Degradation at 550°C of cast chromium-rich cobalt-based alloys exposed to a H_2O-O_2-CO_2-HCl gaseous mixture

摘要

Cobalt-based superalloys may be considered as other refractory alloys based on nickel or iron for applications in waste-to-energy (WtE) boilers. Knowing their generally lower high temperature oxidation and corrosion resistance by comparison to nickel alloys for example, it can be informative to study how such cobalt alloys may behave in contact with hot atmospheres as complex as what can be met in an WtE boiler. In this work a rather complex cobalt-based alloy (Co-10Ni-30Cr-0.5C-7.5Ta) and a more simple Ta-free one (Co-10Ni-30Cr-0.5C) were synthesized by conventional casting. They were cut and prepared as polished samples, then exposed to a {18% H_2O, 8%O_2, 5%CO_2, 1100ppm HCl} gas mixture at 550°C during 260 hours, in a special lab-scale pilot, directly in the complex gaseous atmosphere for the first one, and immersed in synthetic ashes (NaCl) under the same complex gaseous atmosphere for the second one. After test the Ta-containing sample was subjected to X-ray diffraction and SEM observations. A {10 to15μm}-fhick oxide scale is formed on surface. It seemingly grew both externally (probable cationic diffusion) and internally (probable anionic diffusion), as demonstrated by the shape of the initial TaC carbides still existing in the most internal half of the oxide thickness (still TaC in the alloy part but tantalum/tantalum&chromium oxides in the internal part of the oxide scale). This oxide scale was not chromia but Co(Ni)O in the most external part of the scale and CoCr_2O_4 (spinel) in its most internal part. XRD also showed the presence of NaCl and residual CaSO_4, probably coming from previous ashes tests performed in the lab-scale. These observations are different from previous results obtained for the same alloy but oxidized for about 50 hours at 1000°C in synthetic dry air (external oxide made of chromia (Cr_2O_3), spinel (CoCr_2O_4) and CrTaO_4 films, development of a TaC-free zone in subsurface). Concerning the second alloy (TaC-free) immersed in ashes no protective oxide layer seemed developing (dissolution in the molten salts?) and an internal oxidation/chloridation of the chromium carbides network over a 500μm-depfh from surface happened, maybe through an active oxidation mechanism.