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A density functional theory study on influence of 3d alloying elements on electrochemical properties of cobalt-base alloys
Computational Materials Science 68 (2013) 206–211
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A density functional theory study on influence of 3d alloying elements on electrochemical properties of cobalt-base alloys Bai Lin Lü a, Guo Qing Chen b,⇑, Wen Long Zhou b, Hui Su a a b
School of Mechanical Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, PR China School of Materials Science and Engineering, Dalian University of Technology, Dalian, Liaoning 116085, PR China
a r t i c l e
i n f o
Article history: Received 8 April 2012 Received in revised form 9 August 2012 Accepted 22 October 2012 Available online 26 November 2012 Keywords: First principles Density functional theory Adsorption Cobalt-base alloys Corrosion Chemical potential
a b s t r a c t The paper systematically studies the impact of the 3d transition metals Ti, V, Cr, Mn, Fe, Ni and Cu on electrochemical stability of non-passivated cobalt-base alloys {0 0 0 1} surface by evaluating the chemical potential and the electrode potential shift relative to pure cobalt metal using density functional theory calculations. Cr, Fe, Mn, and V are found to make the Co atoms more stable on the {0 0 0 1} surfaces of the corresponding alloys compared to pure Co {0 0 0 1} surfaces, whereas Ti, Ni, and Cu make Co atoms much less stable. Among all the considered alloying elements, chrome is the most beneficial to the stability enhancement of alloys. Furthermore, the effects of water and hydroxyl adsorption on the electrochemical stability are considered. It is found that the surface adsorption properties may be considerably modified by introducing the Cr atoms. Our results indicate that water or hydroxyl adsorption destabilizes both the Co–Cr alloy and pure Co surface. However, the Co–Cr alloy surface is still more stable than the pure Co surface in the presence of adsorbed water, while the pure Co surface is more stable than the Co– Cr alloy surface in the presence of adsorbed hydroxyl. Our calculation reveals that the electrochemical corrosion property of the Co–Cr alloy is insensitive to water adsorption and sensitive to hydroxyl adsorption in comparison with the pure Co metal. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The largest use for cobalt-base alloys is in the area of wear resistant components and/or applications because of their outstanding wear resistance [1]. In heat resistant applications, cobalt-base alloys are not as widely used as nickel-base alloys, however due to their superior properties to resist sulfidation [2,3], hot corrosion, and their strength at temperatures above those at which the precipitates in the nickel-base alloys dissolve, cobalt-base heat resistant alloys nevertheless play an important role [1]. They can often find applications in the nuclear power, the chemical and petrochemical, medical and food processing industries [4]. Owing to their excellent wear resistance, fatigue strength, and biocompatibility, cobalt-base alloys have been used as orthopedic implants [5]. Resistance to various forms of wear and high strength over a wide range of temperatures are the attributes of cobalt as an alloy base. However, the excellent corrosion resistance of cobalt base alloys arises mainly from the active alloying elements such as chromium in the alloys, which reacts with water and/or oxygen to form stable passive oxide film on the surface of alloys acting as an effective barrier to separate the substrate alloys from the corrosive
⇑ Corresponding author. Tel./fax: +86 411 84709967. E-mail address: [email protected] (G.Q. Chen). 0927-0256/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2012.10.031
environment and thus protecting the substrate alloys from further corrosion processes [6]. However, the passive film does not afford complete protection of the substrate alloys, e.g., in tribocorrosion systems or contacting certain species (e.g., Cl, Br), where the passive film can often be damaged or even removed, as a result, exposing the active substrate alloy surface to the corrosive environment. Therefore, it is necessary to investigate the corrosion behaviors of the alloys in the absence of passive films for both scientific research and engineering practice. Nevertheless, because the surfaces of cobalt-base alloys usually have passive films and thus the surfaces without the passive film are difficult to create, it is difficult to carrying out the experiment on the non-passivated cobaltbase alloy surface. Now, computational materials science has become a powerful supplement to experiments [7], a lot of studies on corrosion behaviors using first principle method have been carried out [8–12]. Greeley and Nørskov proposed a simple thermodynamic formalism for estimating the electrode potentials at which surface alloy dissolution becomes favorable using density functional theory [8]. The electrode potential shift of alloys relative to pure metal can be taken as a measure of the electrochemical stability and tendency to dissolution of metal alloys in comparison to pure metal surfaces [9,13,14]. Many researches on corrosion properties based on Greeley and Nørskov’s method obtain satisfactory results [9,10]. In addition, Taylor et al. evaluated the corrosion properties of Cu38 nanoparticle and the (1 1 1), (1 1 3) surfaces in
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terms of surface-cohesive energy, dissociation energies of environmental species such as O2 and H2O, and the recombination energy for adsorbed hydrogen atoms by examining the elementary steps which relate to the fundamental corrosion processes: surface vacancy formation, water dissociation, oxygen dissociation, and hydrogen recombination [15]. One of typical approaches to alleviating or minimizing pitting corrosion is to upgrade the materials of construction, for instance addition of alloying elements, overalloying of welds, and lining with high alloy [1]. In this work, we will carry out the systematic theoretical investigation on the corrosion behavior of non-passivated cobalt-base alloy surface doping different 3d transition metals Ti, V, Cr, Mn, Fe, Ni, and Cu by calculating the electrode potential change. Furthermore, the influence of the adsorbed water molecules on corrosion behavior of the surface is also examined. Because corrosive environments are usually aqueous, the surface in the presence of adsorbed water molecules are closer to the realistic situation. In aqueous corrosion reaction, water usually dissociates and oxides metal or alloy surface, thus in this work, we also investigate the surface with adsorbed hydroxyl groups.
2. Computational methods and models Our first-principles calculations based on the density functional theory (DFT) are performed using the CASTEP code (Cambridge Sequential Total Energy Package) [16] with Vanderbilt-type ultrasoft pseudo potentials to describe the ionic potentials and a plane-wave expansion of the wave functions [17]. The generalized gradient approximation (GGA) in the parametrization by Perdew, Burke and Ernzerhof (PBE) was applied in our calculations [18]. The cut-off energy of plane wave is set to 400 eV and integrations in Brillouin zone are performed using Monkhorst–Pack [19] special k points generated with a 6 6 1 mesh parameters grid. The Methfessel–Paxon method was employed to determine electron occupancies with a smearing width of 0.1 eV. The convergence criteria for structure optimization and energy calculation are set to the following: (a) SCF tolerance of 1.0 106 eV/atom; (b) energy tolerance of 1.0 105 eV/atom; (c) displacement tolerance of 1.0 103 Å; (d) force tolerance of 0.03 eV/Å. Because Co, Ni, Mn and Fe are magnetic elements, spin polarization was taken into account, in order to take into account the spin polarization effect, for all calculations spin-polarized (unrestricted) calculations are performed. Pure cobalt metal exists in two allotropes, a high temperature allotrope a with face-centered-cubic (fcc) crystal structure, stable at higher temperatures up to the melting point (1495 °C), and a low-temperature allotrope e with hexagonal-close-packed (hcp) crystal structure, stable at temperatures below 417 °C [20,21]. Cobalt crystal has four different crystalline planes: {0 0 0 1}, {1 0 1 0}, {1 1 2 0} and {1 0 1 2}. In this work, the simulated structures are a hexagonal close-packed for cobalt-base alloys. The close-packed plane {0 0 0 1} of the hcp structure is used to simulate surface system. Based on a compromise between the calculation accuracy and efficiency, in our work, the {0 0 0 1} surface is modeled by a periodic array of four layer-thick metal atom slabs, separated by a vacuum region equivalent to 15 Å to ensure no significant interaction between the slabs. Structural parameters for isolated water and hydroxy molecules are obtained by performing calculations in large periodically repeated cubic boxes, with sides larger than 15 Å. In the investigation of the interactions of the metal surface with water molecules and hydroxyl groups, to achieve a balance between computational tractability and a realistic representation of the adsorption surface, a p (2 2) unit cell geometry (16 metallic atoms per super cell) is employed with a single water molecule or hydroxyl group located on one side of the slab to simulate the
207
adsorbed system, which was also widely used in the previous theoretical investigation on the molecule chemisorption’s on transition metal surfaces [22–26]. Only one water molecule or hydroxyl group per super cell is adsorbed on one side of the slab to reduce lateral interactions (forming hydrogen bonds) with neighbor water molecules or hydroxyl groups in the adjacent cells as the distances between them in the x and y directions are in the range 5.0 Å (thus, the coverage is set to 0.25 monolayer (ML)). In the optimizations of substrate structures, the first two layers from the bottom of the slab are fixed, whilst the other two layers are fully relaxed to their lowest energy configurations with respect to all remaining degrees of freedom. The fixed layers are set to their bulk bond distances according to their optimized lattice constants which are determined from bulk calculations. For the optimizations of the adsorption systems, the water molecule or hydroxyl group and the slab surface layers are allowed to relax simultaneously, whilst as mentioned above some slab layers are fixed to their bulk optimized lattice constants. Based on the above calculation parameters, some structure parameters of H2O molecules and substrate Co atoms have been calculated: the bond length of O–H and the bond angle of H–O–H are 0.978 Å and 104.3°, respectively, which are in good agreement with the experimental values of 0.958 Å and 104.5° [27]; The calculated lattice constant of Co (a = 2.48 Å, c = 4.00 Å) is consistent with the experimental value (a = 2.51 Å, c = 4.07 Å) [28]. The slab model assumption is tested by increasing the number of metal layers to five, and the differences were less than 0.001 Å for bond lengths and less than 0.01 eV for surface energies. All these indicate that the computational scheme used in this work is reliable. 3. Results and discussion 3.1. Segregation It is known that surface segregation, that is, the chemical composition at the surface of an alloy may differ from the composition in the bulk, is of vital importance in all of surface chemistry as it may enhance or suppress desirable and undesirable chemical reactions [29]. For the segregation model, we only investigated the two cases for alloying atoms in the surface and sub-surface of metal surface. This is because the atoms only in the first two surface layers mainly influence the electrochemical stability and reactivity and the other subsurface layers have little effect that can be neglected [9,30,31]. The first case of our segregation model is that a Co atom in the topmost layer of pure Co {0 0 0 1} in per super cell (2 2) is substituted by a alloying atom M, producing a Co–M (M = Ti, V, Cr, Mn, Fe, Ni, and Cu) alloy; the second case is that a Co atom in the second layer of pure Co {0 0 0 1} in per super cell (2 2) is substituted by a alloying atom M, producing a antisegregated Co–M alloy. The segregating behavior of the alloying element is determined by its substitutional formation energy in each structure: after the structure optimization, the configuration with lower substitutional formation energy in the two cases should be thermodynamically more stable. The substitutional formation energy (Hform) of element M can be written as
Hform ¼ Etot ðxECo þ yEM Þ
ð1Þ
where Etot denotes the total energy with the substituting element M, ECo and EM are the total energy per atom of the pure bulk element for Co and the alloying element M, respectively, x and y are the number of Co and the alloying element M in the supercell. Our calculated results show that Ni and Cu segregate towards the surface of the host, while Ti, V, Cr, Mn, and Fe prefer to remain
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in the interior of the host (antisegregation). The segregation directions of V, Cr, Mn, Ni, and Cu are in line with ones by Ruban et al. [29]. However, in the calculated results by Ruban, the segregation direction of Ti is to the surface, Fe is of no segregation [29]. We increase the unit cell sizes, layer thickness, k points, cutoff energy, however the results remains unchanged. Therefore, in the following calculation, we use our above calculated results. 3.2. Electrochemical potential shift Now, we investigate the corrosion property of cobalt-base alloys using the most stable structures obtained in Section 3.1. The electrode potential shifts DU for Co–M {0 0 0 1} relative to pure cobalt metal for the reaction Co2+ + 2e = Co electrode reaction are calculated by the following formula [8,9]:
DU ¼ U CoM U Co ¼
lCo—M lCo
ð2Þ
2e
in which lCo and lCo–M are the chemical potentials of Co atoms in pure Co and in Co–M alloy, respectively. UCo and UCo–M are the electrode potential of Co atoms in pure Co in Co–M alloy relative to the standard hydrogen electrode, respectively. A surface metal atom is removed from the slab, producing a new slab with a surface cavity. The chemical potential can be approximately evaluated obtained by calculating the total energies of the optimized original and new slabs with vacancies, Eslab and E0slab [9]:
l
Eslab E0slab DN
ð3Þ
where DN is the number of cavities. It should be noted that since E0slab varies with the position of the removed Co atom, we evaluate the chemical potentials using the configuration with the lowest total slab energy. The calculation results of the chemical potential difference (Dl = lM,alloy lM,pure) and the electrode potential shift DU of the Co–M (M = Ti, V, Cr, Mn, Fe, Ni, and Cu) relative to pure cobalt metal are listed in Table 1. Our calculations indicate that the electrode potential shift (DU) follows the order: Cr > Fe > Mn > V > Co > Ni > Ti > Cu, according to Table 1. The first four alloying elements: Cr, Fe, Mn, and V make the surface Co atoms more stable on the {0 0 0 1} surfaces of the corresponding alloys than they are in pure Co {0 0 0 1} surfaces, because the values of the electrode potential shift are positive, indicating that the dissolution of Co atoms from Co–Cr surfaces is delayed to higher potentials relative to pure cobalt metal, whereas the last three alloying elements: Ni, Ti, and Cu make Co much less stable against dissolution, due to the values of the electrode potential
Table 1 Chemical potential difference (Dl) and electrode potential shift (DU) in the absence and presence of water.a
a
{0 0 0 1} Surface
Interlayer distance d12 (Å)
Dl (eV)
DU (V)
Co–Cr Co–Fe Co–Mn Co–V Pure Co Co–Ni Co–Ti Co–Cu Co (0.25 ML H2O) Co–Cr (0.25 ML H2O) Co (0.25 ML OH) Co–Cr (0.25 ML OH)
1.945 1.953 1.954 1.958 1.965 1.979 1.987 2.004 1.969 1.946 1.984 1.962
0.070 0.065 0.055 0.026 0 0.151 0.272 0.275 0.182 0.083 0.598 0.618
0.035 0.033 0.027 0.012 0 0.075 0.136 0.137 0.091 0.042 0.299 0.309
d12 refers to the distance between the first and second layer.
