Behavior in oxidation at elevated temperature of {25Cr, 0.4C-6Ta}-containing Ni and Co-based cast alloys versus their proportion in nickel and cobalt

A series of six alloys from Ni-25Cr-0.4C-6Ta to Co-25Cr-0.4C-6Ta with a progressive addition of Co in substitution to Ni were elaborated. They were tested for 24h at 1237 degrees C in laboratory air. The corrosion products formed around the samples and the deterioration of the subsurface were analyzed to characterize the high temperature oxidation behavior of these alloys. Three of the studied alloys, the nickel-richest ones, well behaved until 24h at this temperature. Their external oxide scales were essentially composed of chromia. In contrast, locally, the three cobalt-richest alloys were starting to oxidize catastrophically. The significant worsening of the oxidation behavior from the 27wt% Co-containing alloy to the 41wt% Co-containing one suggests that a critical Co content exists between these two values.