Abstract
The diffusion and trapping of hydrogen in high purity (99.999 wt%), polycrystalline aluminum was investigated using both isothermal and constant heating rate desorption techniques. Constant heating rate desorption was used to elucidate the effects of microstructural trap states on the apparent diffusivity of hydrogen in aluminum. Isothermal desorption of hydrogen from annealed, uniformly charged, cylindrical specimens was bulk diffusion controlled. Diffusion data exhibited simple Arrhenius behavior between 30 and 600°C and is described with 95% confidence by D=1.75±0.15×10 −8( m 2/s ) exp{−16.2±1.5( kJ/mol)/RT}. Constant heating rate desorption spectra revealed three distinct trapping states with desorption energies 15.3±4.8, 43.5±17.5, and 84.8±32.2 kJ/mol. These trapping states are associated with interstitial lattice sites, dislocations, and vacancies, respectively. Identification, quantification, and understanding the role of these trap states in permeation and desorption experiments helps explain the large scatter in reported values of the diffusivity of hydrogen in aluminum.
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