kamachi Mudali). http://dx.doi.org/10.1016/j.intermet.2017.06.002 Received 24 March 2017; Received in revised form 23 May 2017; Accepted 6 June 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.
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a reproducible surface. Platinum and Ag/AgCl (3 M KCl) were used as counter and reference electrode, respectively. The open circuit potential (OCP) of the sample was monitored for 1 h. After measuring the OCP, potentiodynamic polarization tests were carried out at the scan rate of 0.00017 V/s from 0.2 V below OCP to 2.0 V. Both the OCP and potentiodynamic polarization experiments were performed at least twice, for checking the reproducibility of the results.
TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Mo in H2SO4 and NaCl solutions [22]. Materials exhibiting excellent corrosion resistance, high strength and formability are desirable for the fabrication of dissolver components of reprocessing plant for the processing of spent nuclear fuels by aqueous route. The search for advanced alloys with properties better than the existing conventional materials is a motivating factor among the researchers. The corrosion behavior of the HEAs in boiling 11.5 M nitric acid, which is the dissolution medium for the spent nuclear fuels from fast breeder reactors [23] is not reported in the literature. In addition, the effect of fluoride ions in boiling nitric acid on the corrosion behavior of the dissolver vessels handling Pu-rich mixed oxide fuels [24] need proper investigation. Owing to the single phase structure as well as good mechanical and formability properties of TaNbHfZrTi HEA [9,10], it can be considered as a candidate material for the fabrication of critical components in reprocessing plants. Apart from the application point of view, the role of alloying elements in the TaNbHfZrTi multi-component HEA contributing to its passivation behavior is less understood. In the present study, the performance of TaNbHfZrTi HEA was investigated in boiling nitric acid with and without fluoride ions. Potentiodynamic polarization test was used to characterize the passivation behavior of TaNbHfZrTi alloy in 11.5 M HNO3 as well as 11.5 M HNO3 + 0.05 M NaF at room temperature. The corrosion rate of TaNbHfZrTi was determined by weight loss method in 11.5 M HNO3 as well as fluorinated nitric acid media under boiling conditions. Scanning electron microscopic (SEM) and X-ray photoelectron spectroscopic (XPS) analyses were performed to corroborate the results of the studies pertaining to passivation and corrosion behavior of this alloy.
2.4. Measurement of corrosion rate The experimental set-up used for corrosion test in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at boiling conditions (120 °C) is described elsewhere [25]. A cold finger condenser was used to reflux the acid vapors into the test solution. In the same set-up, the corrosion experiments at room temperature were also conducted. The TaNbHfZrTi HEA sample surface was wet ground in SiC emery paper up to 1200 grit. The polished samples were degreased with acetone, dried and weighed before immersing them into the test solutions. The samples were suspended into the solution through a Teflon thread for a minimum exposure period of 48 h. Based on the extent of corrosion, the experiments were performed for five periods of 48 h (i.e. total exposure period of 240 h) with fresh solutions being used in each period as described in ASTM A262 Practice C method [26]. The immersion tests were carried out at least twice for all the conditions studied. After corrosion testing, the samples were removed, cleaned carefully and the change in weight was measured in a micro balance with resolution of 0.0001 g. From the weight loss experiments, the corrosion rates were calculated using Eq. (1).
Corrosion rate = 2. Experimental procedure
8.76 × 10 4 × W dAt
(1)
where, corrosion rate is expressed in mm/yr, W is weight loss in g, d is the theoretical density of the alloy (11.82 g/cm3), A is the exposed surface area in cm2 and t is the exposed time in h.
2.1. Alloy preparation The TaNbHfZrTi equimolar alloy was prepared using a vacuum arc melting system (M/s. Edmund Buhler GmbH, Germany). A total weight of 5 g of the high purity (99.99%) Ta, Nb, Hf, Zr, and Ti elements were melted together on the water-cooled copper hearth of the arc melting system. Prior to melting, the arc melting chamber was evacuated to a vacuum level 0.035 Pa and back-filled with high purity argon. To promote homogeneity, the ingot was melted at least five times and was flipped between each melting.
2.5. Surface characterisation The surface morphology of the TaNbHfZrTi HEA samples subjected to potentiodynamic polarization and weight loss studies was investigated using SEM, in secondary electron mode at an accelerating voltage of 30 kV. To analyze the surface chemistry of TaNbHfZrTi HEA before and after exposure to 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at boiling conditions, XPS was employed. For recording the spectra, SPECS Surface Nano Analysis GmbH, Germany, XPS was used with Al-Kα excitation. The data were processed by SpecsLab2 software and analyzed by CasaXPS software.
2.2. Phase analysis and microstructural characterization The arc melted TaNbHfZrTi ingot was characterized for phase analysis by X-ray Diffraction technique (XRD; INEL, France) using monochromatic Cu Kα radiation. For metallographic observation, the TaNbHfZrTi sample was wet ground in SiC emery paper up to 1200 grit and followed by final polishing in colloidal silica suspension to obtain a smooth mirror-like surface finish. To reveal the microstructure, the polished sample was chemical etched with the Kroll-reagent (2 ml HF, 3 ml HNO3 and 100 ml distilled H2O) for 10 s. The etched sample's surface was analyzed using scanning electron microscopy (SEM; SNE3000M, Korea).
3. Results and discussion 3.1. Phase analysis and microstructure The XRD pattern of the as-cast, TaNbHfZrTi sample is shown in Fig. 1a and the diffraction peaks matched with body centered cubic (bcc) phase with the corresponding planes of (110), (200) and (211). Since no additional peaks were present, it could be concluded that the TaNbHfZrTi alloy ingot was of single bcc phase and the calculated lattice parameter was about 3.404 Å, which is in good agreement with that of the reported value [9]. The SEM micrograph (Fig. 1b) revealed a typical dendrite microstructure for the as-cast TaNbHfZrTi HEA [9].
2.3. Potentiodynamic polarization studies To understand the passivation behavior of TaNbHfZrTi HEA in both 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at room temperature, potentiodynamic polarization experiments were carried out using the electrochemical system (Autolab, The Netherlands). For polarization measurements, a standard three electrode cell with the test sample as working electrode with an exposed area of 1 cm2 was employed. The working electrode's surface was prepared by wet ground using SiC paper up to 1200 grit and cleaned in distilled water and acetone to have
3.2. Potentiodynamic polarization behavior of TaNbHfZrTi HEA Fig. 2 shows the potentiodynamic polarization behavior of TaNbHfZrTi in 11.5 M HNO3, and 11.5 M HNO3 + 0.05 M NaF. From Fig. 2, the corrosion potential, Ecorr and corrosion current density Icorr were obtained using Tafel analysis [27]. The Ecorr values were found to 124
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Fig. 1. (a) XRD pattern and (b) SEM microstructure of the TaNbHfZrTi HEA.
current behavior of the alloy exhibited in fluorinated nitric acid should not be considered as passive behavior; rather, it is a “pseudo-passive” behavior of the TaNbHfZrTi alloy in 11.5 M HNO3 + 0.05 M NaF medium at room temperature. 3.3. Corrosion rates of TaNbHfZrTi in boiling nitric acid and fluorinated nitric acid The corrosion rates of TaNbHfZrTi HEA in 11.5 M HNO3 at room temperature (27 °C) and under boiling condition (120 °C) for 48 and 240 h are given in Table 1. The corrosion rates were insignificant at both the temperatures investigated, indicating that the weight loss before and after the exposure period was negligible. SEM analysis on the TaNbHfZrTi HEA revealed an un-attacked surface morphology when exposed to boiling nitric acid (Fig. 4) for 240 h. The corrosion rates of TaNbHfZrTi HEA in 11.5 M HNO3 + 0.05 M NaF medium at room temperature (27 °C) and boiling condition (120 °C) for 48 h are also given in Table 1. In comparison to nitric acid, the corrosion rates increased in fluorinated nitric acid at both the temperatures and the values were quite significant at the boiling condition. As shown in Fig. 5a, the fluoride ions attacked the HEA severely, resembling a “dry land of drought conditions”. The surface of the corrosion tested sample in boiling fluorinated nitric acid appeared to be loosely packed like powder. The loosely bound powder (Fig. 5a) on the top surface could be easily removed using an adhesive tape and the underneath surface was observed to be a predominantly smooth surface, as shown in Fig. 5b. This implies that the film formed on the surface of TaNbHfZrTi sample during exposure to boiling fluorinated nitric acid was unstable and un-protective, which could have resulted in the high corrosion rates.
