Abstract
The permeation of hydrogen through the passivating oxide film on iron has been studied by a modulation method as a function of cathodic charging potential, anodic passivation potential and temperature. The diffusivity of hydrogen in the oxide film is determined from the measured phase shift between the input hydrogen concentration and the hydrogen flux at the passivated side of the specimen. The diffusivity of hydrogen in the oxide is found to increase with increasing hydrogen content in the oxide produced either by increasing the input concentration of hydrogen or by decreasing the passivating potential. The electric field across the oxide, and the potential drop and the hydrogen concentration ratio across the metal-oxide interface are evaluated. The concentration ratio is explained in terms of the interfacial potential drop and the ratio of activity coefficients of hydrogen in the two phases.
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