KINETIC-STUDIES OF COKE FORMATION AND REMOVAL ON HP40 IN CYCLED ATMOSPHERES AT HIGH-TEMPERATURES

An austenitic FeNiCr alloy, HP40Nb, has been preoxidized and subsequen tly exposed to an alternating carburizing/oxidizing/carburizing atmosp here. During the oxidation at 1000 degrees C a thick Cr2O3 layer was f ormed which partly spalled off during cooling to room temperature, in this way chromium depleted areas resulted at the surface. The carburiz ing and reducing condition was established by a C2H6/C2H4H2 mixture at a temperature of 850 degrees C while the oxidation for decoking was c onducted in air at 800 degrees C. The exposure times were relatively s hort, 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 degrees C, during the second exposure to the carburizing atmosphere much more catalytic cok e formation was observed compared to the first exposure. The coke form ation was initiated by the reduction of (Fe,Ni,Cr)-spinels formed in t he oxidizing atmosphere. The reduction of the oxides gives rise to the formation of (Fe,Ni)-particles which show strong catalytic activity t owards coke formation.