Investigation of the initial stage of iron oxidation in the air. Part 1. Temperature range 250-400 ºС

Abstract

Iron oxidation in the case of formation of thin oxide films on the surface of iron plates as a result of their oxidation in air by a logarithmic law has been studied. Experimental studies of kinetics and thermodynamics of the oxidation of chemically pure iron in a horizontal tubular quartz glass furnacehave been performed. The samples were heated by a bifilar winding at both ends of the tube. The temperature was measured by the calibrated chromel-alumel thermocouples. The time of occurrence of colors was averaged over three plate, on each of which at least 4-5 measurements were taken. The experimental determination of the thickness of the oxide film depending on time is based on the method of occurrence of variability colors on the surface of the oxidized iron plate. With an increase in the thickness of the oxide filmits color as a result of light's interference will consistently vary according to the series: yellow, orange, red, purple, violet and blue.As a result of the study, it was found that oxidation at 253 ° C significantlydiffers from the results obtained at temperatures of 300-375 ° C. The oxidation at 253 ° C is more complex, since two types of oxides are formed. It has been assumedthat the rate of oxidation depends on the index of the faces of the crystal lattice. Dependences of the thickness of the film (by color) on the time of its formation in a heated furnace at a constant temperaturehave been obtained. It has been shown that in the temperature range 250 - 400 oС the law of the change in the thickness of the film is logarithmic. In this case, the oxidation constant in this law changes with the temperature according to the Arrhenius law. For the first time, an oxidation constant for pure iron was evaluated at the initial oxidation stage. Its dependence on the temperature was obtained and the corresponding values of the activation energy were found.