ANODIC IRON-OXIDE FILMS AND THEIR EFFECT ON THE HYDROGEN PERMEATION THROUGH STEEL

The formation and hydrogen transport properties of the anodic iron oxide film formed in 0.1 N NaOH has been investigated at T = 308 K by means of the electrochemical hydrogen permeation technique. A constant hydrogen fugacity was achieved at the input side of steel membranes through a gas phase charging at P(H-2) = 0.101 MPa on a palladized surface. The iron oxide has been allowed to grow at the exit (detection) side by omitting the usual palladium plating at this surface. In steady-state permeation, the iron oxide film behaves as a series impedance for the hydrogen transport. A mechanism of diffusion enhanced by an electric field is compatible with the experimental results. An estimation of hydrogen diffusion coefficient in the oxide phase was made based on this model. A prolonged reduction-oxidation cycle results in a modified oxide film at the exit site, which increases the apparent hydrogen permeation current of the membranes up to four times above those with palladium-plated exit surfaces, thus suggesting that the presence of hydrogen within the film causes a partial loss of the passivating properties of the film and an increase in the corrosion current.