The oxidation of a Co–15 wt% yttrium alloy in 1 atm O2 at 600–800°C

The oxidation of a cobalt-based alloy containing 15 wt% yttrium (Y) as well as of pure yttrium has been studied in 1 atm of pure oxygen at 600–800°C. The alloy always corrodes faster than pure cobalt and follows the parabolic rate law only approximately, forming an external scale of cobalt oxide which in its inner section contains particles of yttrium oxide. Beneath the external scales, a region of internal oxidation of yttrium having a depth much larger than predicted using the known solubility and diffusivity of oxygen in pure cobalt is also present. This anomalous depth is attributed to enhanced oxygen diffusion in the alloy along preferential paths associated with the localized internal oxidation of yttrium. No yttrium depletion is observed in the alloy beneath the front of internal oxidation. The scaling behavior of this alloy is interpreted by taking into account the limited solubility of yttrium in cobalt and the presence of the intermetallic compound Co17 Y2.