Oxidation behaviour at 1100°C in air of 25 wt-% Cr-containing cobalt-based alloys containing high quantities of hafnium carbides

Abstract

Hafnium, usually added to improve the high temperature oxidation resistance of alloys, allowed obtaining HfC carbides which are very efficient for the creepresistance. For that Hf must be added in particularly high quantities which may possibly influence the oxidation behavior. Three HfC-strengthened cast cobalt alloys were studied all along thermogravimetry tests at 1100°C. They were compared to similar but Hf-free ternary alloys. The mass variation were plotted according to {m × dm/dt = f(-m)} to specify all kinetic oxidation constants, and versus temperature to study the oxidation beginning during heating and the scale spallation during cooling. The presence of many HfC carbides obviously influences the high temperature oxidation: mass gain occurring sooner during heating, faster isothermal mass gains but better behavior in oxide scale spallation during cooling. This deterioration of oxidation behavior must be corrected to hope benefiting from the high creepresistance brought by this new type of strengthening.

Effect of NbC Addition on the High-Temperature Oxidation Resistance of Co- and Ni-Based Chromium-Rich Alloys

“…For exemple the kp value for Ni-25Cr-0.5C at 1100°C [9] is 5.5 ×10 -12 g².cm -4 .s -1 . For the Co-base alloy, there is a factor of 10 between the kp values obtained for CoNb and the corresponding Nb-free alloy [16,17]. The observations are the same than above with the total mass gain data.…”

“…In the cases where the oxide layer spalled off, the lost mass is practically the double of mass gain during the oxidation during 46h, therefore the spallation is too important in all cases. During the thermogravimetric tests carried out by Berthod et al of the Co-25Cr0.5C alloy at 1100°C [17] and 1200°C [16], a spallation of oxide layers was noted during the cooling but it is not the case in the present study with the Nb-containing alloy. The addition of Nb seems to improve the adhesion of oxide layer at 1100°C and 1200°C.…”

“…Contrarily to the Ni-based alloys, at lower temperatures, the mass gains are not parabolic for the Co-based alloys. Previous works indicated that the mass gains after 50h for the Co-based reference alloy are approximately 5×10 -3 g/cm² at 1100°C [17] and 1.1×10 -2 g/cm² at 1200°C [16] and at 1100°C the mass gain curve is not parabolic. Therefore, the total mass gain for the Nb-containing cobalt alloy is greater than for the Nb-free one.…”

“…This is not the case for the Nb-containing alloy where the formation of NbCrO 4 had obstructed the quick formation of chromia. For the Nb-free Co-based alloy oxidized at 1100°C [17] and 1200°C [16], the appearance of oxide layer is alike the Nb-containing alloy. An outer scale of CoO is present.…”

“…For the CoNb and CoNbR alloys, the oxide layers are thick especially at higher temperature, it is due at the formation of CoO and spinel oxide which are porous oxides with a lot of defaults (gaps). In regarding the SEM images of oxide layers of ternary alloy in literature, the thicknesses of the oxide scales are about 20 µm and about 320 µm for the oxidized sample at 1100°C [17] and 1200°C [16] respectively but the oxide layers have spalled for this ternary alloy. Despite the spallation, the oxide layer at 1200°C is thick because CoO and spinel oxide are also formed.…”