Fig. 2. Potentiodynamic polarization curves of TaNbHfZrTi HEA in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at room temperature.
be 0.682 ± 0.07 and 0.115 ± 0.024 V respectively in nitric and fluorinated nitric acid. It is worth mentioning that the Ecorr values followed a similar trend to that of OCP. In concentrated nitric acid, oxidizing agents such as nitrous acid and nitrogen peroxide would be produced due to the autocatalytic reduction [28]. The highly oxidizing nature of 11.5 M HNO3 medium is attributed to the ennoblement of Ecorr value. The Ecorr value of the TaNbHfZrTi HEA was lowered in fluorinated nitric acid when compared to nitric acid solution. The Icorr values measured were 0.2 ± 0.05 and 2.1 ± 0.8 μA/cm2, respectively in nitric and fluorinated nitric acid. The low Icorr value is attributed to the high corrosion resistance of the TaNbHfZrTi HEA in nitric acid at room temperature. The high electronegativity and aggressive nature of fluoride ions could have shifted the Ecorr value to active region, resulting in the higher Icorr value. Above the Ecorr, the current density values remained almost constant until 2 V for both environments studied. In general, the constant current region is defined as the passive region. However, for the TaNbHfZrTi HEA in 11.5 M HNO3 + 0.05 M NaF medium, the current density values were at least an order of magnitude higher than that in nitric acid. The samples after polarization studies in nitric acid medium exhibited an un-attacked surface, as revealed in the SEM image (Fig. 3a), indicating a high corrosion resistance of the alloy in nitric acid. After the polarization studies in fluorinated nitric acid, a slightly attacked morphology (Fig. 3b) was observed. At higher magnification, the attacked surface displayed flaky morphology of the film as shown in Fig. 3c, confirming the un-stable nature of the film formed in fluorinated nitric acid. Thus, the constant
3.4. XPS analysis of TaNbHfZrTi exposed to boiling nitric acid XPS analysis was performed to characterize the composition of the native and passive films formed on the TaNbHfZrTi HEA. The surface film formed in air (before immersion in nitric acid) is referred to as ‘native film’ and the one formed during immersion in boiling 11.5 M HNO3 for 240 h is referred to as ‘passive film’. The spectra of the native and passive films showed peaks corresponding to carbon (C 1s), oxygen (O 1s), tantalum (Ta 4f), niobium (Nb 3d), hafnium (Hf 4f), zirconium (Zr 3d) and titanium (Ti 2p). For both the films, the C 1s peak corresponded to unavoidable contamination of carbon formed during sample handling and the O 1s peaks, originated from the oxygen bonded to metals. The spectra of all the alloying elements in the native film of the TaNbHfZrTi HEA are shown in Fig. 6 and the corresponding binding energy values of their chemical states (oxide or metallic) are given in Table 2. For the native film, the de-convoluted Ta 4f spectrum (Fig. 6a) revealed two sets of doublet peaks at the respective binding energy 125
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Fig. 3. SEM images of TaNbHfZrTi HEA after potentiodynamic polarization studies at room temperature: (a) 11.5 M HNO3; (b–c) 11.5 M HNO3 + 0.05 M NaF; (c) magnified image of the attacked region marked in (b).
values (Table 2), corresponding to Ta existing in both +5 oxidation state (Ta2O5) as well as in metallic form [29]. In the de-convoluted Nb 3d core level region, two doublet peaks and two singlet peaks were fitted as shown in Fig. 6b. The doublet peaks corresponded to those of Nb in metallic state and in +5 oxidation state (Nb2O5) with respect to their binding energies [30]. The two singlet peaks observed (Fig. 6b) were for Hf metal and HfO2 at the respective binding energies of 211.2 and 214.2 eV, corresponding to the photoelectron peaks of Hf 4d5/2 [31] that overlapped in the Nb 3d region. On the other hand, in Fig. 6c, the de-convoluted Hf 4f spectrum showed two doublet peaks corresponding to metallic Hf and HfO2 [31–33]. Similarly, the de-convoluted high resolution spectrum in the Zr 3d region (Fig. 6d) revealed two sets of doublet peaks corresponding to the +4 oxidation state (ZrO2) and metallic state of Zr [34]. Interestingly, the de-convoluted high resolution spectrum in the Ti 2p region exhibited seven peaks as shown in Fig. 6e. The peak observed at the binding energy value of 466.8 eV corresponding to Ta 4p1/2 photoelectron peak [31], which overlapped in the Ti region, was not considered for the calculation of concentration. The other peaks with respect to the binding energies shown in Table 2, corresponded to Ti in different oxidation states namely +4 (TiO2), +3 state (Ti2O3) and in metallic state [32,35]. It is evident from the XPS analysis that the airformed native film of TaNbHfZrTi HEA comprised both oxide and metallic species, as shown in Table 2. To calculate the concentration of oxide and metallic states, only the photoelectron lines of Ta 4f, Nb 3d, Hf 4f, Zr 3d and Ti 2p was used as they correspond to high probability of ionization leads to high intensity. The area under these de-convoluted peaks corresponds to the concentrations and was corrected for the sensitivity factor of the respective photoelectron lines. It is worth mentioning that the observed overlapping peaks of photoelectron lines
Table 1 Corrosion rates of TaNbHfZrTi HEA in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF calculated from weight loss measurements at 27 °C and 120 °C. Environment
Temperature (°C)
Time (h)
Corrosion rate (mm/ y)
11.5 M HNO3
27 120 120 27 120
48 48 240 48 48
Negligible 0.004 0.002 ± 0.001 0.27 ± 0.06 2.2 ± 0.35
11.5 M HNO3 + 0.05 M NaF
Fig. 4. SEM images of the TaNbHfZrTi HEA exposed to boiling 11.5 M HNO3 for 240 h.
Fig. 5. SEM images of the TaNbHfZrTi HEA exposed to boiling 11.5 M HNO3 + 0.05 M NaF for 48 h: (a) top-surface; (b) underneath surface, obtained after removing the loosely adhered particles of the top-surface shown in (a).
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Fig. 6. XPS spectra of the de-convoluted peaks of all the alloying elements on the air-formed native film of TaNbHfZrTi HEA: (a) Ta 4f; (b) Nb 3d; (c) Hf 4f; (d) Zr 3d and (e) Ti 2p.
with low intensity (ionization of other orbital electron) is included only for obtaining a good fitting during de-convolution and not considered for estimating the concentration. The cumulative oxide concentration was about 52.3% and the balance was the metallic concentration. Further, the air formed native film was predominantly composed of ZrO2 and HfO2, when compared to the oxides of other alloying elements Ta, Nb and Ti as shown in Table 2. The spectra of Ta 4f, Nb 3d, Hf 4f, Zr 3d and Ti 2p, recorded for the passive film of the TaNbHfZrTi HEA, formed in nitric acid are presented in Fig. 7 and the respective binding energy values of their oxide states are given in Table 2. Only one set of doublet peaks corresponding to Ta2O5, Nb2O5, HfO2, ZrO2 and TiO2 were observed for the passive film, as shown in Fig. 7a–e, respectively. Nevertheless, in Fig. 7e, apart from the oxide state (TiO2) of Ti 2p peaks, the overlapped peaks observed at the binding energy values of 467.3 and 472 eV corresponded to Ta 4p1/
2 and Nb 3s, respectively [31]. Metallic states were not observed for any of the alloying elements, implying that the surface of the passive film was completely in oxide state. It could be confirmed from Table 2 that the passive film was significantly enriched with Ta2O5, followed by Nb2O5 and the concentrations of HfO2, ZrO2 and TiO2 were more or less equal. The XPS analysis revealed that the composition of the passive film formed in boiling nitric acid is entirely different to that of air-formed oxide film, as the acid medium is highly oxidizing in nature. Metal ions generally hydrolyse in aqueous solutions to form their corresponding metal hydroxides and oxides. In the present study, since the 11.5 M nitric acid solution is highly acidic, TaNbHfZrTi alloy would not form hydroxides [36]. Formation of corresponding metal oxides of the constituent elements of the alloy would be the competing reactions that occur in boiling nitric acid. The values computed for the standard
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Table 2 Concentrations of the oxide and metallic states of the alloying elements present in the native film (air-formed) and passive film (formed in boiling nitric acid for 240 h) of the TaNbHfZrTi HEA. Element
Photo electron lines
Ta
4f7/2; 4f5/2
Nb
3d5/2; 3d3/2
Hf
4f7/2; 4f5/2
Zr
3d5/2; 3d3/2
Ti
2p3/2; 2p1/2
Oxidation/Chemical state
Binding Energy (eV)
5+
Ta (Ta2O5) Ta 5+ Nb (Nb2O5) Nb Hf4+ (HfO2) Hf Zr4+ (ZrO2) Zr Ti4+ (TiO2) Ti3+ (Ti2O3) Ti
Native film
Passive film
Native Film
Passive film
27.2; 29 22.1; 24 208.2; 210.9 202.5; 205.3 17.9; 19.6 14.4; 16.1 183.4; 185.7 178.9; 181.3 459.6; 465.3 457.7; 463.5 454.1; 459.5
26.3; 28.2 – 207.3; 210.1 – 16.6; 18.2 – 182; 184.4 – 458.5; 464.6 – –
7.3 8.7 5.5 14.1 13.1 8.9 17.8 10.7 5.1 3.5 5.3
74 – 12.8 – 4.1 – 4.9 – 4.1 – –
in Table 3. In the F 1s spectrum (Fig. 9a), the peak at the binding energy value of 685.6 eV was attributed to the F bonded with Hf as HfF4 [31,38], and the value of 686.7 eV could be associated with Zr as ZrF4 and ZrOF2 [39,40]. De-convoluting both ZrF4 and ZrOF2 states at F 1s spectra was difficult, as the difference in their binding energies was small. It is expected that in the highly oxidizing fluorinated nitric acid environment, the ZrOF2 formed would be further converted to ZrF4 [40]. The Ta 4f spectrum (Fig. 9d) recorded on the underneath surface showed four sets of doublet peaks at different binding energies (Table 3) revealing four different chemical states corresponding to Ta2O5, TaO2, TaO and metallic Ta [29]. The de-convoluted Nb 3d spectrum (Fig. 9e) show a total of seven peaks, with three doublets at the binding energies (Table 3) corresponding to Nb2O5, NbO2 and NbO [30] and one overlapped peak corresponding to Hf 4d5/2 (HfO2) at the binding energy value of 214.3 eV [31]. Similarly, the de-convoluted high resolution spectrum (Fig. 9f) of Ti 2p region showed three sets of doublet peaks at the binding energies (Table 3) corresponding to TiO2, Ti2O3 and TiO [32,35] along with one overlapped peak corresponding to Ta 4p1/2 at the binding energy value of 466.9 eV [31]. The XPS analysis of the underneath surface revealed that the elements Ta, Nb and Ti were present in their corresponding stable as well as lower oxidation states, as shown in Table 3. The concentrations of the stable oxides Ta2O5, Nb2O5 and TiO2 present in the underneath surface of the fluorinated nitric acid exposed sample were lower than that present at the top surface (Table 3). In fluorinated nitric acid medium, two major competing reactions that can occur are the formation of respective metal oxides and metal fluorides of the alloying elements present in the TaNbHfZrTi alloy. The reactions for the formation of the corresponding metal oxides and their standard Gibbs energy values [37] at 120 °C are given below.
