Hydrogen permeation through oxide and passive films on iron

Hydrogen permeation through thin films of Fe_I–Y O on iron and through chemically polished iron were investigated by the sensitive electrochemical technique. The oxide was formed on the exit side of the sample membrane. The hydrogen arriving at the iron/oxide interface is in an atomic or protonic state which renders the hydrogen uptake by the oxide possible. The wustite films were formed by oxidation in a H₂O-H₂-atmosphere. The dependence of the hydrogen permeation current on temperature and film thickness and different degrees of nonstoichiometry in Fe_I–Y O was studied. Hydrogen permeation through these oxides is possible, but very low permeation coefficients have been found, of the magnitude of at 25°C. The diffusion coefficient of diffusible hydrogen was determined to be about 4 · 10^?10 cm²/s. Measurements of the potential dependence of permeation across the film indicate that hydrogen migrates in the oxide as a charged particle (proton). In the case of the passive surface film on iron formed by chemical polishing, the dependence of the permeation current on temperature and anodic potential was measured. The electrochemical behaviour of the film was studied by cyclic voltametry. Electron transfer reactions were investigated by means of the hexacyanoferrate (II/III) redox system. Further information on the film composition were obtained by Auger electron spectroscopy. On the one hand, electron transfer across the film can occur, but on the other hand, the film is nearly impermeable for hydrogen, even if the hydrogen is in the atomic or protonic state. Cyclic voltamograms show the formation of an oxygen adsorption layer on the film in a range of anodic potential.