High-temperature oxidation of zirconium alloys with magnetron sputtered chromium coatings

Abstract

Loss-of-coolant accidents (LOCAs) often lead to dire consequences. That’s why the global community are actively engaged in research aimed at creating accident tolerant fuels. One of the areas of such research is preserving zirconium alloys as the material for fuel-element claddings by creating corrosion-resistant coatings that would be in contact with the coolant. Chromium could be a promising material to be used in such protective coatings as chromium oxide (Cr2O3) serves as an effective barrier for oxygen both in normal operation and in case of LOCA. Chromium coating hinders oxygen diffusion into the metal substrate and thus prevents embrittlement and failure of the fuel-element cladding. This paper describes the results of research studies that have been carried out in recent years and that look at the resistance of magnetron sputtered chromium coatings to high-temperature oxidation in water vapour up to 1,500 oC. The paper demonstrates advantages and drawbacks of chromium coatings in hightemperature oxidation conditions. The focus is on understanding how regimes of magnetron sputtering influence the resistance of chromium coatings to high-temperature oxidation and how the structure and phase state of chromium coatings are related to their properties. The authors describe the optimal regimes of magnetron sputtering for obtaining dense coatings and examine the effect of the substrate temperature and the bias voltage on the structure and density of resulting coatings. High Power Impulse Magnetron Sputtering (HiPIMS) serves as an effective technique that helps enhance the density of coatings. The conclusion drawn is that in order to broaden the temperature range in which chromium coatings can effectively protect zirconium alloys from failure up to 1500 oC, there should be a diffusion barrier between the surface of the fuel-element cladding and the chromium coating. The authors would like to thank B. A. Kalin, supervisor of this project, who had passed away before this publication was made. The staff of the Laboratory of Ion-Plasma and Ion-Beam Machining of Materials, a part of Department No. 9 at the National Research Nuclear University MEPhI, devote this paper to his memory.