shift are negative, indicating that the alloy surface is unstable and easy for the dissolution of Co atoms from alloy surfaces compared to the pure cobalt metal. We find that among all the considered alloying elements, chrome is the most beneficial to the stability enhancement of alloys. In general, chrome is believed as the most effective element to improve the corrosion resistance of cobalt-base alloys. This also indicate that our calculated results are credible. According to Table 1, we see that the electrode potential shift increasing is accompanied by the interlayer distance d12 decreasing. This indicates that stronger the interactions between the surface and sub-layer atoms become, higher the stability of Co atoms in the surface against dissolution. 3.3. Adsorption of water molecular and hydroxyl group 3.3.1. Adsorption of water molecular Because the metals or alloys are usually in aqueous corrosive environments, the alloy surface adsorbed water molecules are closer to the realistic situation. Thus in the following discussion, we investigate the electrochemical stability of pure Co {0 0 0 1} and Co–Cr {0 0 0 1} alloy surfaces under 0.25 ML of adsorbed molecular water. The adsorption energy is calculated as the difference between the total energy of the system of the adsorbed molecule on the metal surface with the sum of the total energies of the clean metal slab and that of the corresponding free molecule according to following [32,33]:
Eads ¼ Eadsorbate-surface ðEsurface þ Efree
molecule Þ
ð4Þ
where Eadsorbate-surface, Esurface, and Efree molecule are the total energies of the adsorption system, the clean metal surface, and the isolated adsorbate, respectively. In this definition, a negative value of Eads indicates an exothermic adsorption process. There are four possible highly symmetric adsorption positions for water molecule on the Co {0 0 0 1} surface: on-top, bridge, fcc and hcp hollow site. The experimental structure parameter of water molecule is adopted, and a water molecule is placed parallel to the {0 0 0 1} surface for initial position according as the reported results of other metals [34]. The optimized structural parameters and adsorption energies at each site are given in Table 2. The top site is found to be the most stable adsorption site with the dipole vector parallel to the surface, with adsorption energy of 0.36 eV, while each of the other sites is strongly disfavored with adsorption energies of less 0.2 eV, and the fcc hollow site is the least favorable on. These results are consistent with the reported theoretical and experimental results of other metals [30,34–36]. The strongest interaction of water with the Co atom in the surface at the top is also supported by the shortest distance between water oxygen and the Co (dO–surface). In addition, the O–H bonds of the adsorbed molecule are 0.01 (top site) and 0.004 Å (other adsorption sites) longer than those of the gas phase water molecule, respectively, reflecting their weakening due to bonding with the surface. From Table 2, it can be seen that when water molecules are adsorbed on the surface, the interlayer distance d12 for the top site is increased, and ones for the other sites decreased, compared to the respective clean surfaces. This shows that for the top, the interactions of the surface and sub-layer atoms become weaker, decreasing the stability of Co atoms in the surface against dissolution. For the Co–Cr alloy, because of the presence of Cr in the sublayer, additional adsorption sites are identified on the {0 0 0 1} surface. Two top sites, three bridge sites, two hcp hollow sites, and two fcc hollow sites are investigated in this work, as shown in Fig. 1. The optimized structural parameters and adsorption energies at various sites are given in Table 3. Our calculations indicate
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B.L. Lü et al. / Computational Materials Science 68 (2013) 206–211 Table 2 Water molecule adsorption energies (Eads) on the pure Co {0 0 0 1} and optimized structural parameters for each adsorption site and gas-phase water molecule. O location
Eads (eV)
Interlayer distance d12 (Å)
Bond length (Å) dO–surface
dO–H
Top Bridge fcc hcp Clean surface Gas phase water
0.36 0.18 0.15 0.16 – –
1.969 1.960 1.961 1.962 1.965 –
2.181 2.830 2.711 2.841 – –
0.988 0.982 0.982 0.982 – 0.978
It may be concluded that the adsorbed water molecule decrease the stability of Co atoms in the metal and alloy surface. However, the Co–Cr alloy surface still shows enhanced stability with reference to pure Co under 0.25 ML of water: The potential shift of Co–Cr alloy is 0.05 V higher than one of pure Co. The positive values of the shifts mean that the dissolution of Co atoms from Co–Cr surfaces is delayed to higher potentials than that from the pure Co metal. This indicates that the Co–Cr alloy surface is more stable against dissolution in comparison with the pure Co surface under 0.25 ML of adsorbed water. The value of the potential shift is 0.077 V for the surface of Co–Cr alloy, referenced to its electrode potential in the absence of water adsorption. This value is higher 0.014 V than that (0.091 V) for the surface of pure Co metal referenced to its electrode potential without water adsorption, showing that the electrochemical corrosion property of the Co–Cr alloy is insensitive to water molecular adsorption relative to the pure Co metal.
Fig. 1. Adsorption sites on Co–Cr {0 0 0 1}, Black – Co, Grey – Cr.
Table 3 Water molecule adsorption energies (Eads) on Co–Cr {0 0 0 1} and optimized structural parameters for each adsorption site as illustrated in Fig. 1 and gas-phase water molecule. O location
Top1 Top2 Bridge1 Bridge2 Bridge3 fcc1 fcc2 hcp1 hcp2 Clean surface Gas phase water
Eads (eV)
0.21 0.25 0.08 0.13 0.07 0.04 0.13 0.06 0.07 – –
Interlayer distance d12 (Å)
1.954 1.946 1.940 1.943 1.944 1.944 1.942 1.945 1.943 1.945 –
Bond length (Å) dO–surface
dO–H
2.208 2.164 2.904 2.863 2.867 2.737 2.700 2.873 2.727 – –
0.986 0.988 0.981 0.982 0.982 0.982 0.983 0.982 0.982 – 0.978
that the top site are still the most favorable with the dipole vector parallel to the surface, with the largest adsorption energy (0.25 eV), the shortest dO–surface, the longest dO–H and the largest interlayer distance, analogous to pure Co {0 0 0 1} surfaces. Introducing Cr atoms into the Co {0 0 0 1} system considerably alter the surface adsorption properties: the water adsorption energy on the alloy surface is much weaker 0.11 eV than one on the pure metal surface.
3.3.2. Electrochemical potential shift under adsorbed water Now, we investigate the potential shifts of pure Co and Co–Cr alloy surface for the Co2+ + 2e = Co electrode reaction under 0.25 ML of adsorbed water. For each super cell, removal of the Co atom in the surface would produce a new slab with the lowest total energy. According to Table 1, our calculations result in a negative electrode potential shift (0.091 and 0.042 V) for both the pure Co and Co–Cr alloy, with respect to the clean pure Co surface, indicating that the surfaces are less stable than the pure Co surface without water adsorption.
3.3.3. Hydroxyl adsorption In aqueous corrosion reaction, water usually dissociates and oxides metal or alloy surface. The splitting of water can lead to the formation of adsorbed hydroxyl and concomitant proton production. In this work, for simplicity, the effect of hydroxyl dissociation to yield adsorbed atomic oxygen is not considered [8]. In the following dissolution analyses, we incorporate the electrochemical stability of pure Co {0 0 0 1} and Co–Cr {0 0 0 1} alloy surfaces under 0.25 ML of adsorbed hydroxyl. The four possible highly symmetric adsorption positions for hydroxyl on the Co {0 0 0 1} surface: on-top, bridge, fcc and hcp hollow site are considered as initial ones. The optimized structural parameters and adsorption energies at each site are given in Table 4. It can be seen that on the pure Co {1 1 1}, hydroxyl prefers the threefold site (fcc hollow one) with the axis of the molecule perpendicular to the surface, with the largest adsorption energy of 4.29 eV (the hcp hollow site 4.13 eV, the top site 3.56 eV). The optimization result of the initial bridge site deviates from the bridge site and coincides with the fcc hollow site, thus the bridge site does not appear in Table 4. Not like weak water adsorption, the adsorption of hydroxyl to cobalt is strong due to the tendency for the undercoordinated oxygen atom to pair up with the surface atoms, thus forming a chemical bond.
