A DFT study on corrosion mechanism of steel bar under water-oxygen interaction

Computational Materials Science 171 (2020) 109265

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A DFT study on corrosion mechanism of steel bar under water-oxygen interaction

T

Zheng Chena, Yumei Nonga, Jianhua Chenb, , Ye Chenb, , Bo Yua ⁎

⁎

a

Key Laboratory of Disaster Prevention and Structural Safety of China Ministry of Education, School of Civil Engineering and Architecture, Guangxi University, Nanning 530004, China b Guangxi Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China

ARTICLE INFO

ABSTRACT

Keywords: Reinforcement corrosion Iron surface Adsorption Density functional theory (DFT)

Corrosion of steel bar is a major problem influencing the long-term performance of reinforced concrete structures. It is known that both oxygen and water involve in the initiation of corrosion of steel bars and determine the formation of the final corrosion products. In this paper, the interaction of oxygen and water on the Fe (1 0 0) surface have been simulated by using density functional theory (DFT), to study the mechanism of steel bar corrosion and the formation process of the corrosion product (FeOOH). The results show that the preadsorbed water on the iron surface has little effect on the O2 adsorption, but the adsorption of O2 weakens the interaction between H2O and iron surface. On the other hand, the preadsorbed oxygen strengthens the adsorption of H2O via the formation of hydrogen bond between the O atom of O2 and the H atom of H2O. Strong interaction of oxygen and water promotes the dissociation of O2 and H2O molecules on the Fe (1 0 0) surface, and forms a Fe-O and two Fe-OH groups on the iron surface. Moreover, it is found that the Fe-OH group is unstable, and reacts easily with the newly introduced O atom to form the FeOOH, which confirms that FeOOH is the main corrosion product of steel bar instead of Fe-OH. This work could provide a microscopic insight into the mechanism of reinforcement steel corrosion.

1. Introduction

1 Cathodic reaction: O2 + H2 O + 2e 2

The reinforced concrete structure was widely used in many structures such as bridges, buildings, viaducts, dikes, submarine tunnels, large offshore platforms and so on, and the reinforcement steel is an important part of reinforced concrete structure. The corrosion of reinforcement is one of the main factors affecting mechanical property [1–3], durability [1,4] and safety [1,5] of reinforced concrete structure. Therefore, reinforcement corrosion has been highly concerned in the field of corrosion and civil engineering materials. It is widely reported that the presence of moisture and oxygen supports the corrosion [1,6,7,9], and moisture and oxygen together help in the formation of more OH− thereby producing more rust component [1]. The essence of reinforcement corrosion is the electrochemical process [1,8,10], in which the actual dissolution of iron takes place at the anode, and two electrons of iron are set free. At the steel surface, oxygen and water react together, as cathode, obtain the electrons to form OH− ions. The reaction can be expressed by the following equations:

Currently, the electrochemical mechanism of reinforcement corrosion have been studied widely via the electrochemical methods including corrosion potential measurements [9,10], linear polarization resistance measurements (LPR) [11,12] and electrochemical impedance spectroscopy (EIS) [13,14]. These studies have shown that an increase in the concentration of O2 around the steel bar may lead to an increase in the corrosion potential of the steel bar, accelerate the corrosion current density, and cause corrosion easier to occur [10]. Moreover, the content of moisture will have the effect of reducing the electrical resistivity, making charge transfer easier [11]. Electrochemical methods have focused on the effects of oxygen and water on the potential difference between two electrodes, which can roughly reveal the enhanced corrosion mechanism at the macroscopic scale, but it is difficult to reveal the corrosion mechanism at the microscopic level. In order to obtain more microscopic information of chemical and physical changes on the iron surface, the oxygen and water adsorption on the iron surface have been studied using several analytical techniques including Angle-

Anodic reaction:Fe

⁎

2e

Fe 2 +

(1)

Corresponding authors. E-mail addresses: [email protected] (J. Chen), [email protected] (Y. Chen).

https://doi.org/10.1016/j.commatsci.2019.109265 Received 10 May 2019; Received in revised form 31 August 2019; Accepted 3 September 2019 0927-0256/ © 2019 Elsevier B.V. All rights reserved.

2OH

(2)

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Fig. 1. Adsorption configurations for a single O2 molecule on the Fe (1 0 0) surface. The map is shown in lateral view. Numbers are bond lengths in angstroms (Å). Eads (kJ/mol) represents the adsorption energies of the optimized configurations.

resolved photoemission spectroscopy (ARPES) [15,16], low-energy electron diffraction (LEED) [17,19–21,24,27], Auger electron spectroscopy (AES) [19–21,27], scanning tunneling microscopy (STM) [22,23], electron energy loss spectroscopy (EELS) [18,19,24], spin-polarized secondary electron emission (SP-SEE) [25,26] and high resolution electron energy loss spectroscopy spectrum (HREELS) [24,28]. However, the bonding information of adsorbate-surface is usually missing by these experimental methods, which makes it difficult to achieve a better understanding of the mechanism of iron corrosion under the oxygen and water interaction at the molecular/atomic level. Density functional theory (DFT) is a powerful tool in quantum chemistry calculations, which has been widely used to study the reaction mechanisms at molecular/atomic scale [29], such as the adsorption of oxygen or water on iron surface. For the adsorption of O2 on iron surface, P. Bloński et al. [30,31] investigated the electronic, magnetic and structural properties of atomic and molecular oxygen adsorbed on the Fe (1 0 0) surface by using density functional theory (DFT) combined with a thermodynamics formalism, and reported that the dissociation of O2 occurs on the Fe (1 0 0) surface. Tan et al. [32] studied the O absorption on Fe (1 1 0) by considering not only the change of atomic and electronic structures of the substrate induced by O adsorption but also of the electronic properties of adsorbate O itself. For the adsorption of H2O on the iron surface, Jung et al. [33] studied the single water molecule adsorption on the Fe (1 0 0) surface by using DFT, indicating a weak molecule-surface interaction, which is consistent

