ON THE MECHANISM OF HYDROGEN TRANSPORT THROUGH THE PASSIVE OXIDE FILM ON IRON

Hydrogen permeation experiments have been performed to investigate the hydrogen transport through the passive oxide film formed on the exit surface of iron membranes. Charging procedure, membrane thickness and thermal treatment were selected to allow fast stepwise changes between well-defined values of hydrogen input fugacity at the metal/oxide interface. The oxide was formed by anodic polarization of the iron exit surface in 0.1 N NaOH solution. Hydrogen permeation was studied at different stages of the film growth process, at times ranging from 150 to 3 x 10(5) s from the beginning of anodic polarization. The temperature range was 298 K less-than-or-equal-to T less-than-or-equal-to 348 K. The metallic phase can be regarded as an iron-hydrogen alloy where sudden changes in hydrogen contents can be achieved. Both steady state hydrogen flow and iron flow through the oxide film (oxide build-up current) can be described in terms of ion jumps between potential wells under the influence of a high electric field (hopping model). An ionic charge ratio of migrating species Z(H)/Z(fe) of ca 0.3 was experimentally found.