Table 4 Hydroxyl groups adsorption energies (Eads) on the pure Co {0 0 0 1} and optimized structural parameters for each adsorption site and free hydroxyl. O location
Eads (eV)
Interlayer distance d12 (Å)
Top fcc hcp Clean surface Free hydroxyl
3.56 4.29 4.13 – –
1.978 1.984 1.984 1.965 –
Bond length (Å) dO–surface
dO–H
1.824 1.307 1.315 – –
0.984 0.977 0.978 – 0.988
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Similar to the water adsorption, for the strongest interaction of hydroxyl with the Co atom in the surface (at the fcc hollow site), the distance between hydroxyl oxygen and the surface (dO–surface) is the shortest and the interlayer distance d12 is the largest compared to other adsorption sites. However, in contrast to water adsorption, at all the adsorption sites the O–H bonds of the adsorbed hydroxyl is shorter than that of the free one and that at the fcc hollow site is the shortest, reflecting its strengthening due to bonding with the surface. The increasing of the interlayer distance d12 upon adsorption compared to the clean surface show that the interactions of the surface and sub-layer atoms become weaker, decreasing the stability of Co atoms in the surface against dissolution. For hydroxyl adsorption on the Co–Cr {0 0 0 1} surface, similar to the water adsorption, the nine initial adsorption positions: two top sites, three bridge sites, two hcp hollow sites, and two fcc hollow sites are investigated, as shown in Fig. 1. The optimized structural parameters and adsorption energies at various sites are given in Table 5. Analogous to pure metal, the optimization result of all the initial bridge sites deviate from the bridge sites and coincide with the fcc hollow sites, thus the bridge sites are not in Table 5. We see that the fcc hollow site are still the most favorable, with the largest adsorption energy (4.16 eV), the shortest dO–surface, the shortest dO–H, and the largest d12 like pure Co {0 0 0 1} surfaces. Similar to the water adsorption, Cr atoms in the Co–Cr alloy considerably change the surface adsorption properties: the hydroxyl adsorption energy on the alloy surface is much weaker 0.13 eV than one on the pure metal surface. 3.3.4. Electrochemical potential shift under adsorbed hydroxyl The calculation results of the potential shifts of Co atoms on pure Co and Co–Cr alloy surface for the Co2+ + 2e = Co electrode reaction under 0.25 ML of adsorbed hydroxyl are given in Table 1. We see a negative electrode potential shift (0.299 and 0.309 V) for both the pure Co and Co–Cr alloy with respect to the clean pure Co surface, indicating that the surfaces are less stable than the pure Co surface without hydroxyl adsorption. Similar to the water adsorption, the adsorbed hydroxyl decrease the stability of Co atoms in the metal and alloy surface. However, contrary to the water adsorption, the pure Co metal surface shows enhanced stability with reference to Co–Cr alloy under 0.25 ML of hydroxyl: the potential shift of Co–Cr alloy is 0.1 V lower than one of pure Co. This indicates that the pure Co surface is more stable in comparison with the Co–Cr alloy surface under 0.25 ML of adsorbed hydroxyl. For the Co–Cr alloy surface, the value of the potential shift is 0.344 V referenced to its electrode potential in the absence of hydroxyl adsorption. This value is lower 0.045 V than that (0.299 V) for the pure Co metal surface referenced to its electrode potential without hydroxyl adsorption, showing that contrary to the results of the water adsorption, the electrochemical corrosion property of the Co–Cr alloy is sensitive to hydroxyl adsorption relative to the pure Co metal.
Table 5 Hydroxyl adsorption energies (Eads) on the Co–Cr {0 0 0 1} surface and optimized structural parameters for each adsorption site and free hydroxyl. O location
Top1 fcc1 fcc2 hcp1 hcp2 Clean surface Free hydroxyl
Eads (eV)
3.44 4.16 3.77 4.02 3.99 – –
Interlayer distance d12 (Å)
1.958 1.962 1.938 1.962 1.958 1.945 –
Bond length (Å) dO–surface
dO–H
1.820 1.339 1.477 1.390 1.433 – –
0.983 0.976 0.978 0.978 0.980 – 0.988
4. Summary and conclusions In conclusion, our calculated results show that Ni and Cu segregates towards the surface of the host, while Ti, V, Cr, Mn, and Fe prefer to remain in the interior of the host (antisegregation). Cr, Fe, Mn, and V make the surface Co atoms more stable in the {0 0 0 1} surfaces of the corresponding alloys than they are in pure Co {0 0 0 1} surfaces, whereas Ni and Cu make Co much less stable against dissolution. Among all the considered alloying elements, chrome is the most beneficial to the stability enhancement of alloys. The most favorable adsorption site for a water molecule on the pure Co {0 0 0 1} and Co–Cr {0 0 0 1} surfaces is on top of Co atoms, with the dipole vector parallel to the surface, with the highest adsorption energy, the shortest dO–surface, the largest interlayer distance d12 and the longest dO–H. For a hydroxyl group on the pure Co {0 0 0 1} and Co–Cr {0 0 0 1} surfaces, the most favorable adsorption site is at the fcc hollow ones, with the axis of the molecule perpendicular to the surface, with the highest adsorption energy, the largest interlayer distance d12, the shortest dO–surface and the shortest dO–H. Introducing Cr atoms into the Co {0 0 0 1} system considerably alters the surface adsorption properties. The water or hydroxyl adsorption energy on the alloy surface is much weaker than one on the pure metal surface. It is found that the adsorbed water molecule or hydroxyl group decreases the stability of Co atoms in the metal and alloy surface. However, the Co–Cr alloy surface still shows enhanced stability with reference to pure Co under 0.25 ML of water, while under 0.25 ML of hydroxyl, the pure Co surface is more stable than the Co–Cr alloy. Our calculations also revealed that the electrochemical corrosion property of the Co–Cr alloy is insensitive to water molecular adsorption and sensitive to hydroxyl adsorption in comparison with the pure Co metal. Acknowledgments Financial support from the National Basic Research Program of China (973 Program, Grant No. 2009CB724305) and Program for New Century Excellent Talents in University (No. NCET-10-0278) is gratefully acknowledged. References [1] J.R. Davis, ASM Specialty Handbook: Nickel Cobalt, and Their Alloys, ASM International, Materials Park, OH, 2000. [2] A. Petric, K.T. Jacob, Metall. Mater. Trans. A 16 (1985) 503–510. [3] G.Y. Lai, Resistance to carburization of various heat-resistant alloys, in: M.F. Rothman (Ed.), High-Temperature Corrosion in Energy Systems, Proceedings of the TMS-AIME Symposium, Metallurgical Society of AIME, Warrendale, PA, 1985, p. 551. [4] J. Chen, X.Y. Li, T. Bell, H. Dong, Wear 264 (2008) 157–165. [5] Y. Yan, A. Neville, D. Dowson, S. Williams, Tribol. Int. 39 (2006) 1509–1517. [6] I. Milosev, H.H. Strehblow, Electrochim. Acta 48 (2003) 2767–2774. [7] G.P.M. Leyson, W.A. Curtin, L.G. Hector, C.F. Woodward, Nat. Mater. 9 (2010) 750–755. [8] J. Greeley, J.K. Nørskov, Electrochim. Acta 52 (2007) 5829–5836. [9] Y.G. Ma, P.B. Balbuena, J. Phys. Chem. C 112 (2008) 14520–14528. [10] Y.G. Ma, P.B. Balbuena, J. Electrochem. Soc. 157 (2010) B959–B963. [11] G.E. Ramirez-Caballero, Y.G. Ma, R. Callejas-Tovar, P.B. Balbuena, Phys. Chem. Chem. Phys. 12 (2010) 2209–2218. [12] J. Greeley, Electrochim. Acta 55 (2010) 5545–5550. [13] X.P. Wang, R. Kumar, D.J. Myers, Electrochem. Solid State Lett. 9 (2006) A225– A227. [14] K. Yasuda, A. Taniguchi, T. Akita, T. Ioroi, Z. Siroma, Phys. Chem. Chem. Phys. 8 (2006) 746–752. [15] C.D. Taylor, M. Neurock, J.R. Scully, J. Electrochem. Soc. 155 (2008) C407. [16] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045–1097. [17] D. Vanderbilt, Phys Rev. B: Condens. Matter Mater. Phys. 41 (1990) 7892– 7895. [18] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868. [19] H.J. Monkhorst, J.D. Pack, Phys Rev. B: Condens. Matter Mater. Phys. 13 (1976) 5188–5192.
B.L. Lü et al. / Computational Materials Science 68 (2013) 206–211 [20] V. Kuzucu, M. Ceylan, H. Celik, I. Aksoy, J. Mater. Process. Technol. 74 (1998) 137–141. [21] P. Crook, Properties and Selection: Non-ferrous Alloys and Special-Purpose Materials, 10th ed., vol. 2, ASM International, USA, 1993. [22] A. Kokalj, N. Bonini, S.d. Gironcoli, C. Sbraccia, G. Fratesi, S. Baroni, J. Am. Chem. Soc. 128 (2006) 12448–12454. [23] B. Bunnik, G. Kramer, J. Catal. 242 (2006) 309–318. [24] A. Bukoski, H.L. Abbott, I. Harrison, J. Chem. Phys. 123 (2005) 094707. [25] A. Anghel, D. Wales, S. Jenkins, D. King, Phys. Rev. B: Condens. Matter 71 (2005) 113410. [26] A. Stroppa, K. Termentzidis, J. Paier, G. Kresse, J. Hafner, Phys. Rev. B: Condens. Matter 76 (2007) 195440. [27] D.R. Lide, Handbook of Chemistry and Physics, 80th ed., CRC, London, 1999. [28] A. Taylor, R.W. Floyd, Acta Cryst. 3 (1950) 285–289.
