Kinetic studies of coke formation and removal on HP40 in cycled atmospheres at high temperatures

An austenitic FeNiCr alloy, HP40Nb, has been preoxidized and subsequently exposed to an alternating carburizing/oxidizing/carburizing atmosphere. During the oxidation at 1000°C a thick Cr2O3 layer was formed which partly spalled off during cooling to room temperature, in this way chromium depleted areas resulted at the surface. The carburizing and reducing condition was established by a C2H6/C2H4/H2 mixture at a temperature of 850°C while the oxidation for decoking was conducted in air at 800°C. The exposure times were relatively short, respectively 90/30/180 minutes. During the first exposure of the preoxidized alloy to the carburizing atmosphere, coke formation took place, and underneath the coke layer the alloy was carburized, however, only locally. After the decoking in air at 800°C, during the second exposure to the carburizing atmosphere much more catalytic coke formation was observed compared to the first exposure. The coke formation was initiated by the reduction of (Fe,Ni,Cr)-spinels formed in the oxidizing atmosphere. The reduction of the oxides gives rise to the formation of (Fe,Ni)-particles which show strong catalytic activity towards coke formation.