Hydrogen Absorption Mechanism into Iron in Aqueous Solution Including Metal Cations by Laser Ablation Tests and First-principles Calculations

Abstract

In order to clarify the effect of metal cations (Zn²⁺, Mg²⁺, Na⁺) in aqueous solution on hydrogen absorption into iron, the amount of hydrogen absorption from iron surface was measured by electrochemical tests with a laser ablation. Moreover, in order to obtain the basic mechanism of hydrogen absorption with adsorption of metal cation, we obtained the adsorption potential of the adsorbed atom and the electronic state around the adsorbed atom using first principles calculations. Peak value of permeation current and the time until the current reached the peak value decreased and elongated in the order of NaCl, MgCl₂, and ZnCl₂ solutions. Also, by first-principles calculations adsorption strength of each metal atom increased in the order of Na < Mg < Zn. It was suggested that dissolution of Fe is inhibited due to formation of dense metal layer in the solution including the metal cation which has large adsorption strength to Fe surface like Zn, and finally permeation current may have been reduced.

1. Introduction

Hydrogen embrittlement of steel is phenomenon of degradation of steel due to hydrogen atoms absorbed from the external environment. From the viewpoint of ensuring safety of structures using steel, it is important to understand mechanisms of hydrogen embrittlement of steels. A lot of researches have been conducted on the relationship between amount of absorbed hydrogen and embrittlement of steels so far. For example, Nagumo measured position and amount of hydrogen trapped in steel using thermal desorption gas spectroscopy.¹⁾ Wang et al. conducted low strain rate tensile test on various types of high-strength martensite steels charged with hydrogen to obtain relationship between amount of absorbed hydrogen and brittle fracture.²⁾ Also, there have been many researches focusing on hydrogen absorption from the environment. For example, Omura et al. conducted hydrogen permeation test on low-alloy steel coated with Zn, weathering alloy elements and sulfide film, and obtained relation between kinds of film and hydrogen permeation.³⁾ Fuji et al. conducted a hydrogen permeation test on low-alloy steel in solution containing hydrogen sulfide gas and obtained effect of hydrogen sulfide on hydrogen penetration.⁴⁾ Fushimi et al. conducted hydrogen permeation test by a Micro-capillary combined with a Devanathan-Stachurski cell to obtain relation between diffusion coefficient of hydrogen and grain boundary in steels.⁵⁾ Despite various studies, detailed mechanism of hydrogen embrittlement is not understood yet. In particular, there are many unknown basic mechanisms of hydrogen permeation in aqueous solutions.

On the other hand, Sakairi et al. conducted electrochemical hydrogen permeation test to the steel plate whose surface was ablated by a laser, obtained difference in amount of hydrogen penetration before and after the ablation, and discussed on anodic and cathodic reactions at the surface.⁶⁾ By the laser irradiation to the steel surface, the oxide film at the laser irradiation site is peeled off and the steel substrate is exposed to the solution. This means that the ablated site would be an anodic site and the non-ablated site would be a cathodic site, and that anodic site and cathodic site can be separated completely by the laser irradiation. For these reasons, we think to be able to investigate the changes of the active dissolution state and the surface state at anodic site by the electrochemical test with the lase ablation technique. Figure 1 shows schematic drawing for separation of anodic and cathodic sites by laser irradiation.

Fig. 1.

Schematic drawing for separation of anodic and cathodic sites by laser irradiation.

The previous study reported that the presence of metal cations in aqueous solution suppressed corrosion of carbon steel.⁷⁾ In this study, we focused on the possibility that hydrogen absorption into steel was also suppressed by the presence of metal cations. The amount of hydrogen permeating into iron in contact with solutions containing different metal cations was measured by the electrochemical tests with the laser ablation, and the relation between metal cations in aqueous solution and hydrogen absorption was obtained. In order to understand the basic mechanism of suppression of hydrogen absorption from a viewpoint of atomic level, the first-principles calculations with solution approximation to the iron-aqueous solution interface system were conducted and electronic state near the interface was obtained. Relation between adsorption mechanism of metal atoms at iron surface in aqueous solution obtained by first-principles calculations and amount of hydrogen absorption obtained by experiments was investigated.

