Influence of Ti and Co/Ni Ratio on the Oxidation at 1200 °C of Chromium-Containing {Ni, Co}-Based Cast Alloys
Résumé
This work consists in an investigation of the possible influence that titanium may be have on the oxidation of {nickel and/or cobalt}-based chromium-rich alloys, and in the affirmative way the aspects of this behavior may be affected. 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, more than 1 wt.% Ti was introduced in the chemical composition, more precisely 1.6 wt.% in order to reach the atomic equivalence with the carbon present (0.4wt.% C). To amplify the oxidation process, the oxidation tests carried out in laboratory air were run at the constant temperature of 1200°C during a rather long time (170h). The surface states and cross-sections were characterized by XRD, electron microscopy and EDS analyses. The results demonstrates that all the alloy (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.
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