Equivalent hydrogen fugacity during electrochemical charging of nickel single crystal: comparison with gaseous hydrogen charging

The application of a cathodic potential to a metallic structure disadvantages the corrosion process (anodic dissolution) and fosters the hydrogen reactions of adsorption, absorption, diffusion and trapping, the latter being able to lead to embrittlement. The hydrogen adsorption and absorption can be produced in metals by gaseous or electrochemical charging. In aqueous environment, the Hydrogen Evolution Reaction (HER) and Hydrogen Absorption Reaction (HAR) most often share the common intermediate: adsorbed hydrogen. The hydrogen adsorption is a complex phenomenon and its approach, in terms of mechanism, depends on various parameters such as: the metal nature (thermal and mechanical history), the surface and subsurface state (crystallinity, presence of defects, surface relaxation and reconstruction, oxide scale formation, surface energy, cathodic current density or imposed potential) and the nature of the surrounding environment.

The HER is an important electrochemical reaction in aqueous environment, whether it is acid or basic. Depending on the potential swept range, HER occurs in three steps:

• « Volmer step »: Electrochemical hydrogen adsorption associated with the hydrated proton discharge (H⁺);
• « Heyrovsky step »: recombination of an adsorbed hydrogen with a hydrated proton (H⁺) and electrochemical desorption in H₂ form;
• « Tafel step »: recombination between two adsorbed hydrogen and chemical desorption in H₂ form.

The aim of the present study is to formalize the link between the conditions of cathodic charging (overpotential, hydrogen flux, surface coverage) ^[1] and the equivalent pressure (fugacity) of dihydrogen for nickel (100) single crystals in term of hydrogen concentration at the surface by using thermodynamic framework ^[2,3] and assuming a steady state in absorption and diffusion.

The hydrogen reactions at the surface of nickel in alkaline solution were dominated by the Volmer-Heyrovsky mechanism at room temperature. The fugacity-real gas pressure equivalence had been established for a fugacity less than 400 atm, an overpotential between -1.2 V and -1 V and a hydrogen concentration less than 7 wt.ppm with a thermodynamic approach in the Volmer domain. This equivalence considers in the gaseous case a perfect unoxidized surface.

The equivalence conversion model thus established will be challenged by gaseous hydrogen charging at room temperature. The native oxide layer NiO on the hydrogen absorption process will be evaluated.