Steam Oxidation of High-Chromium Ferritic Steels Containing Palladium

Abstract

Abstract The steam oxidation behavior of 9Cr ferritic steel containing Pd was studied in comparison with that of modified 9Cr-1Mo steel (P91). It was found that the oxidation of 9Cr steel containing 3 mass% Pd, the surface of which was finished with 320-grit silicon carbide (SiC) paper, was suppressed by the formation of a protective oxide layer when it was exposed to steam at 650°C. This experimental result is unusual in high-Cr ferritic steels, including mod.9Cr-1Mo. A similar suppression of oxidation was also observed in specimens of mod.9Cr-1Mo steel that were coated with Pd or Pt. When the steel containing Pd was exposed to steam at 650°C for 50 h, the thickness of the oxide layer became ∼0.3 μm. Analyses of the oxide layer by thin film x-ray diffraction, transmission electron microscopy (TEM), and electron diffraction suggest that the protective layer was made of (Fe, Cr)2O3 with a rhombohedral structure. A supply of Cr from the metal matrix because of the residual strain in the surface region of sp...