with the results of Freitas et al. [34]. Eder et al. [35] investigated the initial stages of the oxidation of the Fe (1 0 0) and Fe (1 1 0) surfaces by the adsorption of water by using ab initio local-spin-density functional calculations. The above studies mainly focused on the adsorption of oxygen or water on iron surface, and there are few reports about the simulation of oxygen and water co-adsorbed on the iron surface. For instance, Hung et al. [24] used the experimental methods to study the adsorption of water on oxygen pre-exposed iron surface and explain the interaction of water and preadsorbed oxygen. In addition, Wang et al. [36] confirmed that the presence of pre-adsorbed oxygen atom could significantly promote H2O dissociation on the iron surface by DFT. However, no similar research has been found on the simulation of iron corrosion mechanism under the reaction of water and oxygen molecules. In this paper, the adsorption of single H2O molecule, single O2 molecule and coadsorption of water and oxygen on the Fe (1 0 0) surface at 0.25 ML coverage for each adsorbate were simulated by density functional theory (DFT). The interaction mechanism of water and oxygen on the iron surface and the formation process of FeOOH (main product of steel bar corrosion [37,48–51]) were studied at the atomic scale. The results could give an insight into the nature of iron corrosion and help to explain the subsequent interfacial reactions for the corrosion of reinforcement.

Table 1 Mulliken charge of O atoms and Fe atoms on the Fe (1 0 0) surface before and after dissociated O2 adsorption.

All calculations were performed in the framework of Cambridge Serial Total Energy Package (CASTEP) developed by Payne et al. [38]. The electron-correlation functional applied was the generalized gradient approach (GGA) of PW91 [39–41] to study the adsorption of O2,

Atom O1 O2 Fe4 Fe2 Fe1 Fe3

Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption

s

p

d

Total

Charge (e)

1.88 1.90 1.87 1.90 0.80 0.58 0.80 0.57 0.80 0.58 0.80 0.57

4.13 4.64 4.12 4.66 0.60 0.53 0.62 0.55 0.63 0.52 0.62 0.49

0.00 0.00 0.00 0.00 6.57 6.53 6.56 6.49 6.56 6.55 6.56 6.54

6.00 6.54 6.00 6.56 7.97 7.64 7.98 7.61 7.98 7.65 7.98 7.60

0.00 −0.54 0.00 −0.56 0.03 0.36 0.02 0.39 0.02 0.35 0.02 0.40

2. Computational methods

Table 2 Bonds length of interacting Fe atoms with the adjacent Fe atoms before and after O2 adsorption. Bond

Fe5-Fe1 Fe5-Fe2 Fe5-Fe3 Fe5-Fe4

The atomic label is shown in Fig. 1(e).

Length (Å) Before

After

2.4166 2.4064 2.4064 2.3898

2.4871 2.5178 2.4546 2.4145

The atomic label is shown in Fig. 1(e).

2

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Fig. 2. PDOS of O2 atom of O2 molecule and Fe4 atom of iron surface before and after O2 adsorption on Fe (1 0 0) surface.

340 eV was used throughout, and the Brillouin zone was sampled with Monkhorst and Pack special k-points of a 4 × 4 × 1 grid for all electronic structure calculations [43]. The testing results indicate that the cut off energy and the k-points mesh are sufficient for the present system. For self-consistent electronic minimization, the Pulay Density Mixing method was employed and with the convergence tolerance of 2.0 × 10−6 eV/atom. The convergence criteria for structure optimization and energy calculation were set to: (a) energy tolerance of 2.0 × 10−5 eV/atom, (b) maximum force tolerance of 0.03 eV/Å, and (c) maximum displacement tolerance of 0.001 Å. The transition states (TS) of all reaction pathways were searched by using the complete LST/ QST method [44]. A linear synchronous transit (LST) optimization was performed first, and then quadratic synchronous transit (QST) maximization was used to obtain TS approximation. From the maximization point, another conjugate gradient minimization was performed. The cycle is repeated until a stationary point was located. The Fe (1 0 0) surface, as a simple low-index face, has long been used as a prototypical substrate for studying the interaction of water and oxygen molecules with iron surfaces [30,33]. Surface was cleaved on the basis of the optimized bulk structure. We constructed a (2 × 2) Fe (1 0 0) surface with 5 atomic layers and 15 Å vacuum slab. The two outermost atomic layers of the substrate were allowed to relax while the three bottom-most atomic layers of the substrate were fixed to the bulk coordinates in the adsorption calculations. The computed lattice parameter for the bulk iron is 2.812 Å, which is closed to the experimental value of 2.866 Å [45].

H2O, and H2O/O2 on the iron surface. Spin-polarized basis set were considered in all calculations. Valence electron configurations considered in the study included Fe 3d64s2, O 2s22p4 and H 1s1. The interactions between valence electrons and ionic core were represented by ultrasoft pseudopotentials [42]. The plane-wave cut off energy of

Fig. 3. PDOS of Fe4-O2 bond formed after O2 adsorption on Fe (1 0 0) surface.

3

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Fig. 4. Adsorption configurations for a single H2O molecule and dissociated H2O on the Fe (1 0 0) surface. The map is shown in lateral view. Numbers are bond lengths in angstroms (Å). Eads (kJ/mol) represents the adsorption energies of the optimized configurations.

The optimizations of O2 and H2O molecules were performed in a 10 × 10 × 10 Å cubic cell, respectively. The calculated d O-H and ∠H-O-H of optimized H2O are 0.975 (0.978) Å and 104.87° [46], respectively, which are closed to the experimental values of 0.958 Å and 104.5°. For the optimization of O2, the calculated d O-O is 1.24 Å, which is approximate to the experimental value of 1.209 Å [47]. Adsorption energy can be expressed by the following equation:

3. Results and discussion 3.1. Adsorption of single O2 molecule on the clean Fe (1 0 0) surface

where ETS is the energy of transition state, EIS is the energy of reactant (initial state) and EFS is the energy of product (final state).

