Hydrogen permeation barrier – Recognition of defective barrier film from transient permeation rate

Hydrogen permeation barrier films often exhibit lower efficiency than anticipated. The cause could be defects in the barrier film, high permeability of the defect-free (dense) barrier film, or a combination of both. It is very difficult to point at and quantify the responsible mechanism since the defects can be of submicrometer dimensions and very sparsely populated. This study addresses the recognition of the defects in the hydrogen permeation barrier films using the hydrogen permeation rate transient evolution analysis. For this purpose a mathematical model of the steady-state and transient hydrogen permeation through the membrane coated either with a defective or a defect-free barrier film was developed for the diffusion limited permeation regime. Analysis shows that a defective barrier film might be recognized only in a transient permeation experiment. The effective diffusion coefficient of the membrane with the defect-free barrier film is variable and depends mainly on the ratio of diffusion coefficients in the film and the substrate. Contrary to this, the transient permeation only through pinholes has a constant value of the effective diffusion coefficient. Result of the study is an experimentally useful criterion when and how the permeation through the defects in the barrier layer can be recognized and its extent determined.