2. Experimental and Computational Details

2.1. Experiments

A carbon steel coupon with size of 30×15×1 mm was used as a specimen. The chemical composition of the carbon steel is shown in Table 1. Both sides of each specimen were ground and flattened by mechanical polishing finally using colloidal silica particles of 40 nm in size. The hydrogen detection side was electroplated with a nickel layer of 1.0 μm in thickness by cathodic polarization at 4.0 mA cm⁻² with time of 12 min in a Watt bath (0.312 kmol m⁻³ NiSO₄ and 0.781 kmol m⁻³ H₃BO₃) at 323 K.

Table 1. Chemical composition of carbon steel (mass%).

C	Si	Mn	P	S	Al	N	Fe
0.001	0.01	0.01	0.001	0.006	0.002	0.002	bal.

In order to separate anodic and cathodic sites on a hydrogen absorption side of the specimen, the oxide film on hydrogen absorption side was locally removed using pulse laser, and formed a bare substrate area with size of about 1.5 mm². A microelectrochemical cell was used to measure the amount of permeated hydrogen. Figure 2 shows a schematic diagram of the cell used in experiments. Pt wires were used for counter electrode and reference electrode, and the specimen was set as working electrode. Hydrogen detection side was contacted with 1 kmol m⁻³ NaOH aqueous solution, and was polarized to −30 mV vs. Pt for oxidizing permeated hydrogen quickly. After the current stabilized at 5 nA or less, the hydrogen absorption side cell was filled with the solution among NaCl, MgCl₂, and ZnCl₂ aqueous solutions with a Cl⁻ concentration of 0.1 kmol m⁻³. After 600 s from the solution injection, focused pulse-laser beam was irradiated for 7 s, and the hydrogen that passed through the specimen and reached the detection surface was measured as permeation current.

Fig. 2.

Schematic drawing of electrochemical cell for hydrogen permeation test.

2.2. Calculation

Figure 3 represents schematic view of changes in electrostatic potential and Fermi level due to metal atom adsorption on Fe. There could have a relationship between the adsorption potential and bond strength between adsorbed atom and Fe surface because the behavior of chemical reaction of adsorbed atom at Fe surface should be changed with the change in the Fermi level. Electronic states of solids were calculated by spin polarized density-functional-theory (DFT).⁸⁾ Quantum ESPRESSO package was used for DFT calculation.^9,10) DFT calculations were performed using ultrasoft pseudopotential.^11,12) Simulation system was created using iron slab structure with (110) orientation and [4×4] unit cells using effective-screening-medium (ESM) method.¹³⁾ Slab thickness was set to 5 atomic layers. Spaces above and below the slab were 12 Å and 8 Å, respectively (1 Å=10¹⁰ m). For exchange correlation effect, generalized gradient approximation (GGA) of Perdew, Burke, Ernzerhof was used.^14,15) K point was used by 3×3×1 Monkhorst-Pack mesh sampling.¹⁶⁾

Fig. 3.

Schematic view of potential difference between iron with cation far from surface and with adsorbed atom.

In order to consider the effect of solution on iron surface, reference interaction site model (RISM), which is one of the classical solvent approximations, was applied above the slab.¹⁷⁾ The thickness of RISM region above the slab was set to about 53 Å, which is enough value to converge potential oscillation caused by electronic state at metal surface. In the analysis, Fe and each of three atoms such as Na, Mg and Zn, that may be chemically bonded, were treated in the first-principles calculation and other solvents were considered as RISM elements, because only non-chemical bonding interaction is considered in RISM calculations. In RISM calculation, 1 kmol m⁻³ NaCl solution was assumed, and aqueous solution having a temperature of 300 K composed of 55.5 kmol m⁻³ H₂O, 1 kmol m⁻³ Na⁺ and Cl⁻ RISM elements was adopted. Although the concentration of the species in the solution were different between the calculations and the experiments because of reducing calculation time, that should not affect to the behavior of adsorption because only non-chemical bonding is considered in RISM calculation as mentioned above. For closure equation in RISM, Kovalenko-Hirata model was adopted.¹⁸⁾ The cutoff energy of the distribution function of RISM element was set to 160 Ry (1 Ry=13.6 eV), which value was set as a condition for converging the internal energy of the system. The convergence criterion of the correlation functions in RISM equations was set to 10⁻⁶ Ry. The simple point charge (SPC) model was adopted for the classical force field parameters of H₂O molecule in RISM.¹⁹⁾ For Na⁺ and Cl⁻ in RISM, the parameters obtained by Smith et al. were adopted.²⁰⁾ Lennard-Jones parameters (ε_ij and σ_ij) between different types of RISM elements were set according to the Lorentz-Berthelot rule.²¹⁾ The Lennard-Jones parameters for connecting quantum atoms and RISM elements were set as follows. For the adsorbed atomic species Na, Mg, and Zn, the parameters obtained by Li et al. were adopted.²²⁾ For Fe constituting the slab, parameters were obtained by fitting to the interaction energy between the iron slab surface and an Ar atom. Figure 4 shows a schematic diagram of the ESM-RISM calculation model.