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[29] A.V. Ruban, H.L. Skriver, J.K. Norskov, Phys. Rev. B: Condens. Matter 59 (1999) 15990–16000. [30] P. Hirunsit, P.B. Balbuena, Surf. Sci. 603 (2009) 912–920. [31] A. Dianat, J. Zimmermann, N. Seriani, M. Bobeth, W. Pompe, L.C. Ciacchi, Surf. Sci. 602 (2008) 876–884. [32] S. Sanyal, U.V. Waghmare, J.A. Ruud, Appl. Surf. Sci. 257 (2011) 6462–6467. [33] P.P. Dholabhai, R. Atta-Fynn, A.K. Ray, Phys. B: Conden. Matter 403 (2008) 4269–4280. [34] J.B. Li, S.L. Zhu, Y. Li, F.H. Wang, Phys. Rev. B: Condens. Matter 76 (2007) 235433. [35] A. Michaelides, V.A. Ranea, P.L. de Andres, D.A. King, Phys. Rev. Lett. 90 (2003) 216102. [36] A. Michaelides, A. Alavi, D.A. King, J. Am. Chem. Soc. 125 (2003) 2746–2755.
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A density functional theory study on influence of 3d alloying elements on electrochemical properties of cobalt-base alloys Bai Lin Lü a, Guo Qing Chen b,⇑, Wen Long Zhou b, Hui Su a a b
School of Mechanical Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, PR China School of Materials Science and Engineering, Dalian University of Technology, Dalian, Liaoning 116085, PR China
a r t i c l e
i n f o
Article history: Received 8 April 2012 Received in revised form 9 August 2012 Accepted 22 October 2012 Available online 26 November 2012 Keywords: First principles Density functional theory Adsorption Cobalt-base alloys Corrosion Chemical potential
a b s t r a c t The paper systematically studies the impact of the 3d transition metals Ti, V, Cr, Mn, Fe, Ni and Cu on electrochemical stability of non-passivated cobalt-base alloys {0 0 0 1} surface by evaluating the chemical potential and the electrode potential shift relative to pure cobalt metal using density functional theory calculations. Cr, Fe, Mn, and V are found to make the Co atoms more stable on the {0 0 0 1} surfaces of the corresponding alloys compared to pure Co {0 0 0 1} surfaces, whereas Ti, Ni, and Cu make Co atoms much less stable. Among all the considered alloying elements, chrome is the most beneficial to the stability enhancement of alloys. Furthermore, the effects of water and hydroxyl adsorption on the electrochemical stability are considered. It is found that the surface adsorption properties may be considerably modified by introducing the Cr atoms. Our results indicate that water or hydroxyl adsorption destabilizes both the Co–Cr alloy and pure Co surface. However, the Co–Cr alloy surface is still more stable than the pure Co surface in the presence of adsorbed water, while the pure Co surface is more stable than the Co– Cr alloy surface in the presence of adsorbed hydroxyl. Our calculation reveals that the electrochemical corrosion property of the Co–Cr alloy is insensitive to water adsorption and sensitive to hydroxyl adsorption in comparison with the pure Co metal. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The largest use for cobalt-base alloys is in the area of wear resistant components and/or applications because of their outstanding wear resistance [1]. In heat resistant applications, cobalt-base alloys are not as widely used as nickel-base alloys, however due to their superior properties to resist sulfidation [2,3], hot corrosion, and their strength at temperatures above those at which the precipitates in the nickel-base alloys dissolve, cobalt-base heat resistant alloys nevertheless play an important role [1]. They can often find applications in the nuclear power, the chemical and petrochemical, medical and food processing industries [4]. Owing to their excellent wear resistance, fatigue strength, and biocompatibility, cobalt-base alloys have been used as orthopedic implants [5]. Resistance to various forms of wear and high strength over a wide range of temperatures are the attributes of cobalt as an alloy base. However, the excellent corrosion resistance of cobalt base alloys arises mainly from the active alloying elements such as chromium in the alloys, which reacts with water and/or oxygen to form stable passive oxide film on the surface of alloys acting as an effective barrier to separate the substrate alloys from the corrosive
⇑ Corresponding author. Tel./fax: +86 411 84709967. E-mail address: [email protected] (G.Q. Chen). 0927-0256/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2012.10.031
environment and thus protecting the substrate alloys from further corrosion processes [6]. However, the passive film does not afford complete protection of the substrate alloys, e.g., in tribocorrosion systems or contacting certain species (e.g., Cl, Br), where the passive film can often be damaged or even removed, as a result, exposing the active substrate alloy surface to the corrosive environment. Therefore, it is necessary to investigate the corrosion behaviors of the alloys in the absence of passive films for both scientific research and engineering practice. Nevertheless, because the surfaces of cobalt-base alloys usually have passive films and thus the surfaces without the passive film are difficult to create, it is difficult to carrying out the experiment on the non-passivated cobaltbase alloy surface. Now, computational materials science has become a powerful supplement to experiments [7], a lot of studies on corrosion behaviors using first principle method have been carried out [8–12]. Greeley and Nørskov proposed a simple thermodynamic formalism for estimating the electrode potentials at which surface alloy dissolution becomes favorable using density functional theory [8]. The electrode potential shift of alloys relative to pure metal can be taken as a measure of the electrochemical stability and tendency to dissolution of metal alloys in comparison to pure metal surfaces [9,13,14]. Many researches on corrosion properties based on Greeley and Nørskov’s method obtain satisfactory results [9,10]. In addition, Taylor et al. evaluated the corrosion properties of Cu38 nanoparticle and the (1 1 1), (1 1 3) surfaces in
B.L. Lü et al. / Computational Materials Science 68 (2013) 206–211
terms of surface-cohesive energy, dissociation energies of environmental species such as O2 and H2O, and the recombination energy for adsorbed hydrogen atoms by examining the elementary steps which relate to the fundamental corrosion processes: surface vacancy formation, water dissociation, oxygen dissociation, and hydrogen recombination [15]. One of typical approaches to alleviating or minimizing pitting corrosion is to upgrade the materials of construction, for instance addition of alloying elements, overalloying of welds, and lining with high alloy [1]. In this work, we will carry out the systematic theoretical investigation on the corrosion behavior of non-passivated cobalt-base alloy surface doping different 3d transition metals Ti, V, Cr, Mn, Fe, Ni, and Cu by calculating the electrode potential change. Furthermore, the influence of the adsorbed water molecules on corrosion behavior of the surface is also examined. Because corrosive environments are usually aqueous, the surface in the presence of adsorbed water molecules are closer to the realistic situation. In aqueous corrosion reaction, water usually dissociates and oxides metal or alloy surface, thus in this work, we also investigate the surface with adsorbed hydroxyl groups.
