Influence of pre-oxidation on filamentary carbon deposition on 20Cr25Ni stainless steel

The results of a pre-oxidation heat treatment at 930°C in Ar/H2/H2O environments on a Si-bearing Nb-stabilised 20Cr25Ni austenitic steel are presented. The heat treatment was conducted under low pO2, achieved by the introduction of controlled amounts of moisture in to the gas. The atmosphere promoted the formation of a continuous, dense, adherent, protective surface scale composed of Cr2O3 and MnCr2O4 with a thin Si-rich oxide at the oxide alloy interface. Samples with different oxide layer thicknesses were produced and further exposed at 700°C, to a gas of nominal composition CO2/1%CO/1000 vpm C2H4 for 4 hours. This gas mixture has a carbon activity greater than unity and readily forms filamentary carbon. This is catalysed by nickel particles formed intrinsically from the alloy during the early stages of oxidation of an un-protected surface. The oxide layers produced, as a result of the pre-oxidation process, could suppress carbon deposition onto the alloy; a significant reduction of carbon deposit was noted with an oxide of 125 nm thickness and no deposit was found on the sample with an oxide thickness of 380 nm. The depth of
depletion of chromium from the alloy correlated to the thickness of the oxide formed during the preoxidation heat treatment. No additional chromium depletion of the alloy occurred during the 4-hour deposition stage.