National Research Nuclear University “MEPhI”, Moscow, Russia:

M. G. Isaenkova, Professor, the chair “Physical problems of material science”, Doctor of Physical and Mathematical Sciences, e-mail: isamarg@mail.ru
Yu. A. Perlovich, Professor, Doctor of Physical and Mathematical Sciences
S. D. Stolbov, Post-graduate Student
K. E. Klyukova, Post-graduate Student
V. A. Fesenko, Scientific Researcher

Vacuum Technology Laboratory LLC, Zelenograd, Moscow, Russia:
E. V. Berlin, General Designer

This paper compares the corrosion behavior of two chromium coatings obtained by different technological conditions. Depending on the process characteristics of creating coatings, a different structure was formed, which characterized by different orientations and grain sizes. The main differences of the investigated coatings were as follows: coating No. 1 was 9–10 μm thick, characterized by 0.2 μm columnar crystals and {111} <112> strict orientation throughout its thickness; coating No. 2 had a thickness of 12–14 μm, the size of the columnar crystals was 0.5 μm and their orientation was {111} <112> and {100} <001>. The coatings also differed in the level of compressive tangential macrostresses on the outer surface: in the first one, 1000 MPa, and in the second, only 490 MPa, which indicated the possibility of cracks presence in it. The presence of cracks was confirmed by metallographic images. Oxidation of cladding tubes with chromium coatings was carried out by their annealing in air in the temperature range of 400–1150 ^oC for 1 hour. As a result of electron microscopic study of over the cross section elements distribution of oxidized samples, the main differences in the coatings oxidation kinetics obtained in different modes were established. In the initial state, both coatings interact with the substrate without the formation of intermediate phases, the chromium layer is replaced by a layer of zirconium alloy. In both cases, the transition zone is 2–3 microns. After annealing in air at a temperature of 1100 ^oC for 1 h, an intermetallic layer (Zr, Nb)Cr₂ with a thickness of 2–3 μm is formed between the coating and the substrate. Intermetallide identified by synchrotron diffraction coating study. The first coating changes its thickness to 5–6 microns, while on the surface it is not possible to detect the oxide phase. In the case of the second coating oxidation, a 3–4 μm layer of the oxide phase is observed, easily identified by X-ray and electron microscopic methods, which leads to a reduction in the thickness of the chrome coating. The pore distribution is also interesting: in the case of the first coating, the pores are located at the boundary of the intermetallic and chrome layers, and in the case of the second coating, the pores are distributed over the cross section of the chromium layer and in the oxide layers. The oxide layer thickness from the inner surface of the cladding tube is about 250 microns. The chrome coating, in the absence of cracks in it, reliably protects the cladding tube from oxygen. In the absence of cracks in the chrome coating, it reliably protects the cladding tube from oxygen. The presence of pores is due to differences in the temperature expansion of layers consisting of different metals.
The present work was carried out under Governmental Support of Competitive Growth Program of NRNU MEPhI (agreement No. 02.a03.21.0005).

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