Thermogravimetric Study of Oxide Spallation for Chromium-Rich Cast Cobalt-Based and Iron-Based Alloys Oxidized at High Temperature

During temperature cycles, metallic alloys for high temperature applications are usually oxidized with formation of an external protective oxide scale, but they loose it during the cooling. This problem of oxide spallation can be studied by specific tests of cyclic oxidation but first indications can be provided by analyzing the cooling parts of thermogravimetry curves. This possibility was studied in this work for simple {Co or Fe} – 30 wt.% Cr alloys containing between 0 and 0.8 wt.% of carbon, and for two of these alloys after addition of about 5 wt.% of tantalum. These alloys, elaborated by foundry process, were exposed to oxidation at 1000, 1100 or 1200°C for 50 hours, with recording of their mass gain during heating, isothermal dwell and cooling. The analysis of the cooling part of the thermogravimetry files allowed specifying several data characterizing the spallation of the external oxide, such as the temperature at which the phenomenon begins and the rate with which the mass decreases because of the loss of oxide. Metallographic data about oxidation and spallation were obtained by examination of cross-sections of the oxidized samples. Oxide spallation, characterized by its beginning’s temperature and its corresponding mass loss rate, was obviously more severe for the cobalt alloys than for the iron ones, for the carbon-free alloys than for carbides-containing ones and for tantalum-containing alloys than for the tantalum-free ones. Most of these effects were explained.