Electrochimica Acta 255 (2017) 230–238 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Research Paper Hydrostatic pressure effects on the kinetic parameters of hydrogen evolution and permeation in Armco iron a a a a a, b c X.L. Xiong , H.X. Ma , X. Tao , J.X. Li , Y.J. Su *, Q.J. Zhou , Alex A. Volinsky a Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), University of Science and Technology Beijing, Beijing 100083, China b Research Institute, Baoshan Iron & Steel Co. Ltd, Shanghai 201900, China c Department of Mechanical Engineering, University of South Florida, Tampa FL 33620, USA A R T I C L E I N F O A B S T R A C T Article history: – Received 11 July 2017 Armco iron samples are exposed to pressures of 0.1 30 MPa to study hydrogen permeation during Received in revised form 22 September 2017 potentiostatic charging. Elecrochemical impedance spectroscopy (EIS) is performed to analyze the Accepted 28 September 2017 electrode/electrolyte interface reaction. According to EIS and first-principles calculations, the hydrostatic Available online 29 September 2017 pressure decreases the distance between the adsorbed hydrogen atoms and the sample surface, which induces hydrogen atom diffusion into the subsurface by decreasing the diffusion barrier energy. Under Keywords: high pressure, hydrogen molecules gather at the sample entry side and inhibit the hydrogen Iron recombination reaction. Thus, the amount of adsorbed hydrogen and the hydrogen subsurface coverage Hydrogen permeation increase. Hydrostatic absorption © 2017 Published by Elsevier Ltd. 1. Introduction the permeation rate. Based on the surface effect model [8], Xiong et al. experimentally demonstrated that the hydrostatic pressure Deep-sea water can induce severe hydrogen-induced cracking directly affected the hydrogen surface adsorption. The hydrogen (HIC) of metallic materials and structures [1], which stimulated concentration at the subsurface of the A514 steel membrane several studies of the hydrostatic pressure effects on HIC. Olsen increased with hydrostatic pressure [9]. However, the effects of the et al. demonstrated that the hydrostatic pressure could signifi- hydrostatic pressure on each kinetic parameter of HER remain cantly increase the hydrogen concentration in duplex stainless unclear. steels and super martensitic steels when immersed in a 3.5% NaCl During the past decade, models of hydrogen permeation have electrolyte with aluminum anodes. Thus, HIC occurs more easily in been developed, allowing to obtain kinetic parameters for the HER. deep-sea environment [2]. Therefore, research of the diffusion and These models show that hydrogen diffuses into metal in alkaline hydrogen evolution reaction (HER) mechanisms in steels under solutions in the following reaction [8,10–12]: hydrostatic pressure is essential. À À k þ þ 1 þ ð Þ H2O M e MHads OH 1 Experiments of the pressure effects on hydrogen diffusion mechanisms in metals have been performed by many researchers Here, M represents the metal electrode, and MHads represents a since 1966. Woodward et al. [3] and Nanis et al. [4] claimed that the hydrogen atom adsorbed on the metal surface. Parts of the hydrostatic pressure increased the permeation rate, but the adsorbed hydrogen atoms recombine into a hydrogen molecule via diffusivity remained unchanged. However, Smirnova et al. [5] the simple Tafel chemical recombination reaction: and Blundy et al. [6,7] found that the permeation flux was k MH þ MH 2 H2 þ 2M ð2Þ independent from the hydrostatic pressure if the electrolyte was ads ads stirred at the entry side. The authors hypothesized that the or by the Heyrovsky electrochemical reaction: permeation rate increased with the hydrogen partial pressure at À À k þ þ 3 þ þ ð Þ the entry side and that hydrostatic pressure did not directly affect MHads H2O e H2 OH M 3 In alkaline solution, the coupled discharge-chemical desorption occurs at lower overpotential, and slow discharge-fast electro- chemical desorption occurs at higher overpotential [12]. Adsorp- * Corresponding author. tion is followed by absorption of some adsorbed hydrogen atoms. E-mail address: [email protected] (Y.J. Su). https://doi.org/10.1016/j.electacta.2017.09.181 0013-4686/© 2017 Published by Elsevier Ltd. X.L. Xiong et al. / Electrochimica Acta 255 (2017) 230–238 231 Thus, hydrogen atoms diffuse into the subsurface immediately and ic can be obtained from the experimental data. When a suitable under the electrode surface as follows: Eapplied for hydrogen charging is chosen, HER follows the coupled discharge-recombinationpffiffiffiffi mechanism [12]. Thus, linear relation- þ $ þ ð Þ ð Þ MHads Msubsurface Msurface MHabs subsurface 4 ships between ir=i1icexpðFah=RTÞ and i1, and between pffiffiffiffi Then, the absorbed hydrogen atoms diffuse from the subsurface ln ir=i1 and i1, can be obtained. The kinetic parameters of into the