Oxidation behavior of porous Ni- 4 and 8 wt %Cr alloys in air at 1273K

Abstract

Oxidation behavior of porous Ni-4 and 8 wt% Cr pellets prepared by conventional powder metallurgy was investigated. The pellets were oxidized in air at 1273K for 100 h. The oxidation kinetics were measured by using gravimetric method. The oxide layer morphology and, the surfaces topography and the cross-section microstructure of the oxidized pellets were investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and X-ray energy dispersive analysis. The oxidation rate was initially rapid and then gradually decreased until realizing a constant rate after oxidation time of about 40 h for the 4 wt %Cr pellets with linear rate constant of about 8x10-8 (g/cm2)/sec. For the 8 wt %Cr pellets, the linear rate constant was about 20x10-8 (g/cm2).sec which was achieved after 72 h. Porous nickel oxide scales were formed on the 4 and 8 wt %Cr substrates. Finally, the growth of the oxide scale was explained on the basis of inward migration of molecular oxygen through the porous oxide scale; accordingly, the newly oxide atomic layers were added at the scale/metal interface.