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Abstract
Metal dusting is a type of high temperature corrosion in which Fe-, Ni- or Co-base alloys are disintegrated from bulk materials into metal or carbide particles in a coke deposit formed in strongly carburising gas atmospheres (carbon activity ac > 1) at elevated temperatures (400-800°C). Mechanism study reveals that dusting occurs only when graphite grows into the metal or carbide interior, disrupting its structure and converting it to the dust. Therefore, prevention of dusting requires a surface barrier to carbon entry. The aim of this project is to investigate the effect of nano-crystalline structure on carburization and metal dusting of nickel-chromium alloys.
Nano-crystalline Ni-Cr structure was prepared by either sputtering coating or electro-plating, which was then reacted in 47%CO-47%H2-6%H2O gas for 50 h or pre-oxidised first in 47%Ar-47%H2-6%H2O gas for 24 h and then in carburizing gas for 50 h at 650°C. The coarse grained Ni-Cr alloys were also reacted in the same reaction conditions for the comparison. In general, increasing Cr content reduced weight gain kinetics. For sputtering coated Ni-Cr alloys, without pre-oxidation, Ni-5Cr experienced a significant metal dusting. Higher Cr content, Ni-10Cr and 20Cr alloys, however, appeared little attack due to the formation of chromia scales on the surface and along the interfaces
among the columns. Both alloys formed other reaction products inside of columnar coating scales, with fine graphite deposits for Ni-10Cr and relatively large Cr7C3 precipitates for Ni-20Cr. For electroplated samples, the rates of weight gain performed a slow increase first, followed by an accelerated increase. Pre-oxidation before dusting significantly improved the resistance of alloy against carburization and metal dusting for all cases. This effect is especially significant for low Cr content Ni-5Cr and for nano-coated alloys as fine grain size accelerates chromium diffusion and therefore the protective oxide scale formation. The effect of chromium content on protective chromia
scale formation and chromium carbide formation is analysed using Wagner’s analysis and product solubility calculation. The prediction is basically in agreement with the experimental observation.