Corrosion by Carbon and Nitrogen: Metal Dusting, by Hans Jurgen Grabke, Michael Schutze

By Hans Jurgen Grabke, Michael Schutze

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7 wt%. The BF TEM image in Fig. 16a is a cross-section of the 13Cr steel taken from an area where a pit was present. Fractured M7C3 type carbides in carbon deposit were observed at the 13Cr steel surface. These carbides are stable and enriched in Cr with at least about 25 wt% Cr. The BF TEM image in Fig. 16b is taken from the outer surface of the carbon deposit which is in contact with the gas phase. Profuse amounts of filamentous carbon, whose growth has been catalyzed by metal particles, are observed.

4 Metal dusting corrosion of 20Cr steel Coupons of arc melted 20Cr steel were tested in the temperature range 900– 1100 ∞F (482–593 ∞C). Coupons subjected to 50CO : 50H2 gas mixtures at 900 ∞F (482 ∞C) for 48 h showed regions of localized corrosion and regions of a Cr-rich surface oxide film similar to observations on the 13Cr steel. At a higher temperature of 1100 ∞F (593 ∞C), however, only a small amount of carbon was found on the surface and no pits were observed. The secondary electron images obtained by the FIB system in Fig.

Similar results were obtained at the other temperatures. The effect of grinding on the oxide composition was clearly demonstrated by Auger sputter profiles, taken after 240 h exposure at 600 ∞C (Fig. 12). On the most resistant steels relatively thin scales had grown: 110 nm on the 12CrMoV steel and 45 nm on steel 304, composed of Mn and Cr oxide, nearly free of iron. This kind of scale is impermeable for carbon, the carbon signal decreases to zero from the surface into the oxide. For the etched specimens the oxide scale is thicker and is composed of Fe and Cr oxide in the case of steel 304, even Ni is in the oxide, which is most probably a (Fe,Cr,Ni)3O4 spinel.

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