2. Computational methods and models Our first-principles calculations based on the density functional theory (DFT) are performed using the CASTEP code (Cambridge Sequential Total Energy Package) [16] with Vanderbilt-type ultrasoft pseudo potentials to describe the ionic potentials and a plane-wave expansion of the wave functions [17]. The generalized gradient approximation (GGA) in the parametrization by Perdew, Burke and Ernzerhof (PBE) was applied in our calculations [18]. The cut-off energy of plane wave is set to 400 eV and integrations in Brillouin zone are performed using Monkhorst–Pack [19] special k points generated with a 6 6 1 mesh parameters grid. The Methfessel–Paxon method was employed to determine electron occupancies with a smearing width of 0.1 eV. The convergence criteria for structure optimization and energy calculation are set to the following: (a) SCF tolerance of 1.0 106 eV/atom; (b) energy tolerance of 1.0 105 eV/atom; (c) displacement tolerance of 1.0 103 Å; (d) force tolerance of 0.03 eV/Å. Because Co, Ni, Mn and Fe are magnetic elements, spin polarization was taken into account, in order to take into account the spin polarization effect, for all calculations spin-polarized (unrestricted) calculations are performed. Pure cobalt metal exists in two allotropes, a high temperature allotrope a with face-centered-cubic (fcc) crystal structure, stable at higher temperatures up to the melting point (1495 °C), and a low-temperature allotrope e with hexagonal-close-packed (hcp) crystal structure, stable at temperatures below 417 °C [20,21]. Cobalt crystal has four different crystalline planes: {0 0 0 1}, {1 0 1 0}, {1 1 2 0} and {1 0 1 2}. In this work, the simulated structures are a hexagonal close-packed for cobalt-base alloys. The close-packed plane {0 0 0 1} of the hcp structure is used to simulate surface system. Based on a compromise between the calculation accuracy and efficiency, in our work, the {0 0 0 1} surface is modeled by a periodic array of four layer-thick metal atom slabs, separated by a vacuum region equivalent to 15 Å to ensure no significant interaction between the slabs. Structural parameters for isolated water and hydroxy molecules are obtained by performing calculations in large periodically repeated cubic boxes, with sides larger than 15 Å. In the investigation of the interactions of the metal surface with water molecules and hydroxyl groups, to achieve a balance between computational tractability and a realistic representation of the adsorption surface, a p (2 2) unit cell geometry (16 metallic atoms per super cell) is employed with a single water molecule or hydroxyl group located on one side of the slab to simulate the
207
adsorbed system, which was also widely used in the previous theoretical investigation on the molecule chemisorption’s on transition metal surfaces [22–26]. Only one water molecule or hydroxyl group per super cell is adsorbed on one side of the slab to reduce lateral interactions (forming hydrogen bonds) with neighbor water molecules or hydroxyl groups in the adjacent cells as the distances between them in the x and y directions are in the range 5.0 Å (thus, the coverage is set to 0.25 monolayer (ML)). In the optimizations of substrate structures, the first two layers from the bottom of the slab are fixed, whilst the other two layers are fully relaxed to their lowest energy configurations with respect to all remaining degrees of freedom. The fixed layers are set to their bulk bond distances according to their optimized lattice constants which are determined from bulk calculations. For the optimizations of the adsorption systems, the water molecule or hydroxyl group and the slab surface layers are allowed to relax simultaneously, whilst as mentioned above some slab layers are fixed to their bulk optimized lattice constants. Based on the above calculation parameters, some structure parameters of H2O molecules and substrate Co atoms have been calculated: the bond length of O–H and the bond angle of H–O–H are 0.978 Å and 104.3°, respectively, which are in good agreement with the experimental values of 0.958 Å and 104.5° [27]; The calculated lattice constant of Co (a = 2.48 Å, c = 4.00 Å) is consistent with the experimental value (a = 2.51 Å, c = 4.07 Å) [28]. The slab model assumption is tested by increasing the number of metal layers to five, and the differences were less than 0.001 Å for bond lengths and less than 0.01 eV for surface energies. All these indicate that the computational scheme used in this work is reliable. 3. Results and discussion 3.1. Segregation It is known that surface segregation, that is, the chemical composition at the surface of an alloy may differ from the composition in the bulk, is of vital importance in all of surface chemistry as it may enhance or suppress desirable and undesirable chemical reactions [29]. For the segregation model, we only investigated the two cases for alloying atoms in the surface and sub-surface of metal surface. This is because the atoms only in the first two surface layers mainly influence the electrochemical stability and reactivity and the other subsurface layers have little effect that can be neglected [9,30,31]. The first case of our segregation model is that a Co atom in the topmost layer of pure Co {0 0 0 1} in per super cell (2 2) is substituted by a alloying atom M, producing a Co–M (M = Ti, V, Cr, Mn, Fe, Ni, and Cu) alloy; the second case is that a Co atom in the second layer of pure Co {0 0 0 1} in per super cell (2 2) is substituted by a alloying atom M, producing a antisegregated Co–M alloy. The segregating behavior of the alloying element is determined by its substitutional formation energy in each structure: after the structure optimization, the configuration with lower substitutional formation energy in the two cases should be thermodynamically more stable. The substitutional formation energy (Hform) of element M can be written as
Hform ¼ Etot ðxECo þ yEM Þ
ð1Þ
where Etot denotes the total energy with the substituting element M, ECo and EM are the total energy per atom of the pure bulk element for Co and the alloying element M, respectively, x and y are the number of Co and the alloying element M in the supercell. Our calculated results show that Ni and Cu segregate towards the surface of the host, while Ti, V, Cr, Mn, and Fe prefer to remain
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in the interior of the host (antisegregation). The segregation directions of V, Cr, Mn, Ni, and Cu are in line with ones by Ruban et al. [29]. However, in the calculated results by Ruban, the segregation direction of Ti is to the surface, Fe is of no segregation [29]. We increase the unit cell sizes, layer thickness, k points, cutoff energy, however the results remains unchanged. Therefore, in the following calculation, we use our above calculated results. 3.2. Electrochemical potential shift Now, we investigate the corrosion property of cobalt-base alloys using the most stable structures obtained in Section 3.1. The electrode potential shifts DU for Co–M {0 0 0 1} relative to pure cobalt metal for the reaction Co2+ + 2e = Co electrode reaction are calculated by the following formula [8,9]:
DU ¼ U CoM U Co ¼
lCo—M lCo
ð2Þ
2e
in which lCo and lCo–M are the chemical potentials of Co atoms in pure Co and in Co–M alloy, respectively. UCo and UCo–M are the electrode potential of Co atoms in pure Co in Co–M alloy relative to the standard hydrogen electrode, respectively. A surface metal atom is removed from the slab, producing a new slab with a surface cavity. The chemical potential can be approximately evaluated obtained by calculating the total energies of the optimized original and new slabs with vacancies, Eslab and E0slab [9]:
l
Eslab E0slab DN
ð3Þ
where DN is the number of cavities. It should be noted that since E0slab varies with the position of the removed Co atom, we evaluate the chemical potentials using the configuration with the lowest total slab energy. The calculation results of the chemical potential difference (Dl = lM,alloy lM,pure) and the electrode potential shift DU of the Co–M (M = Ti, V, Cr, Mn, Fe, Ni, and Cu) relative to pure cobalt metal are listed in Table 1. Our calculations indicate that the electrode potential shift (DU) follows the order: Cr > Fe > Mn > V > Co > Ni > Ti > Cu, according to Table 1. The first four alloying elements: Cr, Fe, Mn, and V make the surface Co atoms more stable on the {0 0 0 1} surfaces of the corresponding alloys than they are in pure Co {0 0 0 1} surfaces, because the values of the electrode potential shift are positive, indicating that the dissolution of Co atoms from Co–Cr surfaces is delayed to higher potentials relative to pure cobalt metal, whereas the last three alloying elements: Ni, Ti, and Cu make Co much less stable against dissolution, due to the values of the electrode potential
Table 1 Chemical potential difference (Dl) and electrode potential shift (DU) in the absence and presence of water.a
a
{0 0 0 1} Surface
Interlayer distance d12 (Å)
Dl (eV)
DU (V)
Co–Cr Co–Fe Co–Mn Co–V Pure Co Co–Ni Co–Ti Co–Cu Co (0.25 ML H2O) Co–Cr (0.25 ML H2O) Co (0.25 ML OH) Co–Cr (0.25 ML OH)
1.945 1.953 1.954 1.958 1.965 1.979 1.987 2.004 1.969 1.946 1.984 1.962
0.070 0.065 0.055 0.026 0 0.151 0.272 0.275 0.182 0.083 0.598 0.618
0.035 0.033 0.027 0.012 0 0.075 0.136 0.137 0.091 0.042 0.299 0.309
d12 refers to the distance between the first and second layer.