bulk metal: the HER can be calculated based on Eqs. (7)–(9). MH ð subsurfaceÞ ! MH ðbulkÞ ð5Þ abs abs Since HER occurs at the metal surface/electrolyte interface, it is necessary to provide further insight into the pressure effect on the The absorption of hydrogen into metals shown above is the first structure of the double-layer capacitance. The notion of double- and necessary step in hydrogen embrittlement [8,10,13]. Some layer capacitance was first mentioned by Gouy [18], Chapman [19] researchers considered the surface effects at the hydrogen entry and Stern [20]. However, this model did not consider the ion side during electrochemical hydrogen permeation [8,14–16]. By volume and hydrostatic pressure generated by the Coulomb force. measuring kinetic parameters of the HER, one can illustrate the Both models proposed by Shapovalov [21] and Dreyer [22] claimed pressure effects on reactions (1) through (5). that the electrostatically generated pressure was not negligible; it To measure kinetic parameters of the HER, general derivation of could be several hundred MPa near the electrode surface and the Iver-Pickering-Zamenzadeh (IPZ) model [17] was adopted in dramatically decrease with the distance from the electrode this paper. This model, proposed by Al-Faqeer et al., is applicable surface, which proved that the hydrostatic pressure of the for the discharge-recombination process of hydrogen evolution in electrolyte (approximately 30 MPa) could affect the ion distribu- electrode/electrolyte systems under the Frumkin and the Langmuir tion in the double layer. The double-layer thickness can change the adsorption conditions. place of the HER, so the pressure obviously affects the adsorption According to the Frumkin adsorption isotherm, the free energy and absorption of hydrogen atoms. However, this effect cannot be of the species adsorption is correlated with the species coverage as fi follows: directly observed. Combined with the EIS analysis results, rst- principles calculations based on the density functional theory o o D ¼ D þ u ð Þ Gu G0 f RT 6 (DFT) were used to qualitatively explain the effects of the change in o o double-layer thickness on the HER kinetics under different D D where Gu and G0 are standard free energies of the adsorption at hydrostatic pressures. Based on the DFT, the hydrogen adsorption coverage u and zero coverage (u=0), respectively; R is the gas and diffusion in metals were simulated in several studies [23–27]. constant; T is the absolute temperature; f is a dimensionless factor, Here, a minimum energy path of hydrogen diffusion into the Fe which describes the deviation from the ideal Langmuir behavior. subsurface was obtained using the well-known climbing nudged o The change in DGu is represented by the fRT quantity. elastic-band (CNEB) calculation method [28]. Then, using the To evaluate kinetic parameters of the HER, Eqs. (7)–(9) were Debye model [29], we calculated the Gibbs free energy of each derived based on the original IPZ model [14–16] and the Frumkin structure in the hydrogen diffusion path, which diffused from the adsorption in Eq. (6): surface adsorption into the (100) Fe subsurface, as obtained by the pffiffiffiffi rffiffiffiffiffi 0 ah CNEB. This energy path enables us to relate the energy barrier for ir F k2io 1 ¼ À ð Þ icexp 1 i1 7 hydrogen diffusion into the Fe subsurface and the position for HER. i1 RT F k Fk Finally, the hydrostatic pressure effect on the hydrogen diffusion into Fe can be qualitatively described. ! ! pffiffiffiffi rffiffiffiffiffi This paper analyzes the effects of the hydrostatic pressure on ir k21 af the kinetics of HER and electrode/electrolyte interface based on the ln ¼ ln þ i1 ð8Þ i1 F k Fk electrochemical hydrogen permeation and electrochemical im- pedance spectroscopy. In addition, the DFT and the Debye models calculations are used to qualitatively explain the pressure effect on i1 = Fku (9) hydrogen diffusion into the Fe subsurface. where i1 and ir are the current densities of the steady-state and adsorbed hydrogen atom combination; ic is the charging current 2. Materials and methods density; k2 is the kinetic parameter in Eq. (2); a is the transfer coefficient; F is the Faraday’s constant; h is the overpotential 2.1. Tested materials (h = Eapplied – Ecorr; Eapplied is the applied cathodic potential; Ecorr is the corrosion potential). Armco iron was used in this work with the following composition in wt%: 0.025 C; 0.006 Mn; 0.01 Mo; 0.001 Si. The À Á 0 e þ i ¼ Fk C ¼ i = 1 À u ð10Þ o 1 H o H permeation and potentiodynamic polarization test samples had a e circular shape with 1.2 cm radius and were exposed to the solution. u Here, io is the exchange current density of the HER; His the To avoid the edge effects and ensure one-dimensional diffusion, þ hydrogen surface coverage at equilibrium; CH is the hydrogen ion the samples were 0.5 Æ 0.01 mm thick.