Hydrogen Diffusivity During Corrosion of High-Purity Aluminum

The effective diffusivity of hydrogen in as-annealed (550 degrees C for 5 h) 99.999% purity polycrystalline aluminum was determined using Al/Pd bilayer membranes following the procedure of K. R. Hebert. Bulk diffusion control was verified by the foil thickness variation method. The room temperature diffusivity of hydrogen was determined to be 1.6 +/- 0.4 x 10(-10) cm(2)/s during initial hydrogen charging in 0.01 M sodium hydroxide (NaOH) solution. The effective diffusivity of hydrogen increased by two orders of magnitude during the second hydrogen charging and approaches the literature for gas phase egress from precharged material. This difference was interpreted to be the result of extensive trapping in freshly annealed foils and high residual trap site occupancy after initial precharging. The activation energy for lattice hydrogen egress was analyzed through constant heating rate thermal desorption spectra. This activation energy was 17.0 +/- 0.6 kJ/mol. The experimental results also demonstrated that the measured hydrogen diffusivity is sensitive to the NaOH concentration speculated to be related to its influence on hydrogen production rate, hydrogen overpotential, and solid-state hydrogen concentration.