Regularities of Vacuum Oxidation of Iron in the Range of Low-Temperature Passivation According to the Data of Spectral Ellipsometry

Abstract

The methods of spectral ellipsometry and nanotomography are used to study the kinetics of formation of oxide layer phase components (magnetite and hematite) on the iron surface under the conditions of vacuum treatment, in the region of low-temperature gas passivation manifestation. In the course of oxidation at the temperature of 300°C and vacuum treatment at 1 Torr, an island and, then, a solid layer of magnetite grows on the surface of iron. Further oxidation results in growth of α-Fe₂O₃ in the form of plates at the magnetite–gas boundary depthward into magnetite. It forms an island film consisting of hematite microcrystallites on the surface of magnetite when this magnetite surface is coated. Island coalescence occurs under longterm oxidation exposure, which leads to formation of a solid layer consisting of hematite microcrystallites with thin intergrain boundaries. Here, a “puzzle” surface structure is observed, in which crystallite boundaries approximately correspond to their neighbors and, therefore, result in complete coating of the surface. Such a layer efficiently hinders oxygen diffusion, which passivates the metal and prevents formation of a thick magnetite layer.