Effect of surface nitriding on the durability of a Ti-6Al-4V alloy in high temperature pressurized liquid water
Résumé
The use of titanium alloys in the primary circuit of pressurised water nuclear reactors (PWR) is gaining interest due to their low neutron activation combined with high mechanical properties and good corrosion resistance in PWR primary water. These alloys surface can be nitrided to improve their wear resistance. The aim of the present work is therefore to determine the effect of nitriding on the durability of a Ti-6Al-4V (Grade 5) in primary water.
Nitrided and as-received samples were exposed to simulated primary water (pure water with 2.25 wt ppm of dissolved hydrogen at 300 °C, 150 bar and at 350 °C, 175 bar) in stainless steel autoclaves and corrosion loops up to several weeks. Tensile loads were applied to some specimens in order to evaluate chemo-mechanical coupling.
The near-surface microstructure evolution as a function of exposure time was thoroughly characterised by electron microscopy (SEM and TEM) along with image analysis, EDS, WDS, XPS, XRD and µ-Raman spectroscopy. Additionally, weighing, hydrogen analysis and microhardness testing were performed.
The nitrided samples exhibited a 2-μm thick surface layer of titanium nitrides and a nitrogen diffusion layer, which increased the surface hardness as expected.
After exposure to primary water, both the nitrided and as-received samples developed a continuous TiO2 layer on top of which grew micrometric FeTiO3 crystallites and sub-micrometric TiO2 crystallites. The nitrided specimens presented a lower weight gain and a lower coverage of the surface by crystallites, suggesting that the nitriding treatment improves the overall corrosion resistance of the material. Hydrogen uptake was also quantified for each exposure duration. Samples subjected to high tensile stress exhibited cracks in the nitrogen-containing layer, which increased the surface exposed to water and the local oxidation. All these results will be discussed and corresponding oxide formation mechanisms will be proposed.