This article described the protective properties of Cr coatings with a barrier layer composed of ZrO2/Cr multilayers deposited onto E110 zirconium alloy. The coatings with a ZrO2/Cr multilayer thickness of 100, 250, and 750 nm and single-layer (1.5 µm) ZrO2 barrier were obtained by multi-cathode magnetron sputtering in Ar + O2 atmosphere. Then, cracking resistance and oxidation behavior were studied under conditions of thermal cycling (1000 °C) in air and high-temperature oxidation at 1200–1400 °C in a water steam. The role of the ZrO2/Cr multilayers and multilayer thickness on cracking resistance of the experimental coatings and oxidation resistance of the coated E110 alloy was discussed. It was shown that the coatings with more quantity of the ZrO2/Cr multilayers have higher cracking resistance, but such types of samples have a large amount of coating spallation under thermal cycling. The high-temperature steam oxidation (1200–1400 °C) demonstrated that interfaces of the ZrO2/Cr multilayers can act as a source of cavities formed by the Kirkendall mechanism that results in accelerating Cr–Zr interdiffusion for Cr-coated E110 alloy.
As the materials for fuel claddings for water-cooled reactors, binary and multicomponent zirconium alloys zirconium-niobium (Zr-Nb) (E110, M5), zirconium-tin-iron (Zr-Sn-Fe) (Zircaloy-2,-4), and zirconium-niobium-tin-iron (Zr-Nb-Sn-Fe) (E635, ZIRLO®) most commonly are used. Improvement extends, in particular, by varying their composition by niobium, tin, and iron. At the same time, importance is given to ensuring the safe operation of the fuel rods not only in normal conditions but also in emergency situations such as a loss of coolant accident. In this paper, we present the research results of studies of the influence of alloying elements niobium, tin, and iron on corrosion resistance and embrittlement at high-temperature steam oxidation of fuel claddings based on the alloy types Zr-xNb (x = 1.0 ÷ 2.5) and Zr-xNb-ySn-zFe (x = 0.6 ÷ 2.4; y = 0.24 ÷ 1.1; z = 0.18 ÷ 0.34) made by using zirconium sponge. We conducted high-temperature steam oxidation tests at temperatures of 1,000°C, 1,100°C, and 1,200°C with continuous measurement of the weight gain during the experiment and subsequent cooling in steam with rate ∼20°C/s. We conducted studies of kinetics of high-temperature oxidation in steam, structural-phase state changes, alloying elements distribution, absorbed hydrogen content, and residual ductility after rapid cooling of the oxidized specimens. We revealed the difference in the oxidation kinetics of the materials studied, which decreased as the oxidation temperature increased. Likewise, an increase of the tin content in the alloy influenced a specimen's breakaway oxidation intensification at 1,000°C. This shows that the mechanical properties of fuel claddings oxidized under the same conditions depend on their alloying composition, and the hydrogen fraction is absorbed by the specimen and formed by the oxidation of ZrO2, α-Zr(O), and ex-β layer structures, which in turn varies depending on the alloy composition. The increase of niobium, tin, and iron content in the zirconium alloy leads to a decrease in the residual ductility of fuel claddings after high-temperature steam oxidation. According to obtained results, among the materials studied, Zr-1.0Nb alloy is the most resistant to embrittlement during high-temperature steam oxidation at temperatures of 1,000–1,200°C.
A new technological process of manufacturing ingots for fuel rod claddings made of the electrolytic zirconium-based E110 alloy, which guarantees the removal of residual fluorine impurities in the ingot to less than the 1 ppm level, has been developed in order to ensure claddings' resistance to breakaway oxidation in high-temperature steam. The new technological process involves using electron beam melting by optimized modes as the first melting of the ingot and then at least two optimized vacuum arc melts. Pilot tubes for fuel rod claddings made of such ingots are not susceptible to breakaway oxidation in steam at a temperature of 1,000°C for 5,000 s to the oxidation state of 24% equivalent cladding reacted samples, which is similar to the behavior of fuel rod claddings made of the E110 alloy based on zirconium sponge and that meets the relevant safety criterion of a loss-of-collant design accident.
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