Carbon influence on hydrogen absorption and adsorption on Fe-C alloy surfaces

Highlights

• Effect of carbon on hydrogen adsorption and absorption on Fe-C alloys was studied.

• Carbon doping surfaces are more unstable for hydrogen to adsorb on, hydrogen surface coverage was reduced.

• Carbon doping reduces the thermodynamic energy difference and diffusion barriers, making hydrogen penetration into the bulk more facile.

Abstract

Electrochemical experiments and first-principle calculations were conducted in this study to investigate the impact of carbon doping on the adsorption and absorption on Fe-C alloy surfaces. The generalized Iyer-Pickering-Zawenzaden model was adopted to determine the kinetics related to hydrogen adsorption/absorption. Discharge rate constant k₁ was found increased with the presence of carbon, suggesting the promotion of the H₂O ionization process and more hydrogen atoms were generated on the surface. Meanwhile, the recombination rate constant k₂ decreased on carbon doping surface, indicating recombination process of hydrogen atoms were inhibited. Results also showed the hydrogen surface coverage dropped with increasing carbon content. Additionally, EIS results indicate that the distance between the adsorbed hydrogen atoms and the sample surfaces is reduced by carbon doping. All experimental results are in accordance with the first-principle calculation results which demonstrate that the presence of carbon elevated the binding energy for hydrogen adsorption while lowering the energy barrier for diffusion. Thus, carbon solutes reduce the stability for hydrogen atoms to adsorb on the surface and facilitate their diffusion into the bulk.

Introduction

Hydrogen embrittlement (HE) of metallic materials is a very serious issue in the modern industry, and hydrogen uptake from the environment into the metal surface is the first and essential step in HE [1,2]. Till then, one of the most prevailing ways in the anti HE material design is through the addition of alloying elements, which plays an important role in altering both the mechanical properties and HE sensitivity, as well as providing cost control [3], [4], [5], [6], [7]. Alloy elements doping on the sample surface also have profound impact on the entrance of hydrogen, and will eventually affect the HE behavior [8,9]. Therefore, it is of great significance in designing HE resistant materials by investigating the interaction between alloy elements with hydrogen on material surface.

Carbon, as the most important alloying element in steel, is of vital importance in determining the basic properties of steels. Many previous works have focused on the effect of carbon addition on HE behaviors, but there are significant discrepancies between studies. Some researchers believe that the increasing content of carbon could increase the solubility of hydrogen in a material [10] and the material's sensitivity to cracking [11,12]. However, others showed that the increase in carbon content could be beneficial to HE resistance [13,14]. These controversial opinions mainly originate from the different microstructures of the tested materials.

To address this problem, investigations have been performed that concentrated on various microstructures. Chan et al. [15] conducted a series of experiments addressing the relationship between hydrogen content and its diffusivity in samples with different carbon contents. In a pearlite/ferrite structure, the pearlite/ferrite interface provided the most important hydrogen trapping sites, and the hydrogen concentration reached a maximum when the carbon content reached 0.69 wt.% [16]. Then, in martensite containing carbon from 0.23 wt.% to 0.93 wt.%, the concentration of trapped hydrogen increased with the carbon content, and that high carbon martensite trapped the most hydrogen because of its twin sites and high dislocation density [17]. Furthermore, as-quenched martensite has been shown to capture the greatest quantity of hydrogen [18]. Escobar et al. [19] conducted thermal desorption spectroscopy (TDS) and blister mapping tests on Fe-C alloys ranging from 0.2 wt.% to 0.4 wt.% with three different microstructures: bainite, martensite and pearlite. Their results indicated that as-quenched martensite contained the most hydrogen, and that in all structures, the hydrogen concentration increases with increasing carbon content.

The abovementioned studies have done intensive and excellent works on carbon-related hydrogen trapping phenomenon in different microstructures, which mostly manifest in the bulk effect of carbon addition. However, hydrogen penetration first starts from the surface of the materials, and the addition of carbon in materials may also impact the hydrogen behaviors on sample surfaces. To date, little work was performed to address this issue.

