Ellipsometry of the passivation film on iron in neutral solution

Abstract

Ellipsometric and electrochemical measurements have been made of the passivation film on iron during its potentiostatic anodic formation and galvanostatic cathodic reduction in boric-acid-sodium-borate buffer solution. The complex refractive index estimated ellipsometrically of the cathodically reduced iron surface is identical with that of iron, indicating absence of any oxide films on it. The index of the passivation film comes close to that of spinel-type iron oxide, Fe 3O 4 or γ-Fe 2O 3. The passivation occurs at a minimum film thickness of about 9 Å and the film thickens up to 50 Å at the onset potential of oxygen evolution. The film thickness is linearly related to the oxidation potential in a limited potential region, which extrapolates to a film thicker than a monolayer even at zero anode overvoltage. The distribution of the potential in the film is discussed by a non-uniform film model consisting of an inner layer of intrinsic, anhydrous ferric oxide and an outer layer of hydrous, semiconducting ferric oxide. Ellipsometric measurements also reveal that a surface film is formed on actively dissolving iron, and electrochemical measurements disclose transpassivation to occur above a critical potential in the oxygen-evolution region.