Ni-20Cr合金の高温酸化速度に及ぼす酸素圧の影響

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  • Effects of Oxygen Pressure on the Oxidation Kinetics of a Ni-20Cr Alloy at High Temperatures
  • Ni 20Cr ゴウキン ノ コウオン サンカ ソクド ニ オヨボス サンソア

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The oxidation of a Ni-20Cr alloy was studied at temperatures between 1273 and 1423K under various oxygen partial pressures by mass gain measurements, microstructure observations with optical and scanning electron microscopies, X-ray diffraction and electron probe microanalysis. Oxygen pressures were obtained using air and CO-CO2 mixtures with various ratios, ranging from 2.1×104Pa to the pressures below the dissociation pressure of NiO. Above the dissociation pressure of NiO, the oxidation rate was affected by oxygen pressure in the early stage. The amount of NiO initially formed on the alloy decreased with lowering oxygen pressure. In the later stage, the growth of the oxide scale obeyed the parabolic rate law, and optical micrographs showed the formation of the compact oxide layers. These two results suggested that the oxidation was controlled by the diffusion of Cr ions through the Cr2O3 scale. The parabolic rate constants were independent of oxygen pressure, which was explained by the n-type or intrinsic defect structure of Cr2O3.

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