The adsorption configurations and adsorption energies are shown in Fig. 1(a)–(f). The adsorption energies of O2 molecule parallel to the top, bridge and hollow sites are −190.65 kJ/mol, −268.67 kJ/mol and −532.56 kJ/mol, respectively, and the adsorption energies of O2 molecule vertical to the top, bridge and hollow sites are −139.63 kJ/mol, −149.09 kJ/mol and −176.25 kJ/mol, respectively. It is noted that all adsorption energies are negative and with great absolute value, indicating that O2 molecule could easily adsorb on the clean Fe (1 0 0) surface. In addition, the adsorption energy for O2 adsorbed in parallel to the hollow site is the lowest (−532.56 kJ/mol), suggesting that this site is more favorable for adsorption. It is found from Fig. 1(e) that the two O atoms of O2 molecule interact with the four surface Fe atoms, forming two Fe-O bonds with lengths of 1.775, 1.782, 1.819 and 1.795 Å, respectively. Moreover, the distance of O–O bond is 2.801 Å, which is much longer than the bond length of O2 molecule (1.24 Å), indicating that a complete dissociation of oxygen occurs on the Fe (1 0 0) surface. The Mulliken charge of adsorbed O atoms and Fe atoms on the Fe (1 0 0) surface are listed in Table 1. After adsorption, the Mulliken charge of O 2s and 2p orbitals both increase, in which that of O 2p

Table 3 Mulliken charge of H atoms, O atom and Fe atoms on the Fe (1 0 0) surface before and after dissociated H2O adsorption.

Table 4 Bonds length of interacting Fe atoms with the adjacent Fe atoms before and after dissociated H2O adsorption.

Eads = Eadsorbates / surface

Esurface

(3)

Eadsorbates

where Eads is the adsorption energy, Eadsorbates/surface is the total energy of the surface with adsorbate molecules, Esurface is the total energy of iron surface, and Eadsorbates is the energy of water or oxygen molecules which are calculated in the cell with the same cell size and k-point grid as used in the surface. The energy barrier (Eb) and reaction energy (Er) of the reaction pathway is defined as follow:

Eb = ETS

EIS

(4)

Er = EFS

EIS

(5)

Atom H1 H2 O Fe4 Fe3

Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption

s

p

d

Total

Charge (e)

0.47 1.29 0.48 0.59 1.89 1.84 0.80 0.54 0.80 0.55

0.00 0.00 0.00 0.00 5.16 4.97 0.60 0.60 0.62 0.59

0.00 0.00 0.00 0.00 0.00 0.00 6.57 6.53 6.56 6.56

0.47 1.29 0.48 0.59 7.05 6.81 7.97 7.66 7.98 7.70

0.53 −0.29 0.52 0.41 −1.05 −0.81 0.03 0.34 0.02 0.30

Bond

Fe3-Fe7 Fe3-Fe6 Fe4-Fe7 Fe4-Fe6 Fe4-Fe5 Fe4-Fe8 Fe3-Fe5 Fe3-Fe8

Length (Å) Before

After

2.3998 2.4056 2.3952 2.3925 2.3898 2.3925 2.4064 2.4006

2.4993 2.5530 2.5191 2.5798 2.6854 2.6216 2.5561 2.5013

The atomic label is shown in Fig. 4(f).

The atomic label is shown in Fig. 4(f).

4

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Fig. 5. PDOS of O, H1 atoms of H2O and Fe4 atom of iron surface before and after dissociated H2O adsorption on Fe (1 0 0) surface.

orbital increases more obviously, indicating that the O 2p orbital obtains more electrons. It is known from the molecular orbital of O2, KK( 2s ) 2 ( 2s ) 2 ( 2p) 2 ( 2p) 4 ( 2p)2 that the 2p orbital of O2 has a nearly vacant antibonding orbital 2p . The Mulliken charge of O1 and O2 atoms changes from 0 e to −0.54 e and −0.56 e, respectively. For Fe atoms, the Mulliken charge of Fe s, p, d orbitals all decrease. The interaction between O2 molecule and Fe (1 0 0) surface is via the Fe atoms donate electrons to the O atoms, and the bonds between Fe atoms and O atoms are ionic. The change of bonds length between interacting Fe atoms and the adjacent Fe atoms before and after O2 adsorption are listed in Table 2. It is found that the lengths of these Fe-Fe bonds become elongated, suggesting that the adsorption of O2 molecule weakens the bond between surface iron atoms. Fig. 2 shows the partial density of states (PDOS) of the interacted O (labeled as O2) and Fe ((labeled as Fe4) atoms before and after O2 adsorption on Fe (1 0 0) surface, in which the α and β represent the spin-up and spin-down, respectively. After adsorption, it is noticed from Fig. 2 that the O2 atom changes from a low spin state to a spin-polarization state, and the 2p orbital of O2 atom shifts to the lower energy and become delocalization. The DOS for Fe4 atom shifts to the higher energy part and the state of Fe 3d peak broadens in the range of −5 to 0 eV, which indicates that Fe4 atom involves in the interaction with O2 molecule. Fig. 3 shows the PDOS of Fe4-O2 bond after O2 adsorption on Fe (1 0 0) surface. The bonding states between Fe 3d and O 2p orbitals are located in the range of −6 to −4.5 eV and the antibonding states are located in the range of −4.5 to −0 eV.

3.2. Adsorption of single H2O molecule on the clean Fe (1 0 0) surface The optimized configurations are shown in Fig. 4(a)–(f). The adsorption energies of H2O molecule parallel to the top, bridge and hollow sites are −35.16 kJ/mol, −27.68 kJ/mol and −23.49 kJ/mol, respectively, and the adsorption energies of H2O molecule vertical to the top and bridge sites are −26.43 kJ/mol and −4.72 kJ/mol, respectively. These results show that the most stable adsorption site for molecular water is parallel to the top site of Fe (1 0 0) surface, as shown in Fig. 4(a). It is found that the calculated adsorption energies of molecular water at different sites are rather small, suggesting a relatively weak interaction between molecular water and Fe (1 0 0) surface, which is in agreement with the investigation of Jung et al. [35]. The adsorption energy of the dissociated water on the Fe (1 0 0) surface is −101.66 kJ/mol, which indicates that the adsorption of dissociated water is stronger than molecular water. It is noted from Fig. 4(f) that the dissociated water H1 atom interacts with two surface Fe atoms with the distances of 1.695 and 1.691 Å, and the O atom of OH group interacts with two surface Fe atoms with the distances of 1.949 and 1.983 Å. Here, hydrogen atoms in the dissociated water and OH group were labeled as H1 and H2, respectively. The Mulliken charge of dissociated H2O and the interacting Fe atoms are listed in Table 3. For dissociated water, the charge of H1 1s orbital increases from 0.47 e to 1.29 e, while that of H2 increases from 0.48 e to 0.59 e, and the charge of O 2p orbital decreases from 5.16 e to 4.97 e. For Fe atoms, the Mulliken charge of Fe 4s orbitals decrease. Surface Fe atoms donate electrons to the H1 atom. Meanwhile, 5

