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High Temperature Corrosion of Cast Alloys in Exhaust Environments.

High Temperature Corrosion of Cast Alloys in Exhaust Environments.

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in detail the oxidation of two cast stainless steels in synthetic diesel and gasoline exhaust gases. One alloy was ferritic (Fe18Cr1.4Nb2.1Mn0.32C) and one austenitic (Fe20Cr9Ni1.9Nb2.7W0.47C). Polished sections were exposed, mostly for 50 h, at temperatures between 650 and 1,050 °C. The oxidation product was characterized by means of SEM/EDX, AES, and XRD. Inter-dendritic non-Cr carbides initiated thick oxides. The ferritic steel formed a rather thin and adherent oxide scale at all temperatures. It consisted of (Mn, Cr) oxide on top of Cr 2 O 3 and, starting at 850 °C, a thin silica film at the metal–oxide interface. Chromium depletion triggered dissolution of carbides providing Cr to the oxide. Water vapor did not accelerate the attack since the outer (Mn, Cr)... spinel oxide reduced the Cr evaporation. The austenitic grade was very sensitive to water vapor. Chromium segregation directed pitting to the dendrites up to 950 °C whereas uniform catastrophic oxidation occurred at 1,050 °C.