The objective of this work was to investigate the kinetics of the two-phase oxide film growth on the α-Fe surface at temperatures of 650–750 K. We experimentally determined that the film thickness (h)–time oxidation (τ) relationship in the range denoted above is a logarithmic function, whereas Cabrera and Mott’s theory gives a square law of film growth. In our work, analytical treatment of experimental data was made based on this theory, but we propose that self-deceleration of the film growth is caused not by attenuation of the electric intensity in the film because of an increase of h but by the shielding influence of the space charge of diffusing ions and electrons in that oxide film. With that purpose in view, the Debye shielding distance for plasma substance state in the oxide film was taken into consideration. The logarithmic law of oxide film growth was derived. Estimated calculations of this law’s parameters were made that quantitatively correspond with literature data. The results obtained were used in developing the surface oxidation technology of electric steel.
Every year, the steelmaking industry produces millions of tons of slags resulting in pollution to the environment. Among the waste, secondary metals and scales rich in iron oxides are also thrown away. There is a need to treat the steel waste in a reasonably way to protect the environment and proposing new cheap technologies for producing advanced materials. In this study we report the morphological and structural characterization of waste scales generated during roll milling steel process at JSC “Arcelor Mittal Temirtau”. The raw slag and annealed at 1000 °C were measured by X-ray diffraction (XRD), scanning electron microscopy adapted with energy dispersive X-ray (SEM- EDX), magnetometry and Mössbauer Spectroscopy (MS). Fe and O were detected by EDX as main chemical elements and Si, S, Ca, Mg, C and Al as minimal elemental composition. XDR for the raw sample revealed α-Fe2O3 (hematite) and Fe3O4 (magnetite) as principal and secondary phase, respectively; whereas monophasic α-Fe2O3 is detected for the scales annealed at 1000 °C. Magnetometry measurements show the Verwey transition for the raw sample and the Morin transition for the annealed at 1000 °C; those are fingerprints for the presence of magnetite and hematite, respectively. MS measurements for the raw sample consist of 6 small peaks of absorption and a broad two-lines absorption peak in the central part. The doublets are associated to the hyperfine parameters belonging to wustite. Magnetite is related to the hyperfine parameters for two sextets in octahedral Fe2.5+ and tetrahedral Fe3+sites and a small sextet that resembles the Mössbauer parameters of α-Fe2O3. Only a well crystallized and weakly ferromagnetic sextet confirm the presence of α-Fe2O3 phase for the sample annealed at 1000 °C due to thermal oxidation.
In this work we present a model for the surface oxide film growth considering the influence of space charge. The space charge field E sp is assumed proportional to the charge of moving metal ions and electrons in the oxide layer. The surface charge field E ox decreases as the Cabrera and Motts' oxide thickness X grows to its limit X 1 (E ox = V M /X). E ox remains constant for further growth of the oxide film (E ox = V M /X 1 ). The obtained equation for the growing rate of the oxide film covers two stages. The first stage is characterized by a negligible space charge and is described by the typical inverse logarithmic law. During transition from thin to thick film, the oxidation growth rate is described by a direct logarithmic law which is confirmed by many experiments. At the end of this stage, the drift of metal ions is replaced by their diffusion that leads to parabolic law.
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