Trapping of Hydrogen Absorbed in Aluminum during Corrosion

A model for hydrogen capture in aluminum during aqueous corrosion is presented, incorporating near-surface trapping of H atoms at vacancies produced by metal dissolution. Vacancy-hydrogen interactions are described by a simple non-interacting thermodynamic model incorporating binding of multiple H atoms at vacancies, with energetics derived from first-principles calculations. At large absorption rates, submicron-thickness near-surface layers containing elevated vacancy-hydrogen defects concentrations are predicted, consistent with prior experimental observations. The defect layers arise because of the high sensitivity of the vacancy-hydrogen defect concentration to hydrogen chemical potential, owing to interactions of multiple H atoms with vacancies. Vacancy-hydrogen interactions therefore lead to self-concentration of hydrogen near corroding surfaces, at levels orders of magnitude higher than the H interstitial concentration. The elevated hydrogen concentration explains observations of hydride formation during corrosion, and may be relevant to hydrogen-based microscopic degradation mechanisms. The model predictions are quantitatively consistent with results of hydrogen permeation experiments.