Gibbs energies of reaction [37] leading to the formation of the most stable oxides of the metals constituting the HEA at the boiling temperature of nitric acid (120 °C) are given in Eqs. (2)–(6).
2Ta + 2HNO3 ↔ Ta2O5 + H2 O + N2; ΔGr0/kJ mol−1 = − 2000 2Nb + 2HNO3 ↔ Nb2 O5 + H2 O+
N2; ΔGr0/kJ
mol−1 = − 1856
(2) (3)
3Hf + 4HNO3 ↔ 3HfO2 + 4NO + 2H2 O; ΔGr0/kJ mol−1 = − 1018 (4)
3Zr + 4HNO3 ↔ 3ZrO2 + 4NO + 2H2 O; ΔGr0/kJ mol−1 = −999
(5)
3Ti + 4HNO3 ↔ 3TiO2 + 4NO + 2H2 O; ΔGr0/kJ mol−1 = − 846
(6)
Atomic Concentration (%)
It is obvious from Eqs. (2)–(6) that the most negative value for ΔGr0 is for the formation of Ta2O5 (Eq. (2)). This implies that formation of Ta2O5 is highly favorable and is followed by the formation of Nb2O5, HfO2, ZrO2 and TiO2. This argument is consistent with the XPS results. Based on the above observation, the low corrosion rate values, insignificant surface attack and formation of passive film comprising predominantly of Ta2O5 confirmed the superior corrosion resistance of TaNbHfZrTi HEA in boiling nitric acid. 3.5. XPS characterisation of TaNbHfZrTi exposed to boiling fluorinated nitric acid To understand the corrosion behavior of TaNbHfZrTi HEA in boiling fluorinated nitric acid exposed for 48 h, XPS investigation was carried out on the top surface and underneath surface, corresponding to the SEM images reproduced in Fig. 5a and b, respectively. The powder particles formed on the top surface were collected and mounted on the conductive carbon tape. The XPS spectra obtained for the powder sample, shown in Fig. 8a–c corresponded to Ta 4f, Nb 3d and Ti 2p, respectively in their stable oxide states of Ta2O5 [29], Nb2O5 [30] and TiO2 [32,35], as indicated by the binding energy values given in Table 3. A background signal only was observed for Zr 3d and Hf 4f, as shown in Fig. 8d and e, respectively, indicating the absence of both elements at the top surface. On the underneath surface, F 1s peak (Fig. 9a) was observed along with the other peaks corresponding to Hf 4f (Fig. 9b), Zr 3d (Fig. 9c), Ta 4f (Fig. 9d), Nb 3d (Fig. 9e) and Ti 2p (Fig. 9f). The binding energy value given in Table 3 for the de-convoluted Hf 4f doublet peak (Fig. 9b) corresponded to HfF4 [31,38] and this value is different from that for HfO2 [31–33]. For the underneath surface of the sample exposed to fluorinated nitric acid, the de-convoluted high resolution spectrum of Zr 3d region (Fig. 9c) showed three sets of doublet peaks corresponding to three chemical environments of Zr. Apart from the ZrO2 doublet peak, the peaks corresponding to ZrOF2 and ZrF4 states were observed at the respective binding energy values [39,40] as given
2Ta + 5NaF + 5HNO3 ↔ Ta2O5 + 5NaNO2 + 5HF; ΔGr0/kJ mol−1 (7)
= −1656 2Nb + 5NaF + 5HNO3 ↔ Nb2 O5 + 5NaNO2 + 5HF; ΔGr0/kJ mol−1
(8)
= −1512 Hf + 2NaF + 2HNO3 ↔ HfO2 + 2NaNO2 + 2HF; ΔGr0/kJ mol−1
(9)
= −959
Zr + 2NaF + 2HNO3 ↔ ZrO2 + 2NaNO2 + 2HF; ΔGr0/kJ mol−1 = − 940 (10)
Ti + 2NaF + 2HNO3 ↔ TiO2 + 2NaNO2 + 2HF;
ΔGr0/kJ
mol−1 = −787 (11)
Similarly, the reactions corresponding to the formation of metal fluorides and their standard Gibbs energy values calculated [37] at 128
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Fig. 7. XPS spectra of the de-convoluted peaks of all the alloying elements on the passive film formed on TaNbHfZrTi HEA when exposed to boiling 11.5 M HNO3 for 240 h: (a) Ta 4f, (b) Nb 3d, (c) Hf 4f, (d) Zr 3d and (e) Ti 2p.
Ti + 6NaF + 6HNO3 ↔ TiF4 + 4NaNO3 + 2NaNO2 + 2HF + 2H2 O; ΔGr0
120 °C are given in Eqs. 12–16.
/kJ mol−1 = − 915
2Ta + 12NaF + 12HNO3 ↔ 2TaF5 + 12NaNO3 + 5H2 + 2HF; ΔGr0 /kJ
mol−1
= −569
ΔGr0
The values given in Eqs. (7)–(11) indicate that the formation of the most stable oxides is favored in the following order: Ta2O5 > Nb2O5 > HfO2 > ZrO2 > TiO2. The XPS analysis confirmed that the top surface of the TaNbHfZrTi alloy was predominantly composed of Ta2O5, Nb2O5 and TiO2, whereas the underneath surface consisted of the stable and lower oxidation states of Ta, Nb, Ti, Zr and Hf. The absence of HfO2 and ZrO2 on the top surface suggests that both Hf and Zr are undesirable alloying elements for the passivation of the HEA in fluorinated nitric acid and these elements are preferred for the formation of their respective fluorides. The standard Gibbs energy values for the reactions (12) to (16) reveal that the formation of favorable
(12)
2Nb + 12NaF + 12HNO3 ↔ 2NbF5 + 12NaNO3 + 2HF + 5H2 ; ΔGr0 /kJ mol−1 = −478
(13)
Hf + 6NaF + 6HNO3 ↔ HfF4 + 4NaNO3 + 2NaNO2 + 2HF + 2H2 O; ΔGr0/kJ mol−1 = −1183
(14)
Zr + 6NaF + 6HNO3 ↔ ZrF4 + 4NaNO3 + 2NaNO2 + 2HF + 2H2 O; ΔGr0/kJ mol−1 = −1162
(16)
(15) 129
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Fig. 8. XPS spectra of the de-convoluted peaks of the top-surface (Fig. 5a) of TaNbHfZrTi HEA when exposed to boiling 11.5 M HNO3 + 0.05 M NaF for 48 h: (a) Ta 4f, (b) Nb 3d, (c) Ti 2p, (d) Zr 3d and (e) Hf 4f.
fluorides follows the order: HfF4 > ZrF4 > TiF4 > TaF5 > NbF5. The presence of HfF4 and ZrF4 in the underneath surface was confirmed by XPS analysis. The fluoride states of other elements namely Ti, Ta and Nb were not observed and it is likely that they were not formed at all. Otherwise, it is also possible that these fluorides were formed and subsequently dissolved in the 11.5 M HNO3 + 0.05 M NaF solution at boiling condition. As the fluoride ion is highly electronegative in nature, it has a strong tendency to attack even the metal oxides formed on the surface of the TaNbHfZrTi HEA, which leads to severe corrosion. The metal oxides on the surface in turn, will get converted to their corresponding metal fluorides instantaneously as the Gibbs energy
change [37] for the reactions (17) to (21) indicate the feasibility for the formation of fluorides from the respective metal oxides. However, the concentration of the oxide species on the top surface of the TaNbHfZrTi HEA will depend on the rate of conversion of metal oxides to metal fluorides.