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Fig. 6. PDOS of Fe4-O, O-H2 and Fe4-H1 bonds formed after dissociated H2O adsorption.

electrons transfer from Fe atoms to OH group to form Fe-OH on the Fe (1 0 0) surface. Table 4 shows the bonds length of interacting Fe atoms with the adjacent Fe atoms before and after dissociated H2O adsorption. It is found that the lengths of Fe-Fe bonds all increase, indicating that the Fe-Fe bonds weaken after dissociated H2O adsorption. Fig. 5 shows the PDOS of O, H1 atom of H2O and Fe4 atom of surface before and after the dissociated H2O adsorption on Fe (1 0 0) surface. For O atom, the 2p orbital shifts to the lower energy, and the broadening of states of O 2p occurs upon adsorption, which indicates that the O atom from dissociated H2O interacts with iron surface. It is noted that the O atom remains a low spin state after adsorption. For H1

atom, the states of 1s orbital shift to the lower energy, and these states located on the conduction band become depleted, implying H1 atom receives electrons, which is agreement with the analysis of Mulliken charge. The change of the states of Fe 3d orbital are small, indicating that the interactions between surface Fe4 atom and the dissociated H, Table 5 Mulliken charge of H atoms, O atoms and Fe atoms on the water-preadsorbed Fe (1 0 0) surface before and after O2 adsorption. Atom H1 H2 O

(w)

O(IIO) O(IO) Fe4 Fe2 Fe1 Fe3

Fig. 7. Adsorption configuration of O2 molecule on the water-preadsorbed Fe (1 0 0) surface.

s

p

d

Total

Charge (e)

Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption

0.57 0.57 0.56 0.60 1.86 1.84 1.88

0.00 0.00 0.00 0.00 4.98 4.96 4.13

0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.57 0.57 0.56 0.60 6.85 6.80 6.00

0.43 0.43 0.44 0.40 −0.85 −0.80 0.00

After adsorption Before adsorption

1.88 1.87

4.77 4.12

0.00 0.00

6.65 6.00

−0.65 0.00

After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption

1.91 0.65 0.57 0.76 0.60 0.76 0.54 0.76 0.62

4.58 0.66 0.49 0.65 0.65 0.67 0.52 0.69 0.55

0.00 6.54 6.46 6.58 6.61 6.56 6.46 6.58 6.55

6.49 7.85 7.52 7.98 7.86 7.98 7.51 8.03 7.71

−0.49 0.15 0.48 0.02 0.14 0.02 0.49 −0.03 0.29

The atomic label is shown in Fig. 7.

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Table 6 Bonds length of relevant atoms before and after O2 adsorption on waterpreadsorbed Fe (1 0 0) surface. Bonds

O(w)-H2 O(w)-H1 Fe2-O(w) Fe4-Fe8 Fe2-Fe8 Fe3-Fe8 Fe1-Fe8

Length (Å) Before

After

0.9823 0.9823 2.2477 2.4127 2.4331 2.3827 2.4176

1.0150 0.9920 2.0567 2.5359 2.3614 2.3864 2.4769

The atomic label is shown in Fig. 7.

Fig. 9. Adsorption configuration of H2O molecule on the oxygen-preadsorbed Fe (1 0 0) surface.

OH group are weak. The PDOS of bonding atoms of the dissociated H2O and Fe (1 0 0) surface are shown in Fig. 6. It is found that the bonding and antibonding states of Fe4-O bond are composed of Fe 3d and O 2p orbitals, where bonding states locate between −6.5 and −4 eV and the antibonding states locate in the range of −4 to 0 eV. The bonding states of Fe4-H1 bond derive from the contribution of H 1s and Fe 3d orbitals, and the bonding and antibonding states of Fe4-H1 bond are much weaker than that of Fe4-O bond, suggesting that the interaction between Fe4 and H1 atoms is weaker than Fe4 and O atoms.

been conducted, as well as, the possible products of simultaneous coadsorption of H2O and O2 on the Fe (1 0 0) surface were considered. The optimized configuration of O2 molecule adsorbed on the waterpreadsorbed Fe (1 0 0) surface is shown in Fig. 7, in which the O atom in H2O was labeled as O(w) and two O atoms in O2 molecule were labeled as O(IO) and O(IIO) , respectively. The adsorption energy of O2 molecule adsorbed on the water-preadsorbed Fe (1 0 0) surface is −562.81 kJ/ mol, which is similar to that on the clean Fe (1 0 0) surface (−532.56 kJ/mol). Moreover, the dissociation of O2 molecule also occurs on the water-preadsorbed Fe (1 0 0) surface. The results suggest that the preadsorbed H2O molecule has little effect on the O2 adsorption on the Fe (1 0 0) surface. The Mulliken charge of H atoms, O atoms and Fe atoms on the water-preadsorbed Fe (1 0 0) surface before and after O2 adsorption are shown in Table 5. It is found that the Mulliken charge of H2 1s orbital, O(IO) 2p orbital and O(IIO) 2p orbital all increase, indicating that these atoms obtain the electrons, while the interacting Fe

3.3. Effect of interaction of water and oxygen on the corrosion of iron surface In order to investigate the effect of interaction of water and oxygen on the corrosion of iron surface, the simulations of the adsorption of O2 molecule on the water-preadsorbed Fe (1 0 0) surface and the adsorption of H2O molecule on the oxygen-preadsorbed Fe (1 0 0) surface have

Fig. 8. PDOS of Fe4-Ow bond before and after O2 adsorption on water-preadsorbed Fe (1 0 0) surface.

