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Effect of a high hafnium content on the high temperature oxidation of chromium-rich cast alloys. Part 6: Isothermal oxidation and scale spallation of Co-based alloys

Patrice Berthod, Elodie Conrath

The cooling parts of the mass gain curves of chromium-rich cobalt-based alloys, the oxidation behaviours of which were studied during heating up to 1000 or 1100°C in the first part of this work, were analysed in terms of average isothermal oxidation rate and resistance against oxide spallation. Itwas found that the presence of 4 to 8 wt.% of hafnium was slightly detrimental for the isothermal oxidation behaviour since the mass gain rates were more or less higher than for the hafnium-free ternary alloys added to the work for comparison. In contrast, the gainful effect on the resistance to oxide spallation at cooling was systematically observed. As for the analogous iron-based and nickel-based alloys earlier studied in the same conditions, hafnium demonstrated its beneficial effect in this field. For the isothermal oxidation a critical Hf content under which Hf is gainful and beyond which it becomes deleterious, is proposed.