Formation of Iron Oxides by Surface Oxidation of Iron Plate

Oxidation of iron plates (alpha-phase) at high temperatures and in atmospheric conditions was monitored. The composition of oxidation products was analyzed with XRD, Raman and Mossbauer spectroscopies, whereas the morphologies of oxidation products were inspected by FE-SEM. The oxidation products formed at 300 and 400 degrees C consisted dominantly of magnetite and small fractions of hematite, whereas at 500 and 600 degrees 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 degrees C and vertically grown hematite spikes against the lower oxidation layers at 600 degrees C. Oxidation products formed at 800 degrees C consisted of wustite (Fe1-xO) as the dominant phase, nonstoichiometric magnetite (Fe3-xO4) and hematite (alpha-Fe2O3) 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 alpha-Fe to Fe1-xO and its transformation to Fe3-xO4 that further transforms to alpha-Fe2O3, probably via a short-lived gamma-Fe2O3 (maghemite) phase