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The adsorption of H2O molecule on oxygen-preadsorbed Fe (1 0 0) surface has also been carried out and the optimized configuration is shown in Fig. 9. The calculated adsorption energy is −51.02 kJ/mol, which is slightly more negative than that on clean Fe (1 0 0) surface (−35.16 kJ/mol). Compare with the water adsorption on the clean and oxygen-preadsorbed Fe (1 0 0) surface, it is suggested that the distance between water O atom and surface Fe atom on the water-preadsorbed surface is 2.310 Å, which is slightly longer than that of on the clean surface (2.248 Å). Meanwhile, a hydrogen bond is forming between the adsorbed O atom and water H atom, thereby strengthening the adsorption of H2O molecule on the oxygen-preadsorbed Fe (1 0 0) surface. The Mulliken charge of H atoms, O atoms and Fe atoms on the oxygen-preadsorbed Fe (1 0 0) surface are listed in Table 7. It is noted that the Mulliken charge of H 1s orbitals increase, while that of water O(w) 2p orbital decreases, which indicates that H atoms obtain electrons and water O(w) atom loses electrons. On the other hand, the charges of 2p orbitals of O(IO) and O(IIO) atoms both increase, indicating that those atoms obtain electrons after H2O adsorption. For the surface Fe atoms, Fe4 and Fe1 atoms lose electrons, while Fe3 and Fe2 atoms obtain electrons. The bonds length of these atoms are listed in Table 8, it is shown that the lengths of bonds O(w)-H1 and O(w)-H2 both increase, indicating that the two O–H bonds become weaker. Fig. 10 shows PDOS of Fe 4 O(IIO) bond before and after H2O adsorption on oxygen-preadsorbed Fe (1 0 0) surface. It is found that the 3d orbitals of Fe4 atom become broaden in contract to that of clean surface Fe atom, indicating that Fe atom involves in the interaction with O2 molecule and H2O molecule. After H2O adsorption, the change of bonding states of Fe 4 O(IIO) is small, implying that the adsorption of H2O has little effect on Fe 4 O(IIO) bond. Moreover there is a weak bonding between Fe4 and Ow locates at −4.5 to −4 eV. In order to further investigate the effect of the interaction of H2O and O2 molecule on the corrosion of iron surface, the possible products of coadsorption of O2 and H2O on the Fe (1 0 0) surface were simulated considering the dissociation of H2O into H and OH groups, as well as OH dissociates into O and H groups, and the decomposition of O2 molecule into two O radicals. All possible dissociation reaction pathways of coadsorption of H2O and O2 on Fe (1 0 0) surface are listed in Table 9. The adsorption energies for all products were calculated. Since the optimized configurations for Path 4, Path 5 and Path 7 are similar, the final products of the seven paths can be divided into five configurations as shown in Fig. 11. It is noted that the adsorption energies of the five final product configurations are all negative, indicating that these products are likely to be formed. Among them, the third type of products (Fig. 11(c)) has the most negative adsorption energy (−594.81 kJ/mol), indicating that the third type of products is the most energetically favorable and the formation process of the most stable products was investigated. It can be seen from Fig. 11(c) that after coadsorption of water and oxygen on the Fe (1 0 0) surface, the water molecule dissociates into H and OH groups, and the oxygen molecule dissociates into two O atoms, and then H atom in H2O molecule combines with O atom in O2 molecule to form an OH group, which interacts with a surface iron atom to form a Fe-OH (labeled as Fe2 O(IIO) H 2 ). Meanwhile, the OH radical dissociated from water molecule bonds with a surface Fe atom to form a Fe-OH group (labeled as Fe4-O (w)H1). In addition, the oxygen O atom bonds with two iron atoms to form a Fe-O group (labeled as Fe1 O(IO ) Fe3). The Mulliken charge of H atoms, O atoms and the interacted Fe atoms before and after H2O and O2 coadsorption on Fe (1 0 0) surface are listed in Table 10. For the Fe4-O(w)H1 group, the charge of Fe4 is changed from 0.03 e to 0.37 e, and the charge of O (w) is changed from −1.05 e to −0.77 e and that of H1 is varied from 0.53 e to 0.43 e, which suggests that the surface Fe4 atom donates electrons to the water

Table 7 Mulliken charge of H atoms, O atoms and Fe atoms on the oxygen-preadsorbed Fe (1 0 0) surface before and after H2O adsorption. Atom H1 H2 O

(w)

O(IIO) O(IO) Fe4 Fe3 Fe1 Fe2

s

p

d

Total

Charge (e)

Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption

0.47 0.55 0.48 0.56 1.89 1.90 1.90

0.00 0.00 0.00 0.00 5.16 4.67 4.66

0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.47 0.55 0.48 0.56 7.05 6.56 6.56

0.53 0.45 0.52 0.44 −1.05 −0.56 −0.56

After adsorption Before adsorption

1.89 1.90

4.70 4.64

0.00 0.00

6.59 6.54

−0.59 −0.54

After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption

1.86 0.58 0.52 0.57 0.59 0.58 0.57 0.57 0.59

5.00 0.53 0.55 0.49 0.56 0.52 0.53 0.55 0.56

0.00 6.53 6.44 6.54 6.54 6.55 6.50 6.49 6.55

6.86 7.64 7.52 7.60 7.68 7.65 7.61 7.61 7.7

−0.86 0.36 0.48 0.40 0.32 0.35 0.39 0.39 0.30

The atomic label is shown in Fig. 9. Table 8 Bonds length of relevant atoms before and after H2O adsorption on oxygenpreadsorbed Fe (1 0 0) surface. Bonds

Length (Å)

O(w)-H2 O(w)-H1 Fe 4 O(IIO)

Fe2

O(IIO)

Fe3

O(IO)

Fe1

O(IO)

Fe4-Fe5 Fe2-Fe5 Fe1-Fe5 Fe3-Fe5

Before

After

0.9779 0.9750 1.7945

0.9858 0.9802 1.9375

1.8189

1.7351

1.7820

1.7679

1.7752

1.8588

2.4145 2.5178 2.4871 2.4546

2.5346 2.4457 2.4880 2.4388

The atomic label is shown in Fig. 9.