Fig. 4.

Schematic drawing of calculation model.

In order to evaluate an adsorption strength of each metal atom to iron surface, adsorption potential E_ad, at which adsorption and ionization of metal atom were balanced, was derived using following equation,   

$$E_{\text{ad}}=-\frac{\mathrm{\Delta }G}{nF}$$(1)

where G, n, and F represent Gibbs free energy, the number of reacted electrons, and Faraday constant, respectively. ΔG represents the difference in free energy before and after the reaction.   

$$\mathrm{\Delta }G=G\left({\text{Fe}}_{\text{slab}}+{\text{M}}_{\text{ad}}\right)-\left\{G\left({\text{Fe}}_{\text{slab}}\right)+G\left({\text{M}}^{\text{n}+}\right)\right\}$$(2)

where M represents the investigated metal species (= Na, Mg, Zn). The adsorption potential obtained by Eq. (1) is not a potential based on hydrogen electrode potential (V vs. SHE) or silver-silver chloride electrode potential (V vs. SSE) which were often used in experimental measurement, but a potential based on the potential at a position sufficiently far from the surface (V vs. Φ_s).^23,24)

In order to obtain detailed mechanism of adsorption of metal atoms on iron surface, electron density difference between before and after metal atom adsorption at potential of zero-charge Δρ was derived using following equation, and chemical bonding properties of adsorbed atom were investigated,   

$$\mathrm{\Delta }\rho ={\rho }_{\text{FeSlab}+\text{M}}-\left({\rho }_{\text{FeSlab}}+{\rho }_{\text{M}}\right)$$(3)

where ρ_{FeSlab + M}, ρ_FeSlab, and ρ_M represent electron density of iron slab with metal atom adsorption, that of iron slab, and that of metal atom, respectively. The electron density difference is one of the parameters for investigating the bonding state between atoms, and the positive value between atoms represents the rise of amount of electrons which contribute chemical bonding.

The ESM-RISM calculation was actually performed to determine the absorption site of each metal species on the Fe surface as the site with the lowest energy. Figure 5 shows a typical example for a site of the adsorbed metal atom. The free energy of isolated ion in solution was obtained by DFT calculations on ion in the RISM consisting of 20×20×106 Å. In this study, DFT internal energy was adopted instead of free energy (G≈E).

Fig. 5.

Schematic drawing of hollow adsorption site on Fe(110) surface.

3. Results

3.1. Hydrogen Permeation Current

Figure 6 shows the change in hydrogen permeation current over time after laser irradiation in each solution. After the irradiation, the permeated current increased and showed a peak in all solutions. After that, it gradually decreased and was closed to the value before the irradiation. On the other hand, the peak value of permeation current and the time until the current reached the peak value decreased and elongated in the order of NaCl, MgCl₂, and ZnCl₂ solutions.

Fig. 6.

Changes in hydrogen permeation current after laser irradiation in solutions containing 0.1 kmol m⁻³ Cl⁻.

3.2. Adsorption Potentials of Metal Atoms on Iron Surface

Tables 2 and 3 show the adsorption potential of each metal atom and the adsorption distance between the Fe surface and each metal atom by first-principles calculation, respectively. The adsorption potential increased in order of Mg ~ Na < Zn, and the adsorption distance decreased in order of Na < Mg < Zn. Since the adsorption distance becomes short when the binding force is large generally, the adsorption potential could have correlation with the binding force. Figure 7 shows comparison of the peak current obtained by experiments with each calculated value; (a) the peak current vs the adsorption potential (b) the peak current vs the adsorption distance. Both of the comparison data had a relatively good correlation between the experimental and the calculation values.

