In this paper, the results of a systematic study of the effect of temperature and COS concentration on the formation of carbon deposits on a 20Cr25Ni austenitic stainless steel under a depositing gas of 1000 vppm ethene in 1%CO/CO 2 have been presented. Carbon deposits were found over the temperature range of 550-725°C after 4 h exposure. COS additions to the ethene-bearing gas inhibited 123Oxid Met (2017) 87:667-678 DOI 10.1007 the deposition process, but the effect was concentration and temperature dependent. The results have been interpreted in terms of the oxidation process and the adsorption of sulphur on the catalytic Ni-rich particles.

The results of a pre-oxidation heat treatment at 930 °C in Ar/H 2 /H 2 O environments on a Si-bearing Nb-stabilised 20Cr25Ni austenitic steel are presented. The heat treatment was conducted under low pO 2 , achieved by the introduction of controlled amounts of moisture into the gas. The atmosphere promoted the formation of a continuous, dense, adherent, protective surface scale composed of Cr 2 O 3 and MnCr 2 O 4 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 CO 2 /1%CO/1000 vpm C 2 H 4 for 4 h. This gas mixture has a carbon activity greater than unity and readily forms filamentary carbon on the nonpre-oxidised alloy. This is catalysed by nickel particles formed intrinsically from the alloy during the early stages of oxidation of the unprotected surface. The oxide layers produced, as a result of the pre-oxidation process, could suppress carbon deposition onto the alloy; a significant reduction in 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 with the thickness of the oxide formed during the pre-oxidation heat treatment, but the chromium concentration at the oxide/metal interface remained at ~ 15-16 wt% and considered to be sufficient to reform a protective layer in the event of mechanical damage to the original. No additional chromium depletion of the alloy occurred during the 4-h deposition stage.

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Five batches of pyrolytic-carbon coated uranium carbide particles were exposed to partial pressures of water vapor ranging from 20 to 635 mm Hg at temperatures of 700 to ll00°C. All batches of coated particles were prepared by Minnesota Mining and Manufacturing Company, had laminar type of coating, and contained cores consisting principally of uranium dicarbide. The rate of reaction of the pyrolytic-carbon coatings with water vapor was determined,from weight losses. The extent of failure of the coatings during exposure was established by microscopic examination and leaching of the particles with 8M HN0 3 • Differences in the protectiveness of coatings present on the various batches of particles are attributable, at least in part, to two types of attack by water vapor, a localized or pitting type and a more general type. Rapid failure of the coatings is indicated at 1100°C. The coatings on some batches of particles remained protective at temperatures of 800 to l000°C. I.

Carbon deposition on 20Cr25Ni steel at 700C has been investigated in CO 2-based gases containing 0.7 vol%, 1.0 vol% and 1.4 vol% CO, each with ~1000vpm C 2 H 4 , at 700°C. Filamentary carbon deposits were formed at the two lowest CO contents but not at 1.4 vol% CO. TEM analyses have shown that the different CO levels change the propensity for deposition by altering the ratio of Ni:Fe in the nanometre sized catalytic particles produced during oxidation.