Burn-off of filamentous carbon and subsequent re-deposition on a 20Cr25Ni austenitic steel

Filamentary carbon deposits can form on a 20Cr/25Ni (mass%) steel in a gas of high carbon activity even when the oxygen potential is sufficiently high to oxidise (of the main alloy constituents) both chromium and iron. These filaments are catalysed by metallic nickel which remains unoxidised but exists as small 10nm particles within the porous oxide layer formed in the early stages of oxidation. The objective of the present work was to examine the re-deposition process when the initial deposit has been burned off. As in the previous work, carbon deposits were produced by exposure of specimens at 550 degrees C to a gas mixture of CO2/1% CO/1000 vppmC(2)H(4) at 1-atmosphere total pressure. Specimen examination was performed in a FEGSEM in sufficient detail that the specimen area could be relocated in subsequent examinations. Bum-off exposures were performed at 450 degrees C in Ar/20% O-2. It was found that after 51 minutes bum-off exposure, considerable loss of carbon had occurred but that re-deposition occurred during subsequent exposure to the high-carbon environment. In contrast, a 2-hour burn-off exposure resulted in no further significant formation of carbon filaments during subsequent exposures. This effect of catalyst deactivation is related to the oxide formed during the bum-off stage.