atoms lose electrons after the adsorption of O2 molecule, which indicates that the water-preadsorbed Fe (1 0 0) surface can be oxidized by O2 molecule. The bonds length of these atoms before and after O2 adsorption are listed in Table 6. The O–H bonds of preadsorbed water are stretched after O2 adsorption due to the hydrogen bond between oxygen O and water H atoms. The lengths of Fe-Fe bonds all increase after O2 adsorption, indicating that the adsorbed O2 molecule weakens the surface Fe-Fe bonds. Fig. 8 shows the PDOS of Fe4-Ow bond before and after O2 adsorption on water-preadsorbed Fe (1 0 0) surface. It is found that the 3d orbitals of Fe2 and Fe4 atoms both broaden after O2 adsorption, indicating that Fe atoms take part in the interaction with adsorbed oxygen. After O2 adsorption, the Ow atom of H2O changes from a low spin state to a spin-polarization state, and the antibonding state of FeOw bond becomes stronger (in the range of −4.5 eV to 0 eV), which suggests that the adsorption of O2 molecule weakens the Fe-Ow bond. It could be predicted that the Fe-Ow bond will be more easily broken in further reactions. Moreover, O2 molecule interacts with surface Fe atom to form a Fe O(IIO) bond, in which the bonding states locate in the range of −6 to −4 eV and the antibonding states locate between −4 and 1 eV.

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Fig. 10. PDOS of Fe 4

O(IIO) bond before and after H2O adsorption on oxygen-preadsorbed Fe (1 0 0) surface.

Fe2 O(IIO) bonds, which belong to Fe-OH groups, the antibonding states of these two bonds locating between −5 eV and 0.5 eV are relative strong, while their bonding states in the range of −8 eV to −5 eV are very weak, which indicates that the Fe 4 O(w) and Fe2 O(IIO) bonds of Fe-OH groups are weaker than Fe3 O(IO) bond of Fe-O group, and the Fe-O group is more stable than Fe-OH group on the steel bar surface. The transition states searching of isolated H2O molecule and O2 molecule, as well as coadsorbed H2O-O2 react on the Fe (1 0 0) surface were performed, and the reaction steps are illuminated in Fig. 13. It can be found that O2 molecule dissociates into two O atoms via TS1 by overcoming a low energy barrier of 1.47 kJ/mol, indicating that O2 molecule dissociates easily on the Fe (1 0 0) surface, and the reaction energy is −461.65 kJ/mol. The length of OI(O) -OII (O) bond increases from 1.240 Å in initial O2 molecule to 1.277 Å in TS1. For H2O molecule, the dissociation of H2O molecule into H + OH via TS2 needs to overcome a high energy barrier of 110.46 kJ/mol, which indicates that H2O molecule is more difficult to dissociate on the Fe (1 0 0) surface compared to O2 molecule, and the reaction energy is −57.26 kJ/mol. As shown in Fig. 13(b), In TS2, the distance between O atom and H1 atom increases to 1.445 Å, which is longer than the bond length of O–H1 in H2O molecule (0.982 Å). For the co-adsobed H2O-O2, the reaction path of Fe + H + OH + O2 → 2FeOH + FeO which was the most stable pathway (path 3) obtained from Section 3.3 was considered. As shown in Fig. 13(c), on the basis of the H2O dissociation, the O2 molecule dissociates and reacts with surface iron atoms to form two FeOH groups and a FeO group through TS3. In TS3, the bond length of OI(O) -OII (O) increases from 1.240 Å to 1.279 Å and that of Fe-H2 increases from 1.689 Å to 1.701 Å. The energy barrier and reaction energy of this reaction are 10.46 kJ/mol and −455.74 kJ/mol, respectively, indicating that the reaction path of Fe + H + OH + O2 → 2FeOH + FeO occurs easily on the Fe (1 0 0) surface.

Table 9 Possible dissociation reaction paths of H2O and O2 co-adsorbed on Fe (1 0 0) surface. Reaction paths

Initial state

Possible dissociation paths

Path Path Path Path Path Path Path

Fe + H2O + O2

Fe + O + O + H2O Fe + O + H2O2 Fe + H + OH + O2 Fe + H + O + H + O2 Fe + O + O + H + OH Fe + O + OH + OH Fe + O + O + H + H + O

1 2 3 4 5 6 7

OH group. For the Fe2 O(IIO) H 2 group, the charge of Fe2 is changed from 0.02 e to 0.32 e, and that of O(IIO) is changed from 0 e to −0.75 e, indicating that there is a large amount of electrons transfer between the surface Fe atom and oxygen O(IIO) atom, and thereby results in an ionic bond characteristic of Fe2 O(IIO) H 2 . For Fe1 O(IO ) Fe3 group, it is noted that the charge of Fe1 and Fe3 changes from 0.02 e to 0.44 e and 0.40 e, respectively, and that of O(IO) changes from 0 e to −0.55 e, which suggests that both Fe1 and Fe3 atoms donate electrons to the oxygen O(IO) atom. The bonds length of interacting Fe atoms with the adjacent Fe atoms before and after the coadsorption of H2O and O2 molecule are shown in Table 11. It is found that the bond lengths of the interacting Fe atoms with the adjacent Fe atoms are obviously extended, indicating that the coadsorption of H2O and O2 could weaken the interaction between Fe-Fe bonds. The PDOS of Fe3 O(IO) , Fe 4 O(w) and Fe2 O(IIO) bonds formed after H2O-O2 coadsorption on Fe (1 0 0) surface are shown in Fig. 12. It is found that the bonding states of Fe-O bond are mainly from the contribution of O 2p and Fe 3d orbitals. For Fe3 O(IO) bond, the bonding states locate between −6 and −4 eV and the antibonding states locate in the range of −4 to 0 eV. For the Fe 4 O(w) and 9