Table 2. Adsorption potential of atoms.

Adsorbed atom	Na	Mg	Zn
Adsorption potential (V vs. Φs)	4.29	4.22	4.67

Table 3. Distance between adsorbed atom and nearest Fe atom.

Adsorbed atom	Na	Mg	Zn
Distance (Å)	3.00	2.77	2.23

Fig. 7.

Comparison of experimental data and calculation data, (a) peak current vs. adsorption potential, (b) peak current vs. distance between adsorbed atom and nearest Fe atom.

3.3. Electron Density Difference between before and after Metal Atom Adsorption

Figure 8 shows a contour plot of electron density difference around the adsorbed atom and the Fe atoms at potential of zero charge. Figure 9 shows the distribution of electron density difference between the adsorbed atom and the nearest Fe atom. The horizontal axis indicates a relative position of electron, r_Fe–M/L_Fe–M, where L_Fe–M is the distance between the adsorbed atom and the Fe atom, and r_Fe–M is the position of electron from the Fe atom. As shown in these figures, a maximum of electron density difference between adsorption atom and Fe atom increased in order of Na < Mg < Zn. This order was good agreement with that of the adsorption potential and reverse with that of the peak of permeation current.

Fig. 8.

Contour maps of electron density difference around each adsorbed atom of (a) Na, (b) Mg and (c) Zn.

Fig. 9.

Distribution of electron density difference between the adsorbed atom and the nearest Fe atom. L_Fe–M is the distance between the atoms, and r_Fe–M is the position of electron from the Fe atom.

4. Discussion

In our previous study,^7,25) relatively dense metal layer was formed on surface of specimen immersed in solution containing Zn²⁺, and amount of corrosion products on surface was small, whereas there was relatively rough metal layer in solution containing Mg²⁺, and large amount of corrosion products existed. Metal layer was not formed in the solution containing Na⁺. In this study, from electron density difference at potential of zero-charge for each metal atom, the chemical bonding to Fe atom was strong for Zn, moderate for Mg and weak for Na. These indicate that, metal atom was adsorbed on the Fe surface, and the order of binding force was Na < Mg < Zn. Therefore, we have concluded that for the solution including metal cation which has large binding force to Fe atom like Zn²⁺, dissolution of Fe may have been inhibited due to formation of dense metal layer by strong chemisorption, and finally permeation current may have been reduced. Figure 10 shows schematic view of mechanism of hydrogen absorption into iron in solution containing metal cation.

Fig. 10.

Schematic view of mechanism of hydrogen absorption, (a) atom with weak binding force to Fe atom, (b) atom with strong binding force to Fe atom.

Generally, Zn atoms would be dissolved from the Fe surface because Zn has a lower electrode potential than Fe. On the other hand, chemical adsorption of Zn atoms to Fe surface was observed by first-principles calculation. It can explain that the Zn layer was formed on the Fe surface which observed in previous study.^7,25) The consideration suggests that the adsorption rate of Zn was faster than the dissolution rate. The future direction of this study will be the investigation of adsorption rate of metal atom and dissolution rate of metal layer by experiments and first-principles calculations.

5. Conclusions

Relation between metal cations in aqueous solution and hydrogen absorption into iron was practically investigated by hydrogen permeation test using laser ablation, and theoretically investigated in the viewpoint of electronic state analysis using first-principles calculations. A permeation current was decreased in the order of NaCl, MgCl₂ and ZnCl₂ solution. According to the first-principles calculations, adsorption potential and binding force to Fe atom increased in the order of Na, Mg and Zn in aqueous solution. It is suggested that cation in aqueous solution which has large bond strength forms a dense metal layer on the Fe surface, suppresses anodic reaction on the surface, and thereby reduces hydrogen absorption into iron.

Acknowledgment

The authors would like to thank Dr. M. Otani, National Institute of Advanced Industrial Science and Technology, for useful discussions. This research was conducted using the supercomputer SGI ICE X in the Japan Atomic Energy Agency.

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