Formation of iron oxides by surface oxidation of iron plate

Abstract

Oxidation of iron plates (α-phase) at high temperatures and in atmospheric conditions was monitored. The composition of oxidation products was analyzed with XRD, Raman and Mössbauer spectroscopies, whereas the morphologies of oxidation products were inspected by FE-SEM. The oxidation products formed at 300 and 400 °C consisted dominantly of magnetite and small fractions of hematite, whereas at 500 and 600 °C hematite was the dominant phase, as shown by XRD. In all these samples Raman spectra showed the presence of hematite in the outer oxidation layer. FE-SEM analysis showed the formation of nanowires at 500 °C and vertically grown hematite spikes against the lower oxidation layers at 600 °C. Oxidation products formed at 800 °C consisted of wüstite (Fe 1-xO) as the dominant phase, nonstoichiometric magnetite (Fe 3-xO 4) and hematite (α-Fe 2O 3) in small fractions. The surface of these oxidation layers showed a hierarchical microstructure, as well as the hexagonal hematite rods vertically grown against the lower oxidation layers. The formation of the oxidation products can be considered a process which includes the oxidation of α-Fe to Fe 1-xO and its transformation to Fe 3-xO 4 that further transforms to α-Fe 2O 3, probably via a short-lived γ-Fe 2O 3 (maghemite) phase.