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Fig. 11. Optimized configurations of possible products of H2O and O2 coadsorption on Fe (1 0 0) surface. Table 10 Mulliken charge of H atoms, O atoms and Fe atoms on the Fe (1 0 0) surface before and after coadsorption of H2O and O2 molecule. Atom H1 H2 O

(w)

O(IIO) O(IO) Fe4 Fe2 Fe1 Fe3

s

p

d

Total

Charge (e)

Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption

0.47 0.59 0.48 0.59 1.89 1.85 1.88

0.00 0.00 0.00 0.00 5.16 4.91 4.13

0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.47 0.59 0.48 0.59 7.05 6.77 6.00

0.53 0.43 0.52 0.44 −1.05 −0.77 0.00

After adsorption Before adsorption

1.85 1.87

4.90 4.12

0.00 0.00

6.75 6.00

−0.75 0.00

After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption

1.90 0.80 0.59 0.80 0.60 0.80 0.55 0.80 0.56

4.65 0.60 0.57 0.62 0.6 0.63 0.53 0.62 0.50

0.00 6.57 6.48 6.56 6.48 6.56 6.48 6.56 6.54

6.55 7.97 7.63 7.98 7.68 7.98 7.56 7.98 7.60

−0.55 0.03 0.37 0.02 0.32 0.02 0.44 0.02 0.40

Table 11 Bonds length of interacting Fe atoms with the adjacent Fe atoms before and after the coadsorption of H2O and O2 molecules. Bonds

Length (Å)

Fe7-Fe1 Fe7-Fe2 Fe7-Fe3 Fe7-Fe4

Before

After

2.4189 2.3998 2.3998 2.3952

2.5339 2.4686 2.3853 2.5668

The atomic label is shown in Fig. 11(c).

steel bar corrosion. On the basis of water-oxygen coadsorption on the Fe (1 0 0) surface, further O atom was introduced on the iron surface to simulate the further corrosion of iron. The optimized configuration is shown in Fig. 14(c), in which the new introduced O atom was labeled as O(n). It is found that the introduced O atom interacts with the Fe-OH group to form a FeOOH group with the adsorption energy of −295.54 kJ/mol. The formation process of FeOOH can be expressed as Eq. (6). The previous experimental studies have detected that there are a large amount of FeOOH species on the surface of corroded steel bar, and it has been proved that the FeOOH is the main product of steel bar corrosion [48–51].

The atomic label is shown in Fig. 11(c).

3.4. Formation of FeOOH on the Fe (1 0 0) surface

Fe

The discussions above have shown that the Fe-OH groups are unstable, and it is inferred that the Fe-OH group is not the final product of

O + 2Fe

OH + O

FeOOH + Fe

OH + Fe

O

(6)

The Mulliken charge of H atoms, O atoms and Fe atoms before and after introducing O atom adsorbed on the Fe (1 0 0) surface which have 10

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Fig. 12. PDOS of Fe3

O(IO) , Fe4 - O(w) and Fe2

O(IIO) bonds formed after H2O-O2 coadsorption on Fe (1 0 0) surface.

been oxidized by O2 and H2O molecule are listed in Table 12. It is noted that the charge of O(n) atom changes from 0 e to −0.38 e and that of Fe4 atom changes from 0.37 e to 0.41 e, in addition, the charge of OH group also decreases. It is suggested that further oxidation occurs on the iron surface via the introduced O atom obtains electrons and surface FeOH group donates electrons. During the corrosion process of iron, water plays a significant role by providing the hydroxyl for the formation of FeOOH. PDOS of newly formed Fe4-O(n) bond is shown in Fig. 15, it is noted that the bonding states of Fe4-O(n) bond shift to the lower energy compare to that of Fe4-O bond formed after coadsorption of oxygen and water, indicating that the Fe4-O(n) bond is more stable than Fe4-O bond. It could concluded that with further oxidation, another Fe-OH group may also be oxidized into FeOOH. The results are consistent with the in situ observation experiments, suggesting that Fe-OH groups could be observed in the early corrosion stage, and these Fe-OH groups are subsequently transformed into FeOOH groups as the main products of steel bar corrosion [52].

hydrogen bond between the adsorbed O atom and water H atom with the adsorption energy of −51.02 kJ/mol, which is more negative than that of single H2O adsorbed on Fe (1 0 0) surface (−35.16 kJ/mol). However, the pre-adsorbed water molecule on the Fe (1 0 0) surface has little effect on the adsorption of subsequent oxygen molecule, and it is found that the dissociation of O2 molecule both occur on the clean and water-preadsorbed Fe (1 0 0) surface. And the subsequent adsorption of O2 could weaken the bonding of surface iron atom to the pre-adsorbed water. Therefore, the present of O2 molecule would influence the interaction between H2O molecule and iron surface. Furthermore, the coadsorption of oxygen and water molecules on the Fe (1 0 0) surface was conducted and the possible products on the iron surface were investigated. It is found that the adsorbed O2 molecule dissociates into two O atoms and H2O molecule dissociates into OH and H groups, and one of oxygen O atoms bonds with water H atom, and then forming a Fe-O group and two Fe-OH groups on the Fe (1 0 0) surface. DOS analysis indicates that the Fe-O group is more stable than Fe-OH group, which implies the Fe-O bond of Fe-OH group would be broken easily in the further corrosion reaction. The results of TS search show that the energy barrier for the interaction of H2O and O2 with Fe (1 0 0) surface to form two FeOH groups and a FeO group is low, indicating that this reaction occur easily on the Fe (1 0 0) surface. On the basis of water-oxygen coadsorption on the Fe (1 0 0) surface, further O atom was introduced on the iron surface. The results indicate that the introduced O atom interacts with one Fe-OH group and forming a FeOOH specie which is considered as the main product of

4. Conclusion To study the initiation corrosion mechanism of reinforcement steel, the adsorptions of oxygen and water molecule on the Fe (1 0 0) surface were simulated by density-functional theory. It is observed that the pre-adsorbed oxygen molecule could strengthen the adsorption of H2O on the iron surface by forming a

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Fig. 13. Schematic energy diagram for the dissociation of O2 molecule (a), H2O molecule (b) and co-adsorbed H + OH + O2 (c) on the Fe (1 0 0) surface. Energies are in kJ/mol. 12

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Fig. 14. Process of the formation of FeOOH.

reinforcement corrosion, and the DOS analysis suggests that the newly formed Fe-O bond in FeOOH is more stable than the Fe-O bond in FeOH group produced by water-oxygen co-adsorption, which is beneficial to the further oxidation of iron surface. By DFT simulation, this study confirms theoretically that FeOOH is the main corrosion product of steel bar instead of Fe-OH groups. The detailed understanding of the formation process of FeOOH helps to find a way to block this process (for example, modification of concrete components, use of rust inhibitors, etc.), and thereby to hinder the corrosion of steel bars and could provide guidance for studying the corrosion resistance of steel bars.

