The energetics and mobility of neutral H in alumina were studied by using ab initio density-functional calculations. The mobility of H was studied in corundum (α-Al2O3) as well as in liquid alumina. By using static as well as molecular-dynamics calculations, and applying classical transition-state theory, the temperature-dependent diffusivity of H in α-Al2O3 was deduced to be described by:

D (m2/s) = 2.17 x 10-7 exp[-1.24(eV)/kT]

The corresponding diffusivity of H in liquid/amorphous alumina, as deduced directly from ab initio molecular dynamics calculations, was described by:

D (m2/s) = 8.71 x 10-7 exp[-0.91(eV)/kT]

The computed diffusivity compared well with experimental data. It was concluded that the diffusion of neutral H through bulk alumina was a good approximation to the mechanism for H mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina was therefore thought to be realistic at higher temperatures.

First-Principles Study of Hydrogen Diffusion in α-Al2O3 and Liquid Alumina. A.B.Belonoshko, A.Rosengren, Q.Dong, G.Hultquist, C.Leygraf: Physical Review B, 2004, 69[2], 024302 (6pp)