Influence of Ti and Co/Ni Ratio on the Oxidation at 1200 °C of Chromium-Containing {Ni, Co}-Based Cast Alloys

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 TiO2 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.