Ta2O5 + 10NaF + 10HNO3 ↔ 2TaF5 + 10NaNO3 + 5H2 O; ΔGr0 /kJ mol−1 = −159
(17)
Nb2 O5 + 10NaF + 10HNO3 ↔ 2NbF5 + 10NaNO3 + 5H2 O; ΔGr0 /kJ mol−1 = −140
(18)
Table 3 Concentrations of the oxide, fluoride and metallic states of the alloying elements present in the top and underneath surfaces of the TaNbHfZrTi HEA when exposed to boiling fluorinated nitric acid for 48 h. Element
Photo electron lines
Ta
4f7/2; 4f5/2
Nb
3d5/2; 3d3/2
Hf Zr
4f7/2; 4f5/2 3d5/2; 3d3/2
Ti
2p3/2; 2p1/2
Oxidation/Chemical state
5+
Ta (Ta2O5) Ta4+ (TaO2) Ta2+ (TaO) Ta Nb5+ (Nb2O5) Nb4+ (NbO2) Nb2+ (NbO) Hf4+ (HfF4) Zr4+ (ZrO2) Zr4+ (ZrOF2) Zr4+ (ZrF4) Ti4+ (TiO2) Ti3+ (Ti2O3) Ti2+ (TiO)
Binding Energy (eV)
Atomic Concentration (%)
Top Surface
Underneath Surface
Top Surface
Underneath Surface
26.3; 28.2 – – – 207.4; 210.1 – – – – – – 459; 464.8 – –
26.7; 28.6 25.1; 27 23.6; 25.6 22.4; 24.4 207.9; 210.6 205.9; 208.7 204.4; 207 18.1; 19.7 183.2; 185.6 184; 186.4 185.7; 188.2 459.3; 465 457.7; 463.3 456; 461.7
51.6 – – – 34.7 – – – – – – 13.7 – –
22.2 5 3.8 1.1 11.3 9 11.2 9 6.3 4.6 1.2 7.5 5.6 2.2
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Fig. 9. XPS spectra of the de-convoluted peaks of the underneath surface (Fig. 5b) of TaNbHfZrTi HEA when exposed to boiling 11.5 M HNO3 + 0.05 M NaF for 48 h: (a) F 1s, (b) Hf 4f, (c) Zr 3d, (d) Ta 4f, (e) Nb 3d, and (f) Ti 2p.
HfO2 + 4NaF + 4HNO3 ↔ HfF4 + 4NaNO3 + 2H2 O; ΔGr0/kJ mol−1
refractory HEA containing Zr and Hf may be avoided and alloys free from these elements shall be developed.
(19)
= −225
4. Conclusion
ZrO2 + 4NaF + 4HNO3 ↔ ZrF4 + 4NaNO3 + 2H2 O; ΔGr0/kJ mol−1 (20)
= −223 TiO2 + 4NaF + 4HNO3 ↔ TiF4 + 4NaNO3 + 2H2 O; = − 128
ΔGr0/kJ
High-entropy alloys comprising refractory elements are expected to exhibit high corrosion resistance, as they are single phase materials; however, based on the results of the present work the performance of the HEAs solely depends on the nature of oxide film and the corrosive environment. The TaNbHfZrTi HEA showed a spontaneous passivation and a pseudo-passive behavior in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF solutions respectively, as evident from the potentiodynamic polarization studies at room temperature. The corrosion rate was insignificant and negligible when exposed to boiling 11.5 M HNO3 for 240 h, and SEM investigation did not show any sign of corrosion attack
mol−1 (21)
The high corrosion rate and the corroded morphology due to the formation of ZrF4, ZrOF2 and HfF4 as well as the formation of oxides with lower oxidation states such as Ta4+ (TaO2), Ta2+ (TaO), Nb4+ (NbO2), Nb2+ (NbO), Ti3+ (Ti2O3) and Ti2+ (TiO) confirmed that the film formed on the TaNbHfZrTi HEA in fluorinated nitric acid was unprotective. Hence, for the service in fluorinated nitric acid medium, 131
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on the TaNbHfZrTi HEA. XPS analysis confirmed that the low corrosion rate in boiling 11.5 M HNO3 was attributed to the formation of a protective passive film which predominantly composed of Ta2O5 in contrast to the presence of ZrO2 and HfO2 in air-formed native film. Since the TaNbHfZrTi HEA exhibited high corrosion resistance in boiling 11.5 M HNO3, it has a great potential for service in nitric acid. In contrast to nitric acid environment, the corrosion rate of TaNbHfZrTi HEA in boiling 11.5 M HNO3 + 0.05 M NaF solution was found to be 2–3 orders of magnitude higher than that in nitric acid. SEM studies revealed severely corroded morphology of the TaNbHfZrTi HEA in boiling fluorinated nitric acid. XPS investigations confirmed the presence of ZrF4, ZrOF2 and HfF4 along with the formation of un-protective oxides of Ta, Nb and Ti, which could be correlated to the severe corrosion behavior of TaNbHfZrTi HEA in fluorinated nitric acid. Thus, for service in fluorinated nitric acid, refractory HEA containing suitable alloying elements which facilitate the formation of a protective passive layer against fluoride ions have to be developed. Acknowledgements Mr. Avinash Kumar, Corrosion Science and Technology Division, IGCAR, is acknowledged for his technical support during the preparation of the high entropy alloy. References [1] M.H. Tsai, J.W. Yeh, High-entropy alloys: a critical review, Mater. Res. Lett. 2 (2014) 107–123. [2] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [3] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375 (2004) 213–218. [4] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299–303. [5] J.Y. He, W.H. Liu, H. Wang, Y. Wu, X.J. Liu, T.G. Nieh, Z.P. Lu, Effects of Al addition on structural evolution and tensile properties of the FeCoNiCrMn high-entropy alloy system, Acta Mater. 62 (2014) 105–113. [6] Y. Dong, K. Zhou, Y. Lu, X. Gao, T. Wang, T. Li, Effect of vanadium addition on the microstructure and properties of AlCoCrFeNi high entropy alloy, Mater. Des. 57 (2014) 67–72. [7] O.N. Senkov, G.B. Wilks, D.B. Miracle, C.P. Chuang, P.K. Liaw, Refractory highentropy alloys, Intermetallics 18 (2010) 1758–1765. [8] O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys, Intermetallics 19 (2011) 698–706. [9] O.N. Senkov, J.M. Scotta, S.V. Senkova, D.B. Miracle, C.F. Woodward, Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy, J. Alloys Compd. 509 (2011) 6043–6048. [10] O.N. Senkov, S.L. Semiatin, Microstructure and properties of a refractory high-entropy alloy after cold working, J. Alloys Compd. 649 (2015) 1110–1123. [11] O.N. Senkov, C.F. Woodward, Microstructure and properties of a refractory NbCrMo0.5Ta0.5TiZr alloy, Mater. Sci. Eng. A 529 (2011) 311–320. [12] Y. Zhang, X. Yang, P.K. Liaw, Alloy design and properties optimization of highentropy alloys, JOM-J Min. Met. Mat. S 64 (2012) 830–838. [13] M.G. Poletti, S. Branz, G. Fiore, B.A. Szost, W.A. Crichton, L. Battezzati, Equilibrium high entropy phases in X-NbTaTiZr (X - Al, V, Cr and Sn) multiprincipal component alloys, J. Alloys Compd. 655 (2016) 138–146. [14] O.N. Senkov, S.V. Senkova, C. Woodward, D.B. Miracle, Low-density, refractory multi-principal element alloys of the Cr–Nb–Ti–V–Zr system: microstructure and phase analysis, Acta Mater. 61 (2013) 1545–1557.
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Corrosion behavior and surface film characterization of TaNbHfZrTi high entropy alloy in aggressive nitric acid medium
MARK
J. Jayaraja,∗, C. Thinaharana, S. Ningshena, C. Mallikaa, U. Kamachi Mudalib a b
Corrosion Science and Technology Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, 603 102, India Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, 603 102, India
A R T I C L E I N F O
A B S T R A C T
Keywords: A. High-entropy alloys B. Corrosion C. Casting D. Microstructure F. Scanning electron microscopy Spectroscopic methods
Corrosion behavior of TaNbHfZrTi high-entropy alloy (HEA) was investigated in nitric and fluorinated nitric acid at ambient (27 °C) and boiling (120 °C) conditions. The alloy passivated spontaneously during potentiodynamic polarization in 11.5 M HNO3 at ambient condition. The corrosion rate was negligible in boiling 11.5 M HNO3, exposed for 240 h. Scanning electron microscopic (SEM) studies did not show any significant corrosion attack. The high corrosion resistance of TaNbHfZrTi HEA was attributed to its single phase bcc structure. X-ray photoelectron spectroscopic (XPS) analysis revealed that the protective passive film formed in boiling nitric acid was predominantly composed of Ta2O5, in contrast to the presence of ZrO2 and HfO2 in air-formed native film. Potentiodynamic polarization studies indicated a pseudo-passivation behavior of the HEA in 11.5 M HNO3 + 0.05 M NaF at ambient condition. In boiling fluorinated nitric acid, SEM images of TaNbHfZrTi HEA displayed a severely corroded morphology indicating the instability of the metal-oxides of the alloying elements. XPS investigations confirmed the presence of ZrF4, ZrOF2 and HfF4 along with un-protective oxides of Ta, Nb and Ti on the film, resulting in decreased corrosion resistance of TaNbHfZrTi HEA in fluorinated nitric acid.
