Corrosion of epitaxial Fe(0 0 1) films studied with CEMS and AFM

Abstract

Epitaxial Fe(0 0 1) films of a high crystalline and surface quality, as checked in situ with Auger electron spectroscopy, low energy electron diffraction and STM, were grown on MgO(0 0 1). Such films show an exceptional corrosion resistance remaining metallic for years at the ambient atmosphere, except of a 5–10 nm surface layer, as found using the conversion electron Mössbauer spectroscopy (CEMS). For more systematic corrosion studies, 50 nm iron films were prepared of the 57 Fe isotope. As-prepared Fe films, exposed to the atmosphere did not show clear morphological and compositional changes, which could be associated with the oxide formation, as checked with contact AFM in air and with CEMS. After exposure of the film to HNO 3 or HCl vapors corrosion was observed with AFM. It resulted in two different types of surface modifications. The first one was seen as a homogenous granular layer with the height amplitude of about 15 nm. Corrosion-induced changes of the second type have a localized character resulting in micrometer sized porous protrusions. The corrosion products were identified as γ-FeOOH. Controlled oxidation of a surface iron layer to epitaxial Fe 3O 4 passivated the film.