It was recalled that time-lag techniques were a powerful means for determining H diffusion coefficients in metals, but that the quality and reliability of such measurements depended strongly upon an exact characterization of the experimentally established boundary conditions as used for the mathematical evaluation of the measured time-lag curves. A change in the inner and/or outer boundary conditions could create another type of time-lag experiment, requiring a different solution to the diffusion equation. In many cases, the metal foil was coated with surface layers which had H solubilities and diffusivities that were different to those in the metal. The surface layers governed the boundary conditions at the interface between the metal and the surface layer. In these cases, solutions to the diffusion equation for multi-layer geometries had to be found, and complete analytical solutions usually did not exist. However, numerical calculations permitted these problems to be solved. The results of such calculations were presented for double, AB, and triple, ABA, layer-systems for cases where A or B controlled the time-lag, and also for situations intermediate between those extremes. The simulations could also describe transitions from one type of time-lag experiment to another.

Multi-Layer Problems in Time-Lag Measurements Applied to Metal-Hydrogen Systems. H.Zuchner, L.Opara, H.Barlag: Journal of Alloys and Compounds, 1999, 293-295, 282-8