1. Introduction Recently, a novel class of advanced structural materials, called High-Entropy Alloys (HEAs) is gaining significant importance in the field of Materials Science [1,2]. Compared to conventional alloys containing one principal element, HEAs are usually composed of multiple elements with equimolar or near equimolar elemental fractions, which form predominantly single solid solution phase [1,2]. The four core effects of HEAs are high entropy effect, distorted lattice, sluggish diffusion, and cocktail effect [1,2]. Owing to these effects, HEAs exhibit solid solution strengthening with better mechanical, wear, corrosion resistance properties and they also possess excellent microstructural stability at high temperatures [1,2]. Initially, the development of HEAs was focused mainly on alloying elements containing Fe, Ni, Co, Cr, Mn, Cu, Ti, V and Al [3–6]. Senkov and co-workers [7,8] developed refractory WTaNbMo and WTaNbMoV HEAs that exhibited high yield strength even at an elevated temperature of 1600 °C. Subsequently, Senkov et al. [9] decreased the density of the HEA by replacing W and Mo with Ti, Hf and Zr and developed TaNbHfZrTi HEA with improved specific strength. This TaNbHfZrTi HEA exhibited homogeneous plastic flow and up to 40% compression strain with marked strain hardening at
∗
room temperature. The excellent ductility of TaNbHfZrTi alloy in compression allowed the material to be produced in thin sheet by cold rolling process [10]. The as-cast microstructure of TaNbHfZrTi HEA consisted of dendritic colonies whereas the cold rolled and annealed sample consisted of an equiaxed structure [9,10]. Both the as-cast and cold-rolled TaNbHfZrTi HEA exhibited a single-phase with bcc structure [9,10]. Consequently, researchers concentrated on developing several refractory elements based high-entropy alloys such as NbCrMo0.5Ta0.5TiZr [11], TiZrNbMoV [12], Al(V)NbTaTiZr [13], CrNbTiVZr [14], AlHfNbTaTiZr [15], HfMoNbTaTiZr [16] and MoWAlCrTi [17]. Most of the studies on these HEAs were focused on the phase formation, microstructure and mechanical properties [11–16] and oxidation behavior [17,18]. The advantage of the mechanical properties of the HEAs could not be fully exploited since their corrosion resistance was not understood properly. Exhaustive reviews on the corrosion resistant HEAs based on non-refractory elements are available in literature, where the effect of alloying elements, processing methods, heat-treatment and corrosion behavior, mainly in H2SO4 and NaCl environments were discussed and compared with the conventional engineering alloys [19–21]. Only limited studies have been reported for the corrosion behavior of refractory HEAs such as TiZr0.5NbCr0.5,
Corresponding author. E-mail addresses: [email protected], [email protected] (J. Jayaraj), [email protected] (C. Thinaharan), [email protected] (S. Ningshen), [email protected] (C. Mallika), [email protected] (U. Kamachi Mudali). http://dx.doi.org/10.1016/j.intermet.2017.06.002 Received 24 March 2017; Received in revised form 23 May 2017; Accepted 6 June 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.
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a reproducible surface. Platinum and Ag/AgCl (3 M KCl) were used as counter and reference electrode, respectively. The open circuit potential (OCP) of the sample was monitored for 1 h. After measuring the OCP, potentiodynamic polarization tests were carried out at the scan rate of 0.00017 V/s from 0.2 V below OCP to 2.0 V. Both the OCP and potentiodynamic polarization experiments were performed at least twice, for checking the reproducibility of the results.
TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Mo in H2SO4 and NaCl solutions [22]. Materials exhibiting excellent corrosion resistance, high strength and formability are desirable for the fabrication of dissolver components of reprocessing plant for the processing of spent nuclear fuels by aqueous route. The search for advanced alloys with properties better than the existing conventional materials is a motivating factor among the researchers. The corrosion behavior of the HEAs in boiling 11.5 M nitric acid, which is the dissolution medium for the spent nuclear fuels from fast breeder reactors [23] is not reported in the literature. In addition, the effect of fluoride ions in boiling nitric acid on the corrosion behavior of the dissolver vessels handling Pu-rich mixed oxide fuels [24] need proper investigation. Owing to the single phase structure as well as good mechanical and formability properties of TaNbHfZrTi HEA [9,10], it can be considered as a candidate material for the fabrication of critical components in reprocessing plants. Apart from the application point of view, the role of alloying elements in the TaNbHfZrTi multi-component HEA contributing to its passivation behavior is less understood. In the present study, the performance of TaNbHfZrTi HEA was investigated in boiling nitric acid with and without fluoride ions. Potentiodynamic polarization test was used to characterize the passivation behavior of TaNbHfZrTi alloy in 11.5 M HNO3 as well as 11.5 M HNO3 + 0.05 M NaF at room temperature. The corrosion rate of TaNbHfZrTi was determined by weight loss method in 11.5 M HNO3 as well as fluorinated nitric acid media under boiling conditions. Scanning electron microscopic (SEM) and X-ray photoelectron spectroscopic (XPS) analyses were performed to corroborate the results of the studies pertaining to passivation and corrosion behavior of this alloy.
2.4. Measurement of corrosion rate The experimental set-up used for corrosion test in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at boiling conditions (120 °C) is described elsewhere [25]. A cold finger condenser was used to reflux the acid vapors into the test solution. In the same set-up, the corrosion experiments at room temperature were also conducted. The TaNbHfZrTi HEA sample surface was wet ground in SiC emery paper up to 1200 grit. The polished samples were degreased with acetone, dried and weighed before immersing them into the test solutions. The samples were suspended into the solution through a Teflon thread for a minimum exposure period of 48 h. Based on the extent of corrosion, the experiments were performed for five periods of 48 h (i.e. total exposure period of 240 h) with fresh solutions being used in each period as described in ASTM A262 Practice C method [26]. The immersion tests were carried out at least twice for all the conditions studied. After corrosion testing, the samples were removed, cleaned carefully and the change in weight was measured in a micro balance with resolution of 0.0001 g. From the weight loss experiments, the corrosion rates were calculated using Eq. (1).
Corrosion rate = 2. Experimental procedure
8.76 × 10 4 × W dAt
(1)
where, corrosion rate is expressed in mm/yr, W is weight loss in g, d is the theoretical density of the alloy (11.82 g/cm3), A is the exposed surface area in cm2 and t is the exposed time in h.
2.1. Alloy preparation The TaNbHfZrTi equimolar alloy was prepared using a vacuum arc melting system (M/s. Edmund Buhler GmbH, Germany). A total weight of 5 g of the high purity (99.99%) Ta, Nb, Hf, Zr, and Ti elements were melted together on the water-cooled copper hearth of the arc melting system. Prior to melting, the arc melting chamber was evacuated to a vacuum level 0.035 Pa and back-filled with high purity argon. To promote homogeneity, the ingot was melted at least five times and was flipped between each melting.
2.5. Surface characterisation The surface morphology of the TaNbHfZrTi HEA samples subjected to potentiodynamic polarization and weight loss studies was investigated using SEM, in secondary electron mode at an accelerating voltage of 30 kV. To analyze the surface chemistry of TaNbHfZrTi HEA before and after exposure to 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at boiling conditions, XPS was employed. For recording the spectra, SPECS Surface Nano Analysis GmbH, Germany, XPS was used with Al-Kα excitation. The data were processed by SpecsLab2 software and analyzed by CasaXPS software.
2.2. Phase analysis and microstructural characterization The arc melted TaNbHfZrTi ingot was characterized for phase analysis by X-ray Diffraction technique (XRD; INEL, France) using monochromatic Cu Kα radiation. For metallographic observation, the TaNbHfZrTi sample was wet ground in SiC emery paper up to 1200 grit and followed by final polishing in colloidal silica suspension to obtain a smooth mirror-like surface finish. To reveal the microstructure, the polished sample was chemical etched with the Kroll-reagent (2 ml HF, 3 ml HNO3 and 100 ml distilled H2O) for 10 s. The etched sample's surface was analyzed using scanning electron microscopy (SEM; SNE3000M, Korea).
3. Results and discussion 3.1. Phase analysis and microstructure The XRD pattern of the as-cast, TaNbHfZrTi sample is shown in Fig. 1a and the diffraction peaks matched with body centered cubic (bcc) phase with the corresponding planes of (110), (200) and (211). Since no additional peaks were present, it could be concluded that the TaNbHfZrTi alloy ingot was of single bcc phase and the calculated lattice parameter was about 3.404 Å, which is in good agreement with that of the reported value [9]. The SEM micrograph (Fig. 1b) revealed a typical dendrite microstructure for the as-cast TaNbHfZrTi HEA [9].
