ON THE MECHANISM OF HYDROGEN TRANSPORT THROUGH THE PASSIVE OXIDE FILMON 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 int erface. The oxide was formed by anodic polarization of the iron exit s urface in 0.1 N NaOH solution. Hydrogen permeation was studied at diff erent 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 sud den changes in hydrogen contents can be achieved. Both steady state hy drogen flow and iron flow through the oxide film (oxide build-up curre nt) can be described in terms of ion jumps between potential wells und er the influence of a high electric field (hopping model). An ionic ch arge ratio of migrating species Z(H)/Z(fe) of ca 0.3 was experimentall y found.