Modeling Trapping of Hydrogen Absorbed into 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.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
1. A. Turnbull, in Gaseous Hydrogen Embrittlement of Materials in Energy Technologies,R. P. Gangloff and B. P. Somerday, Eds., Woodhead, Oxford p. 89 (2012).
2. G. A. Young and J. R. Scully, Acta Mater., 46,6337 (1998).
3. Y. Fukai, J. Alloys Compd., 356-357, 263 (2003).
4. H. K. Birnbaum et al., J. Alloys Compd, 253,260 (1997).
5. L. Ismer, M. S. Park, A. Janotti, C. G. Van de Walle, Phys. Rev. B, 80,184110 (2009).
6. R. Nazarov, T. Hickel, J. Neugebauer, Phys. Rev. B, 89, 144108 (2014).
7. K. R. Hebert, Electrochim. Acta, 168,199 (2015).
8. G. A. Young, J. R. Scully, Metall. Mater. Trans. A., 33,101 (2002).
9. S. Adhikari, K. R. Hebert, J. Electrochem. Soc., 155, C16 (2008).
10. S. Adhikari, J. H. Ai, K. R. Hebert, K. M. Ho, C. Z. Wang, Electrochim. Acta,, 55,5326 (2010).
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