The influence of oxide layers on the initiation of carbon deposition on stainless steels

This paper examines the role played by oxide layers in the deposition of carbon on two 20Cr-25Ni-Nh-stabilized austenitic steels containing either zero or 0.56 wt% Si. Selective preoxidation, at 550 degreesC in Ar/H-2/H2O, was used to produce regions covered by either chromia or silica or, in the silicon-free alloy, to be bare metal. Deposition was performed, also at 550 degreesC, in a CO2/1% CO/1000 vppm C2H4 gas mixture having an estimated carbon activity very much greater than unity. This gas also had an oxygen potential sufficient to form magnetite, but not nickel oxide. It was found that, even at these high carbon activities, none of the oxides formed could catalyze carbon deposition and that this occurred only when the gas had direct access to the alloy substrate. The carbon filaments formed were found, by high-resolution electron microscopy, to be solid, have a turbostratic structure, and to contain at least one nickel particle at their tips. The source of these nickel particles is the alloy substrate and a mechanism is proposed for their formation.