2.3. Potentiodynamic polarization studies To understand the passivation behavior of TaNbHfZrTi HEA in both 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at room temperature, potentiodynamic polarization experiments were carried out using the electrochemical system (Autolab, The Netherlands). For polarization measurements, a standard three electrode cell with the test sample as working electrode with an exposed area of 1 cm2 was employed. The working electrode's surface was prepared by wet ground using SiC paper up to 1200 grit and cleaned in distilled water and acetone to have
3.2. Potentiodynamic polarization behavior of TaNbHfZrTi HEA Fig. 2 shows the potentiodynamic polarization behavior of TaNbHfZrTi in 11.5 M HNO3, and 11.5 M HNO3 + 0.05 M NaF. From Fig. 2, the corrosion potential, Ecorr and corrosion current density Icorr were obtained using Tafel analysis [27]. The Ecorr values were found to 124
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Fig. 1. (a) XRD pattern and (b) SEM microstructure of the TaNbHfZrTi HEA.
current behavior of the alloy exhibited in fluorinated nitric acid should not be considered as passive behavior; rather, it is a “pseudo-passive” behavior of the TaNbHfZrTi alloy in 11.5 M HNO3 + 0.05 M NaF medium at room temperature. 3.3. Corrosion rates of TaNbHfZrTi in boiling nitric acid and fluorinated nitric acid The corrosion rates of TaNbHfZrTi HEA in 11.5 M HNO3 at room temperature (27 °C) and under boiling condition (120 °C) for 48 and 240 h are given in Table 1. The corrosion rates were insignificant at both the temperatures investigated, indicating that the weight loss before and after the exposure period was negligible. SEM analysis on the TaNbHfZrTi HEA revealed an un-attacked surface morphology when exposed to boiling nitric acid (Fig. 4) for 240 h. The corrosion rates of TaNbHfZrTi HEA in 11.5 M HNO3 + 0.05 M NaF medium at room temperature (27 °C) and boiling condition (120 °C) for 48 h are also given in Table 1. In comparison to nitric acid, the corrosion rates increased in fluorinated nitric acid at both the temperatures and the values were quite significant at the boiling condition. As shown in Fig. 5a, the fluoride ions attacked the HEA severely, resembling a “dry land of drought conditions”. The surface of the corrosion tested sample in boiling fluorinated nitric acid appeared to be loosely packed like powder. The loosely bound powder (Fig. 5a) on the top surface could be easily removed using an adhesive tape and the underneath surface was observed to be a predominantly smooth surface, as shown in Fig. 5b. This implies that the film formed on the surface of TaNbHfZrTi sample during exposure to boiling fluorinated nitric acid was unstable and un-protective, which could have resulted in the high corrosion rates.
Fig. 2. Potentiodynamic polarization curves of TaNbHfZrTi HEA in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF at room temperature.
be 0.682 ± 0.07 and 0.115 ± 0.024 V respectively in nitric and fluorinated nitric acid. It is worth mentioning that the Ecorr values followed a similar trend to that of OCP. In concentrated nitric acid, oxidizing agents such as nitrous acid and nitrogen peroxide would be produced due to the autocatalytic reduction [28]. The highly oxidizing nature of 11.5 M HNO3 medium is attributed to the ennoblement of Ecorr value. The Ecorr value of the TaNbHfZrTi HEA was lowered in fluorinated nitric acid when compared to nitric acid solution. The Icorr values measured were 0.2 ± 0.05 and 2.1 ± 0.8 μA/cm2, respectively in nitric and fluorinated nitric acid. The low Icorr value is attributed to the high corrosion resistance of the TaNbHfZrTi HEA in nitric acid at room temperature. The high electronegativity and aggressive nature of fluoride ions could have shifted the Ecorr value to active region, resulting in the higher Icorr value. Above the Ecorr, the current density values remained almost constant until 2 V for both environments studied. In general, the constant current region is defined as the passive region. However, for the TaNbHfZrTi HEA in 11.5 M HNO3 + 0.05 M NaF medium, the current density values were at least an order of magnitude higher than that in nitric acid. The samples after polarization studies in nitric acid medium exhibited an un-attacked surface, as revealed in the SEM image (Fig. 3a), indicating a high corrosion resistance of the alloy in nitric acid. After the polarization studies in fluorinated nitric acid, a slightly attacked morphology (Fig. 3b) was observed. At higher magnification, the attacked surface displayed flaky morphology of the film as shown in Fig. 3c, confirming the un-stable nature of the film formed in fluorinated nitric acid. Thus, the constant
3.4. XPS analysis of TaNbHfZrTi exposed to boiling nitric acid XPS analysis was performed to characterize the composition of the native and passive films formed on the TaNbHfZrTi HEA. The surface film formed in air (before immersion in nitric acid) is referred to as ‘native film’ and the one formed during immersion in boiling 11.5 M HNO3 for 240 h is referred to as ‘passive film’. The spectra of the native and passive films showed peaks corresponding to carbon (C 1s), oxygen (O 1s), tantalum (Ta 4f), niobium (Nb 3d), hafnium (Hf 4f), zirconium (Zr 3d) and titanium (Ti 2p). For both the films, the C 1s peak corresponded to unavoidable contamination of carbon formed during sample handling and the O 1s peaks, originated from the oxygen bonded to metals. The spectra of all the alloying elements in the native film of the TaNbHfZrTi HEA are shown in Fig. 6 and the corresponding binding energy values of their chemical states (oxide or metallic) are given in Table 2. For the native film, the de-convoluted Ta 4f spectrum (Fig. 6a) revealed two sets of doublet peaks at the respective binding energy 125
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Fig. 3. SEM images of TaNbHfZrTi HEA after potentiodynamic polarization studies at room temperature: (a) 11.5 M HNO3; (b–c) 11.5 M HNO3 + 0.05 M NaF; (c) magnified image of the attacked region marked in (b).
values (Table 2), corresponding to Ta existing in both +5 oxidation state (Ta2O5) as well as in metallic form [29]. In the de-convoluted Nb 3d core level region, two doublet peaks and two singlet peaks were fitted as shown in Fig. 6b. The doublet peaks corresponded to those of Nb in metallic state and in +5 oxidation state (Nb2O5) with respect to their binding energies [30]. The two singlet peaks observed (Fig. 6b) were for Hf metal and HfO2 at the respective binding energies of 211.2 and 214.2 eV, corresponding to the photoelectron peaks of Hf 4d5/2 [31] that overlapped in the Nb 3d region. On the other hand, in Fig. 6c, the de-convoluted Hf 4f spectrum showed two doublet peaks corresponding to metallic Hf and HfO2 [31–33]. Similarly, the de-convoluted high resolution spectrum in the Zr 3d region (Fig. 6d) revealed two sets of doublet peaks corresponding to the +4 oxidation state (ZrO2) and metallic state of Zr [34]. Interestingly, the de-convoluted high resolution spectrum in the Ti 2p region exhibited seven peaks as shown in Fig. 6e. The peak observed at the binding energy value of 466.8 eV corresponding to Ta 4p1/2 photoelectron peak [31], which overlapped in the Ti region, was not considered for the calculation of concentration. The other peaks with respect to the binding energies shown in Table 2, corresponded to Ti in different oxidation states namely +4 (TiO2), +3 state (Ti2O3) and in metallic state [32,35]. It is evident from the XPS analysis that the airformed native film of TaNbHfZrTi HEA comprised both oxide and metallic species, as shown in Table 2. To calculate the concentration of oxide and metallic states, only the photoelectron lines of Ta 4f, Nb 3d, Hf 4f, Zr 3d and Ti 2p was used as they correspond to high probability of ionization leads to high intensity. The area under these de-convoluted peaks corresponds to the concentrations and was corrected for the sensitivity factor of the respective photoelectron lines. It is worth mentioning that the observed overlapping peaks of photoelectron lines
Table 1 Corrosion rates of TaNbHfZrTi HEA in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF calculated from weight loss measurements at 27 °C and 120 °C. Environment
Temperature (°C)
Time (h)
Corrosion rate (mm/ y)
11.5 M HNO3
27 120 120 27 120
48 48 240 48 48
Negligible 0.004 0.002 ± 0.001 0.27 ± 0.06 2.2 ± 0.35
11.5 M HNO3 + 0.05 M NaF
Fig. 4. SEM images of the TaNbHfZrTi HEA exposed to boiling 11.5 M HNO3 for 240 h.
Fig. 5. SEM images of the TaNbHfZrTi HEA exposed to boiling 11.5 M HNO3 + 0.05 M NaF for 48 h: (a) top-surface; (b) underneath surface, obtained after removing the loosely adhered particles of the top-surface shown in (a).
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Fig. 6. XPS spectra of the de-convoluted peaks of all the alloying elements on the air-formed native film of TaNbHfZrTi HEA: (a) Ta 4f; (b) Nb 3d; (c) Hf 4f; (d) Zr 3d and (e) Ti 2p.
