Multi-layered silicides coating for vanadium alloys for generation IV reactors

The halide-activated pack-cementation technique was employed to fabricate a diffusion coating that is resistant both to isothermal and to cyclic oxidation in air at 650 °C on the surface of the V–4Cr–4Ti vanadium alloy that is a potential core component of future nuclear systems. A thermodynamic assessment determined the deposit conditions in terms of master alloy, activator, filler and temperature. The partial pressures of the main gaseous species (SiCl4, SiCl2 and VCl2) in the pack were calculated with the master alloy Si and the mixture VSi2 + Si. The VSi2 + Si master alloy was used to limit vanadium loss from the surface. The obtained coating consisted of multi-layered VxSiy silicides with an outer layer of VSi2. This silicide developed a protective layer of silica at 650 °C in air and was not susceptible to the pest phenomenon, unlike other refractory silicides (MoSi2, NbSi2). We suggest that VSi2 exhibits no risk of rapid degradation in the gas fast reactor (GFR) conditions.