shift are negative, indicating that the alloy surface is unstable and easy for the dissolution of Co atoms from alloy surfaces compared to the pure cobalt metal. We find that among all the considered alloying elements, chrome is the most beneficial to the stability enhancement of alloys. In general, chrome is believed as the most effective element to improve the corrosion resistance of cobalt-base alloys. This also indicate that our calculated results are credible. According to Table 1, we see that the electrode potential shift increasing is accompanied by the interlayer distance d12 decreasing. This indicates that stronger the interactions between the surface and sub-layer atoms become, higher the stability of Co atoms in the surface against dissolution. 3.3. Adsorption of water molecular and hydroxyl group 3.3.1. Adsorption of water molecular Because the metals or alloys are usually in aqueous corrosive environments, the alloy surface adsorbed water molecules are closer to the realistic situation. Thus in the following discussion, we investigate the electrochemical stability of pure Co {0 0 0 1} and Co–Cr {0 0 0 1} alloy surfaces under 0.25 ML of adsorbed molecular water. The adsorption energy is calculated as the difference between the total energy of the system of the adsorbed molecule on the metal surface with the sum of the total energies of the clean metal slab and that of the corresponding free molecule according to following [32,33]:
Eads ¼ Eadsorbate-surface ðEsurface þ Efree
molecule Þ
ð4Þ
where Eadsorbate-surface, Esurface, and Efree molecule are the total energies of the adsorption system, the clean metal surface, and the isolated adsorbate, respectively. In this definition, a negative value of Eads indicates an exothermic adsorption process. There are four possible highly symmetric adsorption positions for water molecule on the Co {0 0 0 1} surface: on-top, bridge, fcc and hcp hollow site. The experimental structure parameter of water molecule is adopted, and a water molecule is placed parallel to the {0 0 0 1} surface for initial position according as the reported results of other metals [34]. The optimized structural parameters and adsorption energies at each site are given in Table 2. The top site is found to be the most stable adsorption site with the dipole vector parallel to the surface, with adsorption energy of 0.36 eV, while each of the other sites is strongly disfavored with adsorption energies of less 0.2 eV, and the fcc hollow site is the least favorable on. These results are consistent with the reported theoretical and experimental results of other metals [30,34–36]. The strongest interaction of water with the Co atom in the surface at the top is also supported by the shortest distance between water oxygen and the Co (dO–surface). In addition, the O–H bonds of the adsorbed molecule are 0.01 (top site) and 0.004 Å (other adsorption sites) longer than those of the gas phase water molecule, respectively, reflecting their weakening due to bonding with the surface. From Table 2, it can be seen that when water molecules are adsorbed on the surface, the interlayer distance d12 for the top site is increased, and ones for the other sites decreased, compared to the respective clean surfaces. This shows that for the top, the interactions of the surface and sub-layer atoms become weaker, decreasing the stability of Co atoms in the surface against dissolution. For the Co–Cr alloy, because of the presence of Cr in the sublayer, additional adsorption sites are identified on the {0 0 0 1} surface. Two top sites, three bridge sites, two hcp hollow sites, and two fcc hollow sites are investigated in this work, as shown in Fig. 1. The optimized structural parameters and adsorption energies at various sites are given in Table 3. Our calculations indicate
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B.L. Lü et al. / Computational Materials Science 68 (2013) 206–211 Table 2 Water molecule adsorption energies (Eads) on the pure Co {0 0 0 1} and optimized structural parameters for each adsorption site and gas-phase water molecule. O location
Eads (eV)
Interlayer distance d12 (Å)
Bond length (Å) dO–surface
dO–H
Top Bridge fcc hcp Clean surface Gas phase water
0.36 0.18 0.15 0.16 – –
1.969 1.960 1.961 1.962 1.965 –
2.181 2.830 2.711 2.841 – –
0.988 0.982 0.982 0.982 – 0.978
It may be concluded that the adsorbed water molecule decrease the stability of Co atoms in the metal and alloy surface. However, the Co–Cr alloy surface still shows enhanced stability with reference to pure Co under 0.25 ML of water: The potential shift of Co–Cr alloy is 0.05 V higher than one of pure Co. The positive values of the shifts mean that the dissolution of Co atoms from Co–Cr surfaces is delayed to higher potentials than that from the pure Co metal. This indicates that the Co–Cr alloy surface is more stable against dissolution in comparison with the pure Co surface under 0.25 ML of adsorbed water. The value of the potential shift is 0.077 V for the surface of Co–Cr alloy, referenced to its electrode potential in the absence of water adsorption. This value is higher 0.014 V than that (0.091 V) for the surface of pure Co metal referenced to its electrode potential without water adsorption, showing that the electrochemical corrosion property of the Co–Cr alloy is insensitive to water molecular adsorption relative to the pure Co metal.
Fig. 1. Adsorption sites on Co–Cr {0 0 0 1}, Black – Co, Grey – Cr.
Table 3 Water molecule adsorption energies (Eads) on Co–Cr {0 0 0 1} and optimized structural parameters for each adsorption site as illustrated in Fig. 1 and gas-phase water molecule. O location
Top1 Top2 Bridge1 Bridge2 Bridge3 fcc1 fcc2 hcp1 hcp2 Clean surface Gas phase water
Eads (eV)
0.21 0.25 0.08 0.13 0.07 0.04 0.13 0.06 0.07 – –
Interlayer distance d12 (Å)
1.954 1.946 1.940 1.943 1.944 1.944 1.942 1.945 1.943 1.945 –
Bond length (Å) dO–surface
dO–H
2.208 2.164 2.904 2.863 2.867 2.737 2.700 2.873 2.727 – –
0.986 0.988 0.981 0.982 0.982 0.982 0.983 0.982 0.982 – 0.978
that the top site are still the most favorable with the dipole vector parallel to the surface, with the largest adsorption energy (0.25 eV), the shortest dO–surface, the longest dO–H and the largest interlayer distance, analogous to pure Co {0 0 0 1} surfaces. Introducing Cr atoms into the Co {0 0 0 1} system considerably alter the surface adsorption properties: the water adsorption energy on the alloy surface is much weaker 0.11 eV than one on the pure metal surface.
3.3.2. Electrochemical potential shift under adsorbed water Now, we investigate the potential shifts of pure Co and Co–Cr alloy surface for the Co2+ + 2e = Co electrode reaction under 0.25 ML of adsorbed water. For each super cell, removal of the Co atom in the surface would produce a new slab with the lowest total energy. According to Table 1, our calculations result in a negative electrode potential shift (0.091 and 0.042 V) for both the pure Co and Co–Cr alloy, with respect to the clean pure Co surface, indicating that the surfaces are less stable than the pure Co surface without water adsorption.
3.3.3. Hydroxyl adsorption In aqueous corrosion reaction, water usually dissociates and oxides metal or alloy surface. The splitting of water can lead to the formation of adsorbed hydroxyl and concomitant proton production. In this work, for simplicity, the effect of hydroxyl dissociation to yield adsorbed atomic oxygen is not considered [8]. In the following dissolution analyses, we incorporate the electrochemical stability of pure Co {0 0 0 1} and Co–Cr {0 0 0 1} alloy surfaces under 0.25 ML of adsorbed hydroxyl. The four possible highly symmetric adsorption positions for hydroxyl on the Co {0 0 0 1} surface: on-top, bridge, fcc and hcp hollow site are considered as initial ones. The optimized structural parameters and adsorption energies at each site are given in Table 4. It can be seen that on the pure Co {1 1 1}, hydroxyl prefers the threefold site (fcc hollow one) with the axis of the molecule perpendicular to the surface, with the largest adsorption energy of 4.29 eV (the hcp hollow site 4.13 eV, the top site 3.56 eV). The optimization result of the initial bridge site deviates from the bridge site and coincides with the fcc hollow site, thus the bridge site does not appear in Table 4. Not like weak water adsorption, the adsorption of hydroxyl to cobalt is strong due to the tendency for the undercoordinated oxygen atom to pair up with the surface atoms, thus forming a chemical bond.