with low intensity (ionization of other orbital electron) is included only for obtaining a good fitting during de-convolution and not considered for estimating the concentration. The cumulative oxide concentration was about 52.3% and the balance was the metallic concentration. Further, the air formed native film was predominantly composed of ZrO2 and HfO2, when compared to the oxides of other alloying elements Ta, Nb and Ti as shown in Table 2. The spectra of Ta 4f, Nb 3d, Hf 4f, Zr 3d and Ti 2p, recorded for the passive film of the TaNbHfZrTi HEA, formed in nitric acid are presented in Fig. 7 and the respective binding energy values of their oxide states are given in Table 2. Only one set of doublet peaks corresponding to Ta2O5, Nb2O5, HfO2, ZrO2 and TiO2 were observed for the passive film, as shown in Fig. 7a–e, respectively. Nevertheless, in Fig. 7e, apart from the oxide state (TiO2) of Ti 2p peaks, the overlapped peaks observed at the binding energy values of 467.3 and 472 eV corresponded to Ta 4p1/
2 and Nb 3s, respectively [31]. Metallic states were not observed for any of the alloying elements, implying that the surface of the passive film was completely in oxide state. It could be confirmed from Table 2 that the passive film was significantly enriched with Ta2O5, followed by Nb2O5 and the concentrations of HfO2, ZrO2 and TiO2 were more or less equal. The XPS analysis revealed that the composition of the passive film formed in boiling nitric acid is entirely different to that of air-formed oxide film, as the acid medium is highly oxidizing in nature. Metal ions generally hydrolyse in aqueous solutions to form their corresponding metal hydroxides and oxides. In the present study, since the 11.5 M nitric acid solution is highly acidic, TaNbHfZrTi alloy would not form hydroxides [36]. Formation of corresponding metal oxides of the constituent elements of the alloy would be the competing reactions that occur in boiling nitric acid. The values computed for the standard
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Table 2 Concentrations of the oxide and metallic states of the alloying elements present in the native film (air-formed) and passive film (formed in boiling nitric acid for 240 h) of the TaNbHfZrTi HEA. Element
Photo electron lines
Ta
4f7/2; 4f5/2
Nb
3d5/2; 3d3/2
Hf
4f7/2; 4f5/2
Zr
3d5/2; 3d3/2
Ti
2p3/2; 2p1/2
Oxidation/Chemical state
Binding Energy (eV)
5+
Ta (Ta2O5) Ta 5+ Nb (Nb2O5) Nb Hf4+ (HfO2) Hf Zr4+ (ZrO2) Zr Ti4+ (TiO2) Ti3+ (Ti2O3) Ti
Native film
Passive film
Native Film
Passive film
27.2; 29 22.1; 24 208.2; 210.9 202.5; 205.3 17.9; 19.6 14.4; 16.1 183.4; 185.7 178.9; 181.3 459.6; 465.3 457.7; 463.5 454.1; 459.5
26.3; 28.2 – 207.3; 210.1 – 16.6; 18.2 – 182; 184.4 – 458.5; 464.6 – –
7.3 8.7 5.5 14.1 13.1 8.9 17.8 10.7 5.1 3.5 5.3
74 – 12.8 – 4.1 – 4.9 – 4.1 – –
in Table 3. In the F 1s spectrum (Fig. 9a), the peak at the binding energy value of 685.6 eV was attributed to the F bonded with Hf as HfF4 [31,38], and the value of 686.7 eV could be associated with Zr as ZrF4 and ZrOF2 [39,40]. De-convoluting both ZrF4 and ZrOF2 states at F 1s spectra was difficult, as the difference in their binding energies was small. It is expected that in the highly oxidizing fluorinated nitric acid environment, the ZrOF2 formed would be further converted to ZrF4 [40]. The Ta 4f spectrum (Fig. 9d) recorded on the underneath surface showed four sets of doublet peaks at different binding energies (Table 3) revealing four different chemical states corresponding to Ta2O5, TaO2, TaO and metallic Ta [29]. The de-convoluted Nb 3d spectrum (Fig. 9e) show a total of seven peaks, with three doublets at the binding energies (Table 3) corresponding to Nb2O5, NbO2 and NbO [30] and one overlapped peak corresponding to Hf 4d5/2 (HfO2) at the binding energy value of 214.3 eV [31]. Similarly, the de-convoluted high resolution spectrum (Fig. 9f) of Ti 2p region showed three sets of doublet peaks at the binding energies (Table 3) corresponding to TiO2, Ti2O3 and TiO [32,35] along with one overlapped peak corresponding to Ta 4p1/2 at the binding energy value of 466.9 eV [31]. The XPS analysis of the underneath surface revealed that the elements Ta, Nb and Ti were present in their corresponding stable as well as lower oxidation states, as shown in Table 3. The concentrations of the stable oxides Ta2O5, Nb2O5 and TiO2 present in the underneath surface of the fluorinated nitric acid exposed sample were lower than that present at the top surface (Table 3). In fluorinated nitric acid medium, two major competing reactions that can occur are the formation of respective metal oxides and metal fluorides of the alloying elements present in the TaNbHfZrTi alloy. The reactions for the formation of the corresponding metal oxides and their standard Gibbs energy values [37] at 120 °C are given below.
Gibbs energies of reaction [37] leading to the formation of the most stable oxides of the metals constituting the HEA at the boiling temperature of nitric acid (120 °C) are given in Eqs. (2)–(6).
2Ta + 2HNO3 ↔ Ta2O5 + H2 O + N2; ΔGr0/kJ mol−1 = − 2000 2Nb + 2HNO3 ↔ Nb2 O5 + H2 O+
N2; ΔGr0/kJ
mol−1 = − 1856
(2) (3)
3Hf + 4HNO3 ↔ 3HfO2 + 4NO + 2H2 O; ΔGr0/kJ mol−1 = − 1018 (4)
3Zr + 4HNO3 ↔ 3ZrO2 + 4NO + 2H2 O; ΔGr0/kJ mol−1 = −999
(5)
3Ti + 4HNO3 ↔ 3TiO2 + 4NO + 2H2 O; ΔGr0/kJ mol−1 = − 846
(6)
Atomic Concentration (%)
It is obvious from Eqs. (2)–(6) that the most negative value for ΔGr0 is for the formation of Ta2O5 (Eq. (2)). This implies that formation of Ta2O5 is highly favorable and is followed by the formation of Nb2O5, HfO2, ZrO2 and TiO2. This argument is consistent with the XPS results. Based on the above observation, the low corrosion rate values, insignificant surface attack and formation of passive film comprising predominantly of Ta2O5 confirmed the superior corrosion resistance of TaNbHfZrTi HEA in boiling nitric acid. 3.5. XPS characterisation of TaNbHfZrTi exposed to boiling fluorinated nitric acid To understand the corrosion behavior of TaNbHfZrTi HEA in boiling fluorinated nitric acid exposed for 48 h, XPS investigation was carried out on the top surface and underneath surface, corresponding to the SEM images reproduced in Fig. 5a and b, respectively. The powder particles formed on the top surface were collected and mounted on the conductive carbon tape. The XPS spectra obtained for the powder sample, shown in Fig. 8a–c corresponded to Ta 4f, Nb 3d and Ti 2p, respectively in their stable oxide states of Ta2O5 [29], Nb2O5 [30] and TiO2 [32,35], as indicated by the binding energy values given in Table 3. A background signal only was observed for Zr 3d and Hf 4f, as shown in Fig. 8d and e, respectively, indicating the absence of both elements at the top surface. On the underneath surface, F 1s peak (Fig. 9a) was observed along with the other peaks corresponding to Hf 4f (Fig. 9b), Zr 3d (Fig. 9c), Ta 4f (Fig. 9d), Nb 3d (Fig. 9e) and Ti 2p (Fig. 9f). The binding energy value given in Table 3 for the de-convoluted Hf 4f doublet peak (Fig. 9b) corresponded to HfF4 [31,38] and this value is different from that for HfO2 [31–33]. For the underneath surface of the sample exposed to fluorinated nitric acid, the de-convoluted high resolution spectrum of Zr 3d region (Fig. 9c) showed three sets of doublet peaks corresponding to three chemical environments of Zr. Apart from the ZrO2 doublet peak, the peaks corresponding to ZrOF2 and ZrF4 states were observed at the respective binding energy values [39,40] as given
2Ta + 5NaF + 5HNO3 ↔ Ta2O5 + 5NaNO2 + 5HF; ΔGr0/kJ mol−1 (7)
= −1656 2Nb + 5NaF + 5HNO3 ↔ Nb2 O5 + 5NaNO2 + 5HF; ΔGr0/kJ mol−1
(8)
= −1512 Hf + 2NaF + 2HNO3 ↔ HfO2 + 2NaNO2 + 2HF; ΔGr0/kJ mol−1
(9)
= −959
Zr + 2NaF + 2HNO3 ↔ ZrO2 + 2NaNO2 + 2HF; ΔGr0/kJ mol−1 = − 940 (10)
Ti + 2NaF + 2HNO3 ↔ TiO2 + 2NaNO2 + 2HF;
ΔGr0/kJ
mol−1 = −787 (11)
Similarly, the reactions corresponding to the formation of metal fluorides and their standard Gibbs energy values calculated [37] at 128
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Fig. 7. XPS spectra of the de-convoluted peaks of all the alloying elements on the passive film formed on TaNbHfZrTi HEA when exposed to boiling 11.5 M HNO3 for 240 h: (a) Ta 4f, (b) Nb 3d, (c) Hf 4f, (d) Zr 3d and (e) Ti 2p.
Ti + 6NaF + 6HNO3 ↔ TiF4 + 4NaNO3 + 2NaNO2 + 2HF + 2H2 O; ΔGr0
120 °C are given in Eqs. 12–16.
