Passivation oxide films and rust layers on iron

Abstract

The passivation film and the rust layer on iron and steels were compared, both of which are considered not to be very different in composition from each other. (1) The passivation film on iron is formed in a strongly oxidative environment, i.e., under highly anodic potential, while the rust layers on iron and steels are formed by mild oxidation-reduction cycles. On the formation of the rust layer, reduction of iron oxide or hydroxide to low valence below three may be an important step in wet conditions. In the passivation film, the film thickness is limited to a few nm, because the ionic transport through the film follows the high-field assisted ionic migration mechanism, while the rust layer growing with oxidation-reduction cycles reaches several hundred μm thickness. (2) For the passivation film, the anodic potential applied to an iron electrode is sustained by the dielectric layer of space charge, and the impedance response reveals the definite presence of a dielectric layer. The potential drop is distributed in the whole passivation film. For the rust layer, impedance response reveals a resistive property in enough small frequencies. The resistance may be a resistance against ionic diffusion. For protectiveness of the rust layer, an ionic perm-selectivity may be one of the key factors, particularly under exposure to marine environments, where the rust layer exhibiting a cationic perm-selectivity may be more protective.