engleski

The microstructure changes in the protective magnetite film

The protective magnetite film of the system of the boiler tube in the thermal power plant is the product of the water steam and iron at the temperatures above 250°C [1].The incurred film is of grayish black color, uniformly overlapping the inner surface of the tube, and has a high adhesiveness as well as linear coefficient of stretching almost the same as the wall of the tube [2]. While the thermal power plant is operating, the thickness of the protective magnetite film is changing, according to Milobari [3], from m. At the surface of the film the appearance of sludge and accumulation of various sizes and shapes can be noted which in certain moments can cause damage [4]. In practice its appearance can be avoided by continually measuring the electrical conductivity and density of the cooling feed water. In certain time intervals, the thickness of the film is measured and the changes in the microstructures are examined. In this work the changes of the thickness of the film and microstructures after 80000 hours of work of the thermal power plants are investigated. In the investigation the samples were taken along the system of the broiler tubes at the four characteristics places: A, B, C and D as the pieces of the tube. Every part taken from the tube was cut in strips. From the surface of one strip the film was mechanically removed along with the sludge and was prepared for measurements. The second sample of the strip was further cut into plates and directly measured using the Philips diffractometric system using CoKα radiation and the Joel scanning electronic microscope [5, 6]. The results of the measurements were worked out mathematically and point the occurrence of the following changes: the thickness of the protective magnetite film (Figure 1), the concentration of the Fe3O4 phase (Figure 2), and the microstructures (Figure 3) as the system of the boiler tubes are exploited. The changes that appear are caused by the presence of the local corrosion, the intensified oxidation effects of the water steam, ununiformed heating of the tubes as well as the sedimentation of the particles within the film.

protective magnetite film; thickness; concentration of Fe3O4; microstructure

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