The diffusion and trapping of H in 5N-purity polycrystalline material was investigated by using isothermal and constant heating-rate desorption techniques. The latter technique was used to clarify the effects of microstructural trap states upon the apparent diffusivity of H. The isothermal desorption of H from annealed uniformly-charged cylindrical specimens was controlled by bulk diffusion. The data (figure 3) obeyed a simple Arrhenius relationship, at temperatures of between 30 and 600C, and could be described with 95% confidence by:

D (m2/s) = 1.75 x 10-8 exp[-16.2(kJ/mol)/RT]

The constant heating-rate desorption spectra revealed 3 distinct trapping states, with desorption energies of 15.3, 43.5 and 84.8kJ/mol. These trapping states were associated with interstitial lattice sites, dislocations and vacancies, respectively. They were suggested to explain the large scatter in the reported values of H diffusivity in Al.

G.A.Young, J.R.Scully: Acta Materialia, 1998, 46[18], 6337-49

Figure 3

Diffusivity of H in Polycrystalline 5N-Purity Al