Table 12 Mulliken charge of H atom, O atoms and Fe atom on the Fe (1 0 0) surface. Atom H1 O1 O

(n)

Fe4

Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption Before adsorption After adsorption

s

p

d

Total

Charge(e)

0.59 0.57 1.85 1.89 2.00 1.90 0.59 0.57

0.00 0.00 4.91 4.60 4.00 4.48 0.57 0.48

0.00 0.00 0.00 0.00 0.00 0.00 6.48 6.54

0.59 0.57 6.77 6.49 6.00 6.38 7.63 7.59

0.41 0.43 −0.77 −0.49 0.00 −0.38 0.37 0.41

The atomic label is shown in Fig. 14(c).

Fig. 15. PDOS of the original formed Fe4-O bond (after coadsorption of H2O and O2) and the newly formed Fe4-O(n) bond (after introducing new O atom).

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Data availability

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The data sets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. CRediT authorship contribution statement Zheng Chen: Conceptualization, Writing - review & editing, Supervision. Yumei Nong: Formal analysis, Investigation, Writing original draft. Jianhua Chen: Methodology, Resources. Ye Chen: Methodology, Formal analysis, Writing - review & editing. Bo Yu: Resources. Acknowledgements This research was funded by National Natural Science Foundation of People’s Republic of China (NSFC 51864003, 51468004), Guangxi Natural Science Foundation, China (2018GXNSFAA050127, GKAA18242007 & GKAA18118029) and Guangxi Key Laboratory of Processing for Non-ferrous Metal and Featured Materials (GXYSYF1811), China. The authors are thankful for these supports. References [1] S. Ahmad, Reinforcement corrosion in concrete structures, its monitoring and service life prediction—a review, Cem. Concr. Compos. 25 (2003) 459–471. [2] A.A. Almusallam, Effect of degree of corrosion on the properties of reinforcing steel bars, Constr. Build. Mater. 15 (2001) 361–368. [3] I. Fernandez, J.M. Bairán, A.R. Marí, Corrosion effects on the mechanical properties of reinforcing steel bars. Fatigue and σ – ε behavior, Constr. Build. Mater. 101 (2015) 772–783. [4] J. Ožbolt, F. Oršanić, G. Balabanić, Modeling damage in concrete caused by corrosion of reinforcement: coupled 3D FE model, Int. J. Fract. 178 (2012) 233–244. [5] L. Bertolini, Steel corrosion and service life of reinforced concrete structures, Struct. Infrastruct. Eng. 4 (2008) 123–137. [6] R.R. Hussain, Effect of moisture variation on oxygen consumption rate of corroding steel in chloride contaminated concrete, Cem. Concr. Compos. 33 (2011) 154–161. [7] R.R. Hussain, T. Ishida, Influence of connectivity of concrete pores and associated diffusion of oxygen on corrosion of steel under high humidity, Constr. Build. Mater. 24 (2010) 1014–1019. [8] B. Huet, V. L’hostis, G. Santarini, Steel corrosion in concrete: determinist modeling of cathodic reaction as a function of water saturation degree, Corros. Sci. 49 (2007) 1918–1932. [9] D.A. Hausmann, Steel corrosion in concrete. How does it occur? Mater. Protect. 6 (1967) 19–23. [10] M. Raupach, Investigations on the influence of oxygen on corrosion of steel in concrete—Part 1, Mater. Struct. 29 (1996) 174–184. [11] J.N. Enevoldsen, C.M. Hansson, B.B. Hope, The influence of internal relative humidity on the rate of corrosion of steel embedded in concrete and mortar, Cem. Concr. Res. 24 (1994) 1373–1382. [12] T. Yonezawa, V. Ashworth, R.P.M. Procter, Pore solution composition and chloride effects on the corrosion of steel in concrete, Corrosion 44 (1988) 489–499. [13] L. Li, A.A. Sagüés, Chloride corrosion threshold of reinforcing steel in alkaline solutions — open-circuit immersion tests, Corrosion 57 (2001) 19–28. [14] W. Chen, R. Du, C. Ye, Study on the corrosion behavior of reinforcing steel in simulated concrete pore solutions using in situ Raman spectroscopy assisted by electrochemical techniques, Electrochim. Acta 55 (2010) 5677–5682. [15] Y. Sakisaka, T. Komeda, T. Miyano, Angle-resolved photoemission of the c (2×2) and c (3×1) oxygen overlayers on Fe (110), Surf. Sci. 164 (1985) 220–234. [16] A.A. Hezaveh, G. Jennings, D. Pescia, Quenching of exchange splitting in face centred cubic Fe observed by angle resolved photoemission, Solid State Commun. 57 (1986) 329–334. [17] A.J. Pignocco, G.E. Pellissier, Low-energy electron diffraction studies of oxygen adsorption and oxide formation on a (001) iron surface, J. Electrochem. Soc. 112 (1965) 1188–1194. [18] T. Miyano, Y. Sakisaka, T. Komeda, Electron energy-loss spectroscopy study of oxygen chemisorption and initial oxidation of Fe (110), Surf. Sci. 169 (1986) 197. [19] J.P. Lu, M.R. Albert, S.L. Bernasek, The adsorption of oxygen on the Fe (100) surface, Surf. Sci. 215 (1989) 348–362. [20] C. Leygraf, S. Ekelund, A LEED-AES study of the oxidation of Fe (110) and Fe (100), Surf. Sci. 40 (1973) 609–635.

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