Atomistic mechanism for hydrogen outgassing from Al and Cu fcc metal surfaces under thermal treatment: Failure of the diffusion-limited and the recombination-limited models

Traditional outgassing models (recombination limited and diffusion limited) depend on hydrogen diffusivity within the bulk and its concentration on the surface. These macroscopic models are incapable of explaining the experimentally observed hydrogen outgassing processes. To study the hydrogen outgassing mechanism, first-principles electronic-structure calculations and molecular dynamics simulations were performed on hydrogen diffusing through FCC Al and Cu (110) surfaces. Orbital hybridization between surface-atom orbitals and hydrogen $s$-orbitals resulted in the formation of stable and sometimes also metastable hydrogen adsorption sites on the metal surfaces. For a systematic and detailed analysis, the outgassing process was divided into four separate stages: (1) transit through bulk, (2) surfacing, (3) diffusion and recombination, and (4) what we call bubble-up recombination and outgassing. For each stage, pertinent properties (diffusion and energetics) were calculated and exhibited excellent agreement with available experimental measurements. These atomistic methods revealed the significant and direct impact of the hydrogen potential energy surface on hydrogen surface mobility and recombination (molecular formation); in particular, stages 2, 3, and 4 can only be understood by analyzing the potential energy surface. The atomistic approach used in this paper can be generalized to any other H outgassing system.