Six conventionally cast chromium-rich titanium-containing alloys based on cobalt and nickel with various Co/Ni ratios were considered. They were tested in oxidation in air at 1250 °C for 70 h in a thermo-balance. The mass gain curves were exploited to specify different types of kinetic constants as well as several parameters characterizing the oxide spallation occurring during cooling. The obtained results show that, the higher the Ni content, the slower the mass gain and the better the quality of the protective external chromia scale. Secondly, no dependence of the oxide spallation characteristics on the Co content was clearly noted. Globally, the isothermal oxidation behavior becomes better when Ni is more and more present at the expense of Co. Titanium seems to be playing a particular role in the process of oxidation. It notably leads to the presence of an external thin TiO2 continuous scale beyond the chromia scale. The thermogravimetry records were numerically treated to determine the parabolic constant and the chromia volatilization constant. The values of these constants evidenced a double tendency: chromia growth acceleration and chromia volatilization slow-down. These trends are to be confirmed and further investigated.

This work investigates the possible influence that titanium may have on the oxidation of {nickel and/or cobalt}-based chromium-rich alloys. It starts with the elaboration by casting of a series of alloys from pure elements, with a base element ranging from nickel only to cobalt only. To magnify these possible effects of titanium and reach the atomic equivalence with the carbon present (0.4 wt pct C) in the alloys, 1.6 wt pct Ti was introduced in the chemical composition. To amplify the oxidation process, the oxidation tests carried out in laboratory air were run at the constant temperature of 1200 °C for a rather long time (170 hours). The surface states and cross-sections were characterized by XRD, electron microscopy and EDS analyses. The results demonstrate that all the alloys (except the nickel-free cobalt-based one) resisted oxidation rather well without catastrophic evolution due to titanium. The tested Ti content led to significant internal oxidation and to an external selective oxidation producing a thin layer stick on the outer side of the chromia scale. It is supposed that this outermost TiO 2 layer may be beneficial for the oxidation behavior by the possible limitation of the deleterious over-consumption of chromium by chromia re-oxidation/volatilization. This will be later verified by further investigations combining thermogravimetry follow-up of the oxidation rate and analysis of chromium balance sheets.

Six {Ni,Co}-based alloys containing 25wt-%Cr, 0.4wt-%C and 1.6wt-%Ti were cast and subjected to metallographic characterisation before and after exposure for 70 h in air at 1250°C, the highest temperature at which these alloys may be used under low applied stresses. The alloys based mainly on nickel contain principally chromium carbides while TiC is the principal carbide phase in the alloys mainly based on cobalt. The oxidation resistance is the best for the alloys richer in nickel than in cobalt, but titanium has the same effect for all alloys, whatever the base element: after oxidation it is present as an external TiO 2 layer covering chromia. The presence of this outermost more (Ni-based alloys) or less thick (Co-based alloys) TiO 2 scale on the outer face of the external chromia scale, is expected to have a protective role against chromia volatilisation, phenomenon which can be very important at so high temperature.