/kJ mol−1 = − 915
2Ta + 12NaF + 12HNO3 ↔ 2TaF5 + 12NaNO3 + 5H2 + 2HF; ΔGr0 /kJ
mol−1
= −569
ΔGr0
The values given in Eqs. (7)–(11) indicate that the formation of the most stable oxides is favored in the following order: Ta2O5 > Nb2O5 > HfO2 > ZrO2 > TiO2. The XPS analysis confirmed that the top surface of the TaNbHfZrTi alloy was predominantly composed of Ta2O5, Nb2O5 and TiO2, whereas the underneath surface consisted of the stable and lower oxidation states of Ta, Nb, Ti, Zr and Hf. The absence of HfO2 and ZrO2 on the top surface suggests that both Hf and Zr are undesirable alloying elements for the passivation of the HEA in fluorinated nitric acid and these elements are preferred for the formation of their respective fluorides. The standard Gibbs energy values for the reactions (12) to (16) reveal that the formation of favorable
(12)
2Nb + 12NaF + 12HNO3 ↔ 2NbF5 + 12NaNO3 + 2HF + 5H2 ; ΔGr0 /kJ mol−1 = −478
(13)
Hf + 6NaF + 6HNO3 ↔ HfF4 + 4NaNO3 + 2NaNO2 + 2HF + 2H2 O; ΔGr0/kJ mol−1 = −1183
(14)
Zr + 6NaF + 6HNO3 ↔ ZrF4 + 4NaNO3 + 2NaNO2 + 2HF + 2H2 O; ΔGr0/kJ mol−1 = −1162
(16)
(15) 129
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Fig. 8. XPS spectra of the de-convoluted peaks of the top-surface (Fig. 5a) of TaNbHfZrTi HEA when exposed to boiling 11.5 M HNO3 + 0.05 M NaF for 48 h: (a) Ta 4f, (b) Nb 3d, (c) Ti 2p, (d) Zr 3d and (e) Hf 4f.
fluorides follows the order: HfF4 > ZrF4 > TiF4 > TaF5 > NbF5. The presence of HfF4 and ZrF4 in the underneath surface was confirmed by XPS analysis. The fluoride states of other elements namely Ti, Ta and Nb were not observed and it is likely that they were not formed at all. Otherwise, it is also possible that these fluorides were formed and subsequently dissolved in the 11.5 M HNO3 + 0.05 M NaF solution at boiling condition. As the fluoride ion is highly electronegative in nature, it has a strong tendency to attack even the metal oxides formed on the surface of the TaNbHfZrTi HEA, which leads to severe corrosion. The metal oxides on the surface in turn, will get converted to their corresponding metal fluorides instantaneously as the Gibbs energy
change [37] for the reactions (17) to (21) indicate the feasibility for the formation of fluorides from the respective metal oxides. However, the concentration of the oxide species on the top surface of the TaNbHfZrTi HEA will depend on the rate of conversion of metal oxides to metal fluorides.
Ta2O5 + 10NaF + 10HNO3 ↔ 2TaF5 + 10NaNO3 + 5H2 O; ΔGr0 /kJ mol−1 = −159
(17)
Nb2 O5 + 10NaF + 10HNO3 ↔ 2NbF5 + 10NaNO3 + 5H2 O; ΔGr0 /kJ mol−1 = −140
(18)
Table 3 Concentrations of the oxide, fluoride and metallic states of the alloying elements present in the top and underneath surfaces of the TaNbHfZrTi HEA when exposed to boiling fluorinated nitric acid for 48 h. Element
Photo electron lines
Ta
4f7/2; 4f5/2
Nb
3d5/2; 3d3/2
Hf Zr
4f7/2; 4f5/2 3d5/2; 3d3/2
Ti
2p3/2; 2p1/2
Oxidation/Chemical state
5+
Ta (Ta2O5) Ta4+ (TaO2) Ta2+ (TaO) Ta Nb5+ (Nb2O5) Nb4+ (NbO2) Nb2+ (NbO) Hf4+ (HfF4) Zr4+ (ZrO2) Zr4+ (ZrOF2) Zr4+ (ZrF4) Ti4+ (TiO2) Ti3+ (Ti2O3) Ti2+ (TiO)
Binding Energy (eV)
Atomic Concentration (%)
Top Surface
Underneath Surface
Top Surface
Underneath Surface
26.3; 28.2 – – – 207.4; 210.1 – – – – – – 459; 464.8 – –
26.7; 28.6 25.1; 27 23.6; 25.6 22.4; 24.4 207.9; 210.6 205.9; 208.7 204.4; 207 18.1; 19.7 183.2; 185.6 184; 186.4 185.7; 188.2 459.3; 465 457.7; 463.3 456; 461.7
51.6 – – – 34.7 – – – – – – 13.7 – –
22.2 5 3.8 1.1 11.3 9 11.2 9 6.3 4.6 1.2 7.5 5.6 2.2
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Fig. 9. XPS spectra of the de-convoluted peaks of the underneath surface (Fig. 5b) of TaNbHfZrTi HEA when exposed to boiling 11.5 M HNO3 + 0.05 M NaF for 48 h: (a) F 1s, (b) Hf 4f, (c) Zr 3d, (d) Ta 4f, (e) Nb 3d, and (f) Ti 2p.
HfO2 + 4NaF + 4HNO3 ↔ HfF4 + 4NaNO3 + 2H2 O; ΔGr0/kJ mol−1
refractory HEA containing Zr and Hf may be avoided and alloys free from these elements shall be developed.
(19)
= −225
4. Conclusion
ZrO2 + 4NaF + 4HNO3 ↔ ZrF4 + 4NaNO3 + 2H2 O; ΔGr0/kJ mol−1 (20)
= −223 TiO2 + 4NaF + 4HNO3 ↔ TiF4 + 4NaNO3 + 2H2 O; = − 128
ΔGr0/kJ
High-entropy alloys comprising refractory elements are expected to exhibit high corrosion resistance, as they are single phase materials; however, based on the results of the present work the performance of the HEAs solely depends on the nature of oxide film and the corrosive environment. The TaNbHfZrTi HEA showed a spontaneous passivation and a pseudo-passive behavior in 11.5 M HNO3 and 11.5 M HNO3 + 0.05 M NaF solutions respectively, as evident from the potentiodynamic polarization studies at room temperature. The corrosion rate was insignificant and negligible when exposed to boiling 11.5 M HNO3 for 240 h, and SEM investigation did not show any sign of corrosion attack
mol−1 (21)
The high corrosion rate and the corroded morphology due to the formation of ZrF4, ZrOF2 and HfF4 as well as the formation of oxides with lower oxidation states such as Ta4+ (TaO2), Ta2+ (TaO), Nb4+ (NbO2), Nb2+ (NbO), Ti3+ (Ti2O3) and Ti2+ (TiO) confirmed that the film formed on the TaNbHfZrTi HEA in fluorinated nitric acid was unprotective. Hence, for the service in fluorinated nitric acid medium, 131
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on the TaNbHfZrTi HEA. XPS analysis confirmed that the low corrosion rate in boiling 11.5 M HNO3 was attributed to the formation of a protective passive film which predominantly composed of Ta2O5 in contrast to the presence of ZrO2 and HfO2 in air-formed native film. Since the TaNbHfZrTi HEA exhibited high corrosion resistance in boiling 11.5 M HNO3, it has a great potential for service in nitric acid. In contrast to nitric acid environment, the corrosion rate of TaNbHfZrTi HEA in boiling 11.5 M HNO3 + 0.05 M NaF solution was found to be 2–3 orders of magnitude higher than that in nitric acid. SEM studies revealed severely corroded morphology of the TaNbHfZrTi HEA in boiling fluorinated nitric acid. XPS investigations confirmed the presence of ZrF4, ZrOF2 and HfF4 along with the formation of un-protective oxides of Ta, Nb and Ti, which could be correlated to the severe corrosion behavior of TaNbHfZrTi HEA in fluorinated nitric acid. Thus, for service in fluorinated nitric acid, refractory HEA containing suitable alloying elements which facilitate the formation of a protective passive layer against fluoride ions have to be developed. Acknowledgements Mr. Avinash Kumar, Corrosion Science and Technology Division, IGCAR, is acknowledged for his technical support during the preparation of the high entropy alloy. References [1] M.H. Tsai, J.W. Yeh, High-entropy alloys: a critical review, Mater. Res. Lett. 2 (2014) 107–123. [2] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [3] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375 (2004) 213–218. [4] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299–303. [5] J.Y. He, W.H. Liu, H. Wang, Y. Wu, X.J. Liu, T.G. Nieh, Z.P. Lu, Effects of Al addition on structural evolution and tensile properties of the FeCoNiCrMn high-entropy alloy system, Acta Mater. 62 (2014) 105–113. [6] Y. Dong, K. Zhou, Y. Lu, X. Gao, T. Wang, T. Li, Effect of vanadium addition on the microstructure and properties of AlCoCrFeNi high entropy alloy, Mater. Des. 57 (2014) 67–72. [7] O.N. Senkov, G.B. Wilks, D.B. Miracle, C.P. Chuang, P.K. Liaw, Refractory highentropy alloys, Intermetallics 18 (2010) 1758–1765. [8] O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys, Intermetallics 19 (2011) 698–706. [9] O.N. Senkov, J.M. Scotta, S.V. Senkova, D.B. Miracle, C.F. Woodward, Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy, J. Alloys Compd. 509 (2011) 6043–6048. [10] O.N. Senkov, S.L. Semiatin, Microstructure and properties of a refractory high-entropy alloy after cold working, J. Alloys Compd. 649 (2015) 1110–1123. [11] O.N. Senkov, C.F. Woodward, Microstructure and properties of a refractory NbCrMo0.5Ta0.5TiZr alloy, Mater. Sci. Eng. A 529 (2011) 311–320. [12] Y. Zhang, X. Yang, P.K. Liaw, Alloy design and properties optimization of highentropy alloys, JOM-J Min. Met. Mat. S 64 (2012) 830–838. [13] M.G. Poletti, S. Branz, G. Fiore, B.A. Szost, W.A. Crichton, L. Battezzati, Equilibrium high entropy phases in X-NbTaTiZr (X - Al, V, Cr and Sn) multiprincipal component alloys, J. Alloys Compd. 655 (2016) 138–146. [14] O.N. Senkov, S.V. Senkova, C. Woodward, D.B. Miracle, Low-density, refractory multi-principal element alloys of the Cr–Nb–Ti–V–Zr system: microstructure and phase analysis, Acta Mater. 61 (2013) 1545–1557.
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