Structure and mechanical properties of oxide films on zirconium alloys upon oxidation under different conditions

The structure and mechanical properties, as well as the mechanism and kinetics of the destruction of oxide films formed on tube specimens of E110 zirconium alloy based on electrolytic or sponge zirconium upon corrosion testing in an autoclave and high-temperature oxidation in steam (LOCA conditions), were studied with the use of transmission and scanning electron microscopy, microhardness measurements, acoustic emission, and fractography. Oxide films on specimens made of electrolytic zirconium upon testing in an autoclave are found to be composed of extended grains with a thickness of 100 nm, while those on sponge specimens are composed chiefly of equiaxial grains with a diameter of 30 nm. After high-temperature oxidation, the structure of oxide films consists of extended grains of a variable thickness, which increases from the surface to the film-metal boundary from 600 nm to 2.5 μm on specimens made of electrolytic zirconium and from 250 nm to 2 μm on sponge-zirconium specimens. The microhardness of films after testing in an autoclave is 1200 ± 50 HV on electrolytic zirconium and 2000 ± 50 HV on sponge zirconium. The hardness of films on sponge-zirconium specimens upon high-temperature oxidation is 1600 ± 50 HV. The combined analysis of deformation diagrams, fractures, and acoustic emission data showed that the destruction of thin oxide films after testing in an autoclave and that of thick films after high-temperature oxidation begins with the formation of transverse brittle ruptures at the same load of 12–15 MPa. The number of cracks in the films on specimens that underwent high-temperature oxidation increases under loading especially rapidly on electrolytic zirconium.