Modeling Trapping of Hydrogen Absorbed into Aluminum during Corrosion
Authors: Hebert Kurt 
Published: February 2020

Hydrogen absorption is a leading cause of degradation of metal structures, during gas-phase exposures and aqueous corrosion processes. Hydrogen absorption and transport in the metal influence kinetics of degradation, and thus incubation periods before fracture and crack propagation rate. Quantitative models of hydrogen embrittlement include descriptions of hydrogen absorption and diffusion coupled to H-induced failure mechanisms.1 Hydrogen diffusion in metals depends critically on reversible and irreversible trapping of H atoms at defects such as dislocations, precipitates, and vacancies. In the case of the high purity aluminum-hydrogen system of interest here, hydrogen diffusion and thermal desorption measurements both suggest the importance of trapping by vacancies and dislocations.2 Evidence for strong hydrogen interactions with vacancies in many metals has been obtained both experimentally,3,4 and through first-principles density function theory (DFT) calculations.5,6 The equilibrium distribution of hydrogen among traps is governed by the hydrogen chemical potential (μH
) or fugacity in the metal, and the formation energies of the various hydrogen traps.

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.7 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.5 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, resulting from inclusion of 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. Similarly elevated hydrogen concentrations near crack surfaces on Al alloys after testing in humid environments.8 Further, the elevated hydrogen concentration explains observations of hydride formation during corrosion, and may be relevant to hydrogen-based microscopic degradation mechanisms.9 The model predictions are quantitatively consistent with results of hydrogen permeation experiments (Figure 1).10

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Figure 1. Evolution of hydrogen chemical potential on the exit side of an Al membrane, during open-circuit corrosion at an equivalent current density of 1 mA/cm2.7 Calculated results (solid lines) at two H absorption current densities, and experimental results (dashed lines) for corrosion in NaOH solutions at the indicated pH values.10

Figure 1