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The paper presents results of studies on the crystallite sizes of oxide layer formed during a long-term operation on steel at an elevated temperature. This value was determined by a method based on analysis of the diffraction line profile, according to a Scherrer formula. The oxide layer was studied on a surface and a cross-section at the inner site on the pipe inlet, at the fire and counter-fire wall of the tube. X-ray studies were carried out on the inner surface of a tube (in a flowing medium environment), then the layer’s surface was polished and the diffraction measurements repeated to reveal differences in the originated oxides layer. X-ray phase analysis was performed by the use of a SEIFERT 3003 T/T X-ray diffractometer, with a cobalt source of λCo = 0.17902 nm wavelength. X-ray diffraction measurements were performed in the 20÷ 120◦ range of angles with an angular step of 0.1◦. To interpret the results the diffractograms were described by a pseudo Voigt curve using the Analyze software. A computer software and the DHN PDS, PDF4+2009 crystallographic database were used for the phase identification. The results showed that the crystallite sizes have an effect on the properties of the oxide. It has been shown that the outer oxide layer having larger crystallites, is more porous and thus more brittle.

Analysis of Crystallite Size Changes in an Oxide Layer Formed on Steel Used in the Power Industry