In an alkaline solution, hydrogen can interact with the metal surface through the following steps [20,21].

$${\mathrm{H}}_2\mathrm{O}+\mathrm{M}+\mathrm{e}\stackrel{{\mathrm{k}}_1}{\to }\mathrm{M}{\mathrm{H}}_{\text{ads}}+\mathrm{O}{\mathrm{H}}^{-}$$

$$\mathrm{M}{\mathrm{H}}_{\text{ads}}+\mathrm{M}{\mathrm{H}}_{\text{ads}}\stackrel{{\mathrm{k}}_2}{\to }{\mathrm{H}}_2+2\mathrm{M}$$

$$\mathrm{M}{\mathrm{H}}_{\text{ads}}+{\mathrm{H}}_2\mathrm{O}+\mathrm{e}\stackrel{{\mathrm{k}}_3}{\to }{\mathrm{H}}_2+\mathrm{O}{\mathrm{H}}^{-}+\mathrm{M}$$

$$\mathrm{M}{\mathrm{H}}_{\text{ads}}\stackrel{\mathrm{k}}{\to }\mathrm{M}{\mathrm{H}}_{\text{abs}}$$

In Eq. (1), the metal surface is represented by M, and MH_ads refers to hydrogen atoms adsorbed on the surface. Alternatively, some of the MH_ads will either collide with each other chemically via a Tafel step shown by Eq. (2) or combine with the H atom in water molecules through a Heyrovsky step shown by Eq. (3) to form H₂ gas. Some of MH_ads will penetrate the subsurface of the metal, forming absorbed hydrogen atoms, MH_abs, according to Eq. (4).

Iyer et al. proposed the Iyer-Pickering-Zawenzaden (IPZ) model to quantitatively measure hydrogen properties on sample surfaces, mainly surface coverage and the related kinetic parameters [22,23]. The model was then developed by Al-Faqeer as a generalized IPZ model [24], in which the adsorption of hydrogen was described using the Frumkin adsorption energy as shown in Eq. (5).

$$\Delta {G_{\theta }}^{\circ }=\Delta G^{\circ }+fRT\theta$$

In Eq. (5), ΔG_θ^○ refers to the standard free energy at coverage θ, and ΔG^○is the standard free energy at zero coverage. Eq. (5) is adaptable to describe a more general adsorption situation under both Frumkin and Langmuir conditions by introducing the fRT value, which represents the deviation of free adsorption energy between the Frumkin adsorption and ideal Langmuir adsorption. R is the gas constant, T is the temperature, and f is a dimensionless factor.

Then, the kinetic parameters of hydrogen evolution reaction (HER) in Eqs. (1) and (2) can be obtained based on the following equations:

$$\frac{\sqrt{i_r}}{i_{\infty }}{i}_c\exp \left(\alpha \eta \frac{F}{RT}\right)=\sqrt{\frac{k_2}{F}}•\frac{{i_0}^{\prime }}{k}\left(1-\frac{1}{Fk}{i}_{\infty }\right)$$

$$\ln \left(\frac{\sqrt{i_r}}{i_{\infty }}\right)=\ln (\sqrt{\frac{k_2}{F}}•\frac{1}{k})+\frac{\alpha f}{Fk}{i}_{\infty }$$

$$i_{\infty }=Fk\theta$$

$$i_0^{\prime }=F{k}_1{C}_H^{+}=i_0/\left(1-{\theta }_H^e\right)$$

Here, i_∞ and i_c are the steady-state current density and charging current density, respectively, which both can be obtained directly from hydrogen permeation tests. IPZ analysis makes the assumption that the surface recombination reactions are all Tafel type (see Eq. (2)), so that i_r represents the Tafel recombination current density. k₁, k₂, and k are the parameters shown in Eqs. (1), (2) and (4), respectively. α is the transfer coefficient, obtained by the Tafel polarized curve, and F is the Faraday constant. η is the cathodic overpotential, which equals the difference between the applied potential (E_applied) and the corrosion potential (E_corr) [1].

In Eq. (9), i₀ is the exchange current density of the HER, ${\theta }_H^e$ is the equilibrium hydrogen coverage on the surface, and $C_H^{+}$is the hydrogen concentration. Then, the discharge kinetic parameter k₁ can be calculated.

In addition to quantitative models, another widely used technique in revealing hydrogen adsorption/absorption properties on sample surfaces is electrochemical impendence spectroscopy (EIS) [25]. The adsorption and absorption of H can be interpreted by EIS patterns [26,27]. The patterns could be fitted appropriately by equivalent circuits, where each electro element corresponds to one process either in the hydrogen adsorption or absorption reactions [26]. The patterns also give insight into the structure of the double layer capacitor, whose thickness is directly linked to the starting point of the hydrogen evolution reaction (HER).

Currently, atomistic modeling methods such as first principle calculations [28], [29], [30], [31], have also been extensively used to quantitively describe the energy levels and transition barriers along a hydrogen diffusion path, which can provide in-depth information for hydrogen adsorption, absorption, and diffusion processes on metal surfaces. In this paper, IPZ analysis and EIS tests were performed to investigate the electrode/electrolyte interface of Fe-C alloy samples with different carbon contents and the resulting influences on the behaviors of hydrogen atoms on the sample surfaces were revealed. In addition to the experiments, first principle calculations were also used to explore the interaction energetics between hydrogen atoms and Fe(110) surfaces, with or without carbon addition.