Influence of plastic strain on the hydrogen evolution reaction on nickel (100) single crystal surfaces to improve hydrogen embrittlement

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Abstract

Hydrogen-induced embrittlement can be accountable for premature failure of structure in relation with physical and/or chemical processes occurring on material's surface or in the bulk of the material. Hydrogen Evolution Reaction (HER) corresponding to the early step of hydrogen ingress in the material is explored in present study in relation with plastic strain. HER on nickel (100) single crystal in sulphuric acid medium can be related by a Volmer–Heyrovsky mechanism. The corresponding elementary kinetic parameters as symmetry coefficients, activation enthalpies, and number of active sites have been identified via a thermokinetic model using experimental data. These parameters can be affected by defects associated with plastic strain. Irreversible plastic strain modifies the density and the distribution of storage dislocations affecting the surface roughness at atomic scale and generating additional active adsorption sites. Furthermore, surface emergence of mobile dislocations induces the formation of slip bands, which modify the surface roughness and the electronic state of the surface and increases the (111) surface density. The consequence of plastic strain on HER is explored and discussed in relation with both processes.

Introduction

It is generally claimed that Stress Corrosion Cracking (SCC) and Hydrogen Embrittlement (HE) correspond to synergetic effects between corrosion and mechanical stresses [1], [2], [3], [4], [5], [6], [7], [8], [9]. The corrosion process can induce a localized enhanced plasticity which leads to a microscopically and macroscopically brittle cracking in ductile materials. It is the reason why the concept: Corrosion-Strain-Interaction (CSI) is particularly appropriate to describe environment sensitive fracture. Even if the applied stress is of importance for cracking, it will be emphasized that localised interactions between corrosion (anodic dissolution, hydrogen adsorption and absorption etc.), and plastic strain are the key-points of SCC and HE. Although the impact of environment on the mechanical behavior is well documented [10], [11], the influence of plastic strain on the corrosion process is not yet clear [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. A numerous examples of strain effects on both thermochemistry and kinetics of surface reactions have been identified [29], [30]. Several of these studies focused on strain induced by epitaxial metal overlayers, but recently it has been demonstrated that strain from another source namely dislocations that intersect crystal surfaces and create a charged steps likewise causes significant changes in surface reactivity with hydrogen [31].

The study of the Hydrogen Evolution Reaction (HER) on metallic surfaces, or more precisely the initial step of hydrogen adsorption, requires the knowledge of the interactions between hydrogen and the metallic structure: the place of these interactions (adsorption sites), their nature and their energy [32], [33], [34], [35], [36].

Recently a thorough investigation of plastic strain effect on hydrogen evolution reaction (HER) [24], and a first thermodynamic approach proposed to determine the influence of temperature and plastic strain on HER mechanism [25], have been performed on strained and un-strained polycrystalline nickel in 1 M sulphuric acid. A physical base of the Volmer–Heyrovsky mechanism has been identified with the assumption of Langmuir adsorption isotherm. It was found that the dislocations density and distribution modify the hydrogen activity at atomic scale in term of the number of potential sites and the energy associated with adsorption and desorption steps, in relation with the stored dislocations lines emerged at surface from the bulk [24], [25]. Despite these studies, the question of the contribution of the mobile dislocations, which emerge at the surface (slip bands) and modify surface properties (roughness, electronic state etc.), on hydrogen activity at the surface stays unknown. Both expression of plastic strain (emergences of slip bands and dislocation lines) probably affect the hydrogen activity at the surface differently. Thus the aim of the present work is to study the consequences of the plastic strain on the HER processes on nickel (100) single-crystal surface in sulphuric acid medium. Particular attention is paid to the expression of plastic strain on the surface in relation with storage dislocations and mobile dislocations and their consequence at surface sites (emergences of slip bands and dislocation lines).

Section snippets

Material and surface preparation

The material used in this study is a nickel single crystal (100) with a purity of 99.999% and a dislocation density below 1010 m−2, provided by Goodfellow Company. It was obtained by the method of Bridgman–Stockbarger and appears as a cylindrical bar with diameter of 1 cm, whose axis of growth is 〈100〉. Crystallographic orientation is checked using electron back scattering diffraction (EBSD OIMTSL) analysis and SEM-FEG Philips with a resolution of 5 nm.

Obtaining clean representative surfaces at

Experimental results

All the results have been systematically corrected from the ohmic resistance drop. The electrolyte ohmic resistance has been measured by impedance spectroscopy, and is equal to an average value of 2 Ωcm2. Experimental data are represented in Fig. 7, Fig. 8, Fig. 9. Fig. 7 presents the evolution of cathodic current density j developed on: un-strained Ni(100) samples (γp=0), strained and strained with suppression of strain bands Ni(100) specimens (γp=0.50), in H2SO4 1 M, for all the temperatures

Conclusion

In this paper we studied the influence of temperature and the effects of plastic strain on the adsorption and desorption steps associated with the HER on nickel single crystal (1 0 0) in sulphuric acid medium. We prestrained monocrystalline specimens using a tensile loading machine, and then studied by SEM, AFM and TEM the evolution of plastic strain in terms of emergence of slip bands and dislocation density. The plastic strain and temperature significantly influence the cathodic current density

Acknowledgements

The authors thank the Centre Commun d'Analyse of the French federation FR-EDD FR CNRS 3097 of the University of La Rochelle for electron microscopy facilities. Thanks are also due to the Agence Nationale de la Recherche (GIP–ANR–Program ANR DISHYDRO NT09-608446) for financial support.

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