Rokurou Nishimura1, Kiyokatsu Kudo2 and Norio Sato2
1Graduate School, Hokkaido University, Sapporo
The composition of anodic passive film on iron in a neutral borate-buffer solution at pH 8.42 has been studied as a function of potentials ranging from passivation to transpassivation by using ellipsometric and electrochemical techniques.
Ellipsometric measurements reveal that there are three different potential regions with respect to the Δ P- Δ A relationship, i.e. a region more cathodic to +0.245 V (S.H.E.) (region I), a region from +0.245 V (S.H.E.) to the oxygen evolution potential (region II) and a region of transpassivity (region III). It is also found that the anodic passive film consists of a barrier layer and a deposit layer.
The barrier layer thickness increases linearly with the passivating potential in region I and region II, whereas it appears independent of anodic potential in region III. The deposit layer thickness is affected to some extent by the anodic potential.
The film dissolution measurements by means of cathoedic reduction in a solution of pH 6.48 at constant current indicate that the film formed in region I includes Fe2+ ions in both the barrier layer and the deposit layer, while the film formed in region II includes no Fe2+ ions in both layers. The film formed in region III contains no Fe2+ ions in the deposit layer but it contains Fe2+ ions in the barrier layer. Furthermore, there is a layer of concentrated Fe2+ ions at the interface between the barrier layer and the deposit layer in the film formed at potentials more cathodic to +0.8 V (S.H.E.), whereas a depletion layer of iron ions is formed at the barrier/deposit interface at potentials more anodic to +0.8 V (S.H.E.).
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