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 specimen and an increase of the Cr activity due to Pd may have contributed to the formation of the protective oxide layer.
