C. Steurbaut et al., KINETIC-STUDIES OF COKE FORMATION AND REMOVAL ON HP40 IN CYCLED ATMOSPHERES AT HIGH-TEMPERATURES, Werkstoffe und Korrosion, 49(5), 1998, pp. 352-359
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.