Table 4 Hydroxyl groups adsorption energies (Eads) on the pure Co {0 0 0 1} and optimized structural parameters for each adsorption site and free hydroxyl. O location
Eads (eV)
Interlayer distance d12 (Å)
Top fcc hcp Clean surface Free hydroxyl
3.56 4.29 4.13 – –
1.978 1.984 1.984 1.965 –
Bond length (Å) dO–surface
dO–H
1.824 1.307 1.315 – –
0.984 0.977 0.978 – 0.988
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Similar to the water adsorption, for the strongest interaction of hydroxyl with the Co atom in the surface (at the fcc hollow site), the distance between hydroxyl oxygen and the surface (dO–surface) is the shortest and the interlayer distance d12 is the largest compared to other adsorption sites. However, in contrast to water adsorption, at all the adsorption sites the O–H bonds of the adsorbed hydroxyl is shorter than that of the free one and that at the fcc hollow site is the shortest, reflecting its strengthening due to bonding with the surface. The increasing of the interlayer distance d12 upon adsorption compared to the clean surface show that the interactions of the surface and sub-layer atoms become weaker, decreasing the stability of Co atoms in the surface against dissolution. For hydroxyl adsorption on the Co–Cr {0 0 0 1} surface, similar to the water adsorption, the nine initial adsorption positions: two top sites, three bridge sites, two hcp hollow sites, and two fcc hollow sites are investigated, as shown in Fig. 1. The optimized structural parameters and adsorption energies at various sites are given in Table 5. Analogous to pure metal, the optimization result of all the initial bridge sites deviate from the bridge sites and coincide with the fcc hollow sites, thus the bridge sites are not in Table 5. We see that the fcc hollow site are still the most favorable, with the largest adsorption energy (4.16 eV), the shortest dO–surface, the shortest dO–H, and the largest d12 like pure Co {0 0 0 1} surfaces. Similar to the water adsorption, Cr atoms in the Co–Cr alloy considerably change the surface adsorption properties: the hydroxyl adsorption energy on the alloy surface is much weaker 0.13 eV than one on the pure metal surface. 3.3.4. Electrochemical potential shift under adsorbed hydroxyl The calculation results of the potential shifts of Co atoms on pure Co and Co–Cr alloy surface for the Co2+ + 2e = Co electrode reaction under 0.25 ML of adsorbed hydroxyl are given in Table 1. We see a negative electrode potential shift (0.299 and 0.309 V) for both the pure Co and Co–Cr alloy with respect to the clean pure Co surface, indicating that the surfaces are less stable than the pure Co surface without hydroxyl adsorption. Similar to the water adsorption, the adsorbed hydroxyl decrease the stability of Co atoms in the metal and alloy surface. However, contrary to the water adsorption, the pure Co metal surface shows enhanced stability with reference to Co–Cr alloy under 0.25 ML of hydroxyl: the potential shift of Co–Cr alloy is 0.1 V lower than one of pure Co. This indicates that the pure Co surface is more stable in comparison with the Co–Cr alloy surface under 0.25 ML of adsorbed hydroxyl. For the Co–Cr alloy surface, the value of the potential shift is 0.344 V referenced to its electrode potential in the absence of hydroxyl adsorption. This value is lower 0.045 V than that (0.299 V) for the pure Co metal surface referenced to its electrode potential without hydroxyl adsorption, showing that contrary to the results of the water adsorption, the electrochemical corrosion property of the Co–Cr alloy is sensitive to hydroxyl adsorption relative to the pure Co metal.
Table 5 Hydroxyl adsorption energies (Eads) on the Co–Cr {0 0 0 1} surface and optimized structural parameters for each adsorption site and free hydroxyl. O location
Top1 fcc1 fcc2 hcp1 hcp2 Clean surface Free hydroxyl
Eads (eV)
3.44 4.16 3.77 4.02 3.99 – –
Interlayer distance d12 (Å)
1.958 1.962 1.938 1.962 1.958 1.945 –
Bond length (Å) dO–surface
dO–H
1.820 1.339 1.477 1.390 1.433 – –
0.983 0.976 0.978 0.978 0.980 – 0.988
4. Summary and conclusions In conclusion, our calculated results show that Ni and Cu segregates towards the surface of the host, while Ti, V, Cr, Mn, and Fe prefer to remain in the interior of the host (antisegregation). Cr, Fe, Mn, and V make the surface Co atoms more stable in the {0 0 0 1} surfaces of the corresponding alloys than they are in pure Co {0 0 0 1} surfaces, whereas Ni and Cu make Co much less stable against dissolution. Among all the considered alloying elements, chrome is the most beneficial to the stability enhancement of alloys. The most favorable adsorption site for a water molecule on the pure Co {0 0 0 1} and Co–Cr {0 0 0 1} surfaces is on top of Co atoms, with the dipole vector parallel to the surface, with the highest adsorption energy, the shortest dO–surface, the largest interlayer distance d12 and the longest dO–H. For a hydroxyl group on the pure Co {0 0 0 1} and Co–Cr {0 0 0 1} surfaces, the most favorable adsorption site is at the fcc hollow ones, with the axis of the molecule perpendicular to the surface, with the highest adsorption energy, the largest interlayer distance d12, the shortest dO–surface and the shortest dO–H. Introducing Cr atoms into the Co {0 0 0 1} system considerably alters the surface adsorption properties. The water or hydroxyl adsorption energy on the alloy surface is much weaker than one on the pure metal surface. It is found that the adsorbed water molecule or hydroxyl group decreases the stability of Co atoms in the metal and alloy surface. However, the Co–Cr alloy surface still shows enhanced stability with reference to pure Co under 0.25 ML of water, while under 0.25 ML of hydroxyl, the pure Co surface is more stable than the Co–Cr alloy. Our calculations also revealed that the electrochemical corrosion property of the Co–Cr alloy is insensitive to water molecular adsorption and sensitive to hydroxyl adsorption in comparison with the pure Co metal. Acknowledgments Financial support from the National Basic Research Program of China (973 Program, Grant No. 2009CB724305) and Program for New Century Excellent Talents in University (No. NCET-10-0278) is gratefully acknowledged. References [1] J.R. Davis, ASM Specialty Handbook: Nickel Cobalt, and Their Alloys, ASM International, Materials Park, OH, 2000. [2] A. Petric, K.T. Jacob, Metall. Mater. Trans. A 16 (1985) 503–510. [3] G.Y. Lai, Resistance to carburization of various heat-resistant alloys, in: M.F. Rothman (Ed.), High-Temperature Corrosion in Energy Systems, Proceedings of the TMS-AIME Symposium, Metallurgical Society of AIME, Warrendale, PA, 1985, p. 551. [4] J. Chen, X.Y. Li, T. Bell, H. Dong, Wear 264 (2008) 157–165. [5] Y. Yan, A. Neville, D. Dowson, S. Williams, Tribol. Int. 39 (2006) 1509–1517. [6] I. Milosev, H.H. Strehblow, Electrochim. Acta 48 (2003) 2767–2774. [7] G.P.M. Leyson, W.A. Curtin, L.G. Hector, C.F. Woodward, Nat. Mater. 9 (2010) 750–755. [8] J. Greeley, J.K. Nørskov, Electrochim. Acta 52 (2007) 5829–5836. [9] Y.G. Ma, P.B. Balbuena, J. Phys. Chem. C 112 (2008) 14520–14528. [10] Y.G. Ma, P.B. Balbuena, J. Electrochem. Soc. 157 (2010) B959–B963. [11] G.E. Ramirez-Caballero, Y.G. Ma, R. Callejas-Tovar, P.B. Balbuena, Phys. Chem. Chem. Phys. 12 (2010) 2209–2218. [12] J. Greeley, Electrochim. Acta 55 (2010) 5545–5550. [13] X.P. Wang, R. Kumar, D.J. Myers, Electrochem. Solid State Lett. 9 (2006) A225– A227. [14] K. Yasuda, A. Taniguchi, T. Akita, T. Ioroi, Z. Siroma, Phys. Chem. Chem. Phys. 8 (2006) 746–752. [15] C.D. Taylor, M. Neurock, J.R. Scully, J. Electrochem. Soc. 155 (2008) C407. [16] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045–1097. [17] D. Vanderbilt, Phys Rev. B: Condens. Matter Mater. Phys. 41 (1990) 7892– 7895. [18] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868. [19] H.J. Monkhorst, J.D. Pack, Phys Rev. B: Condens. Matter Mater. Phys. 13 (1976) 5188–5192.
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