The Anodic Oxide of Iron: Its Component Layers and Their Properties

Abstract

The anodic oxidation of an iron electrode in a neutral borate electrolyte is studied using a following ellipsometer. The oxide film is found to consist of two layers with different refractive indices: an outer layer of Fe2O3 and a thicker inner layer of Fe3O4 . A thin layer of Fe3O4 is grown on the electrode in the active state, and the electrode passivates when a partial monolayer of Fe2O3 is grown at the Fe3O4‐normalelectrolyte interface. Passive state anodic oxidation causes simultaneous growth of the two layers, the inner layer growing approximately 80% faster than the outer layer. We conclude that the Fe2O3 layer inhibits anodic dissolution of the electrode, and that both the Fe2O3 layer and the portion of the Fe3O4 layer grown in the passive state act as the electrically limiting barrier across which the overpotential appears. The possibility that the two layers grow in a proportion dictated by the transport numbers of mobile defects (as has been proposed for the anodic oxide of tantalum) is discussed.

The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman microspectroscopy and electrochemical polarization. Part II: In situ Raman spectra of the passive film surface during growth by electrochemical polarization

“…Given the expected reactions involving oxidation of Fe(OH) 2 → Fe 3 O 4 and the observation of Fe 3 O 4 in previous studies of the passive film on iron in neutral borate and 1 M alkaline solutions,6, 7, 9, 13, 24 it is somewhat surprising that no Fe 3 O 4 was fitted in the MCR optimization. The absence of Fe 3 O 4 bands may be the result of oxidation of the outer part of Fe 3 O 4 to γ‐Fe 2 O 3 ,8 which would agree with the two‐layer model,44, 45 with an inner layer composed of Fe 3 O 4 and γ‐Fe 2 O 3 with the Fe 3 O 4 adjacent to the metal surface and the γ‐Fe 2 O 3 adjacent to and underneath a layer of a loosely bound outer layer of FeOOH, so that the Fe 3 O 4 is not detected even though it may be present.…”

The Composition of Anodically Formed Iron Oxide Films

“…Staehle and co-workers (5), using Auger analysis, concluded that the film was duplex in nature. This conclusion had also been reached by Ord (6) using ellipsometry. In other work, Kruger (7) suggested that the film contained a small amount of hydrogen and postulated, following Bloom (8), that the outer layer of 7-Fe20~ was stable only if it contained a small amount of H (~0.06%).…”

An evaluation of permeable reactive barrier projects in California

“…As an option for treating chlorinated solvents in groundwater, iron powder showed clear promise as it could dehalogenate these compounds (Ho & Ord, 1972). However, iron powder is limited in how it can be handled, especially in how water can be filtered through it.…”